JP6678205B2 - Magnetic recording media - Google Patents

Description

  The present invention relates to a magnetic recording medium.

  The recording and / or reproduction of information on a magnetic recording medium is usually performed by bringing the surface of the magnetic recording medium (the surface of the magnetic layer) into contact with a magnetic head (hereinafter, also simply referred to as “head”) and sliding. Done.

  In order to continuously and intermittently reproduce information recorded on the magnetic recording medium, sliding between the surface of the magnetic layer and the head is repeated (repeated sliding). It is desirable to suppress the deterioration of the electromagnetic conversion characteristics in such repeated sliding in order to increase the reliability of a magnetic recording medium as a recording medium for data storage. This is because a magnetic recording medium with a small decrease in electromagnetic conversion characteristics during repeated sliding can continue to exhibit excellent electromagnetic conversion characteristics even if reproduction is repeated continuously or intermittently.

  The cause of the deterioration of the electromagnetic conversion characteristics in repeated sliding is that a phenomenon that the distance between the surface of the magnetic layer and the head is widened (called “spacing loss”) occurs. A cause of the spacing loss is that foreign matter derived from the magnetic recording medium adheres to the head while the reproduction is repeated and the surface of the magnetic layer and the head continue to slide. As a countermeasure against the head deposits generated in this way, conventionally, an abrasive has been included in the magnetic layer in order to give the magnetic layer surface a function of removing the head deposits (for example, Patent Document 1). reference).

JP 2005-243162 A

  It is preferable to include an abrasive in the magnetic layer in order to suppress a decrease in electromagnetic conversion characteristics due to a spacing loss caused by a head deposit. However, if it becomes possible to suppress the deterioration of the electromagnetic conversion characteristics further than the level achieved by adding an abrasive to the magnetic layer as conventionally performed, if the magnetic layer is used as a recording medium for data storage, The reliability of the recording medium can be further improved.

  In view of such a situation, an object of the present invention is to provide a magnetic recording medium in which the electromagnetic conversion characteristics are hardly deteriorated even when the magnetic layer surface and the head are repeatedly slid.

One embodiment of the present invention provides
A magnetic recording medium having a magnetic layer containing a ferromagnetic powder and a binder on a non-magnetic support,
The ferromagnetic powder is a ferromagnetic hexagonal ferrite powder, the magnetic layer contains an abrasive,
The peak intensity Int of the (110) plane diffraction peak relative to the peak intensity Int (114) of the (114) plane diffraction peak of the hexagonal ferrite crystal structure obtained by X-ray diffraction analysis of the magnetic layer using the In-Plane method. The intensity ratio of (110) (Int (110) / Int (114); hereinafter, also referred to as “X-ray diffraction (XRD) intensity ratio”) is 0.5 or more and 4.0 or less;
The perpendicular squareness ratio of the magnetic recording medium is 0.65 or more and 1.00 or less, and a logarithmic decay rate (hereinafter, referred to as “logarithmic decay of the magnetic layer surface”) determined by a pendulum viscoelasticity test on the surface of the magnetic layer. Ratio "or simply" logarithmic decay rate. ") Is 0.050 or less;
About.

  In one aspect, the vertical squareness ratio can be 0.65 or more and 0.90 or less.

  In one embodiment, the logarithmic decrement on the surface of the magnetic layer can be 0.010 or more and 0.050 or less.

  In one aspect, the magnetic recording medium may have a nonmagnetic layer containing a nonmagnetic powder and a binder between the nonmagnetic support and the magnetic layer.

  In one embodiment, the magnetic recording medium may have a backcoat 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.

  In one aspect, the magnetic recording medium can be a magnetic tape.

  According to one embodiment of the present invention, it is possible to provide a magnetic recording medium in which the electromagnetic conversion characteristics are hardly deteriorated even when the surface of the magnetic layer and the head are repeatedly slid.

It is explanatory drawing of the measuring method of a logarithmic decrement. It is explanatory drawing of the measuring method of a logarithmic decrement. It is explanatory drawing of the measuring method of a logarithmic decrement. An example of a specific embodiment of a magnetic tape manufacturing process (process schematic diagram) is shown.

  One embodiment of the present invention is a magnetic recording medium having a magnetic layer containing a ferromagnetic powder and a binder on a nonmagnetic support, wherein the ferromagnetic powder is a ferromagnetic hexagonal ferrite powder, and the magnetic layer is polished. Of the diffraction peak of the (110) plane with respect to the peak intensity Int (114) of the (114) plane of the hexagonal ferrite crystal structure determined by X-ray diffraction analysis of the magnetic layer using the In-Plane method. The intensity ratio of the peak intensity Int (110) (Int (110) / Int (114)) is 0.5 or more and 4.0 or less, and the perpendicular squareness ratio of the magnetic recording medium is 0.65 or more and 1.00. And a logarithmic decay rate determined by a pendulum viscoelasticity test on the surface of the magnetic layer is 0.050 or less.

  In the present invention and in the present specification, the “surface of (magnetic layer)” has the same meaning as the surface of the magnetic recording medium on the side of the magnetic layer. Further, in the present invention and the present specification, “ferromagnetic hexagonal ferrite powder” means an aggregate of a plurality of ferromagnetic hexagonal ferrite particles. Ferromagnetic hexagonal ferrite particles are ferromagnetic particles having a hexagonal ferrite crystal structure. Hereinafter, the particles (ferromagnetic hexagonal ferrite particles) constituting the ferromagnetic hexagonal ferrite powder are also described as “hexagonal ferrite particles” or simply “particles”. The term “aggregate” is not limited to an embodiment in which particles constituting the aggregate are in direct contact, but also includes an embodiment in which a binder, an additive, and the like are interposed between particles. The same applies to the present invention and various powders such as the non-magnetic powder in the present specification.

  In the present invention and the present specification, descriptions regarding a direction and an angle (for example, vertical, orthogonal, parallel, and the like) include an allowable error range in the technical field to which the present invention belongs, unless otherwise specified. The error range means, for example, a range of less than a strict angle ± 10 °, preferably within a strict angle ± 5 °, and more preferably within ± 3 °.

The inventors' inference regarding the magnetic recording medium is as follows.
The magnetic layer of the magnetic recording medium contains an abrasive. By including an abrasive in the magnetic layer, a function of removing head deposits can be imparted to the surface of the magnetic layer. However, when the abrasive present on the surface of the magnetic layer and / or near the surface of the magnetic layer does not sink into the inside of the magnetic layer properly due to the force from the head when sliding between the surface of the magnetic layer and the head, It is considered that the head is shaved by the contact with the protruding abrasive (head shaving). It is considered that if the head scraping that occurs in this manner can be suppressed, it is possible to further suppress the deterioration of the electromagnetic conversion characteristics due to the spacing loss.
With respect to the above points, the present inventors have found that the ferromagnetic hexagonal ferrite powder contained in the magnetic layer contains particles that support the abrasive pressed into the magnetic layer and affect the degree of subsidence of the abrasive ( In the following, it is presumed that there are particles that are considered to have no or little effect (hereinafter, also referred to as “the particles of the latter”). The latter particles are considered to be fine particles generated by, for example, chipping of a part of the particles due to a dispersion treatment performed at the time of preparing the composition for forming a magnetic layer. Furthermore, the present inventors speculate that the more such fine particles, the lower the strength of the magnetic layer, although the reason is not clear. The decrease in the strength of the magnetic layer is caused by the fact that foreign substances generated by the magnetic layer surface being scraped when the magnetic layer surface slides on the head (magnetic layer shaving) are interposed between the magnetic layer surface and the head, resulting in a spacing loss. Cause you to
The present inventors have found that among the ferromagnetic hexagonal ferrite powders present in the magnetic layer, the former particles are particles that give a diffraction peak in X-ray diffraction analysis using the In-Plane method, and the latter particles are Is considered to have no diffraction peak or to have a small effect on the diffraction peak due to its fineness. Therefore, based on the intensity of the diffraction peak obtained by the X-ray diffraction analysis of the magnetic layer using the In-Plane method, it supports the abrasive pressed into the magnetic layer and affects the degree of sinking of the abrasive. The present inventors speculate that it is possible to control the presence state of the particles in the magnetic layer, and as a result, it is possible to control the degree of sinking of the abrasive. The present inventors consider the XRD intensity ratio, which will be described in detail later, to be an index relating to this point.
On the other hand, the perpendicular squareness ratio is a ratio of the residual magnetization to the saturation magnetization measured in a direction perpendicular to the surface of the magnetic layer. The smaller the residual magnetization, the smaller the value. Since the latter particles are considered to be fine and difficult to maintain magnetization, it is presumed that the perpendicular direction squareness tends to decrease as the magnetic layer contains more of the latter particles. Therefore, the present inventors believe that the perpendicular squareness ratio can be an index of the abundance of fine particles (the latter particles) in the magnetic layer. As the number of such fine particles increases in the magnetic layer, the strength of the magnetic layer decreases, and foreign matter generated by scraping the magnetic layer surface when sliding between the magnetic layer surface and the head is generated between the magnetic layer surface and the head. It is thought that the tendency of spacing loss to occur due to intervening increases.
The present inventors believe that in the magnetic recording medium, the XRD intensity ratio and the perpendicular squareness ratio each within the above-described ranges contribute to suppressing a decrease in electromagnetic conversion characteristics in repeated sliding. This is because the head shaving can be suppressed mainly by controlling the XRD intensity ratio, and the magnetic layer shaving can be mainly suppressed by controlling the vertical squareness ratio. Is speculating.

  Further, in the above magnetic recording medium, the fact that the logarithmic decay rate determined by the pendulum viscoelasticity test on the surface of the magnetic layer is 0.050 or less contributes to further suppressing the deterioration of the electromagnetic conversion characteristics in repeated sliding. The present inventors consider. This will be further described below.

