阅读说明：本技术 磁带及磁记录回放装置 (Magnetic tape and magnetic recording/reproducing apparatus ) 是由 岩本崇裕 细田英正 笠田成人 黑川拓都 于 2018-09-11 设计创作，主要内容包括：本发明提供一种磁带以及包括该磁带的磁记录回放装置,所述磁带在非磁性支撑体上具有包含强磁性粉末及粘结剂的磁性层,其中,磁性层包含氧化物研磨剂,根据对磁性层的表面照射聚焦离子束来获取的二次离子像求出的上述氧化物研磨剂的平均粒径为0.04μm以上且0.08μm以下,且在磁性层的面内方向上测量的折射率Nxy与在磁性层的厚度方向上测量的折射率Nz的差分的绝对值ΔN为0.25以上且0.40以下。(The magnetic tape has a magnetic layer containing ferromagnetic powder and a binder on a non-magnetic support, wherein the magnetic layer contains an oxide abrasive, the average particle diameter of the oxide abrasive, which is determined from a secondary ion image obtained by irradiating a surface of the magnetic layer with a focused ion beam, is 0.04 [ mu ] m or more and 0.08 [ mu ] m or less, and the absolute value [ Delta ] N of the difference between the refractive index Nxy measured in the in-plane direction of the magnetic layer and the refractive index Nz measured in the thickness direction of the magnetic layer is 0.25 or more and 0.40 or less.)

1. A magnetic tape having a magnetic layer containing ferromagnetic powder and a binder on a nonmagnetic support, wherein,

the magnetic layer comprises an oxide abrasive,

a nonmagnetic layer containing a nonmagnetic powder and a binder is provided between the nonmagnetic support and the magnetic layer,

the thickness of the non-magnetic layer is 0.1 to 1.5 μm,

the oxide abrasive has an average particle diameter of 0.04 to 0.08 [ mu ] m, which is determined from a secondary ion image obtained by irradiating the surface of the magnetic layer with a focused ion beam

An absolute value Δ N of a difference between a refractive index Nxy measured in an in-plane direction of the magnetic layer and a refractive index Nz measured in a thickness direction of the magnetic layer is 0.25 or more and 0.40 or less.

2. The magnetic tape of claim 1, wherein the thickness of the nonmagnetic layer is 0.1 to 1.0 μm.

3. The magnetic tape of claim 1, wherein the thickness of the nonmagnetic layer is 0.1 to 0.7 μm.

4. The magnetic tape of claim 1,

the oxide abrasive is alumina powder.

5. The magnetic tape of claim 1 or 4,

the ferromagnetic powder is ferromagnetic hexagonal ferrite powder.

6. The magnetic tape of claim 1 or 4,

the difference Nxy-Nz between the refractive index Nxy and the refractive index Nz is 0.25 to 0.40.

7. The magnetic tape of claim 1 or 4,

the nonmagnetic support has a back coat layer containing a nonmagnetic powder and a binder on the surface side opposite to the surface side having the magnetic layer.

8. A magnetic recording playback device, comprising:

the magnetic tape of any one of claims 1 to 7; and

a magnetic head is provided.

Technical Field

The present invention relates to a magnetic tape and a magnetic recording/reproducing apparatus.

Background

Magnetic recording media include tape-shaped and disk-shaped ones, and magnetic tapes, which are tape-shaped magnetic recording media, are mainly used for data storage. In general, recording and/or playback of information on a magnetic tape is performed by bringing the surface of the magnetic tape (magnetic layer surface) into contact with and sliding over a magnetic head. As the magnetic tape, a magnetic tape having a structure in which a magnetic layer containing a polishing agent in addition to ferromagnetic powder and a binder is provided on a nonmagnetic support is widely used (for example, refer to patent document 1).

Prior art documents

Patent document

Patent document 1: japanese patent laid-open No. 2005-243162

Disclosure of Invention

Technical problem to be solved by the invention

One of the performances required for a magnetic tape is that the magnetic tape can exhibit excellent electromagnetic conversion characteristics when information recorded on the magnetic tape is reproduced. However, if the magnetic layer surface and/or the magnetic head is scraped when the sliding between the magnetic layer surface and the magnetic head is repeated, a phenomenon (so-called pitch loss) may occur in which the distance between the magnetic layer surface and the playback element of the magnetic head is increased. In this regard, as described in patent document 1, for example, the inclusion of an abrasive in the magnetic layer contributes to the magnetic head cleaning property of the magnetic layer surface by the abrasive. By providing the magnetic head cleaning property to the surface of the magnetic layer, it is possible to suppress the occurrence of a gap loss due to foreign matter generated by scraping the surface of the magnetic layer being interposed between the surface of the magnetic layer and the magnetic head. On the other hand, however, the more the head cleaning property of the magnetic layer surface is improved, the more easily the head is scraped by sliding with the magnetic layer surface, and therefore, the pitch loss still occurs. Such a pitch loss causes a phenomenon in which the electromagnetic conversion characteristics deteriorate when information recorded on the magnetic tape is repeatedly played back (hereinafter, also referred to as "reduction in electromagnetic conversion characteristics at the time of repeated playback").

However, in recent years, magnetic tapes used for data storage purposes are sometimes used in low-temperature and low-humidity environments (for example, in an environment having a temperature of 10 to 15 ℃ and a relative humidity of 10 to 20%) such as data centers in which temperature and humidity are controlled. However, from the viewpoint of reducing the air conditioning cost for managing the temperature and humidity, it is desirable to be able to relax the conditions for managing the temperature and humidity during use or to eliminate the need for management.

In view of the above, the present inventors have studied to relax or eliminate the need to manage the temperature and humidity conditions when using a magnetic tape. As a result, it was found that the electromagnetic conversion characteristics are likely to be degraded when repeatedly playing back under an environment in which the temperature and humidity control conditions are relaxed or in which the temperature and humidity control conditions are not required (hereinafter, referred to as "high-temperature and high-humidity environment"). In addition, the high-temperature and high-humidity environment is, for example, an environment in which the atmospheric temperature is 30 to 45 ℃ and the relative humidity is 65% or more (e.g., 65 to 90%).

Accordingly, an object of the present invention is to provide a magnetic tape in which a decrease in electromagnetic conversion characteristics at the time of repeated playback in a high-temperature and high-humidity environment is suppressed.

Means for solving the technical problem

One aspect of the present invention relates to a magnetic tape having a magnetic layer containing ferromagnetic powder and a binder on a nonmagnetic support, wherein,

the above-mentioned magnetic layer contains an oxide abrasive,

the average particle diameter of the oxide abrasive (hereinafter also referred to as "FIB abrasive diameter") determined from a secondary ion image obtained by irradiating the surface of the magnetic layer with a Focused Ion Beam (FIB) is 0.04 to 0.08 μm,

and an absolute value Δ N of a difference between a refractive index Nxy measured in an in-plane direction of the magnetic layer and a refractive index Nz measured in a thickness direction of the magnetic layer (hereinafter, also referred to as "Δ N of the magnetic layer") is 0.25 or more and 0.40 or less.

In one aspect, the oxide abrasive may be alumina powder.

In one aspect, a difference (Nxy-Nz) between the refractive index Nxy and the refractive index Nz may be 0.25 or more and 0.40 or less.

In one aspect, the ferromagnetic powder may be a ferromagnetic hexagonal ferrite powder.

In one aspect, the magnetic tape may include a nonmagnetic layer containing nonmagnetic powder and a binder between the nonmagnetic support and the magnetic layer.

In one aspect, the magnetic tape may have a back coat layer containing a nonmagnetic powder and a binder on a surface side of the nonmagnetic support opposite to a surface side having the magnetic layer.

Another embodiment of the present invention relates to a magnetic recording/reproducing apparatus including the magnetic tape and the magnetic head.

Effects of the invention

According to an aspect of the present invention, it is possible to provide a magnetic tape capable of suppressing a decrease in electromagnetic conversion characteristics when repeatedly played back in a high-temperature and high-humidity environment. Further, according to an aspect of the present invention, a magnetic recording/reproducing apparatus including the magnetic tape can be provided.

Detailed Description

[ magnetic tape ]

One aspect of the present invention relates to a magnetic tape having a magnetic layer including ferromagnetic powder and a binder on a nonmagnetic support, wherein the magnetic layer includes an oxide abrasive, an average particle diameter (FIB abrasive diameter) of the oxide abrasive, which is obtained from a secondary ion image obtained by irradiating a surface of the magnetic layer with a focused ion beam, is 0.04 μm or more and 0.08 μm or less, and an absolute value Δ N of a difference between a refractive index Nxy measured in an in-plane direction of the magnetic layer and a refractive index Nz measured in a thickness direction of the magnetic layer is 0.25 or more and 0.40 or less.

In the present invention and the present specification, the term "surface (of) a magnetic layer" is the same as the meaning of the surface on the magnetic layer side of a magnetic tape. In the present invention and the present specification, the term "ferromagnetic powder" refers to an aggregate of a plurality of ferromagnetic particles. The term "aggregate" is not limited to a form in which particles constituting an aggregate are in direct contact with each other, and includes a form in which a binder, an additive, or the like is interposed between particles. The above points are also the same for various powders such as nonmagnetic powder in the present invention and the present specification.

In the present invention and the present specification, "oxide abrasive" means a non-magnetic oxide powder having a mohs hardness of more than 8.

In the present invention and the present specification, the FIB polishing slurry diameter is a value obtained by the following method.

(1) Acquisition of secondary ion images

A secondary ion image of a 25 μm square (25 μm. times.25 μm) region on the surface of the magnetic layer of the target magnetic tape, in which the FIB polishing slurry diameter is determined, is obtained by a focused ion beam apparatus. As the focused ion beam device, MI4050 manufactured by HitachiHigh-technologies corporation.

As the beam irradiation conditions of the focused ion beam apparatus when obtaining the secondary ion image, the following are set: acceleration voltage 30kV, current value 133pA (Peak Amp), beam size 30nm, and brightness 50%. The surface of the magnetic layer was not subjected to coating treatment before imaging. Secondary Ion (SI) signals are detected by a secondary ion detector and a secondary ion image is taken. The conditions for capturing the secondary ion image are determined by the following method. The color tone of the image is stabilized by applying ACB (automatic contrast brightness ss) to three portions of the non-image-pickup area on the surface of the magnetic layer (i.e., ACB is applied 3 times), and the contrast reference value and the brightness reference value are determined. The contrast value that is 1% lower than the reference contrast value determined by the ACB and the reference brightness value are used as the shooting conditions. The non-imaging region of the surface of the magnetic layer is selected, and a secondary ion image is imaged under the imaging conditions determined in the above. A secondary ion image of the number of pixels of 2000 pixels × 2000 pixels is acquired by eliminating a portion (a micrometer bar, a cross mark, or the like) indicating the size or the like from the captured image. As a specific example of the imaging conditions, the following examples can be referred to.

