JP2018133120A - Magnetic recording medium and recording / reproducing mechanism thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic recording medium having excellent electromagnetic conversion characteristics and running durability. A magnetic recording medium according to the present invention includes a non-magnetic support and a magnetic layer containing magnetic particles, and is the length of magnetization of a signal recorded on the magnetic layer, which is the length of magnetization of the magnetic layer. The length of the magnetization in the width direction is 1 μm or less, the magnetic particles are made of ε-iron oxide having an average particle diameter of 15 nm or less, and the square ratio in the thickness direction of the magnetic layer is 0.65. As described above, where L is the average thickness of the surface mixed layer formed on the surface of the magnetic layer opposite to the non-magnetic support side, 4 nm ≦ L ≦ 8 nm. [Selection diagram] Fig. 2

Description

  The present invention relates to a high-capacity magnetic recording medium excellent in electromagnetic conversion characteristics and running durability, and a recording / reproducing mechanism for the magnetic recording medium.

  Coating-type magnetic recording media in which a magnetic layer containing magnetic powder and a binder is formed on a non-magnetic support will further improve recording density as the recording / reproducing method shifts from analog to digital. Is required. In particular, this demand is increasing year by year for high-density digital video tapes and computer backup tapes.

  With such an increase in recording density, the recording wavelength has been shortened, and in order to cope with this short wavelength recording, the magnetic powder has been made finer year by year. At present, the ferromagnetic material has an average particle diameter of about 20 nm. Hexagonal ferrite powder has been realized, and a magnetic recording medium using this magnetic powder has been put into practical use (for example, Patent Document 1).

  In order to further improve the recording density of the magnetic recording medium using the ferromagnetic hexagonal ferrite powder, it is necessary to further refine the ferromagnetic hexagonal ferrite powder. However, there is a problem that by further miniaturizing the ferromagnetic hexagonal ferrite powder, the particle volume of the magnetic powder becomes small and is easily affected by thermal fluctuations. For this reason, it is necessary to suppress thermal fluctuations using a magnetic material that has a high coercive force and a large anisotropic energy even if it is made fine.

Under such circumstances, in recent years, ε-Fe ₂ O ₃ has been studied as a new magnetic material for magnetic recording media, and ε has a ferromagnetic property even with an average particle size of 15 nm or less, preferably 10 nm or less. -fe ₂ O ₃ of consisting of a single-phase iron oxide magnetic nanoparticles powder has been proposed (e.g., Patent Document 2). A magnetic recording medium using ε-Fe ₂ O ₃ as a magnetic powder has also been proposed (for example, Patent Documents 3 and 4).

  Further, as prior art documents related to the present invention, Patent Document 5 relating to a method for measuring layer thickness, Patent Document 6 specifying the thickness of the mixed layer on the surface of the magnetic layer, and forming a lubricant layer on the surface of the magnetic layer Patent Document 7 relating to a method for performing the method and Patent Document 8 relating to a method for measuring the spacing.

Japanese Patent Laying-Open No. 2015-91747 JP 2014-224027 A JP 2014-149886 A Japanese Patent Laying-Open No. 2015-82329 JP 2008-128672 A JP 2012-133837 A JP 2014-157644 A JP 2012-43495 A

  As the recording density for increasing the capacity of such magnetic recording media increases, the track density of the magnetic layer is increasing. However, there is a problem that the track width is reduced with the increase in the track density, and as a result, the output characteristics are lowered and the electromagnetic conversion characteristics are also lowered.

  Further, ε-iron oxide can improve the electromagnetic conversion characteristics because it can be made into fine particles having an average particle diameter of 15 nm or less while maintaining a high coercive force. However, when the magnetic particles become fine particles, the strength of the magnetic layer is increased. In addition, since the ε-iron oxide particles are usually spherical, the magnetic layer has a dense structure, and the lubricant contained in the undercoat layer is difficult to move to the outer surface of the magnetic layer. As a result, the magnetic layer There is a problem that the running durability of the vehicle decreases.

  The present invention has been made to solve such problems, and uses ε-iron oxide particles as magnetic particles, and electromagnetic conversion characteristics and running durability even when the track density is increased as the recording density is increased. Provided is a magnetic recording medium excellent in properties.

  The magnetic recording medium of the present invention is a magnetic recording medium comprising a nonmagnetic support and a magnetic layer containing magnetic particles, wherein the magnetic layer has a length of magnetization of a signal recorded in the magnetic layer. The magnetization length in the width direction is 1 μm or less, the magnetic particles are made of ε-iron oxide having an average particle diameter of 15 nm or less, and the squareness ratio in the thickness direction of the magnetic layer is 0. When the average thickness of the surface mixed layer formed on the surface of the magnetic layer opposite to the nonmagnetic support is L, 4 nm ≦ L ≦ 8 nm.

  The magnetic recording medium recording / reproducing mechanism of the present invention includes the magnetic recording medium of the present invention and a TMR head.

  According to the magnetic recording medium of the present invention, ε-iron oxide fine particles having a high coercive force are used as magnetic particles, and the track width is reduced by increasing the track density as the recording density is increased for increasing the capacity. Even in this case, it is possible to provide a magnetic recording medium excellent in electromagnetic conversion characteristics and running durability.