In the present invention and the present specification, the logarithmic decrement on the surface of the magnetic layer is a value obtained by the following method.
1 to 3 are explanatory diagrams of a method of measuring a logarithmic decay rate. Hereinafter, a method for measuring the logarithmic decay rate will be described with reference to these drawings. However, the illustrated embodiment is an exemplification, and does not limit the present invention in any way.
On the sample stage 101 in the pendulum viscoelasticity tester, a part of the magnetic tape (measurement sample) 100 to be measured is placed on a substrate 103 with a measurement surface (magnetic layer surface ) Is placed upward with the fixing tape 105 or the like.
A round bar-shaped cylinder edge 104 with a pendulum is placed on the measurement surface of the measurement sample 100 such that the major axis direction of the cylinder edge is parallel to the longitudinal direction of the measurement sample 100. FIG. 1 shows an example of a state in which the round bar-shaped cylinder edge 104 with a pendulum is placed on the measurement surface of the measurement sample 100 (a state viewed from above). In the embodiment shown in FIG. 1, a holder / temperature sensor 102 is provided so that the surface temperature of the substrate 103 can be monitored. However, this configuration is not essential. In the embodiment shown in FIG. 1, the longitudinal direction of the measurement sample 100 is a direction indicated by an arrow in the drawing, and refers to the longitudinal direction of the magnetic recording medium from which the measurement sample is cut out. As the pendulum 107 (see FIG. 2), a pendulum made of a material (for example, a metal, an alloy, or the like) having a property of being attracted to a magnet is used.
The surface temperature of the substrate 103 on which the measurement sample 100 is mounted is raised to 80 ° C. at a temperature rising rate of 5 ° C./min or less (an arbitrary temperature rising rate is 5 ° C./min or less). The pendulum motion is started (inducing initial vibration) by releasing the attraction between the pendulum 107 and the magnet 106. FIG. 2 shows an example of a state of the pendulum 107 performing a pendulum motion (a state viewed from the side). In the embodiment shown in FIG. 2, in the pendulum viscoelasticity tester, the power supply to the magnet (electromagnet) 106 disposed below the sample stage is stopped (the switch is turned off) and the adsorption is released to start the pendulum motion. Then, the power supply to the electromagnet is restarted (the switch is turned on) and the pendulum 107 is attracted to the magnet 106 to stop the pendulum motion. During the pendulum movement, as shown in FIG. 2, the pendulum 107 repeats its amplitude. From the results obtained by monitoring the displacement of the pendulum by the displacement sensor 108 while the pendulum repeats the amplitude, a displacement-time curve is obtained in which the vertical axis represents the displacement and the horizontal axis represents the elapsed time. An example of the displacement-time curve is shown in FIG. FIG. 3 schematically shows the correspondence between the state of the pendulum 107 and the displacement-time curve. The displacement (time) obtained at the measurement interval after the lapse of 10 minutes or more (any time may be used if the time is 10 minutes or more) is repeated by repeating the stationary (adsorption) and the pendulum motion at a constant measurement interval. The logarithmic decay rate Δ (no unit) is obtained from the following equation, and this value is defined as the logarithmic decay rate of the surface of the magnetic layer of the magnetic tape. The adsorption time of one adsorption is 1 second or more (an arbitrary time may be used if it is 1 second or more), and the interval from the end of the adsorption to the start of the next adsorption is 6 seconds or more (any time is 6 seconds or more). Time is fine.) The measurement interval is a time interval from the start of adsorption to the start of the next adsorption. The humidity of the environment in which the pendulum movement is performed may be any relative humidity as long as the relative humidity is in the range of 40 to 70%. The ambient temperature of the environment in which the pendulum motion is performed may be any temperature as long as it is in the range of 20 to 30 ° C.

In the displacement-time curve, the interval from the minimum to the minimum again is defined as one cycle of the wave. Let n be the number of waves included in the displacement-time curve during the measurement interval, and let An be the difference between the minimal displacement and the maximal displacement in the nth wave. In Figure 3, the displacement of the n-th wave intervals up again becomes minimum of minimum, labeled Pn (e.g. P ₁ for the first _wave, for second P _2, P ₃ for _third) doing. In calculating the logarithmic decay rate, the difference between the minimal displacement and the maximal displacement appearing next to the n-th wave (A _{n + 1} in the above equation, A _{4 in the} displacement-time curve shown in FIG. 3) is also used. Thereafter, the portion where the pendulum 107 is stationary (adsorbed) is not used for counting the number of waves. The portion where the pendulum 107 is stationary (adsorbed) before the maximum displacement is not used for counting the number of waves. Therefore, in the displacement-time curve shown in FIG. 3, the number of waves is three (n = 3).

The logarithmic decrement is considered to be a value that is an indicator of the amount of the adhesive component that is released from the magnetic layer surface and intervenes between the magnetic layer surface and the head when the head and the magnetic layer surface contact and slide. Can be It is presumed that the adhesion and deposition of such an adhesive component on the head during repeated sliding also lead to a spacing loss which causes a reduction in electromagnetic conversion characteristics. On the other hand, when the logarithmic decrement on the surface of the magnetic layer in the magnetic recording medium is 0.050 or less, that is, the state in which the adhesive component is reduced, the adhesive component adheres to the head and deposits. This is considered to contribute to suppressing occurrence of spacing loss. The present inventors presume that this leads to further suppression of the occurrence of spacing loss and deterioration of the electromagnetic conversion characteristics in repeated sliding.
The details of the adhesive component are not clear. The present inventors speculate that the adhesive component may be derived from a resin used as a binder. The details are as follows. As the binder, various resins can be used as described in detail below. The resin is a polymer of two or more polymerizable compounds (including a homopolymer and a copolymer), and a component having a molecular weight lower than the average molecular weight (hereinafter, referred to as a “low molecular weight binder component”). Usually included. Such a low molecular weight binder component may be separated from the magnetic layer surface and intervened between the magnetic layer surface and the head at the time of sliding between the head and the magnetic layer surface, to cause a spacing loss, The present inventors consider. The low molecular weight binder component is considered to have tackiness, and the logarithmic decrement determined by the pendulum viscoelasticity test is such that the low molecular weight binder released from the magnetic layer surface when the magnetic layer surface and the head slide. The present inventors speculate that this may be an indicator of the amount of the component. In one embodiment, in addition to the ferromagnetic hexagonal ferrite powder and the binder, the magnetic layer is formed by coating a composition for forming a magnetic layer containing a hardener on a nonmagnetic support directly or via another layer. , And is formed by performing a curing treatment. The curing treatment here allows a curing reaction (crosslinking reaction) between the binder and the curing agent. However, the present inventors believe that the low molecular weight binder component may have poor reactivity in the curing reaction although the reason is not clear. For this reason, the low molecular weight binder component is unlikely to remain in the magnetic layer and is easily released from the magnetic layer, and the low molecular weight binder component intervenes between the magnetic layer surface and the head when sliding between the magnetic layer surface and the head. The present inventors speculate that this may be one of the reasons for this.

  The above is the inference of the present inventors regarding the fact that deterioration of the electromagnetic conversion characteristics can be suppressed even when the magnetic layer surface and the head repeatedly slide in the magnetic recording medium. However, the present invention is not limited at all by the above presumption. The present specification includes the inference of the present inventors. The present invention is not limited to such inference.

  Hereinafter, the various values will be described in more detail.

[XRD intensity ratio]
The magnetic recording medium contains a ferromagnetic hexagonal ferrite powder in a magnetic layer. The XRD intensity ratio is determined by X-ray diffraction analysis of the magnetic layer containing the ferromagnetic hexagonal ferrite powder using the In-Plane method. Hereinafter, the X-ray diffraction analysis performed using the In-Plane method is also described as “In-Plane XRD”. In-plane XRD is performed by irradiating a magnetic layer surface with X-rays under the following conditions using a thin-film X-ray diffractometer. Magnetic recording media are broadly classified into tape-shaped magnetic recording media (magnetic tapes) and disk-shaped magnetic recording media (magnetic disks). The measurement direction is the longitudinal direction for the magnetic tape and the radial direction for the magnetic disk.
Using Cu source (output 45kV, 200mA)
Scan conditions: 0.05 degree / step, 0.1 degree / min in the range of 20 to 40 degrees
Optical system used: Parallel optical system Measurement method: 2θχ scan (X-ray incident angle 0.25 °)
The above conditions are set values in a thin film X-ray diffractometer. As the thin film X-ray diffraction device, a known device can be used. As an example of the thin film X-ray diffractometer, Rigaku's SmartLab can be mentioned. The sample to be subjected to the In-Plane XRD analysis is a medium sample cut out from the magnetic recording medium to be measured, and it is sufficient that a diffraction peak described later can be confirmed, and the size and shape are not limited.

  X-ray diffraction analysis techniques include thin-film X-ray diffraction and powder X-ray diffraction. While powder X-ray diffraction measures X-ray diffraction of a powder sample, thin-film X-ray diffraction can measure X-ray diffraction of a layer or the like formed on a substrate. Thin film X-ray diffraction is classified into an In-Plane method and an Out-Of-Plane method. The X-ray incidence angle at the time of measurement is in the range of 5.0 to 90.00 ° in the Out-Of-Plane method, whereas it is generally in the range of 0.20 to 0.50 ° in the In-Plane method. . In the present invention and In-Plane XRD in this specification, the X-ray incident angle is 0.25 ° as described above. In the In-Plane method, the X-ray penetration angle is smaller than that of the Out-Of-Plane method, so that the X-ray penetration depth is shallower. Therefore, according to the X-ray diffraction analysis using the In-Plane method (In-Plane XRD), the X-ray diffraction analysis of the surface layer of the sample to be measured can be performed. For a magnetic recording medium sample, X-ray diffraction analysis of the magnetic layer can be performed according to In-Plane XRD. The XRD intensity ratio refers to the (110) plane relative to the peak intensity Int (114) of the diffraction peak of the (114) plane of the hexagonal ferrite crystal structure in the X-ray diffraction spectrum obtained by the In-Plane XRD. Is the intensity ratio (Int (110) / Int (114)) of the peak intensity Int (110) of the diffraction peak. Int is used as an abbreviation for Intensity. In the X-ray diffraction spectrum (vertical axis: Intensity, horizontal axis: diffraction angle 2θχ (degree)) obtained by In-Plane XRD, the diffraction peak of the (114) plane is detected when 2θχ is in the range of 33 to 36 degrees. The diffraction peak of the (110) plane is a peak detected when 2θχ is in the range of 29 to 32 degrees.