(2) Calculation of FIB abrasive diameter

Reading the secondary ion image obtained in the above (1) into image processing software, and performing binarization processing according to the following steps. As the image analysis software, for example, the free software ImageJ can be used.

The color modulation of the secondary ion image obtained in the above (1) is further changed to 8 bits. As for the threshold values used for the binarization processing, the lower limit value is set to 250 gradations and the upper limit value is set to 255 gradations, and the binarization processing is performed by these two threshold values. After the binarization processing, noise component removal processing is performed by image analysis software. The noise component removal processing can be performed by the following method, for example. In the image analysis software ImageJ, noise cut processing noise reduction (Despeckle) was selected, and Size (Size) was set to 4.0-Infinity in the analysis particles (AnalyzeParticle) to remove noise components.

In the binarized image thus obtained, each of the whitish portions was judged as an oxide abrasive, the number of the whitish portions was determined by image analysis software, and the area of each of the whitish portions was determined. From the areas of the whitish portions obtained here, the equivalent circle diameters of the portions are obtained. Specifically, the equivalent circle diameter L is calculated from the obtained area a by (a/pi) ^ (1/2) × 2 ═ L.

The FIB polishing agent diameter is calculated from the FIB polishing agent diameter Σ (Li)/Σ i by performing the process 4 or more times at different portions (25 μm square) on the magnetic layer surface of the target magnetic tape for which the FIB polishing agent diameter is determined. Σ i is the total number of whitish portions observed in the binarized image obtained by performing 4 times. Σ (Li) is the total of the equivalent circle diameters L obtained for each whitish portion observed in the binarized image obtained by performing the binarization 4 times. With regard to the whitish portion, there may be a case where only a part of the portion is included in the binarized image. In this case, Σ i and Σ (Li) are obtained without including this portion.

In the present invention and the present specification, the absolute value Δ N of the difference between the refractive index Nxy measured in the in-plane direction of the magnetic layer and the refractive index Nz measured in the thickness direction of the magnetic layer is a value obtained by the following method.

The refractive index in each direction of the magnetic layer is obtained by spectroscopic ellipsometry using a two-layer model. In order to obtain the refractive index of the magnetic layer by spectroscopic ellipsometry using a two-layer model, the value of the refractive index of a portion adjacent to the magnetic layer was used. Hereinafter, a magnetic tape having a layer structure in which a nonmagnetic layer and a magnetic layer are sequentially laminated on a nonmagnetic support will be described by taking the case of obtaining the refractive indices Nxy and Nz of the magnetic layer as an example. However, the magnetic tape according to one aspect of the present invention may have a layer structure in which a magnetic layer is directly laminated on a nonmagnetic support without interposing a nonmagnetic layer therebetween. In the magnetic tape having this structure, the refractive index in each direction of the magnetic layer is obtained in the same manner as in the following method using a two-layer model of the magnetic layer and the nonmagnetic support. The incident angle described below is an incident angle when the incident angle at the time of normal incidence is 0 °.

(1) Preparation of measurement sample

With respect to a magnetic tape having a back coat layer on a surface of a non-magnetic support on the side opposite to the surface having a magnetic layer, measurement was performed after removing the back coat layer of a measurement sample cut out from the magnetic tape. The removal of the back coat layer can be performed by a known method such as dissolving the back coat layer using a solvent. As the solvent, for example, methyl ethyl ketone can be used. However, any solvent may be used as long as it can remove the back coat layer. The surface of the non-magnetic support after removal of the back coating is surface roughened by a known method so that the reflected light on the surface is not detected in the measurement with an ellipsometer. The surface roughening can be performed, for example, by a method of polishing the surface of the non-magnetic support after removing the back coat layer with sandpaper. With respect to a measurement sample cut out from a magnetic tape not having a back coat layer, surface roughening was performed on the surface of a non-magnetic support on the side opposite to the surface having a magnetic layer.

In order to measure the refractive index of the nonmagnetic layer described below, the magnetic layer was further removed to expose the surface of the nonmagnetic layer. In order to measure the refractive index of the nonmagnetic support described below, the nonmagnetic layer was further removed to expose the surface of the nonmagnetic support on the magnetic layer side. The removal of each layer can be performed by a known method as described for the removal of the back coat layer. The longitudinal direction described below is a direction in the longitudinal direction of the magnetic tape when the magnetic tape is included before the measurement sample is cut out. This also applies to other directions described below.

(2) Refractive index measurement of magnetic layers

Using an ellipsometer, incident angles were set to 65 °, 70 °, and 75 °, and incident light having a beam diameter of 300 μm was irradiated onto the surface of the magnetic layer from the long side direction, thereby measuring Δ (phase difference between s-polarized light and p-polarized light) and Ψ (amplitude ratio between s-polarized light and p-polarized light). The wavelength of incident light is changed in a scale of 1.5nm within a range of 400 to 700nm to perform measurement, and a measurement value is obtained for each wavelength.

The refractive index of the nonmagnetic layer and the thickness of the magnetic layer in each direction were obtained by the following method using the measured values of Δ and ψ of the magnetic layer at each wavelength, and the refractive index of the magnetic layer at each wavelength was obtained by a two-layer model as follows.

The 0 th layer of the substrate as a two-layer model was set as a nonmagnetic layer, and the 1 st layer was set as a magnetic layer. Two-layer models were made considering only the reflection at the interface of air/magnetic layer and magnetic/non-magnetic layer and considering the effect of back reflection without non-magnetic layer. The refractive index of the 1 st layer that best matches the obtained measurement value is found by using least squares fitting. As a slaveThe refractive index Nx of the magnetic layer in the longitudinal direction and the refractive index Nz in the thickness direction of the magnetic layer measured by making incident light enter from the longitudinal direction are obtained from the value at a wavelength of 600nm obtained as a result of the fitting₁。

Except that the direction in which incident light was made incident was set to the width direction of the magnetic tape, the refractive index Ny of the magnetic layer in the width direction and the refractive index Nz of the magnetic layer in the thickness direction measured by making incident light from the width direction were determined as values at a wavelength of 600nm obtained from the fitting result in the same manner as described above₂。

The fitting was performed by the following method.

The "complex refractive index n ═ η + i κ" is typical. Where η is the real part of the refractive index, κ is the extinction coefficient, and i is the imaginary number. At complex dielectric constants ε 1 ═ ε 1+ i ε 2(ε 1 and ε 2 satisfy the relationship of Clalmor-Kroni) and ε 1 ═ η²-κ²Calculating Nx and Nz in relation to 2 eta κ ∈ 2₁When the complex dielectric constant of Nx is set as ε_x＝ε_x1+iε_x2, mixing Nz₁Has a complex dielectric constant of ∈_z1＝ε_z11+iε_z12。

Will epsilon_x2 is a Gaussian, and Nx is determined by setting an arbitrary point where the peak position is 5.8 to 5.1eV and σ is 4 to 3.5eV as a starting point, setting a parameter for shifting the dielectric constant outside the measurement wavelength region (400 to 700nm), and performing least square fitting on the measurement value. Likewise,. epsilon_z12 obtaining Nz by setting an offset parameter with an arbitrary point having a peak position of 3.2 to 2.9eV and a sigma of 1.5 to 1.2eV as a starting point and performing least square fitting on the measured value₁. Ny and Nz₂The same applies to the above. The refractive index Nxy measured in the in-plane direction of the magnetic layer is determined as "Nxy ═ (Nx + Ny)/2". The refractive index Nz measured in the thickness direction of the magnetic layer is taken as "Nz ═ (Nz)₁+Nz₂) And/2' was obtained. From the obtained Nxy and Nz, an absolute value Δ N of a difference therebetween is obtained.

(3) Refractive index measurement of non-magnetic layer

The refractive index of the nonmagnetic layer at a wavelength of 600nm (refractive index in the longitudinal direction, refractive index in the width direction, refractive index in the thickness direction measured by making incident light enter from the longitudinal direction, and refractive index in the thickness direction measured by making incident light enter from the width direction) was determined in the same manner as in the above-described method except for the following points.

The wavelength of incident light is changed in a scale of 1.5nm within a range of 250-700 nm.

A two-layer model of a nonmagnetic layer and a nonmagnetic support was used, and the 0 th layer as the substrate of the two-layer model was set as the nonmagnetic support, and the 1 st layer was set as the nonmagnetic layer. A two-layer model was made considering only the reflection at the interface of the air/nonmagnetic layer and the nonmagnetic layer/nonmagnetic support and considering the effect of back reflection without the nonmagnetic support.

In the fitting, a peak (0.6eV, 2.3eV, 2.9eV, 3.6eV, 4.6eV, 5.0eV, 6.0eV) at 7 is assumed as the imaginary part (. epsilon.2) of the complex dielectric constant, and a parameter for shifting the dielectric constant is set outside the measurement wavelength region (250 to 700 nm).

(4) Refractive index measurement of non-magnetic supports

The refractive index of the nonmagnetic support for obtaining the refractive index of the nonmagnetic layer by the two-layer model at a wavelength of 600nm (the refractive index in the longitudinal direction, the refractive index in the width direction, the refractive index in the thickness direction measured by making incident light enter from the longitudinal direction, and the refractive index in the thickness direction measured by making incident light enter from the width direction) was obtained in the same manner as the above-described method for measuring the refractive index of the magnetic layer except for the following points.

Instead of using a two-layer model, a single-layer model with only surface reflection is used.

By the Cauchy model (n ═ An + Bn/lambda)²N is a refractive index, An and Bn are constants respectively determined by fitting, λ is a wavelength).

The present inventors presume the reason why the magnetic tape described above can suppress the reduction in electromagnetic conversion characteristics when repeatedly played back under a high-temperature and high-humidity environment as follows.

The FIB abrasive diameter is a value that can be used as an index of the state of presence of an oxide abrasive in the magnetic layer, and can be obtained from a secondary ion image obtained by irradiating the surface of the magnetic layer with a Focused Ion Beam (FIB). The secondary ion image is generated by capturing secondary ions generated from the surface of the magnetic layer irradiated with FIB. On the other hand, as a method for observing the existence state of the polishing agent in the magnetic layer, conventionally, a method using a Scanning Electron Microscope (SEM) has been proposed, as described in, for example, paragraph 0109 of japanese patent application laid-open No. 2005-243162 (patent document 1). In the SEM, an electron beam is irradiated on the surface of the magnetic layer, and secondary electrons released from the surface of the magnetic layer are captured to generate an image (SEM image). Due to the difference in the image formation principle, the size of the oxide abrasive obtained from the secondary ion image differs from the size of the oxide abrasive obtained from the SEM image even when the same magnetic layer is observed. As a result of intensive studies, the inventors of the present invention completed the following: the FIB abrasive grain size obtained from the secondary ion image by the method described above is used as a new index of the presence state of the oxide abrasive in the magnetic layer, and the presence state of the oxide abrasive in the magnetic layer is controlled so that the FIB abrasive grain size is 0.04 μm or more and 0.08 μm or less. The present inventors believe that such control of the presence state of the oxide abrasive in the magnetic layer contributes to the ability to suppress a decrease in electromagnetic conversion characteristics upon repeated playback in a high-temperature and high-humidity environment. Specifically, the inventors of the present invention speculate that the FIB abrasive has a diameter of 0.08 μm or less and contributes to suppression of the scraping of the magnetic head, and that the FIB abrasive has a diameter of 0.04 μm or more and contributes to suppression of the scraping of the magnetic head in a high-temperature and high-humidity environment and to imparting head cleanability to the surface of the magnetic layer.