Among the diffraction planes, the (114) plane of the hexagonal ferrite crystal structure is located near the easy axis (c-axis direction) of the ferromagnetic hexagonal ferrite powder particles (hexagonal ferrite particles). The (110) plane of the hexagonal ferrite crystal structure is located in a direction perpendicular to the direction of the easy axis of magnetization.
As the former particles among the hexagonal ferrite particles contained in the magnetic layer exist in a state in which the direction orthogonal to the easy axis direction is closer to the magnetic layer surface, the abrasive is more hexagonal. The present inventors believe that it is difficult for the ferrite particles to sink into the magnetic layer because of being supported by the crystalline ferrite particles. In contrast, as the former particles in the magnetic layer exist in a state where the direction orthogonal to the easy axis direction is more perpendicular to the magnetic layer surface, the abrasive is more difficult to be supported by the hexagonal ferrite powder, The present inventors believe that it easily sinks into the inside of the layer. Further, the present inventors have found that in the X-ray diffraction spectrum obtained by In-Plane XRD, the peak intensity of the diffraction peak of the (110) plane with respect to the peak intensity Int (114) of the diffraction peak of the (114) plane of the hexagonal ferrite crystal structure. As the intensity ratio of the intensity Int (110) (Int (110) / Int (114); XRD intensity ratio) increases, the direction perpendicular to the easy axis direction is closer to the magnetic layer surface. It is inferred that the former particles are included in the magnetic layer in a large amount, and that the smaller the XRD intensity ratio is, the smaller the former particles existing in such a state are in the magnetic layer. When the XRD intensity ratio is 4.0 or less, the former particles, that is, the particles that support the abrasive pressed into the magnetic layer and affect the degree of sinking of the abrasive, support the abrasive too much. However, as a result, it is considered that the abrasive can appropriately sink into the magnetic layer when the head slides on the magnetic layer surface. The present inventors presume that this contributes to reducing the occurrence of head shaving even when the magnetic layer surface and the head repeatedly slide. On the other hand, when the magnetic layer surface slides on the head, the abrasive is appropriately projected on the magnetic layer surface, which contributes to reducing the area where the magnetic layer surface comes into contact with the head (true contact). It is thought that. It is considered that the larger the true contact area is, the stronger the force applied from the head to the magnetic layer surface when sliding between the magnetic layer surface and the head is to cause the magnetic layer surface to be damaged and the magnetic layer surface to be scraped. In this regard, the fact that the XRD intensity ratio is 0.5 or more means that the abrasive particles protrude appropriately to the surface of the magnetic layer when the head slides on the surface of the magnetic layer. The present inventors presume that this indicates that the particles exist in the magnetic layer in a state capable of supporting the abrasive.

  The XRD intensity ratio is preferably 3.5 or less, and more preferably 3.0 or less, from the viewpoint of further suppressing the deterioration of the electromagnetic conversion characteristics. Further, from the same viewpoint, the XRD intensity ratio is preferably 0.7 or more, and more preferably 1.0 or more. The XRD intensity ratio can be controlled, for example, by the processing conditions of the alignment processing performed in the manufacturing process of the magnetic recording medium. As the alignment treatment, it is preferable to perform a vertical alignment treatment. The vertical alignment treatment can be preferably performed by applying a magnetic field perpendicular to the surface of the coating layer of the composition for forming a magnetic layer in a wet state (undried state). As the orientation condition is strengthened, the value of the XRD intensity ratio tends to increase. The processing conditions of the alignment treatment include the magnetic field strength in the alignment treatment. The processing conditions of the alignment processing are not particularly limited. What is necessary is just to set the processing conditions of the alignment processing so that the XRD intensity ratio of 0.5 or more and 4.0 or less can be realized. As an example, the magnetic field strength in the vertical alignment process may be 0.10 to 0.80 T, or may be 0.10 to 0.60 T. As the dispersibility of the ferromagnetic hexagonal ferrite powder in the composition for forming a magnetic layer increases, the value of the XRD intensity ratio tends to increase due to the vertical alignment treatment.

[Vertical squareness ratio]
The perpendicular squareness ratio is a squareness ratio measured in the vertical direction of the magnetic recording medium. The “vertical direction” described with respect to the squareness ratio refers to a direction orthogonal to the surface of the magnetic layer. For example, when the magnetic recording medium is a tape-shaped magnetic recording medium, that is, a magnetic tape, the vertical direction is also a direction orthogonal to the longitudinal direction of the magnetic tape. The vertical squareness ratio is measured using a vibrating sample magnetometer. More specifically, the vertical squareness ratio in the present invention and in the present specification is such that an external magnetic field is applied to a magnetic recording medium at a measurement temperature of 23 ° C. ± 1 ° C. with a maximum external magnetic field of 1194 kA / m (15 kOe) in a vibrating sample magnetometer. In addition, the value is obtained by sweeping under the condition of a scan speed of 4.8 kA / m / sec (60 Oe / sec), and is a value after demagnetizing field correction. The measured value is obtained as a value obtained by subtracting the magnetization of the sample probe of the vibrating sample magnetometer as background noise.

  The perpendicular squareness ratio of the magnetic recording medium is 0.65 or more. The present inventors speculate that the perpendicular squareness ratio of a magnetic recording medium can be an indicator of the abundance of the latter particles (fine particles) described above, which are considered to cause a decrease in the strength of the magnetic layer. ing. It is considered that a magnetic layer having a perpendicular squareness ratio of 0.65 or more in a magnetic recording medium has a high strength due to a small amount of such fine particles, and is hardly abraded by sliding between the magnetic layer surface and the head. It is presumed that since the surface of the magnetic layer is hardly shaved, it is possible to suppress the occurrence of spacing loss due to foreign matter generated by shaving the surface of the magnetic layer and deterioration of electromagnetic conversion characteristics. In order to further suppress the deterioration of the electromagnetic conversion characteristics, the vertical squareness ratio is preferably equal to or greater than 0.68, more preferably equal to or greater than 0.70, and further preferably equal to or greater than 0.73. It is more preferably 0.75 or more. The squareness ratio is 1.00 at the maximum in principle. Therefore, the perpendicular squareness ratio of the magnetic recording medium is 1.00 or less. The vertical squareness ratio may be, for example, 0.95 or less, 0.90 or less, 0.87 or less, or 0.85 or less. However, it is considered that the larger the value of the squareness ratio in the vertical direction is, the smaller the number of the fine particles of the latter in the magnetic layer is, which is preferable from the viewpoint of the strength of the magnetic layer. Therefore, the vertical squareness ratio may exceed the upper limit exemplified above.

  In order to set the vertical squareness ratio to 0.65 or more, it is necessary to suppress generation of fine particles due to partial chipping (chipping) in the step of preparing the composition for forming a magnetic layer. We believe it is preferred. Specific means for suppressing the occurrence of chipping will be described later.

[Logarithmic decay rate]
The logarithmic decay rate obtained by the pendulum viscoelasticity test on the surface of the magnetic layer of the magnetic recording medium is 0.050 or less. This can also contribute to suppressing the deterioration of the electromagnetic conversion characteristics in repeated sliding. From the viewpoint of further suppressing the deterioration of the electromagnetic conversion characteristics in the repeated sliding, the logarithmic decrement is preferably 0.048 or less, more preferably 0.045 or less, and 0.040 or less. Is more preferred. On the other hand, the lower the logarithmic decrement, the more preferable from the viewpoint of suppressing the deterioration of the electromagnetic conversion characteristics in repeated sliding. Therefore, the lower limit is not particularly limited. As an example, the logarithmic decay rate can be, for example, 0.010 or greater, or 0.015 or greater. However, the logarithmic decay rate may be lower than the value exemplified above. The specific mode of the means for adjusting the logarithmic decay rate will be described later.

  Hereinafter, the magnetic recording medium will be described in further detail.

[Magnetic layer]
<Ferromagnetic hexagonal ferrite powder>
The magnetic layer of the magnetic recording medium contains a ferromagnetic hexagonal ferrite powder as a ferromagnetic powder. Regarding ferromagnetic hexagonal ferrite powder, magnetoplumbite type (also called “M type”), W type, Y type and Z type are known as crystal structures of hexagonal ferrite. The ferromagnetic hexagonal ferrite powder contained in the magnetic layer may have any crystal structure. The crystal structure of hexagonal ferrite includes iron atoms and divalent metal atoms as constituent atoms. The divalent metal atom is a metal atom that can be a divalent cation as an ion, and examples thereof include a barium atom, a strontium atom, an alkaline earth metal atom such as a calcium atom, and a lead atom. For example, a hexagonal ferrite containing a barium atom as a divalent metal atom is a barium ferrite, and a hexagonal ferrite containing a strontium atom is a strontium ferrite. The hexagonal ferrite may be a mixed crystal of two or more hexagonal ferrites. An example of a mixed crystal is a mixed crystal of barium ferrite and strontium ferrite.

The activation volume can be used as an index of the particle size of the ferromagnetic hexagonal ferrite powder. “Activation volume” is a unit of magnetization reversal. The activation volume described in the present invention and the present specification is measured using a vibrating sample magnetometer at a magnetic field sweep speed of 3 minutes and 30 minutes of a coercive force Hc measuring section under an environment of an ambient temperature of 23 ° C. ± 1 ° C. It is a value obtained from the following relational expression between Hc and activation volume V.
Hc = 2Ku / Ms {1-[(kT / KuV) ln (At / 0.693)] ^1/2 }
[Where Ku: anisotropy constant, Ms: saturation magnetization, k: Boltzmann constant, T: absolute temperature, V: activation volume, A: spin precession frequency, t: magnetic field inversion time]
As a method for achieving high-density recording, there is a method of reducing the particle size of the ferromagnetic powder contained in the magnetic layer and increasing the filling rate of the ferromagnetic powder in the magnetic layer. In this respect, the activation volume of the ferromagnetic hexagonal ferrite powder is preferably at 2500 nm ³ or less, more preferably 2300 nm ³ or less, and further preferably 2000 nm ³ or less. On the other hand, from the viewpoint of magnetization stability, the activation volume is, for example, preferably 800 nm ³ or more, more preferably 1000 nm ³ or more, and still more preferably 1200 nm ³ or more.