It is considered that the magnetic layer in which the oxide abrasive has a diameter of 0.04 μm or more and 0.08 μm or less has a lower head-cleaning property than the magnetic layer in which the FIB abrasive has a diameter exceeding the above range. Therefore, it is presumed that, without taking some measures, the foreign matter adhering to the magnetic head is not sufficiently removed to cause a pitch loss, and even if the magnetic head can be suppressed from being scraped, the electromagnetic conversion characteristics are degraded. In this regard, the present inventors considered that Δ N obtained by the above-described method is a value that can be an index of the existence state of the ferromagnetic powder in the surface layer region of the magnetic layer. It is assumed that this Δ N is a value influenced by various factors such as the presence state of the binder and the density distribution of the ferromagnetic powder in addition to the orientation state of the ferromagnetic powder in the magnetic layer, and it is considered that the strength of the magnetic layer surface of the magnetic layer in which Δ N is set to 0.25 or more and 0.40 or less by controlling the various factors is high and it is difficult to scrape the magnetic layer by sliding with the magnetic head. The present inventors speculate that this contributes to suppression of scratching of the surface of the magnetic layer in which the FIB polishing agent diameter is in the above-described range, and as a result, suppression of a decrease in electromagnetic conversion characteristics at the time of repeated playback in a high-temperature and high-humidity environment.

However, the above is the presumption of the present inventors and the like, and the present invention is not limited at all.

The magnetic tape will be described in further detail below. In the following, the reduction of the electromagnetic conversion characteristics when repeatedly played back under a high-temperature and high-humidity environment is also simply referred to as "reduction of the electromagnetic conversion characteristics".

< FIB abrasive diameter >

The FIB abrasive has a diameter of 0.04 to 0.08 [ mu ] m, which is determined from a secondary ion image obtained by irradiating the surface of the magnetic layer of the magnetic tape with FIB. It is considered that the FIB abrasive having a diameter of 0.08 μm or less contributes to suppression of the magnetic head from being scraped at the time of repeated playback in a high-temperature and high-humidity environment. Further, it is presumed that when the FIB polishing slurry has a diameter of 0.04 μm or more, the surface of the magnetic layer exhibits head-cleaning properties, and thus contributes to removal of foreign matter generated by scraping of the surface of the magnetic layer during repeated playback in a high-temperature and high-humidity environment. From the viewpoint of further suppressing the decrease in electromagnetic conversion characteristics, the FIB milling agent diameter is preferably 0.05 μm or more, and more preferably 0.06 μm or more. From the same viewpoint, the FIB milling agent preferably has a diameter of 0.07 μm or less. The specific embodiment of the method for adjusting the diameter of the FIB polishing slurry will be described later.

< Δ N of magnetic layer >

The magnetic layer of the magnetic tape has a Δ N of 0.25 to 0.40. It is considered that under a high-temperature and high-humidity environment, the friction coefficient is likely to increase when the magnetic layer surface slides against the magnetic head, and scraping of the magnetic layer surface is likely to occur. This is considered to be a cause of the electromagnetic conversion characteristics being liable to be lowered upon repeated playback under a high-temperature and high-humidity environment. On the other hand, as described above, it is estimated that the strength of the magnetic layer surface of the magnetic layer having Δ N of 0.25 or more and 0.40 or less is high and it is difficult to scrape the magnetic head by sliding. Therefore, regarding the magnetic layer having Δ N within the above range, it is considered that scratching of the surface of the magnetic layer is hard to occur even if information recorded on the magnetic layer is repeatedly reproduced under a high-temperature and high-humidity environment, and it is presumed that this contributes to suppressing a decrease in electromagnetic conversion characteristics. From the viewpoint of further suppressing the reduction in electromagnetic conversion characteristics, Δ N is preferably 0.25 or more and 0.35 or less. Specific modes of the method for adjusting Δ N are described later.

Δ N is the absolute value of the difference between Nxy and Nz. Nxy is a refractive index measured in an in-plane direction of the magnetic layer, and Nz is a refractive index measured in a thickness direction of the magnetic layer. In one mode, Nxy > Nz, and in another mode Nxy < Nz. From the viewpoint of electromagnetic conversion characteristics of the magnetic tape, it is preferable that Nxy > Nz, and therefore, a difference between Nxy and Nz (Nxy-Nz) is preferably 0.25 or more and 0.40 or less.

The magnetic tape will be described in further detail below.

< magnetic layer >

(ferromagnetic powder)

As the ferromagnetic powder contained in the magnetic layer, ferromagnetic powder generally used in the magnetic layer of various magnetic recording media can be used. From the viewpoint of improving the recording density of the magnetic recording medium, it is preferable to use a ferromagnetic powder having a small average particle size as the ferromagnetic powder. From this point of view, it is preferable to use a ferromagnetic powder having an average particle size of 50nm or less as the ferromagnetic powder. On the other hand, the average particle size of the ferromagnetic powder is preferably 10nm or more from the viewpoint of stability of magnetization.

As a preferred specific example of the ferromagnetic powder, a ferromagnetic hexagonal ferrite powder can be cited. The average particle size of the ferromagnetic hexagonal ferrite powder is preferably 10nm or more and 50nm or less, and more preferably 20nm or more and 50nm or less, from the viewpoint of improving the recording density and the stability of magnetization. For details of the ferromagnetic hexagonal ferrite powder, for example, refer to paragraphs 0012 to 0030 of Japanese patent application laid-open No. 2011-225417, paragraphs 0134 to 0136 of Japanese patent application laid-open No. 2011-216149, and paragraphs 0013 to 0030 of Japanese patent application laid-open No. 2012-204726.

As a preferred specific example of the ferromagnetic powder, a ferromagnetic metal powder can be cited. From the viewpoint of improving the recording density and the stability of magnetization, the average particle size of the ferromagnetic metal powder is preferably 10nm or more and 50nm or less, and more preferably 20nm or more and 50nm or less. For details of the ferromagnetic metal powder, for example, reference can be made to paragraphs 0137 to 0141 of Japanese patent application laid-open No. 2011-216149 and paragraphs 0009 to 0023 of Japanese patent application laid-open No. 2005-251351.

In the present invention and the present specification, unless otherwise specified, the average particle size of each powder is a value measured by the following method using a transmission electron microscope.

The powder was photographed with a transmission electron microscope at a magnification of 100000 times, and printed on a printing paper so that the total magnification became 500000 times, to obtain a photograph of particles constituting the powder. The target particle is selected from the obtained photograph of the particle, and the particle profile is followed by a digitizer to measure the size of the particle (primary particle). Primary particles refer to individual particles that are not agglomerated.

The above measurements were performed on 500 particles drawn at random. The arithmetic mean of the particle sizes of the 500 particles thus obtained was taken as the average particle size of the powder. As the transmission electron microscope, for example, a transmission electron microscope H-9000 type manufactured by Hitachi, ltd. The particle size can be measured using known image analysis software, for example, image analysis software KS-400 manufactured by CarlZeiss. Unless otherwise stated, the values relating to the powder size such as the average particle size shown in the examples described below were measured using a transmission electron microscope H-9000 model manufactured by Hitachi, ltd. as a transmission electron microscope and using image analysis software KS-400 manufactured by CarlZeiss as image analysis software.

As a method for collecting the sample powder from the magnetic recording medium for the purpose of measuring the particle size, for example, the method described in paragraph 0015 of japanese patent application laid-open No. 2011-048878 can be employed.

In the present invention and the present specification, unless otherwise specified, regarding the size (particle size) of the particles constituting the powder, the shape of the particles observed in the above-mentioned particle photograph is:

(1) needle-like, spindle-like, columnar (wherein the height is smaller than the maximum major axis of the bottom surface), etc., the major axis length, which is the length of the major axis constituting the particle,

(2) a plate-like or columnar shape (wherein the thickness or height is smaller than the maximum major axis of the plate surface or bottom surface), expressed by the maximum major axis of the plate surface or bottom surface,

(3) the equivalent circle diameter is expressed when the major axis of the constituent particle cannot be determined according to the shape, such as a spherical shape, a polyhedral shape, or an unspecified shape. The equivalent circle diameter is a diameter obtained by a circle projection method.

The average acicular ratio of the powder is an arithmetic average of values obtained by measuring the length of the short axis of the particles, that is, the length of the short axis in the measurement, obtaining the value (length of the long axis/length of the short axis) of each particle, and obtaining the above 500 particles. Here, unless otherwise specified, the short axis length refers to the length of the short axis constituting the particle when the above definition of the particle size is (1), and similarly, the thickness or height when the above definition of the particle size is (2), and (long axis length/short axis length) is regarded as 1 for convenience because the long axis and the short axis are not distinguished from each other when the above definition of the particle size is (3).

Unless otherwise stated, when the shape of the particle is determined, for example, in the case of the above definition (1) of the particle size, the average particle size is the average major axis length, and in the case of the definition (2) of the particle size, the average particle size is the average plate diameter. The average aspect ratio is the arithmetic mean (maximum major axis/thickness or height). In the case of the definition (3) of the particle size, the average particle size is an average diameter (also referred to as an average particle diameter).

In one aspect, the ferromagnetic particles constituting the ferromagnetic powder contained in the magnetic layer may have a plate shape. In the following, ferromagnetic powder composed of plate-like ferromagnetic particles is referred to as plate-like ferromagnetic powder. The average plate ratio of the plate-like ferromagnetic powder can be preferably in the range of 2.5 to 5.0. As the average plate ratio is larger, uniformity of the orientation state of ferromagnetic particles constituting the plate-like ferromagnetic powder tends to be more easily improved by the orientation treatment, and the value of Δ N tends to be larger.

The activation volume can also be used as an index of the particle size of the ferromagnetic powder. The "activation volume" is the unit of magnetization reversal. The activation volume described in the present invention and the present specification is a value obtained from the following relational expression between Hc and the activation volume V, which is measured using a vibration sample type fluxmeter at a magnetic field scanning speed of a coercive force Hc measuring unit for 3 minutes and 30 minutes in an atmosphere temperature of 23 ℃ ± 1 ℃.

Hc＝2Ku/Ms{1-[(kT/KuV)ln(At/0.693)]^1/2}

[ in the above formula, Ku: anisotropy constant, Ms: saturation magnetization, k: boltzmann constant, T: absolute temperature, V: activation volume, a: spin precession frequency, t: magnetic field reversal time

The active volume of the ferromagnetic powder is preferably 2500nm from the viewpoint of improving the recording density³More preferably 2300nm³Hereinafter, more preferably 2000nm³The following. On the other hand, from the viewpoint of stability of magnetization, the activation volume of the ferromagnetic powder is preferably 800nm, for example³Above, more preferably 1000nm³Above, more preferably 1200nm³The above.

The content (filling ratio) of the ferromagnetic powder in the magnetic layer is preferably in the range of 50 to 90 mass%, and more preferably in the range of 60 to 90 mass%. The components of the magnetic layer other than the ferromagnetic powder are at least a binder and an oxide abrasive, and may optionally contain one or more other additives. From the viewpoint of improving the recording density, the magnetic layer preferably has a high filling ratio of the ferromagnetic powder.

(Binder and curing agent)

The magnetic tape is a coating type magnetic tape, and the magnetic layer contains a binder. The binder is more than one resin. The resin may be a homopolymer or a copolymer (copolymer). As the binder contained in the magnetic layer, a resin selected from the following can be used alone or a plurality of resins selected from the following can be used in a mixture: polyurethane resins, polyester resins, polyamide resins, vinyl chloride resins, acrylic resins obtained by copolymerizing styrene, acrylonitrile, methyl methacrylate, and the like, cellulose resins such as nitrocellulose, epoxy resins, phenoxy resins, polyvinyl acetal, polyvinyl alkyl (polyvinyl butyral) resins such as polyvinyl butyral, and the like. Among them, preferred are polyurethane resins, acrylic resins, cellulose resins, and vinyl chloride resins. These resins can also be used as binders in the nonmagnetic layer and/or the back coat layer described later. For the above binder, reference can be made to paragraphs 0029 to 0031 of jp 2010-024113 a. The binder may be a radiation-curable resin such as an electron beam-curable resin. For the radiation curable resin, reference can be made to paragraphs 0044 to 0045 of japanese patent application laid-open publication No. 2011-048878.

As for the average molecular weight of the resin used as the binder, the weight average molecular weight may be, for example, 10,000 or more and 200,000 or less. The weight average molecular weight in the present invention and the present specification means a value obtained by converting a value measured by Gel Permeation Chromatography (GPC) into polystyrene. The following conditions can be mentioned as the measurement conditions. The weight average molecular weight shown in examples described later is a value obtained by converting a value measured under the following measurement conditions into polystyrene.

GPC apparatus: HLC-8120 (manufactured by TOSO CORPORATION)

Pipe column: TSKgelMultiporeHXL-M (manufactured by TOSOHCORPORATION, 7.8mmID (inner diameter (InnerDeiameter). times.30.0 cm)

Eluent: tetrahydrofuran (THF)

In one aspect, as the binder, can be usedWith a binder comprising acidic groups. The acidic groups of the invention and the description herein release H as contained in water or in aqueous solvents (aqueous solvents)⁺And a group capable of dissociating into anions and a form of a salt thereof. Specific examples of the acidic group include a sulfonic acid group, a sulfuric acid group, a carboxyl group, a phosphoric acid group, and salt forms thereof. For example, sulfonic acid groups (-SO)₃H) The salt form is represented by the formula-SO₃M represents a group of an atom (for example, an alkali metal atom) which can become a cation in water or an aqueous solvent. This also applies to the salt forms of the various groups. Examples of the binder containing an acidic group include resins (e.g., urethane resins, vinyl chloride resins, etc.) containing at least one acidic group selected from the group consisting of sulfonic acid groups and salts thereof. However, the resin contained in the magnetic layer is not limited to these resins. In the binder containing an acid group, the amount of the acid group may be, for example, 20 to 500 eq/ton. Eq is an equivalent (equivalent) and is a unit that cannot be converted to SI units. The content of various functional groups such as an acidic group contained in the resin can be determined by a known method depending on the kind of the functional group. The larger the amount of acidic groups used, the larger the value of Δ N tends to be. In the composition for forming a magnetic layer, the binder may be used in an amount of, for example, 1.0 to 30.0 parts by mass, preferably 1.0 to 20.0 parts by mass, based on 100.0 parts by mass of the ferromagnetic powder. The larger the amount of the binder used relative to the ferromagnetic powder, the larger the value of Δ N tends to be.

Further, the curing agent may be used together with a resin that can be used as a binder. In one embodiment, the curing agent may be a thermosetting compound which is a compound that undergoes a curing reaction (crosslinking reaction) by heating, and in another embodiment, the curing agent may be a photocurable compound that undergoes a curing reaction (crosslinking reaction) by light irradiation. By performing the curing reaction in the magnetic layer forming step, at least a part of the curing agent can be contained in the magnetic layer in a state of reacting (crosslinking) with other components such as a binder. This also applies to a layer formed using the composition when the composition used for forming another layer contains a curing agent. The preferred curing agent is a thermosetting compound, with a polyisocyanate being preferred. For the details of the polyisocyanate, refer to paragraphs 0124 to 0125 of Japanese patent application laid-open No. 2011-216149. In the composition for forming a magnetic layer, the curing agent can be used in an amount of, for example, 0 to 80.0 parts by mass with respect to 100.0 parts by mass of the binder, and is preferably used in an amount of 50.0 to 80.0 parts by mass from the viewpoint of improving the strength of the magnetic layer.

(oxide abrasive)

The magnetic tape comprises an oxide abrasive in the magnetic layer. The oxide abrasive is a non-magnetic oxide powder having a mohs hardness of more than 8, preferably a non-magnetic oxide powder having a mohs hardness of 9 or more. Further, the maximum value of the Mohs hardness is 10. The oxide abrasive may be an inorganic oxide powder or an organic oxide powder, and is preferably an inorganic oxide powder. Specifically, the polishing agent may be alumina (Al)₂O₃) Titanium oxide (TiO)₂) Cerium oxide (CeO)₂) Zirconium oxide (ZrO)₂) Etc., among which alumina powder is preferred. In addition, the Mohs hardness of alumina is about 9. As for the alumina powder, reference can also be made to paragraph 0021 of japanese patent laid-open No. 2013-229090. As an index of the particle size of the oxide abrasive, the specific surface area can be used. It is considered that the larger the specific surface area is, the smaller the particle size of the primary particles constituting the oxide abrasive particles is. As the oxide abrasive, it is preferable to use an oxide abrasive having a specific surface area (hereinafter, referred to as "BET specific surface area") of 14m as measured by the BET (Brunauer-Emmett-Teller: Brunauer-Emmett-Teller) method²An oxide abrasive of at least one of the foregoing amounts. From the viewpoint of dispersibility, it is preferable to use a BET specific surface area of 40m²An oxide abrasive of not more than g. The content of the oxide abrasive in the magnetic layer is preferably 1.0 to 20.0 parts by mass, and more preferably 1.0 to 10.0 parts by mass, based on 100.0 parts by mass of the ferromagnetic hexagonal powder.

(additives)

The magnetic layer contains ferromagnetic powder, a binder, an oxide abrasive, and optionally one or more additives. The curing agent may be used as an example of the additive. Examples of the additives that can be contained in the magnetic layer include non-magnetic powder other than the oxide abrasive, a lubricant, a dispersant, a dispersion aid, a mildewproofing agent, an antistatic agent, and an antioxidant. The additives can be suitably selected from commercially available products according to the desired properties or can be produced by a known method and used in an arbitrary amount. For example, as for the lubricant, refer to paragraphs 0030 to 0033, 0035, and 0036 of Japanese patent laid-open No. 2016 and 126817. A lubricant may also be included in the nonmagnetic layer. As for the lubricant that can be contained in the nonmagnetic layer, paragraphs 0030 to 0031, 0034, 0035 and 0036 of Japanese patent laid-open No. 2016-126817 can be referred to. As the dispersant, refer to paragraphs 0061 and 0071 of japanese patent application laid-open No. 2012-133837. The dispersant may be contained in the nonmagnetic layer. As for the dispersant that can be contained in the nonmagnetic layer, refer to paragraph 0061 of japanese patent laid-open No. 2012-133837.

The dispersant is a dispersant for improving the dispersibility of the oxide abrasive. As a compound that can function as such a dispersant, an aromatic hydrocarbon compound having a phenolic hydroxyl group can be cited. "phenolic hydroxyl group" refers to a hydroxyl group directly bonded to an aromatic ring. The aromatic ring included in the aromatic hydrocarbon compound may have a monocyclic ring, a polycyclic ring structure, or a condensed ring structure. From the viewpoint of improving the dispersibility of the polishing agent, an aromatic hydrocarbon compound containing a benzene ring or a naphthalene ring is preferable. The aromatic hydrocarbon compound may have a substituent other than the phenolic hydroxyl group. Examples of the substituent other than the phenolic hydroxyl group include a halogen atom, an alkyl group, an alkoxy group, an amino group, an acyl group, a nitro group, a nitroso group, and a hydroxyalkyl group, and a halogen atom, an alkyl group, an alkoxy group, an amino group, and a hydroxyalkyl group are preferable. The aromatic hydrocarbon compound 1 may have one phenolic hydroxyl group in a molecule, or two, three or more phenolic hydroxyl groups.

A preferred embodiment of the aromatic hydrocarbon compound having a phenolic hydroxyl group includes a compound represented by the following general formula 100.

[ chemical formula 1]

General formula 100

[ in the general formula 100, X¹⁰¹～X¹⁰⁸Two of them are hydroxyl groups, and the other six thereof each independently represent a hydrogen atom or a substituent.]

In the compound represented by the general formula 100, the substitution positions of two hydroxyl groups (phenolic hydroxyl groups) are not particularly limited.

In the compound represented by the formula 100, X¹⁰¹～X¹⁰⁸Two of them are hydroxyl groups (phenolic hydroxyl groups), and the other six thereof each independently represent a hydrogen atom or a substituent. And, X¹⁰¹～X¹⁰⁸In the above formula, all of the moieties other than the two hydroxyl groups may be hydrogen atoms, or some or all of them may be substituents. Examples of the substituent include the substituents described above. As the substituent other than the two hydroxyl groups, one or more phenolic hydroxyl groups may be contained. From the viewpoint of improving the dispersibility of the polishing agent, X is preferably excluded¹⁰¹～X¹⁰⁸The other two hydroxyl groups in (a) are not phenolic hydroxyl groups. That is, the compound represented by the formula 100 is preferably dihydroxynaphthalene or a derivative thereof, and more preferably 2, 3-dihydroxynaphthalene or a derivative thereof. Preferably X¹⁰¹～X¹⁰⁸Examples of the substituent include a halogen atom (e.g., chlorine atom, bromine atom), an amino group, an alkyl group having 1 to 6 (preferably 1 to 4) carbon atoms, a methoxy group, an ethoxy group, an acyl group, a nitro group, a nitroso group, and-CH₂And (4) OH groups.