  The shape of the particles constituting the ferromagnetic hexagonal ferrite powder is obtained by photographing the ferromagnetic hexagonal ferrite powder using a transmission electron microscope at a magnification of 100,000 times and printing it on photographic paper at a total magnification of 500,000 times. In the obtained particle photograph, the outline of the particle (primary particle) is traced by a digitizer and specified. Primary particles refer to independent particles without aggregation. Photographing using a transmission electron microscope is performed by a direct method using a transmission electron microscope at an acceleration voltage of 300 kV. Observation and measurement with a transmission electron microscope can be performed using, for example, a transmission electron microscope model H-9000 manufactured by Hitachi and image analysis software KS-400 manufactured by Carl Zeiss. With respect to the shape of the particles constituting the ferromagnetic hexagonal ferrite powder, “plate” refers to a shape having two opposing plate surfaces. On the other hand, among such particle shapes having no plate surface, a shape having a distinction between a long axis and a short axis is “elliptical”. The major axis is determined as an axis (straight line) that can take the longest particle length. On the other hand, the short axis is determined as the axis having the longest length when the particle length is taken along a straight line orthogonal to the long axis. A shape in which there is no distinction between a major axis and a minor axis, that is, a shape in which major axis length = minor axis length is “spherical”. A shape in which the major axis and the minor axis cannot be specified from the shape is called an indefinite shape. The photographing using a transmission electron microscope for specifying the particle shape is performed without subjecting the powder to be photographed to an orientation treatment. The shape of the ferromagnetic hexagonal ferrite powder used for preparing the composition for forming a magnetic layer and the shape of the ferromagnetic hexagonal ferrite powder contained in the magnetic layer may be any of plate, ellipse, sphere, and amorphous.

  The average particle size of the various powders described in the present invention and the present specification is an arithmetic average of values obtained for 500 randomly extracted particles using the particle photograph taken as described above. . The average particle size shown in the examples described below is a value obtained using a transmission electron microscope H-9000 manufactured by Hitachi as a transmission electron microscope and image analysis software KS-400 manufactured by Carl Zeiss as image analysis software.

  For details of the ferromagnetic hexagonal ferrite powder, see, for example, paragraphs 0134 to 0136 of JP-A-2011-216149.

  The content (filling rate) of the ferromagnetic hexagonal ferrite powder in the magnetic layer is preferably in the range of 50 to 90% by mass, and more preferably in the range of 60 to 90% by mass. The components of the magnetic layer other than the ferromagnetic hexagonal ferrite powder are at least a binder and an abrasive, and may optionally include one or more additives. A high filling ratio of the ferromagnetic hexagonal ferrite powder in the magnetic layer is preferable from the viewpoint of improving the recording density.

<Binder, curing agent>
The magnetic recording medium contains a binder in the magnetic layer. A binder is one or more resins. The resin may be a homopolymer or a copolymer (copolymer). As the binder contained in the magnetic layer, polyurethane resin, polyester resin, polyamide resin, vinyl chloride resin, styrene, acrylonitrile, acrylic resin copolymerized with methyl methacrylate, cellulose resin such as nitrocellulose, epoxy resin, phenoxy resin, Those selected from polyvinyl alkylal resins such as polyvinyl acetal and polyvinyl butyral can be used alone, or a plurality of resins can be mixed and used. Preferred among these are polyurethane resins, acrylic resins, cellulose resins and vinyl chloride resins. These resins can be used as a binder also in a nonmagnetic layer and / or a back coat layer described later. With respect to the above binder, paragraphs 0029 to 0031 of JP-A-2010-24113 can be referred to. The average molecular weight of the resin used as the binder can be, for example, 10,000 or more and 200,000 or less as a weight average molecular weight. In the present invention and the present specification, the weight average molecular weight is a value obtained by converting a value measured by gel permeation chromatography (GPC) into polystyrene. The following conditions can be mentioned as measurement conditions. The weight average molecular weight shown in the examples described below is a value obtained by converting a value measured under the following measurement conditions into polystyrene.
GPC device: HLC-8120 (manufactured by Tosoh Corporation)
Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8 mm ID (Inner Diameter) x 30.0 cm)
Eluent: tetrahydrofuran (THF)

  When forming the magnetic layer, a curing agent may be used together with the resin usable as the binder. In one embodiment, the curing agent can be a thermosetting compound that is a compound in which a curing reaction (crosslinking reaction) proceeds by heating, and in another embodiment, a photocuring in which a curing reaction (crosslinking reaction) proceeds by light irradiation. Compounds. The curing agent can be included in the magnetic layer in a state where at least a part thereof has reacted (cross-linked) with another component such as a binder as the curing reaction proceeds in the process of manufacturing the magnetic recording medium. Preferred curing agents are thermosetting compounds, with polyisocyanates being preferred. For details of the polyisocyanate, refer to paragraphs 0124 to 0125 of JP-A-2011-216149. The curing agent is used in the composition for forming a magnetic layer in an amount of, for example, 0 to 80.0 parts by mass, preferably 50.0 to 80 parts by mass, based on 100.0 parts by mass of the binder. It can be used in an amount of 0 parts by mass.

<Abrasive>
The magnetic recording medium contains an abrasive in the magnetic layer. The abrasive means a nonmagnetic powder having a Mohs hardness of more than 8, preferably a nonmagnetic powder having a Mohs hardness of 9 or more. The abrasive may be a powder of an inorganic substance (inorganic powder) or a powder of an organic substance (organic powder), and is preferably an inorganic powder. The abrasive is more preferably an inorganic powder having a Mohs 'hardness of more than 8, more preferably an inorganic powder having a Mohs' hardness of 9 or more. The maximum value of Mohs hardness is 10 for diamond. Specifically, examples of the abrasive include powders of alumina (Al ₂ O ₃ ), silicon carbide, boron carbide (B ₄ C), TiC, cerium oxide, zirconium oxide (ZrO ₂ ), and diamond. Among them, alumina powder is preferable. For the alumina powder, reference can also be made to paragraph 0021 of JP-A-2013-229090. The specific surface area can be used as an index of the particle size of the abrasive. The larger the specific surface area, the smaller the particle size. It is preferable to use an abrasive having a specific surface area (hereinafter referred to as “BET specific surface area”) of the primary particles measured by the BET (Brunauer-Emmett-Teller) method of 14 m ² / g or more. From the viewpoint of dispersibility, it is preferable to use an abrasive having a BET specific surface area of 40 m ² / g or less. The content of the abrasive in the magnetic layer is preferably 1.0 to 20.0 parts by mass with respect to 100.0 parts by mass of the ferromagnetic hexagonal ferrite powder.

<Additives>
The magnetic layer contains ferromagnetic hexagonal ferrite powder, a binder and an abrasive, and may further contain one or more additives as necessary. As an example of the additive, the above-mentioned curing agent can be used. In addition, examples of additives that can be included in the magnetic layer include nonmagnetic powders, lubricants, dispersants, dispersing aids, fungicides, antistatic agents, antioxidants, and the like. For example, as an example of an additive that can be used for a magnetic layer containing an abrasive, a dispersant described in paragraphs 0012 to 0022 of JP-A-2013-131285 may be used. As a dispersing agent for improving the dispersibility.
Examples of the dispersant include known dispersants such as a carboxy group-containing compound and a nitrogen-containing compound. For example, the nitrogen-containing compound may be any of a primary amine represented by NH ₂ R, a secondary amine represented by NHR ₂ , and a tertiary amine represented by NR ₃ . In the above, R represents an arbitrary structure constituting the nitrogen-containing compound, and a plurality of Rs may be the same or different. The nitrogen-containing compound may be a compound (polymer) having a plurality of repeating structures in the molecule. The present inventors believe that the nitrogen-containing part of the nitrogen-containing compound functions as an adsorption part on the particle surface of the ferromagnetic hexagonal ferrite powder, which is the reason why the nitrogen-containing compound can work as a dispersant. . Examples of the carboxy group-containing compound include fatty acids such as oleic acid. With respect to the carboxy group-containing compound, the present inventors consider that the reason why the carboxy group functions as an adsorption portion to the surface of the ferromagnetic hexagonal ferrite powder particles is that the carboxy group-containing compound can function as a dispersant. I have. It is also preferable to use a carboxy group-containing compound and a nitrogen-containing compound in combination.

  Examples of the nonmagnetic powder that can be included in the magnetic layer include a nonmagnetic powder that can form projections on the surface of the magnetic layer and contribute to control of friction characteristics (hereinafter, also referred to as a “projection forming agent”). As such a nonmagnetic powder, various nonmagnetic powders generally used for a magnetic layer can be used. These may be inorganic powders or organic powders. In one aspect, from the viewpoint of making the friction characteristics uniform, it is preferable that the particle size distribution of the nonmagnetic powder is not a polydispersion having a plurality of peaks in the distribution but a monodispersion showing a single peak. From the viewpoint of availability of monodisperse particles, the nonmagnetic powder is preferably an inorganic powder. Examples of the inorganic powder include powders of metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, metal sulfides, and the like. The particles constituting the nonmagnetic powder are preferably colloidal particles, more preferably inorganic oxide colloidal particles. In addition, from the viewpoint of the availability of monodisperse particles, the inorganic oxide constituting the inorganic oxide colloid particles is preferably silicon dioxide (silica). More preferably, the inorganic oxide colloid particles are colloidal silica (silica colloid particles). In the present invention and the present specification, “colloidal particles” means at least one organic solvent of 100 mL of at least one of methyl ethyl ketone, cyclohexanone, toluene or ethyl acetate, or a mixed solvent containing two or more of the above solvents at an arbitrary mixing ratio. When 1 g is added, it refers to particles that can be dispersed without settling to give a colloidal dispersion. In another aspect, the nonmagnetic powder is also preferably carbon black. The average particle size of the nonmagnetic powder is, for example, 30 to 300 nm, and preferably 40 to 200 nm. In addition, from the viewpoint that the nonmagnetic filler can exert its function better, the content of the nonmagnetic powder in the magnetic layer is preferably 100.0 parts by mass of the ferromagnetic hexagonal ferrite powder. Therefore, the amount is 1.0 to 4.0 parts by mass, and more preferably 1.5 to 3.5 parts by mass.