Further, as for a dispersant for improving the dispersibility of the oxide abrasive, it is also possible to refer to paragraphs 0024 to 0028 of japanese patent application laid-open No. 2014-179149.

In the preparation of the composition for forming a magnetic layer (preferably, in the preparation of an abrasive liquid as described later), a dispersant for improving the dispersibility of the oxide abrasive can be used in a proportion of, for example, 0.5 to 20.0 parts by mass, preferably 1.0 to 10.0 parts by mass, based on 100.0 parts by mass of the abrasive.

Examples of the non-magnetic powder other than the oxide abrasive that the magnetic layer may contain include a non-magnetic powder that can contribute to friction property control by forming a protrusion on the surface of the magnetic layer (hereinafter, also referred to as a "protrusion forming agent"). As the protrusion forming agent, various nonmagnetic powders generally used as a protrusion forming agent in a magnetic layer can be used. These may be inorganic substance powders (inorganic powders) or organic substance powders (organic powders). In one aspect, the particle size distribution of the protrusion forming agent is preferably monodisperse showing a single peak, rather than polydispersion having multiple peaks in the distribution, from the viewpoint of uniformization of the frictional characteristics. The protrusion-forming agent is preferably an inorganic powder from the viewpoint of easy availability of the monodisperse particles. Examples of the inorganic powder include powders of metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, and metal sulfides. The particles constituting the projection forming agent (non-magnetic powder other than the oxide abrasive) are preferably colloidal particles, and more preferably inorganic oxide colloidal particles. In addition, from the viewpoint of easy availability of the monodisperse particles, the inorganic oxide constituting the inorganic oxide colloidal particles is preferably silica (silica). The inorganic oxide colloidal particles are more preferably colloidal silica (silica colloidal particles). In the present invention and the present specification, "colloidal particles" mean particles which can be dispersed without causing precipitation and form a colloidal dispersion when 1g of at least 1 organic solvent is added per 100mL of methyl ethyl ketone, cyclohexanone, toluene, ethyl acetate, or a mixed solvent containing two or more of the above solvents in an arbitrary mixing ratio. In another mode, the protrusion forming agent is also preferably carbon black. The average particle size of the protrusion-forming agent may be, for example, 30 to 300nm, preferably 40 to 200 nm. In addition, from the viewpoint of the ability of the protrusion-forming agent to perform its function more satisfactorily, the content of the protrusion-forming agent in the magnetic layer is preferably 1.0 to 4.0 parts by mass, and more preferably 1.5 to 3.5 parts by mass, based on 100.0 parts by mass of the ferromagnetic powder.

The magnetic layer described above can be provided directly or indirectly on the surface of the nonmagnetic support via the nonmagnetic layer.

< nonmagnetic layer >

Next, the nonmagnetic layer will be described.

The magnetic tape may have a magnetic layer directly on the surface of the nonmagnetic support, or may have a nonmagnetic layer containing nonmagnetic powder and a binder between the nonmagnetic support and the magnetic layer. The nonmagnetic powder contained in the nonmagnetic layer may be an inorganic powder or an organic powder. Further, carbon black or the like can also be used. Examples of the inorganic powder include powders of metals, metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, and metal sulfides. These nonmagnetic powders are commercially available, and can also be produced by a known method. For details thereof, reference can be made to paragraphs 0036 to 0039 of Japanese patent application laid-open No. 2010-024113. The content (filling ratio) of the nonmagnetic powder in the nonmagnetic layer is preferably in the range of 50 to 90 mass%, more preferably in the range of 60 to 90 mass%.

Other details of the binder, additives, and the like of the nonmagnetic layer can be applied to the known techniques related to the nonmagnetic layer. For example, the type and content of the binder, the type and content of the additive, and the like can be applied to the known techniques related to the magnetic layer.

The nonmagnetic layer in the present invention and the present specification also includes a substantially nonmagnetic layer containing, for example, a ferromagnetic powder as an impurity or intentionally containing a small amount of the ferromagnetic powder together with the nonmagnetic powder. Here, the substantially nonmagnetic layer means a layer having a remanent magnetic flux density of 10mT or less, a coercivity of 7.96kA/m (100Oe) or less, or a remanent magnetic flux density of 10mT or less and a coercivity of 7.96kA/m (100Oe) or less. The nonmagnetic layer preferably has no residual magnetic flux density and no coercive force.

< non-magnetic support >

Next, a description will be given of a nonmagnetic support (hereinafter, also simply referred to as a "support").

Examples of the nonmagnetic support include publicly known nonmagnetic supports such as biaxially stretched polyethylene terephthalate, polyethylene naphthalate, polyamide, polyamideimide, and aromatic polyamide. Among them, polyethylene terephthalate, polyethylene naphthalate and polyamide are preferable. These supports may be subjected to corona discharge, plasma treatment, easy adhesion treatment, heat treatment, or the like in advance.

< Back coating >

The magnetic tape may have a back coat layer containing a nonmagnetic powder and a binder on a surface side of the nonmagnetic support opposite to the surface side having the magnetic layer. The back coat layer preferably contains one or both of carbon black and inorganic powder. As for the binder contained in the back coat layer and various additives that may be optionally contained, the publicly known techniques relating to the back coat layer can be applied, and the publicly known techniques relating to the formulation of the magnetic layer and/or the nonmagnetic layer can be applied. For example, as for the back coating layer, reference can be made to the descriptions of paragraphs 0018 to 0020 of Japanese patent application laid-open No. 2006-331625 and columns 4, lines 65 to 5, line 38 of the specification of U.S. Pat. No. 7,029,774.

< various thicknesses >

The thickness of the nonmagnetic support and each layer in the magnetic recording medium will be described below.

The thickness of the non-magnetic support is, for example, 3.0 to 80.0 μm, preferably 3.0 to 50.0 μm, and more preferably 3.0 to 10.0 μm.

The thickness of the magnetic layer can be optimized according to the saturation magnetization of the magnetic head to be used, the head gap length, the frequency band of the recording signal, and the like. The thickness of the magnetic layer is usually 10nm to 100nm, and from the viewpoint of high density recording, it is preferably 20 to 90nm, and more preferably 30 to 70 nm. At least one magnetic layer may be provided, or the magnetic layer may be separated into two or more layers having different magnetic properties, and a structure related to a known multilayer magnetic layer can be applied. The thickness of the magnetic layer when separated into two or more layers is set to the total thickness of these layers.

The thickness of the nonmagnetic layer is, for example, 0.1 to 1.5 μm, preferably 0.1 to 1.0. mu.m.

The thickness of the back coating is preferably 0.9 μm or less, and more preferably 0.1 to 0.7. mu.m.

The thickness of each layer and the nonmagnetic support is determined by exposing a cross section of the magnetic tape in the thickness direction by a known method such as an ion beam or a microtome, and then observing the cross section of the exposed cross section by a Scanning Transmission Electron Microscope (STEM). As a specific example of the thickness measuring method, reference can be made to the description about the thickness measuring method in the examples described later.

< manufacturing Process >

(preparation of composition for Forming Each layer)

The step of preparing the composition for forming the magnetic layer, the nonmagnetic layer or the back coat layer usually includes at least a kneading step, a dispersing step and, if necessary, a mixing step provided before and after these steps. Each step may be divided into two or more stages. The components used in the preparation of the composition for forming each layer may be added at the beginning or in the middle of any step. As the solvent, one or two or more of various solvents generally used in the production of a coating-type magnetic recording medium can be used. As for the solvent, for example, reference can be made to paragraph 0153 of japanese patent application laid-open No. 2011-216149. Further, each component may be added separately in two or more steps. For example, the binder may be separately charged in the kneading step, the dispersing step, and the mixing step for adjusting the viscosity after dispersion. In order to manufacture the magnetic tape, conventionally known manufacturing techniques can be used in various steps. In the kneading step, a device having a strong kneading force such as an open kneader, a continuous kneader, a pressure kneader, or an extruder is preferably used. For details of these kneading processes, reference can be made to Japanese patent application laid-open Nos. H1-106338 and H1-079274. The dispersing machine may be a known dispersing machine. Filtration may be performed at any stage of preparing the composition for forming each layer by a known method. The filtration can be performed by, for example, filter filtration. As the filter used for filtration, for example, a filter having a pore size of 0.01 to 3 μm (for example, a filter made of glass fiber, a filter made of polypropylene, etc.) can be used.

By allowing the oxide abrasive to exist in a finer state in the magnetic layer, the value of the FIB abrasive diameter tends to be small. As one of the methods for making the oxide abrasive exist in the magnetic layer in a finer state, use of a dispersant capable of improving dispersibility of the oxide abrasive can be cited as described above. In order to make the oxide abrasive exist in the magnetic layer in a finer state, it is preferable to use an abrasive having a small particle size, to suppress aggregation of the abrasive and to suppress uneven dispersion of the abrasive in the magnetic layer. One of the methods for dealing with this is to enhance the dispersion condition of the oxide abrasive in the preparation of the composition for forming a magnetic layer. For example, the oxide abrasive is dispersed separately from the ferromagnetic powder in such a manner that the dispersion conditions are enhanced. More specifically, the separate dispersion refers to a method for preparing a composition for forming a magnetic layer by a step of mixing an abrasive liquid (but substantially not including ferromagnetic powder) including an oxide abrasive and a solvent with a magnetic liquid including ferromagnetic powder, a solvent, and a binder. By dispersing and mixing the oxide abrasive and the ferromagnetic powder separately in this manner, the dispersibility of the oxide abrasive in the composition for forming a magnetic layer can be improved. The above "substantially not containing ferromagnetic powder" means that a slight amount of ferromagnetic powder is allowed to exist as an impurity inadvertently mixed, without adding ferromagnetic powder as a constituent component of the polishing liquid. Further, in addition to or together with the split dispersion, the dispersion conditions can be enhanced by arbitrarily combining the dispersion treatment for a long time, the use of a dispersion medium having a small size (for example, the diameter of dispersed beads in the bead dispersion), the high filling of the dispersion medium in the dispersing machine, and the like. Commercially available products can be used as the dispersing machine and the dispersing medium. Further, performing centrifugal separation processing of the polishing slurry can contribute to the following: particles having a size larger than the average particle size and/or agglomerated particles are removed from the particles constituting the oxide abrasive, whereby the oxide abrasive is present in a finer state in the magnetic layer. The centrifugal separation treatment can be performed using a commercially available centrifugal separator. Further, it is preferable that the abrasive liquid is filtered by a filter or the like to remove coarse aggregates in which particles constituting the oxide abrasive aggregate. Removal of such coarse aggregates can also contribute to making the oxide abrasive exist in a finer state in the magnetic layer. For example, filter filtration using a filter having a smaller pore size can contribute to making the oxide abrasive exist in the magnetic layer in a finer state. Further, the dispersibility of the oxide abrasive in the composition for forming a magnetic layer can be improved by adjusting various processing conditions (for example, stirring conditions, dispersion processing conditions, filtration conditions, and the like) after mixing the abrasive liquid and the components for preparing the composition for forming a magnetic layer such as ferromagnetic powder. This portion can also contribute to the oxide abrasive existing in the magnetic layer in a finer state. However, if the oxide abrasive is present in the magnetic layer in an extremely fine state, the FIB abrasive diameter is less than 0.04 μm, and therefore, it is preferable to adjust various conditions for preparing the abrasive liquid so as to achieve a FIB abrasive diameter of 0.04 μm or more and 0.08 μm or less.

Further, regarding Δ N, the longer the dispersion time of the magnetic liquid, the larger the value of Δ N tends to be. This is considered to be because the longer the dispersion time of the magnetic liquid is, the more the dispersibility of the ferromagnetic powder in the coating layer of the composition for forming a magnetic layer is improved, and the more the uniformity of the orientation state of the ferromagnetic particles constituting the ferromagnetic powder by the orientation treatment tends to be improved.

Further, the longer the dispersion time when the various components of the composition for forming a nonmagnetic layer are mixed and dispersed, the larger the value of Δ N tends to be.

The above dispersion time of the magnetic liquid and the dispersion time of the composition for forming a non-magnetic layer may be set so that Δ N of 0.25 to 0.40 can be achieved.

(coating Process)

The nonmagnetic layer and the magnetic layer can be formed by sequentially or simultaneously applying a composition for forming a nonmagnetic layer and a composition for forming a magnetic layer in a plurality of layers. The back coat layer can be formed by applying the composition for forming a back coat layer to the surface of the non-magnetic support opposite to the surface having the non-magnetic layer and the magnetic layer (or the non-magnetic layer and/or the magnetic layer to be provided later). The coating step for forming each layer can be performed in two or more stages. For example, in one embodiment, the composition for forming a magnetic layer can be applied in two or more steps. In this case, the drying treatment may be performed between the two stages of coating steps, or may not be performed. Further, the alignment treatment may be performed between the two stages of coating steps, or the drying treatment may not be performed. For details of the coating for forming each layer, reference can also be made to paragraph 0066 of japanese patent application laid-open No. 2010-231843. In addition, a known technique can be applied to the drying step after the application of each layer-forming composition. As for the composition for forming a magnetic layer, the lower the drying temperature of a coating layer formed by applying the composition for forming a magnetic layer (hereinafter, referred to as "coating layer of composition for forming a magnetic layer" or simply as "coating layer"), the larger the value of Δ N tends to be. The drying temperature may be, for example, an atmospheric temperature at which the drying step is performed, and may be set so that Δ N of 0.25 to 0.40 can be realized.

(other steps)

Known techniques can be applied to various other processes for manufacturing a magnetic tape. For each step, for example, refer to paragraphs 0067 to 0070 of Japanese patent application laid-open No. 2010-231843.

For example, the coating layer of the composition for forming a magnetic layer is preferably subjected to an alignment treatment when the coating layer is in a wet state. From the viewpoint of easily achieving Δ N of 0.25 to 0.40, it is preferable to perform the alignment treatment by arranging the magnet so that a magnetic field is applied perpendicularly to the surface of the coating layer of the composition for forming a magnetic layer (that is, the perpendicular alignment treatment). The intensity of the magnetic field during the alignment treatment may be set so that Δ N of 0.25 to 0.40 can be achieved. When the coating step of the composition for forming a magnetic layer is performed in two or more stages of coating steps, the alignment treatment is preferably performed at least after the last coating step, and more preferably the vertical alignment treatment is performed. For example, when the magnetic layer is formed by the two-stage coating process, the coating layer formed in the second-stage coating process may be subjected to the orientation treatment after the drying process without being subjected to the orientation treatment after the first-stage coating process.

The coating layer of the composition for forming a magnetic layer is preferably subjected to a rolling treatment at an arbitrary stage after drying the coating layer. Regarding the conditions of the rolling treatment, for example, paragraph 0026 of japanese patent application laid-open No. 2010-231843 can be referred to. The higher the rolling temperature (surface temperature of the reduction rolls), the larger the value of Δ N tends to be. The rolling temperature may be set so that Δ N of 0.25 to 0.40 inclusive can be achieved.

As described above, the magnetic tape according to one aspect of the present invention can be obtained. In general, a magnetic tape is accommodated in a tape cartridge, and the tape cartridge is mounted in a magnetic recording and playback apparatus. Since the magnetic recording and reproducing apparatus can perform head tracking servo, a servo pattern can be formed on a magnetic tape by a known method. In the case of reproducing information recorded on a magnetic tape in a magnetic recording/reproducing apparatus, even if the magnetic tape according to one embodiment of the present invention is reproduced in a high-temperature and high-humidity environment without relaxing the temperature and humidity control conditions of the environment in which the magnetic recording/reproducing apparatus is placed, the deterioration of the electromagnetic conversion characteristics during repeated reproduction can be suppressed.

[ magnetic recording/reproducing apparatus ]

One aspect of the present invention relates to a magnetic recording/reproducing apparatus including the magnetic tape and the magnetic head.

In the present invention and the present specification, the "magnetic recording/reproducing device" refers to a device capable of at least one of recording information on a magnetic tape and reproducing information recorded on the magnetic tape. This device is generally referred to as a driver. The magnetic head included in the magnetic recording and reproducing device may be a recording head capable of recording information on the magnetic tape, or may be a reproducing head capable of reproducing information recorded on the magnetic tape. Also, in one aspect, the magnetic recording and playback apparatus can include both a recording head and a playback head as independent magnetic heads. In another aspect, the magnetic head included in the magnetic recording and reproducing apparatus may have a structure in which both the recording element and the reproducing element are provided in one magnetic head. The playback head is preferably a magnetic head (MR head) having a Magnetoresistive (MR) element as a playback element, which can read information recorded on a magnetic tape with good sensitivity. As the MR head, various known MR heads can be used. The magnetic head for recording and/or reproducing information may include a servo pattern reading element. Alternatively, a magnetic head (servo head) provided with a servo pattern read element may be included in the magnetic recording/reproducing device as a magnetic head different from the magnetic head for performing information recording and/or information reproduction.

In the magnetic recording and reproducing device, recording of information on the magnetic tape and reproduction of information recorded on the magnetic tape can be performed by sliding the magnetic layer surface of the magnetic tape in contact with the magnetic head. The magnetic recording/reproducing apparatus may include the magnetic tape according to one aspect of the present invention, and other aspects may be applied with known techniques.

The magnetic recording/reproducing apparatus includes the magnetic tape according to one aspect of the present invention. Therefore, when the playback of the information recorded on the magnetic tape is repeated in a high-temperature and high-humidity environment, the degradation of the electromagnetic conversion characteristics can be suppressed. Further, it is considered that the magnetic layer surface and/or the magnetic head can be suppressed from being scraped when the magnetic layer surface slides against the magnetic head in order to record information on the magnetic tape under a high-temperature and high-humidity environment.

Examples

The present invention will be described below with reference to examples. However, the present invention is not limited to the embodiment shown in the examples. The "parts" and "%" described below are based on mass.

[ example 1]

< preparation of abrasive liquid >

100.0 parts of the oxide abrasive (alumina powder) shown in Table 1 were mixed with 2, 3-dihydroxynaphthalene (Tokyo chemical industry Co., Ltd.) having SO as a polar group in an amount shown in Table 1₃31.3 parts of a 32% solution (solvent is a mixed solvent of methyl ethyl ketone and toluene) of Na-based polyester urethane resin (toyoboco, UR-4800 (polar group amount: 80meq/kg) manufactured by ltd., methyl ethyl ketone and cyclohexanone 1: 1 (mass ratio) 570.0 parts of a mixed solution, and in the presence of zirconium beads (bead diameter: 0.1mm), using a paint stirrerThe dispersion was allowed to proceed for a period of time (bead dispersion time) shown in Table 1. After the dispersion, the dispersion obtained by separating the beads from the dispersion by a sieve was subjected to a centrifugal separation treatment. As the centrifugal separator, CS150GXL manufactured by hitachi kokico, ltd. (S100 AT6 manufactured by hitachi kokico, ltd. using a rotor) was used, and the centrifugal separation treatment was performed AT the rotation speed (rpm) shown in table 1 for the time (centrifugal separation time) shown in table 1. Thereafter, the mixture was filtered through a filter having a pore size shown in table 1, thereby obtaining an alumina dispersion (polishing slurry).

< preparation of composition for Forming magnetic layer >

(magnetic liquid)

100.0 parts of platy ferromagnetic hexagonal barium ferrite powder

(volume of activation and average plate ratio: refer to Table 1)

Containing SO₃Na-based polyurethane resin reference Table 1

(weight average molecular weight: 70,000, SO)₃Amount of Na group: reference table 1)

150.0 parts of cyclohexanone

150.0 parts of methyl ethyl ketone

(abrasive liquid)

6.0 parts of alumina dispersion prepared in the above

(silica Sol (bump former liquid))

2.0 parts of colloidal silica (average particle size: 100nm)

Methyl ethyl ketone 1.4 parts

(other Components)

Stearic acid 2.0 parts

2.0 parts of butyl stearate

2.5 parts of polyisocyanate (CORONATE (registered trademark) manufactured by TOSOHCORPORATION)

(finishing addition solvent)

Cyclohexanone 200.0 parts

Methyl ethyl ketone 200.0 parts

(preparation method)

The magnetic liquid was prepared by dispersing the beads of the various components of the above magnetic liquid in a batch vertical sand mill using the beads as a dispersion medium. The beads were dispersed for the time (magnetic bead dispersion time) shown in Table 1 using zirconia beads (bead diameter: see Table 1).

The magnetic liquid thus obtained, the polishing slurry, the silica sol, other components, and the finishing additive solvent were introduced into a solution stirrer, and stirred at a peripheral speed of 10 m/sec for a time (stirring time) shown in table 1. Thereafter, the composition for forming a magnetic layer was prepared by subjecting the mixture to ultrasonic dispersion treatment at a flow rate of 7.5 kg/min for a time period (ultrasonic treatment time) shown in table 1 by a flow ultrasonic disperser and then filtering the mixture through a filter having a pore size shown in table 1 a number of times shown in table 1.

< preparation of composition for Forming nonmagnetic layer >

A dispersion was obtained by bead dispersing, using a batch vertical sand mill, components other than stearic acid, butyl stearate, cyclohexanone and methyl ethyl ketone among the various components of the composition for forming a nonmagnetic layer (dispersion medium: zirconia beads (bead diameter: 0.1mm), dispersion time: refer to Table 1). Thereafter, the remaining components were added to the obtained dispersion, and the resulting mixture was stirred by a dissolver stirrer. Subsequently, the obtained dispersion was filtered using a filter (pore size: 0.5 μm), to prepare a composition for forming a non-magnetic layer.