  Various additives that can be arbitrarily included in the magnetic layer can be appropriately selected from commercially available products and those manufactured by known methods according to desired properties.

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

[Non-magnetic layer]
Next, the nonmagnetic layer will be described.
The magnetic recording medium may have a magnetic layer directly on the surface of the nonmagnetic support, and has a nonmagnetic layer containing a nonmagnetic powder and a binder between the nonmagnetic support and the magnetic layer. You may. The non-magnetic powder contained in the non-magnetic layer may be an inorganic powder or an organic powder. Further, carbon black or the like can be used. Examples of the inorganic powder include powders of metals, metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, metal sulfides, and the like. These nonmagnetic powders are available as commercial products, and can also be manufactured by a known method. For details, refer to paragraphs 0036 to 0039 of JP-A-2010-24113. The content (filling rate) of the nonmagnetic powder in the nonmagnetic layer is preferably in the range of 50 to 90% by mass, and more preferably in the range of 60 to 90% by mass.

  For other details such as a binder and an additive of the non-magnetic layer, a known technique regarding the non-magnetic layer can be applied. Further, for example, with respect to the type and content of the binder, the type and content of the additive, and the like, a known technique for the magnetic layer can be applied.

  The non-magnetic layer of the present invention and the present specification shall include a substantially non-magnetic layer containing a small amount of ferromagnetic powder together with the non-magnetic powder, for example, as an impurity or intentionally. Here, the substantially non-magnetic layer means that the layer has a residual magnetic flux density of 10 mT or less, a coercive force of 7.96 kA / m (100 Oe) or less, or a residual magnetic flux density of 10 mT or less. A layer having a coercive force of 7.96 kA / m (100 Oe) or less. The nonmagnetic layer preferably does not have a residual magnetic flux density and a coercive force.

[Non-magnetic support]
Next, a non-magnetic support (hereinafter, simply referred to as “support”) will be described.
Examples of the nonmagnetic support include known materials such as polyethylene terephthalate, polyethylene naphthalate, polyamide, polyamide imide, and aromatic polyamide that have been biaxially stretched. Among these, polyethylene terephthalate, polyethylene naphthalate and polyamide are preferred. These supports may be subjected to corona discharge, plasma treatment, easy adhesion treatment, heat treatment, or the like in advance.

[Back coat layer]
The magnetic recording medium may have a backcoat layer containing a nonmagnetic powder and a binder on the surface of the nonmagnetic support opposite to the surface 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 can be optionally contained, a known technique relating to the back coat layer can be applied, and a known technique relating to the formulation of the magnetic layer and / or the non-magnetic layer can be applied. Can also.

[Various thicknesses]
The nonmagnetic support and the thickness of each layer in the magnetic recording medium will be described below.
The thickness of the nonmagnetic 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 band of the recording signal, and the like. The thickness of the magnetic layer is generally from 10 nm to 100 nm, preferably from 20 to 90 nm, more preferably from 30 to 70 nm, from the viewpoint of high density recording. The number of the magnetic layers may be at least one. The magnetic layer may be divided into two or more layers having different magnetic properties, and a known configuration relating to a multilayer magnetic layer can be applied. The thickness of the magnetic layer when it is separated into two or more layers is the total thickness of these layers.

  The thickness of the nonmagnetic layer is, for example, 50 nm or more, preferably 70 nm or more, and more preferably 100 nm or more. On the other hand, the thickness of the nonmagnetic layer is preferably 800 nm or less, more preferably 500 nm or less.

  The thickness of the back coat layer is preferably 0.9 μm or less, more preferably 0.1 to 0.7 μm.

  The thickness of each layer of the magnetic recording medium and the thickness of the nonmagnetic support can be determined by a known thickness measurement method. As an example, for example, after a cross section in the thickness direction of a magnetic recording medium is exposed by a known method such as an ion beam or a microtome, the cross section of the exposed cross section is observed using a scanning electron microscope. Various thicknesses can be obtained as an arithmetic average of the thickness obtained at one location in the thickness direction in the cross-sectional observation, or at two or more randomly selected locations, for example, two locations. Alternatively, the thickness of each layer may be determined as a design thickness calculated from manufacturing conditions.

[Manufacturing process]
<Preparation of composition for forming each layer>
The step of preparing a composition for forming a magnetic layer, a non-magnetic layer, or a back coat layer usually includes at least a kneading step, a dispersing step, and a mixing step provided before and after these steps as necessary. Each step may be divided into two or more steps. The components used for preparing each layer forming composition may be added at the beginning or during any step. As the solvent, one kind or two or more kinds of various kinds of solvents usually used for producing a coating type magnetic recording medium can be used. For the solvent, for example, paragraph 0153 of JP-A-2011-216149 can be referred to. In addition, individual components may be added in two or more steps in a divided manner. For example, the binder may be added separately in the kneading step, the dispersing step, and the mixing step for adjusting the viscosity after dispersion. In order to manufacture the magnetic recording medium, a conventionally known manufacturing technique can be used in various steps. In the kneading step, it is preferable to use one having a strong kneading force, such as an open kneader, a continuous kneader, a pressure kneader, or an extruder. For details of these kneading processes, reference can be made to JP-A-1-106338 and JP-A-1-79274. A well-known disperser can be used. Each layer forming composition may be filtered by a known method before being subjected to the coating step. Filtration can be performed, for example, by filter filtration. As a filter used for filtration, for example, a filter having a pore size of 0.01 to 3 μm (for example, a glass fiber filter, a polypropylene filter, or the like) can be used.

  Regarding the dispersion treatment of the composition for forming a magnetic layer, as described above, it is preferable to suppress the occurrence of chipping. For this purpose, in the step of preparing the composition for forming a magnetic layer, the ferromagnetic hexagonal ferrite powder is dispersed in a two-stage dispersion process, and the ferromagnetic hexagonal ferrite powder is coarsely dispersed in the first stage. It is preferable to perform a second-stage dispersion treatment in which the collision energy applied to the particles of the ferromagnetic hexagonal ferrite powder due to the collision with the dispersed beads after the fine aggregates are crushed is smaller than the first dispersion treatment. According to such a dispersion treatment, it is possible to achieve both improvement in the dispersibility of the ferromagnetic hexagonal ferrite powder and suppression of the occurrence of chipping.

  As a preferred embodiment of the two-stage dispersion treatment, a ferromagnetic hexagonal ferrite powder, a binder and a solvent, a first stage of obtaining a dispersion by performing a dispersion treatment in the presence of the first dispersion beads, The dispersion obtained in the first step, a second step of performing a dispersion treatment in the presence of a second dispersion beads having a smaller bead diameter and density than the first dispersion beads, and it can. Hereinafter, the distributed processing of the above preferred embodiment will be further described.

  In order to enhance the dispersibility of the ferromagnetic hexagonal ferrite powder, the first and second steps described above may be performed as a dispersion treatment before mixing the ferromagnetic hexagonal ferrite powder with other powder components. preferable. For example, when forming a magnetic layer containing an abrasive and the non-magnetic powder, before mixing with the abrasive and the non-magnetic powder, ferromagnetic hexagonal ferrite powder, a binder, a solvent and optionally added additives It is preferable to perform the first step and the second step as a dispersion treatment of a liquid (magnetic liquid) containing

The bead diameter of the second dispersed beads is preferably 1/100 or less, more preferably 1/500 or less, of the bead diameter of the first dispersed beads. Further, the bead diameter of the second dispersed beads can be, for example, 1 / 10,000 or more of the bead diameter of the first dispersed beads. However, it is not limited to this range. For example, the bead diameter of the second dispersed beads is preferably in the range of 80 to 1000 nm. On the other hand, the bead diameter of the first dispersed beads can be, for example, in the range of 0.2 to 1.0 mm.
In the present invention and the present specification, the bead diameter is a value measured by the same method as the method for measuring the average particle size of the powder described above.

The second step is preferably performed under the condition that the second dispersed beads are present in an amount of 10 times or more of the ferromagnetic hexagonal ferrite powder on a mass basis, preferably in an amount of 10 to 30 times. More preferably, it is performed under existing conditions.
On the other hand, the amount of the first dispersed beads in the first stage is also preferably in the above range.

The second dispersed beads are beads having a lower density than the first dispersed beads. The “density” is obtained by dividing the mass (unit: g) of the dispersed beads by the volume (unit: cm ³ ). The measurement is performed by the Archimedes method. The density of the second dispersed beads is preferably not more than 3.7 g / cm ³ , more preferably not more than 3.5 g / cm ³ . The density of the second dispersed beads may be, for example, 2.0 g / cm ³ or more, and may be less than 2.0 g / cm ³ . Preferred second dispersed beads in terms of density include diamond beads, silicon carbide beads, silicon nitride beads, and the like, and preferred second dispersed beads in terms of density and hardness include diamond beads. Can be.
On the other hand, the first dispersion beads, preferably density is 3.7 g / cm ³ greater than the dispersion beads, density is 3.8 g / cm ³ or more dispersing beads more preferably, 4.0 g / cm ³ or more dispersion Beads are more preferred. The density of the first dispersed beads may be, for example, 7.0 g / cm ³ or less, or may be greater than 7.0 g / cm ³ . As the first dispersed beads, it is preferable to use zirconia beads, alumina beads, and the like, and it is more preferable to use zirconia beads.

  The dispersion time is not particularly limited, and may be set according to the type of the disperser used.

<Coating step, cooling step, heat drying step, burnishing step, curing step>
The magnetic layer can be formed by directly applying the composition for forming a magnetic layer on a non-magnetic support, or by sequentially or simultaneously coating the composition for forming a non-magnetic layer. For details of the coating for forming each layer, reference can be made to paragraph 0066 of JP-A-2010-231843.