Non-magnetic inorganic powder: 100.0 parts of alpha-iron oxide

Average particle size (average major axis length): 0.15 μm

Average needle ratio: 7

BET specific surface area: 52m²/g

20.0 parts of carbon black

Average particle size: 20nm

Electron beam curing type vinyl chloride copolymer 13.0 parts

6.0 parts of electron beam-curable polyurethane resin

Stearic acid 1.0 part

1.0 part of butyl stearate

Cyclohexanone 300.0 parts

300.0 parts of methyl ethyl ketone

< preparation of composition for Back coating formation >

The components other than stearic acid, butyl stearate, polyisocyanate and cyclohexanone among the various components of the composition for forming a back coat layer described below were kneaded and diluted by an open kneader to obtain a mixed solution. Then, the obtained mixed solution was subjected to 12-pass dispersion treatment using zirconia beads having a bead diameter of 1.0mm by a horizontal bead mill with a bead packing rate of 80 vol% and a rotor tip circumferential speed of 10 m/sec, with a retention time per 1 pass being 2 minutes. Thereafter, the remaining components were added to the obtained dispersion, and the resulting mixture was stirred by a dissolver stirrer. Next, the obtained dispersion was filtered using a filter (pore size: 1.0 μm), to thereby prepare a back coat layer-forming composition.

Non-magnetic inorganic powder: 80.0 parts of alpha-iron oxide

Average particle size (average major axis length): 0.15 μm

Average needle ratio: 7

BET specific surface area: 52m²/g

20.0 parts of carbon black

Average particle size: 20nm

Vinyl chloride copolymer 13.0 parts

6.0 parts of polyurethane resin containing sulfonate group

3.0 parts of phenylphosphonic acid

155.0 parts of methyl ethyl ketone

Stearic acid 3.0 parts

3.0 parts of butyl stearate

Polyisocyanate 5.0 parts

355.0 parts of cyclohexanone

< manufacture of magnetic tape >

The composition for forming a nonmagnetic layer was applied to a polyethylene naphthalate support and dried, and then irradiated with an electron beam to have an energy of 40kGy at an acceleration voltage of 125kV to form a nonmagnetic layer.

The coating layer is formed by applying the composition for forming a magnetic layer to the surface of the formed nonmagnetic layer. When the coating layer was in a wet state, a magnetic field having a strength shown in table 1 was applied in a direction perpendicular to the surface of the coating layer using an opposing magnet in an atmosphere having an atmosphere temperature (drying temperature) shown in table 1, and a vertical alignment treatment and a drying treatment were performed to form a magnetic layer.

Then, the back coat layer-forming composition was applied to the surface of the support opposite to the surface on which the nonmagnetic layer and the magnetic layer were formed, and dried.

Thereafter, using a rolling roll composed only of a metal roll, a surface smoothing treatment (rolling treatment) was performed with a rolling speed of 80 m/min, a line pressure of 300kg/cm (294kN/m), and a rolling temperature (surface temperature of the rolling roll) set to the temperatures shown in table 1.

Thereafter, heat treatment was performed at an atmospheric temperature of 70 ℃ for 36 hours. After the heat treatment, the magnetic layer was divided into 1/2 inches (0.0127m) in width, and the surface of the magnetic layer was cleaned by a tape cleaning device attached to a device having a slit sheet feeding and winding device so that the nonwoven fabric and the doctor blade were pressed against the surface of the magnetic layer. Thereafter, servo patterns are formed on the magnetic layer by commercially available servo writers.

In this way, the magnetic tape of example 1 was produced.

Examples 2,3 and 5 and comparative examples 1 to 8

Magnetic tapes were produced in the same manner as in example 1, except that various items shown in table 1 were changed as shown in table 1. The oxide abrasives described in table 1 are all alumina powders.

In table 1, in the comparative example in which "non-alignment treatment" is described in the column "formation and alignment of magnetic layer", magnetic tapes were produced without performing alignment treatment on the coating layer of the composition for forming a magnetic layer.

[ example 4]

After the formation of the nonmagnetic layer, the composition for forming a magnetic layer was applied onto the surface of the nonmagnetic layer so that the thickness after drying became 25nm, thereby forming a first coating layer. The first coating layer was passed through an atmosphere having an atmosphere temperature (drying temperature) shown in table 1 without applying a magnetic field, thereby forming a first magnetic layer (non-alignment treatment).

Then, the magnetic layer forming composition was applied onto the surface of the first magnetic layer so that the dried thickness became 25nm, thereby forming a second coating layer. When the second coating layer was in a wet state, a magnetic field having a strength shown in table 1 was applied in a direction perpendicular to the surface of the second coating layer using an opposing magnet in an atmosphere having an atmosphere temperature (drying temperature) shown in table 1, and a vertical alignment treatment and a drying treatment were performed, thereby forming a second magnetic layer.

A magnetic tape was produced in the same manner as in example 1, except that the magnetic layers were formed in multiple layers as described above.

Comparative example 9

After the formation of the nonmagnetic layer, the composition for forming a magnetic layer was applied onto the surface of the nonmagnetic layer so that the thickness after drying became 25nm, thereby forming a first coating layer. When the first coating layer was in a wet state, a magnetic field having a strength shown in table 1 was applied in a direction perpendicular to the surface of the first coating layer using an opposing magnet in an atmosphere having an atmosphere temperature (drying temperature) shown in table 1, and a vertical alignment treatment and a drying treatment were performed, thereby forming a first magnetic layer.

Then, the magnetic layer forming composition was applied onto the surface of the first magnetic layer so that the dried thickness became 25nm, thereby forming a second coating layer. The second coating layer was passed through an atmosphere having an atmosphere temperature (drying temperature) shown in table 1 without applying a magnetic field, thereby forming a second magnetic layer (non-alignment treatment).

A magnetic tape was produced in the same manner as in example 1, except that the magnetic layers were formed in multiple layers as described above.

Comparative example 10

After the formation of the nonmagnetic layer, the composition for forming a magnetic layer was applied onto the surface of the nonmagnetic layer so that the thickness after drying became 25nm, thereby forming a first coating layer. When the first coating layer was in a wet state, a magnetic field having a strength shown in table 1 was applied in a direction perpendicular to the surface of the first coating layer using an opposing magnet in an atmosphere having an atmosphere temperature (drying temperature) shown in table 1, and a vertical alignment treatment and a drying treatment were performed, thereby forming a first magnetic layer.

Then, the magnetic layer forming composition was applied onto the surface of the first magnetic layer so that the dried thickness became 25nm, thereby forming a second coating layer. The second coating layer was passed through an atmosphere having an atmosphere temperature (drying temperature) shown in table 1 without applying a magnetic field, thereby forming a second magnetic layer (non-alignment treatment).

A magnetic tape was produced in the same manner as in comparative example 8, except that the magnetic layers were formed in multiple layers as described above.

Comparative example 11

After the formation of the nonmagnetic layer, the composition for forming a magnetic layer was applied onto the surface of the nonmagnetic layer so that the thickness after drying became 25nm, thereby forming a first coating layer. The first coating layer was passed through an atmosphere having an atmosphere temperature (drying temperature) shown in table 1 without applying a magnetic field, thereby forming a first magnetic layer (non-alignment treatment).

Then, the magnetic layer forming composition was applied onto the surface of the first magnetic layer so that the dried thickness became 25nm, thereby forming a second coating layer. When the second coating layer was in a wet state, a magnetic field having a strength shown in table 1 was applied in a direction perpendicular to the surface of the second coating layer using an opposing magnet in an atmosphere having an atmosphere temperature (drying temperature) shown in table 1, and a vertical alignment treatment and a drying treatment were performed, thereby forming a second magnetic layer.

A magnetic tape was produced in the same manner as in comparative example 6, except that the magnetic layers were formed in multiple layers as described above.

[ evaluation of physical Properties of magnetic tape ]

(1) FIB abrasive diameter

The FIB polishing slurry diameter of each prepared magnetic tape was determined by the following method. As the focused ion beam device, MI4050 manufactured by HitachiHigh-technologies corporation was used, and as the image analysis software, ImageJ, free software, was used.

(i) Acquisition of secondary ion images

The back coating surface of each measurement sample cut out from each prepared magnetic tape was attached to an adhesive layer of a commercially available SEM measurement double-sided carbon tape (double-sided tape in which a carbon film was formed on an aluminum substrate). The adhesive layer on the surface of the double-sided tape opposite to the surface to which the back coat layer was attached to a sample stage of a focused ion beam apparatus. In this way, the measurement sample is disposed on the sample stage of the focused ion beam apparatus such that the magnetic layer surface faces upward.

Without the pre-image-taking coating treatment, the beam of the focused ion beam apparatus was set to an acceleration voltage of 30kV, a current value of 133pA, a beam size of 30nm, and a luminance of 50%, and an SI signal was detected by a secondary ion detector. The color tone of the image is stabilized by applying ACB to three portions of the non-image-pickup region on the surface of the magnetic layer, and the contrast reference value and the luminance reference value are determined. A contrast value that is 1% lower than the reference contrast value determined by the ACB and the reference brightness value are determined as the photographing conditions. An area on the surface of the magnetic layer, which was not imaged, was selected, and imaging was performed under the imaging conditions specified above with a pixel distance (pixelstance) of 25.0 (nm/pixel). The image reading method was photoscan dot × 4_ Dwell Time 15 μ sec (microseconds) (reading Time: 1 minute), and the reading size was set to 25 μm square. Thus, a secondary ion image of a 25 μm square region of the surface of the magnetic layer was obtained. After the scanning of the obtained secondary ion image is completed, the mouse is right-clicked on the read screen, and the format of the document is saved as JPEG in the ExportImage. The number of pixels of the image was confirmed to be 2000 pixels × 2100 pixels, and the cross mark and the micron bar of the read image were canceled, thereby setting the image to 2000 pixels × 2000 pixels.

(ii) Calculation of FIB abrasive diameter

(ii) drag-and-drop (draganddrop) the image data of the secondary ion image acquired in (i) above to the image analysis software ImageJ.

The tone of the image data was changed to 8 bits using image analysis software. Specifically, the Image of the operation menu of the Image analysis software is pressed, and 8bit of Type is selected.

For the binarization processing, the lower limit value 250 gradation and the upper limit value 255 gradation are selected, and the binarization processing based on these two threshold values is performed. Specifically, the Image (Image) is clicked on the operation menu of the Image analysis software, the Threshold (Threshold) for adjustment (adjustment) is selected, the lower limit value 250 and the upper limit value 255 are selected, and then the application (application) is selected. The acquired image was subjected to a Process (Process) of pressing an operation menu of image analysis software, Noise reduction (Despeckle) was selected from Noise (Noise), Size (Size) was set to 4.0-Infinity in analysis particles (AnalyzeParticle), and Noise components were removed.