  In a preferred embodiment, the composition for forming a magnetic layer containing a ferromagnetic hexagonal ferrite powder, a binder, an abrasive, a curing agent and a solvent is applied onto a nonmagnetic support directly or via a nonmagnetic layer. The magnetic layer can be formed through a coating step of forming a layer, a heating drying step of drying the coating layer by heat treatment, and a magnetic layer forming step including a curing step of performing a curing treatment on the coating layer. The magnetic layer forming step preferably includes a cooling step of cooling the coating layer between the coating step and the heating and drying step, and further, between the heating and drying step and the curing step, the surface of the coating layer is burnished ( It is preferable to include a burnishing step of performing burnishing.

Performing the cooling step and the burnishing step in the above-described magnetic layer forming step is considered to be a preferable means for reducing the logarithmic decrement to 0.050 or less. The details are as follows.
Performing the cooling step of cooling the coating layer between the coating step and the heating and drying step involves localizing the adhesive component described above on the surface of the coating layer and / or the surface layer near the surface. It is presumed that it will contribute. This is not because the coating layer of the composition for forming a magnetic layer is cooled before the heating and drying step, so that the adhesive component easily migrates to the surface of the coating layer and / or the surface layer when the solvent evaporates in the heating and drying step. It is thought. However, the reason is not clear. Then, it is considered that the sticky component can be removed by subjecting the surface of the coating layer in which the sticky component is localized to the surface and / or the surface layer to burnishing. It is presumed that performing the curing step after removing the adhesive component in this manner leads to a logarithmic decay rate of 0.050 or less. However, the above is merely an inference and does not limit the present invention in any way.

  As described above, the composition for forming a magnetic layer can be applied sequentially or simultaneously with the composition for forming a nonmagnetic layer. In a preferred embodiment, the magnetic recording medium can be manufactured by successive multilayer coating. The manufacturing process including the sequential multi-layer coating can be preferably performed as follows. The nonmagnetic layer is formed through a coating step of forming a coating layer by coating the composition for forming a nonmagnetic layer on a nonmagnetic support, and a heating and drying step of drying the formed coating layer by heat treatment. Then, a magnetic layer is formed through a coating step of forming a coating layer by coating the composition for forming a magnetic layer on the formed nonmagnetic layer, and a heating and drying step of drying the formed coating layer by a heat treatment. .

  Hereinafter, a specific embodiment of the above manufacturing method will be described with reference to FIG. However, the present invention is not limited to the following specific embodiments.

  FIG. 4 is a process outline showing a specific embodiment of a process for manufacturing a magnetic recording medium having a nonmagnetic layer and a magnetic layer on one surface of a nonmagnetic support in this order, and having a backcoat layer on the other surface. FIG. In the embodiment shown in FIG. 4, the non-magnetic support (elongated film) is continuously fed from the feeding section and wound up by the winding section, and is applied at each section or each zone shown in FIG. By performing various processes such as drying and orientation, a non-magnetic layer and a magnetic layer can be sequentially formed on one surface of the running non-magnetic support by multi-layer coating, and a back coat layer can be formed on the other surface. it can. This manufacturing method is the same as the manufacturing method usually performed for manufacturing a coating type magnetic recording medium, except that a cooling zone is included in the magnetic layer forming step and a burnishing step is performed before the curing processing. Can be.

  On the non-magnetic support sent out from the sending-out section, the non-magnetic layer-forming composition is applied in the first coating section (application step of the non-magnetic layer-forming composition).

  After the coating step, in the first heat treatment zone, the coating layer of the composition for forming a nonmagnetic layer formed in the coating step is heated to dry the coating layer (heat drying step). The heating and drying step can be performed by passing a nonmagnetic support having a coating layer of the composition for forming a nonmagnetic layer through a heating atmosphere. The atmosphere temperature of the heating atmosphere here can be, for example, about 60 to 140 ° C. However, the temperature may be a temperature at which the solvent can be volatilized to dry the coating layer, and the temperature is not limited to the above range. Optionally, a heated gas may be sprayed on the surface of the coating layer. The same applies to the heat drying step in the second heat treatment zone and the heat drying step in the third heat treatment zone, which will be described later.

  Next, in the second coating unit, the composition for forming a magnetic layer is coated on the non-magnetic layer formed by performing the heating and drying step in the first heat treatment zone (the composition of the composition for forming a magnetic layer). Coating process).

  After the coating step, in the cooling zone, the coating layer of the magnetic layer forming composition formed in the coating step is cooled (cooling step). For example, the cooling step can be performed by passing a non-magnetic support having the coating layer formed on the non-magnetic layer in a cooling atmosphere. The ambient temperature of the cooling atmosphere can be preferably in the range of -10 ° C to 0 ° C, and more preferably in the range of -5 ° C to 0 ° C. The time for performing the cooling step (for example, the time from when any part of the coating layer is carried into the cooling zone to when it is carried out (hereinafter, also referred to as “residence time”)) is not particularly limited. Since the value of the logarithmic decay rate tends to decrease as the staying time increases, it is preferable to adjust the logarithmic decay rate by performing a preliminary experiment or the like as necessary so that a logarithmic decay rate of 0.050 or less can be realized. In the cooling step, a cooled gas may be sprayed on the coating layer surface.

  Thereafter, in the embodiment in which the orientation treatment is performed, the orientation treatment of the ferromagnetic hexagonal ferrite powder in the application layer is performed in the orientation zone while the application layer of the composition for forming a magnetic layer is in a wet state. For the orientation treatment, various known techniques including those described in paragraph 0067 of JP-A-2010-231843 can be applied without any limitation. As described above, it is preferable to perform a vertical alignment process as the alignment process from the viewpoint of controlling the XRD intensity ratio. The above description can be referred to for the orientation treatment.

  The coating layer after the orientation treatment is subjected to a heating and drying step in a second heat treatment zone.

  Next, in the third coating section, a composition for forming a back coat layer is applied to the surface of the non-magnetic support opposite to the surface on which the non-magnetic layer and the magnetic layer are formed to form a coating layer. (Step of applying composition for forming back coat layer). Thereafter, in the third heat treatment zone, the coating layer is heat-treated and dried.

  Thus, it is possible to obtain a magnetic recording medium having a coating layer of the composition for forming a magnetic layer which is dried by heating on the nonmagnetic layer on one surface of the nonmagnetic support, and having a backcoat layer on the other surface. it can. The magnetic recording medium obtained here becomes a product magnetic recording medium after performing various processes described later.

  The obtained magnetic recording medium is cut (slit) to the size of the product magnetic recording medium after being wound by a winding section. The slitting can be performed using a known cutting machine.

  The magnetic recording medium that has been slit is heated and dried before performing a curing treatment (heating, light irradiation, etc.) in accordance with the type of curing agent contained in the magnetic layer forming composition. The surface of the coating layer is burnished (burnishing step between the heating and drying step and the curing step). When the burnishing treatment removes the adhesive component cooled in the cooling zone and transferred to the surface and / or surface layer of the coating layer, the logarithmic decay rate becomes 0.050 or less. Speculate. However, as described above, this is merely an inference and does not limit the present invention in any way.

  The burnishing process is a process of rubbing the surface to be treated with a member (for example, a polishing tape, or a grinding tool such as a grinding blade or a grinding wheel), and a burnishing process known for manufacturing a coating type magnetic recording medium. The same can be done. However, burnishing has not been conventionally performed after the cooling step and the heating and drying step and before the curing step. On the other hand, by performing the burnishing process at the above stage, the logarithmic decrement can be reduced to 0.050 or less. This point has been newly found by the present inventors.

  The burnishing treatment is preferably performed by performing one or both of rubbing (polishing) the surface of the coating layer to be treated with a polishing tape and rubbing (grinding) the surface of the coating layer to be treated with a grinding tool. Can be implemented. When the composition for forming a magnetic layer contains an abrasive, it is preferable to use a polishing tape containing at least one abrasive having a higher Mohs hardness than the abrasive. As the polishing tape, a commercially available product may be used, or a polishing tape produced by a known method may be used. As the grinding tool, a known grinding blade such as a fixed blade, a diamond wheel, a rotary blade, or the like, a grinding wheel, or the like can be used. In addition, a wiping process of wiping the surface of the coating layer rubbed with a polishing tape and / or a grinding tool with a wiping material may be performed. For details of a preferable polishing tape, grinding tool, burnishing process and wiping process, reference can be made to paragraphs 0034 to 0048 of JP-A-6-52544, FIG. As the burnishing process is enhanced, the value of the logarithmic decay rate tends to decrease. The burnishing treatment can be strengthened as the abrasive having higher hardness is used as the abrasive contained in the polishing tape, and can be enhanced as the amount of the abrasive in the polishing tape is increased. Further, the higher the hardness of the grinding tool, the stronger the strength. Regarding the burnishing conditions, the burnishing process can be strengthened as the sliding speed between the surface of the coating layer to be treated and a member (for example, a polishing tape or a grinding tool) is increased. The sliding speed can be increased by increasing one or both of the moving speed of the member and the moving speed of the magnetic tape to be processed.

  After the burnishing process (burnishing process), a hardening process is performed on the coating layer of the composition for forming a magnetic layer. In the embodiment shown in FIG. 4, the coating layer of the composition for forming a magnetic layer is subjected to a surface smoothing treatment after the burnishing treatment and before the curing treatment. The surface smoothing treatment is preferably performed by a calendar treatment. For details of the calendar processing, for example, paragraph 0026 of JP-A-2010-231843 can be referred to. The stronger the calendering process, the smoother the magnetic tape surface can be. The calendering can be strengthened by increasing the surface temperature of the calendar roll (calendar temperature) and / or by increasing the calender pressure.

  Thereafter, the coating layer of the composition for forming a magnetic layer is subjected to a curing treatment according to the type of the curing agent contained in the coating layer (curing step). The curing treatment can be performed by a treatment according to the type of the curing agent contained in the coating layer, such as a heat treatment or light irradiation. The curing treatment conditions are not particularly limited, and may be appropriately set according to the formulation of the magnetic layer forming composition used for forming the coating layer, the type of the curing agent, the thickness of the coating layer, and the like. For example, when a coating layer is formed using a magnetic layer forming composition containing a polyisocyanate as a curing agent, the curing treatment is preferably a heat treatment. When a hardening agent is contained in a layer other than the magnetic layer, the hardening reaction of that layer can also be advanced by the hardening treatment here. Alternatively, a curing step may be separately provided. After the curing step, a burnishing treatment may be further performed.

  As described above, a magnetic recording medium according to one embodiment of the present invention can be obtained. However, the above manufacturing method is merely an example, and the XRD intensity ratio, the perpendicular squareness ratio, and the logarithmic decrement of the magnetic layer surface can be controlled to any of the above ranges by any means that can be adjusted. Such an embodiment is also included in the present invention.

  The magnetic recording medium according to one embodiment of the present invention described above can be, for example, a tape-shaped magnetic recording medium (magnetic tape). The magnetic tape is usually accommodated in a magnetic tape cartridge, distributed and used. A servo pattern can be formed on the magnetic tape by a known method in order to enable head tracking servo in a drive. A magnetic tape cartridge is mounted on a drive (also referred to as a magnetic tape device or the like), and the magnetic tape is run in the drive so that the surface of the magnetic tape (the surface of the magnetic layer) and the magnetic head come into contact with each other and slide. Recording and reproduction of information are performed. In order to continuously or intermittently reproduce the information recorded on the magnetic tape, the magnetic tape travels repeatedly in the drive. According to one aspect of the present invention, it is possible to provide a magnetic tape in which the electromagnetic conversion characteristics are hardly deteriorated even when the magnetic layer surface and the head repeatedly slide during the repeated running. Note that the magnetic recording medium according to one embodiment of the present invention is not limited to a magnetic tape. The magnetic recording medium according to one embodiment of the present invention is suitable as various magnetic recording media (magnetic tape, disk-shaped magnetic recording medium (magnetic disk), and the like) used in a sliding type magnetic recording and / or reproducing apparatus. is there. The above-mentioned sliding type device refers to a device in which the surface of the magnetic layer comes into contact with the head and slides when recording information on a magnetic recording medium and / or reproducing the recorded information. Such devices include at least a magnetic tape and one or more magnetic heads for recording and / or reproducing information.

  In the sliding-type device, as the traveling speed of the magnetic tape is increased, the recording of information and the reproduction of the recorded information can be performed in a shorter time. Here, the running speed of the magnetic tape refers to a relative speed between the magnetic tape and the magnetic head, and is usually set by a control unit of the apparatus. As the traveling speed of the magnetic tape increases, the pressure applied to both the surface of the magnetic layer and the magnetic head at the time of contact increases, so that head scraping and / or scraping of the magnetic layer tends to occur. Therefore, it is considered that the higher the traveling speed is, the more easily the electromagnetic conversion characteristics are reduced in repeated sliding. In the field of magnetic recording, high-density recording is required, but as the recording density increases, the effect of signal interference with adjacent bits increases, so that when the spacing loss increases due to repeated sliding, the electromagnetic conversion characteristics will increase. Tends to decrease more easily. As described above, as the high-speed running and the high-density recording progress, the reduction in the electromagnetic conversion characteristics in repeated sliding tends to become more apparent. On the other hand, even in such a case, according to the magnetic recording medium of one embodiment of the present invention, it is possible to suppress a decrease in electromagnetic conversion characteristics in repeated sliding. The magnetic tape according to one embodiment of the present invention is suitable for use in, for example, a sliding-type device in which the traveling speed of the magnetic tape is 5 m / sec or more (for example, 5 to 20 m / sec). Further, the magnetic tape according to one embodiment of the present invention is suitable, for example, as a magnetic tape for recording and reproducing information at a linear recording density of 250 kfci or more. The unit kfci is a unit of linear recording density (cannot be converted to SI unit system). The linear recording density can be, for example, 250 kfci or more, and can be 300 kfci or more. Further, the linear recording density can be, for example, 800 kfci or less, and can be more than 800 kfci.

  Hereinafter, the present invention will be described based on examples. However, the present invention is not limited to the embodiments shown in the examples. The indication of “parts” and “%” described below indicates “parts by mass” and “% by mass” unless otherwise specified. Further, the steps and evaluations described below were performed in an environment at an ambient temperature of 23 ° C. ± 1 ° C. unless otherwise specified.

[Example 1]
The formulation of each layer forming composition is shown below.

<Formulation of composition for forming magnetic layer>
(Magnetic liquid)
Plate-like ferromagnetic hexagonal ferrite powder (M-type barium ferrite): 100.0 parts (activation volume: 1500 nm ³ )
Oleic acid: 2.0 parts Vinyl chloride copolymer (manufactured by Zeon Corporation MR-104): 10.0 parts SO ₃ Na group-containing polyurethane resin: 4.0 parts (weight average molecular weight 70000, SO ₃ Na group: 0.1 part) 07meq / g)
Amine-based polymer (DISPERBYK-102, manufactured by BYK-Chemie): 6.0 parts Methyl ethyl ketone: 150.0 parts Cyclohexanone: 150.0 parts (abrasive liquid)
α-alumina: 6.0 parts (BET specific surface area 19 m ² / g, Mohs hardness 9)
SO ₃ Na group-containing polyurethane resin: 0.6 part (weight average molecular weight 70000, SO ₃ Na group: 0.1 meq / g)
2,3-dihydroxynaphthalene: 0.6 parts cyclohexanone: 23.0 parts (projection forming agent liquid)
Colloidal silica: 2.0 parts (average particle size 80 nm)
Methyl ethyl ketone: 8.0 parts (lubricant and hardener liquid)
Stearic acid: 3.0 parts Stearic acid amide: 0.3 parts Butyl stearate: 6.0 parts Methyl ethyl ketone: 110.0 parts Cyclohexanone: 110.0 parts Polyisocyanate (Coronate (registered trademark) L manufactured by Tosoh Corporation): 3 .0 copies

<Formulation of composition for forming nonmagnetic layer>
Non-magnetic inorganic powder α iron oxide: 100.0 parts (average particle size 10 nm, BET specific surface area 75 m ² / g)
Carbon black: 25.0 parts (average particle size 20 nm)
SO ₃ Na group-containing polyurethane resin: 18.0 parts (weight average molecular weight 70000, SO ₃ Na group content 0.2 meq / g)
Stearic acid: 1.0 part Cyclohexanone: 300.0 parts Methyl ethyl ketone: 300.0 parts

<Formulation of composition for forming back coat layer>
Non-magnetic inorganic powder α iron oxide: 80.0 parts (average particle size 0.15 μm, BET specific surface area 52 m ² / g)
Carbon black: 20.0 parts (average particle size 20 nm)
Vinyl chloride copolymer: 13.0 parts Sulfonate group-containing polyurethane resin: 6.0 parts Phenylphosphonic acid: 3.0 parts Cyclohexanone: 155.0 parts Methyl ethyl ketone: 155.0 parts Stearic acid: 3.0 parts Stearic acid Butyl: 3.0 parts Polyisocyanate: 5.0 parts Cyclohexanone: 200.0 parts

<Preparation of composition for forming magnetic layer>
A composition for forming a magnetic layer was prepared by the following method.
The various components of the magnetic liquid are dispersed for 24 hours using zirconia beads having a bead diameter of 0.5 mm (first dispersion beads, density 6.0 g / cm ³ ) by a batch type vertical sand mill (first step). ) And then using a filter having a pore size of 0.5 µm to prepare Dispersion A. The zirconia beads were used in an amount 10 times the mass of the ferromagnetic hexagonal ferrite powder on a mass basis.
Thereafter, the dispersion A was dispersed in a batch type vertical sand mill using diamond beads having the bead diameter shown in Table 1 (second dispersed beads, density 3.5 g / cm ³ ) for the time shown in Table 1 (second dispersion). ), A dispersion (dispersion B) was prepared by separating the diamond beads using a centrifuge. The following magnetic liquid is the dispersion B thus obtained.
The abrasive liquid is mixed with various components of the above-mentioned abrasive liquid and put into a horizontal bead mill dispersing machine together with zirconia beads having a bead diameter of 0.3 mm, and the bead volume / (abrasive liquid volume + bead volume) becomes 80%. Then, bead mill dispersion treatment was performed for 120 minutes, the treated liquid was taken out, and subjected to ultrasonic dispersion filtration treatment using a flow type ultrasonic dispersion filtration device. Thus, an abrasive solution was prepared.
The prepared magnetic liquid, abrasive liquid, and the remaining components were introduced into a dissolver stirrer, stirred at a peripheral speed of 10 m / sec for 30 minutes, and then subjected to a 3-pass treatment at a flow rate of 7.5 kg / min by a flow ultrasonic disperser. Thereafter, the mixture was filtered through a filter having a pore size of 1 μm to prepare a composition for forming a magnetic layer.

  The activation volume of the ferromagnetic hexagonal ferrite powder described above is a value measured and calculated using the same powder lot as the ferromagnetic hexagonal ferrite powder used for preparing the composition for forming a magnetic layer. . The measurement was performed using a vibration sample type magnetometer (manufactured by Toei Kogyo KK) at a magnetic field sweep speed of 3 minutes and 30 minutes of the coercive force Hc measuring unit, and the activation volume was calculated from the above-described relational expression. The measurement was performed in an environment of 23 ° C. ± 1 ° C.

<Preparation of composition for forming nonmagnetic layer>
The various components of the composition for forming a non-magnetic layer are dispersed for 24 hours using zirconia beads having a bead diameter of 0.1 mm by a batch type vertical sand mill, and thereafter, using a filter having a pore diameter of 0.5 μm. By filtering, a composition for forming a non-magnetic layer was prepared.

<Preparation of composition for forming back coat layer>
After kneading and diluting components other than the lubricant (stearic acid and butyl stearate), polyisocyanate and 200.0 parts of cyclohexanone among the various components of the composition for forming a back coat layer using an open kneader, the mixture is dispersed in a horizontal bead mill. Using a zirconia bead having a bead diameter of 1 mm by a machine, the bead filling rate was 80% by volume, the residence time per pass was 2 minutes at a peripheral speed of the rotor tip of 10 m / sec, and the dispersion treatment was performed for 12 passes. Thereafter, the above-mentioned remaining components were added, the mixture was stirred with a dissolver, and the obtained dispersion was filtered using a filter having a pore size of 1 μm to prepare a composition for forming a backcoat layer.