For the binarized image thus obtained, analysis particles (AnalyzeParticle) were selected from the operation menu of the image analysis software, and the number of white portions and selected areas (Area) (unit: pixels) on the image were obtained. The Area is obtained by converting Area (unit: pixel) into an Area for each whitish portion on the screen by image analysis software. Specifically, in the image obtained by the above-described imaging conditions, 1 pixel corresponds to 0.0125 μm, and therefore the area a [ μm ] is calculated from the area a being area pixel × 0.0125^2²]. Using the area thus calculated, the equivalent circle diameter L was obtained for each whitish portion by the equivalent circle diameter L being (a/pi) ^ (1/2) × 2 being L.

The above-described steps were performed 4 times on different portions (25 μm square) of the surface of the magnetic layer of the measurement sample, and from the obtained results, the FIB abrasive diameter was calculated from the FIB abrasive diameter ∑ (Li)/Σ i.

(2) Non-magnetic support and thickness of each layer

The thicknesses of the magnetic layer, the nonmagnetic support, and the back coat layer of each of the magnetic tapes thus produced were measured by the following methods. As a result of the measurement, in any magnetic tape, the thickness of the magnetic layer was 50nm, the thickness of the nonmagnetic layer was 0.7 μm, the thickness of the nonmagnetic support was 5.0 μm, and the thickness of the back coat layer was 0.5 μm.

The thicknesses of the magnetic layer, the nonmagnetic layer, and the nonmagnetic support measured here were used to calculate the following refractive indices.

(i) Production of sample for Cross-section Observation

According to the method described in sections 0193 to 0194 of jp 2016 a-177851, a cross-sectional observation sample including all regions in the thickness direction from the magnetic layer side surface to the back coating side surface of the magnetic tape was prepared.

(ii) Thickness measurement

The prepared sample was observed by STEM, and a STEM image was taken. The STEM image is a STEM-HAADF (High-angle annular dark field image) image captured at an acceleration voltage of 300kV and a capture magnification of 450000 times, and is captured so that all regions in the thickness direction from the magnetic layer side surface to the back coat side surface of the magnetic tape are included in one image. In the STEM image thus obtained, a straight line connecting both ends of a line segment representing the surface of the magnetic layer is determined as a reference line representing the surface of the magnetic layer side of the magnetic tape. For example, when a STEM image is captured such that the magnetic layer side of the cross-sectional observation sample is positioned above the image and the back coat layer side is positioned below the image, a straight line connecting both ends of the line segment is a straight line connecting an intersection point of the left side of the image (rectangular or square) of the STEM image and the line segment and an intersection point of the right side of the STEM image and the line segment. Similarly, a reference line indicating an interface between the magnetic layer and the nonmagnetic layer, a reference line indicating an interface between the nonmagnetic layer and the nonmagnetic support, a reference line indicating an interface between the nonmagnetic support and the back coat layer, and a reference line indicating a back coat layer side surface of the magnetic tape were determined.

The thickness of the magnetic layer was determined as the shortest distance from a randomly selected one of the portions on the reference line representing the surface of the magnetic layer on the magnetic layer side of the magnetic tape to the reference line representing the interface between the magnetic layer and the nonmagnetic layer. Similarly, the thicknesses of the nonmagnetic layer, the nonmagnetic support, and the back coat layer were determined.

(3) Δ N of magnetic layer

M-2000U manufactured by woollamco, inc. Two-layer or single-layer models were prepared and fitted using WVASE32 manufactured by woollamco, inc.

(i) Refractive index measurement of non-magnetic supports

The measurement sample was cut out from each magnetic tape, the back coat layer of the measurement sample was wiped off with a cloth into which methyl ethyl ketone had penetrated to expose the surface of the nonmagnetic support, and then the surface was roughened with sandpaper so that the reflected light from the exposed surface was not detected in the measurement with an ellipsometer performed thereafter.

After that, the magnetic layer and the nonmagnetic layer of the measurement sample were wiped off and removed with a cloth impregnated with methyl ethyl ketone, and then the surface of the silicon wafer was attached to the surface-roughened surface by static electricity, whereby the measurement sample was disposed on the silicon wafer so that the surface of the nonmagnetic support exposed by removing the magnetic layer and the nonmagnetic layer (hereinafter referred to as "the surface on the magnetic layer side of the nonmagnetic support") faced upward.

Δ and Ψ were measured using an ellipsometer using the magnetic layer side surface of the nonmagnetic support of the measurement sample on which incident light was made incident as described above. Using the obtained measurement value and the thickness of the nonmagnetic support obtained in (2) above, the refractive index of the nonmagnetic support (refractive index in the longitudinal direction, refractive index in the width direction, refractive index in the thickness direction measured by making incident light incident from the longitudinal direction, and refractive index in the thickness direction measured by making incident light incident from the width direction) was obtained by the method described above.

(ii) Refractive index measurement of non-magnetic layer

The measurement sample was cut out from each magnetic tape, the back coat layer of the measurement sample was wiped off with a cloth into which methyl ethyl ketone had penetrated to expose the surface of the nonmagnetic support, and then the surface was roughened with sandpaper so that the reflected light from the exposed surface was not detected in the measurement with a spectroscopic ellipsometer performed thereafter.

After that, the surface of the magnetic layer of the measurement sample was gently wiped with a cloth impregnated with methyl ethyl ketone to remove the magnetic layer and expose the surface of the nonmagnetic layer, and then the measurement sample was placed on the silicon wafer in the same manner as in (i) above.

The surface of the nonmagnetic layer of the measurement sample on the silicon wafer was measured by an ellipsometer, and the refractive index (the refractive index in the longitudinal direction, the refractive index in the width direction, the refractive index in the thickness direction measured by making incident light enter from the longitudinal direction, and the refractive index in the thickness direction measured by making incident light enter from the width direction) of the nonmagnetic layer was obtained by the spectroscopic ellipsometry method according to the method described above.

(iii) Refractive index measurement of magnetic layers

The measurement sample was cut out from each magnetic tape, the back coat layer of the measurement sample was wiped off with a cloth into which methyl ethyl ketone had penetrated to expose the surface of the nonmagnetic support, and then the surface was roughened with sandpaper so that the reflected light from the exposed surface was not detected in the measurement with a spectroscopic ellipsometer performed thereafter.

Then, the measurement sample was placed on the silicon wafer in the same manner as in (i) above.

The surface of the magnetic layer of the measurement sample on the silicon wafer was measured with an ellipsometer, and the refractive index of the magnetic layer (refractive index Nx in the longitudinal direction, refractive index Ny in the width direction, refractive index Nz in the thickness direction measured by making incident light enter from the longitudinal direction) was determined by spectroscopic ellipsometry by the method described above₁And a refractive index Nz in a thickness direction measured by making incident light incident from a width direction₂). From the obtained values, Nxy and Nz are obtained, and the absolute value Δ N of the difference between them is further obtained. In all the magnetic tapes of examples and comparative examples, Nxy was determined to be a value larger than Nz (i.e., Nxy > Nz).

(4) Vertical direction squareness ratio (SQ)

The perpendicular squareness ratio of the magnetic tape is the squareness ratio measured in the perpendicular direction of the magnetic tape. The "vertical direction" described with respect to the squareness ratio refers to a direction perpendicular to the surface of the magnetic layer. For each magnetic tape thus produced, a vibration sample type fluxgate (manufactured by toeiindustryco., ltd.) was used to scan the magnetic tape at a maximum external magnetic field of 1194kA/m (15kOe) and a scanning speed of 4.8 kA/m/sec (60 Oe/sec) at a measurement temperature of 23 ℃ ± 1 ℃ to obtain a vertical squareness ratio. The measurement value is a value corrected for the diamagnetic field and is obtained as a value obtained by subtracting the magnetization of the sample probe of the vibrating sample type magnetic flowmeter as background noise. In one aspect, the perpendicular direction squareness ratio of the magnetic tape is preferably 0.60 or more and 1.00 or less, more preferably 0.65 or more and 1.00 or less. In one embodiment, the ratio of the rectangles in the vertical direction of the magnetic tape may be, for example, 0.90 or less, 0.85 or less, or 0.80 or less, or may be larger than these values.

[ change in electromagnetic conversion characteristics (SNR; Signal-to-Noise-Ratio) after repeated playback in a high-temperature, high-humidity environment (SNR reduction amount) ]

The electromagnetic conversion characteristic (SNR) was measured by the following method using an 1/2-inch (0.0127m) reel tester with a magnetic head fixed thereto.

Recording was performed by setting the head/tape relative speed to 5.5 m/sec, using a MIG (Metal-In-Gap: Metal-containing) head (Gap length 0.15 μm, track width 1.0 μm), and setting the recording current to the optimum recording current for each tape.

The playback head used a GMR (Giant-Magnetoresistive effect) head having an element thickness of 15nm, a shield (shield) interval of 0.1 μm, and a lead width of 0.5 μm. The recording of the signal was performed at a linear recording density of 270kfci, and the playback signal was measured using a spectrum analyzer manufactured by shibassokuco. The unit kfci is a unit of linear recording density (cannot be converted to SI unit system). The signal uses a portion of the signal that is sufficiently stable after the tape run begins. The SNR is taken as the ratio of the output value of the carrier signal to the integrated noise of all spectral bandwidths.

Under the above conditions, the tape length per 1 pass was set to 1,000m, and the tape was reciprocated by 5,000 passes in an atmosphere of an atmospheric temperature of 32 ℃ and a relative humidity of 80% for playback (head/tape relative speed: 6.0 m/sec), thereby measuring the SNR. The difference between the SNR of the 1 st pass and the SNR of the 5,000 th pass (SNR of the 5,000 th pass-SNR of the 1 st pass) was obtained. If the difference is less than-2.0 dB, it can be determined that the tape is a tape that exhibits excellent electromagnetic conversion characteristics required for data backup tapes.

The results are shown in Table 1 (tables 1-1 and 1-2).

From the results shown in table 1, it was confirmed that in the magnetic tapes of examples 1 to 5 in which Δ N of the magnetic layer and the FIB polishing agent diameter were within the ranges described above, the decrease in the electromagnetic conversion characteristics at the time of repeated playback in a high-temperature and high-humidity environment was suppressed.

In general, it is known that the squareness ratio is an index of the existence state of ferromagnetic powder in the magnetic layer. However, as shown in table 1, Δ N is different even for magnetic tapes having the same rectangular ratio in the vertical direction (for example, examples 1 to 3 and comparative example 10). The present inventors considered that this indicates that Δ N is a value influenced by other factors in addition to the existence state of the ferromagnetic powder in the magnetic layer.

Industrial applicability

One aspect of the present invention is applicable to the technical field of various magnetic recording media such as magnetic tapes for data storage.