<Preparation of magnetic tape>
A magnetic tape was manufactured according to the specific embodiment shown in FIG. The details are as follows.
A support made of polyethylene naphthalate having a thickness of 5.0 μm is delivered from the delivery portion, and a composition for forming a nonmagnetic layer is applied to one surface so that the thickness after drying in the first application portion is 100 nm. It was dried in one heat treatment zone (atmospheric temperature 100 ° C.) to form a coating layer.
Thereafter, the composition for forming a magnetic layer was applied on the non-magnetic layer so that the thickness after drying became 70 nm in the second application section, thereby forming an application layer. After the formed coating layer was passed through the cooling zone adjusted to the ambient temperature of 0 ° C. for the stay time shown in Table 1 while the formed coating layer was in the wet state, and the cooling step was performed, the magnetic field having the strength shown in Table 1 was further applied to the orientation zone. The coating was applied in the vertical direction to the surface of the coating layer to perform a vertical alignment treatment, and then dried in a second heat treatment zone (atmospheric temperature 100 ° C.).
Thereafter, in the third coating portion, the back coat layer is formed on the surface of the polyethylene naphthalate support opposite to the surface on which the nonmagnetic layer and the magnetic layer are formed so that the thickness after drying is 0.4 μm. The composition for forming was applied to form a coating layer, and the formed coating layer was dried in a third heat treatment zone (atmospheric temperature 100 ° C.).
After slitting the magnetic tape thus obtained to a width of 1/2 inch (0.0127 m), the surface of the coating layer of the composition for forming a magnetic layer was subjected to burnishing and wiping. The burnishing process and the wiping process are performed by using a commercially available polishing tape (trade name: MA22000, manufactured by FUJIFILM Corporation, abrasive: diamond / Cr ₂₎ as a polishing tape in a processing apparatus having a configuration shown in FIG. A commercially available wiping material (manufactured by Kuraray Co., Ltd.) is used as a wiping material using a commercially available sapphire blade (manufactured by Kyocera Corporation, width: 5 mm, length: 35 mm, tip angle: 60 degrees) using O ₃ / Vengara) (WRP 736). The processing conditions employed in Example 12 of JP-A-6-52544 were employed.
After the burnishing process and the wiping process, at a speed of 80 m / min, a linear pressure of 300 kg / cm (294 kN / m), and a calendar temperature (surface temperature of the calendar roll) of 90 ° C. using a calendar roll composed of only metal rolls. A calendar process (surface smoothing process) was performed.
Then, after performing a heat treatment (curing treatment) for 36 hours in an environment of an atmosphere temperature of 70 ° C., a servo pattern was formed on the magnetic layer by a commercially available servo writer.
Thus, the magnetic tape of Example 1 was obtained.

<Evaluation of a decrease in electromagnetic conversion characteristics (Signal-to-Noise-Ratio; SNR)>
The electromagnetic conversion characteristics of the magnetic tape of Example 1 were measured by the following method using a 1/2 inch (0.0127 meter) reel tester to which the head was fixed.
The running speed (magnetic head / magnetic tape relative speed) of the magnetic tape was a value shown in Table 1. A MIG (Metal-In-Gap) head (gap length 0.15 μm, track width 1.0 μm) was used as a recording head, and the recording current was set to the optimum recording current for each magnetic tape. A GMR (Giant-Magnetoresistive) head having a device thickness of 15 nm, a shield interval of 0.1 μm, and a lead width of 0.5 μm was used as a reproducing head. The signals were recorded at the linear recording densities shown in Table 1, and the reproduced signals were measured with a spectrum analyzer manufactured by Shibasoku. The ratio between the output value of the carrier signal and the integrated noise in the entire spectrum band was defined as SNR. For the SNR measurement, a signal in a portion where the signal was sufficiently stable after running the magnetic tape was used.
Under the above conditions, the SNR was measured after each magnetic tape was reciprocated for 5,000 passes in an environment at a temperature of 40 ° C. and a relative humidity of 80% at 1,000 m per pass. The difference between the SNR of the first pass and the SNR of the 5000th pass (SNR of the 5000th pass-SNR of the 1st pass) was obtained.
The above recording and reproduction were performed by sliding the surface of the magnetic layer of the magnetic tape and the head.

[Examples 2 to 17]
A magnetic tape was produced in the same manner as in Example 1 except that the various items shown in Table 1 were changed as shown in Table 1, and the magnetic conversion characteristics (SNR) reduction of the produced magnetic tape was evaluated.
In Table 1, for the examples in which “None” is described in the column of the dispersed beads and the column of the time, the composition for forming a magnetic layer was prepared without performing the second step in the magnetic liquid dispersion treatment.
In Table 1, in the example of “None” in the column of the magnetic field strength for the vertical alignment treatment, the magnetic layer was formed without performing the alignment treatment.
In Table 1, in the example of “not performed” in the column of the cooling zone stay time and the column of the burnishing process before the curing treatment, the magnetic layer forming step does not include the cooling zone, and the burnishing before the curing treatment is performed. A magnetic tape was manufactured by a manufacturing process without performing the processing and the wiping processing.

  A part of each of the produced magnetic tapes was used for evaluation of deterioration of electromagnetic conversion characteristics (SNR), and another part was used for evaluation of the following physical properties.

[Evaluation of physical properties of magnetic tape]
(1) XRD intensity ratio A tape sample was cut out from each of the magnetic tapes of Examples 1 to 17.
The cut-out tape sample was irradiated with X-rays on the surface of the magnetic layer using a thin-film X-ray diffractometer (SmartLab manufactured by Rigaku Corporation), and In-Plane XRD was performed by the method described above.
From the X-ray diffraction spectrum obtained by In-Plane XRD, the peak intensity Int (114) of the diffraction peak on the (114) plane and the peak intensity Int (110) of the diffraction peak on the (110) plane of the hexagonal ferrite crystal structure were determined. Then, the XRD intensity ratio (Int (110) / Int (114)) was calculated.

(2) Vertical direction squareness ratio For each of the magnetic tapes of Examples 1 to 17, the vertical direction squareness ratio was determined by the method described above using a vibration sample type magnetometer (manufactured by Toei Kogyo Co., Ltd.).

(3) Measurement of Logarithmic Decay Rate of Magnetic Layer Surface As a measuring device, a rigid pendulum type physical property tester RPT-3000W manufactured by A & D Corporation (Pendulum: rigid pendulum FRB-100 manufactured by A & D Corporation, Weight: none, round bar type cylinder edge: R & D Co., Ltd., RBP-040 manufactured by A & D Co., Ltd., substrate: glass substrate, substrate heating rate 5 ° C./min. The logarithmic decay rate of the layer surface was determined.
As the glass substrate, a commercially available slide glass cut into a size of 25 mm (short side) × 50 mm (long side) was used. With the magnetic tape placed on the center of the glass substrate so that the longitudinal direction of the magnetic tape is parallel to the short side direction of the glass substrate, fix the four corners of the magnetic tape on the glass substrate with a fixing tape (Dupont Toray DuPont Kapton Tape). ) And fixed to the glass substrate. Thereafter, the portion of the magnetic tape protruding from the glass substrate was cut. In this way, the measurement sample was fixed and mounted on the glass substrate at four points as shown in FIG. The adsorption time was 1 second, the measurement interval was 7 to 10 seconds, a displacement-time curve was created for the 86th measurement interval, and the logarithmic decay rate was determined using this curve. The measurement was performed in an environment with a relative humidity of about 50%.

  Table 1 shows the above results.

From the results shown in Table 1, Examples 1 to 8 in which the XRD intensity ratio of the magnetic tape, the squareness in the vertical direction, and the logarithmic decrement of the surface of the magnetic layer are respectively in the ranges described above, as compared with Examples 9 to 17, It was confirmed that even when reproduction was repeated by sliding the magnetic layer surface and the head, the electromagnetic conversion characteristics did not decrease much.
Examples 9 to 11 are examples using magnetic tapes having the same physical properties, and have different magnetic tape running speeds and linear recording densities. From the comparison of Examples 9 to 11, it can be confirmed that as the magnetic tape is run at a higher speed and as the linear recording density is increased, the electromagnetic conversion characteristics in repeated sliding become more apparent. Such a decrease in the electromagnetic conversion characteristics in repeated sliding was able to be suppressed in Examples 1 to 8.

  The present invention is useful in the technical field of data storage magnetic recording media such as data backup tapes.

Claims (6)

A magnetic recording medium having a magnetic layer containing a ferromagnetic powder and a binder on a non-magnetic support,
The ferromagnetic powder is a ferromagnetic hexagonal ferrite powder, the magnetic layer includes an abrasive,
The peak intensity Int of the diffraction peak of the (110) plane relative to the peak intensity Int of the (114) plane of the hexagonal ferrite crystal structure obtained by X-ray diffraction analysis of the magnetic layer using the In-Plane method is Int. The intensity ratio of (110), Int (110) / Int (114), is 0.5 or more and 4.0 or less,
The perpendicular recording squareness ratio of the magnetic recording medium is 0.65 or more and 1.00 or less, and the logarithmic decrement obtained on the surface of the magnetic layer by a pendulum viscoelasticity test is 0.050 or less. Medium. The magnetic recording medium according to claim 1, wherein the perpendicular squareness ratio is 0.65 or more and 0.90 or less. The magnetic recording medium according to claim 1, wherein the logarithmic decay rate is 0.010 or more and 0.050 or less. The magnetic recording medium according to claim 1, further comprising a nonmagnetic layer containing a nonmagnetic powder and a binder between the nonmagnetic support and the magnetic layer. The magnetic recording according to claim 1, further comprising a backcoat layer containing a nonmagnetic powder and a binder on a surface of the nonmagnetic support opposite to a surface having the magnetic layer. Medium. The magnetic recording medium according to any one of claims 1 to 5, which is a magnetic tape.