1A is a cross-sectional photograph of the magnetic recording medium, FIG. 1B is a diagram showing a luminance curve of the cross section of the magnetic recording medium shown in FIG. 1A, and FIG. 1C is a diagram showing a differential curve of the luminance curve shown in FIG. 1B. It is. FIG. 2 is a schematic cross-sectional view showing an example of a magnetic recording medium.

(Embodiment of magnetic recording medium)
An embodiment of the magnetic recording medium of the present invention will be described.

  The magnetic recording medium of the present embodiment includes a nonmagnetic support and a magnetic layer containing magnetic particles, the length of the magnetization of a signal recorded in the magnetic layer, and the width direction of the magnetic layer. The length of magnetization is 1 μm or less. The magnetic particles are made of ε-iron oxide having an average particle diameter of 15 nm or less, and the squareness ratio of the magnetic layer in the thickness direction is 0.65 or more. Furthermore, when the average thickness of the surface mixed layer formed on the surface of the magnetic layer opposite to the nonmagnetic support is L, 4 nm ≦ L ≦ 8 nm.

  Since the magnetic particles are made of ε-iron oxide, the coercive force of the magnetic particles does not decrease even if the average particle diameter of the magnetic particles is 15 nm or less in order to make the track width 1 μm or less. The average particle diameter of the magnetic particles made of ε-iron oxide is more preferably 12 nm or less. The lower limit of the average particle diameter of the magnetic particles made of the ε-iron oxide is usually about 8 nm. This is because ε-iron oxide having an average particle diameter of less than 8 nm is not easy to produce.

  Further, since the resolution of the recording magnetization is improved by setting the squareness ratio in the thickness direction of the magnetic layer to 0.65 or more, even better electromagnetic conversion characteristics (SN characteristics) can be achieved even if the track width is 1 μm or less. ) Can be obtained. The squareness ratio is more preferably 0.75 or more.

  Furthermore, since the average thickness L of the surface mixed layer is set to 4 nm ≦ L ≦ 8 nm, the magnetic recording medium of the present embodiment is excellent in electromagnetic conversion characteristics and running durability. The average thickness L of the surface mixed layer is more preferably 7 nm ≦ L ≦ 8 nm. As described above, the ε-iron oxide can be made into fine particles having an average particle diameter of 15 nm or less while maintaining a high coercive force. However, when the fine particles are formed, the strength of the magnetic layer is reduced, and the ε-oxidation is performed. Since iron particles are usually spherical, the magnetic layer has a dense structure, and the lubricant contained in the undercoat layer is difficult to move to the outer surface of the magnetic layer, resulting in a decrease in running durability of the magnetic layer. There is. However, by setting the average thickness L of the surface mixed layer to 4 nm ≦ L ≦ 8 nm, both electromagnetic conversion characteristics and running durability can be improved.

  The ε-iron oxide is usually composed of spherical particles, but is not limited to spherical particles, and may be substantially spherical or elliptical.

  The magnetic recording medium of this embodiment is preferably reproduced by a tunnel type magnetoresistive head (TMR head). This is because the TMR head has high sensitivity and can obtain a high S / N ratio by combining with fine particles of ε-iron oxide.

  The coercive force in the thickness direction of the magnetic layer is preferably 3000 oersted [Oe] or more. This is because by setting the coercive force to 3000 oersted [Oe] or higher, a high reproduction output can be obtained with little self-demagnetization loss even in a short wavelength recording region at high recording density.

  Further, when the spacing of the surface of the magnetic layer after the surface of the magnetic layer is washed with n-hexane is measured by TSA (Tape Spacing Analyzer), the spacing value is 5 nm or more and 12 nm or less. Is preferred. If the spacing value is less than 5 nm, the surface of the magnetic layer becomes too smooth, the contact area between the magnetic head and the magnetic layer increases, the friction coefficient increases, and the durability of the magnetic layer decreases. Tend. On the other hand, when the spacing value exceeds 12 nm, the distance between the magnetic head and the magnetic layer surface becomes too large, and the recording / reproducing characteristics tend to be deteriorated. The spacing value is more preferably 7 nm to 13 nm, and most preferably 8 nm to 11 nm.

  The method for measuring the spacing value and the control method thereof are not particularly limited, and can be performed by, for example, the method described in JP 2012-43495 A (Patent Document 8).

  The average thickness t of the magnetic layer is preferably 30 nm or more and 200 nm or less. Short wavelength recording characteristics can be improved by setting the average thickness of the magnetic layer to 200 nm or less, and servo signals can be recorded by setting the average thickness of the magnetic layer to 30 nm or more. When ε-iron oxide particles are used as the magnetic particles of the present embodiment, the saturation magnetization amount of ε-iron oxide particles is 1/2 to 1/1 compared to the saturation magnetization amount of conventional ferromagnetic hexagonal ferrite particles. Therefore, when recording a servo signal having a long recording wavelength, the average thickness of the magnetic layer needs to be 30 nm or more.

  When the servo signal is not recorded on the magnetic layer, the average thickness of the magnetic layer is preferably 10 nm or more and 50 nm or less. Even if the average thickness of the magnetic layer is 10 nm or more and 50 nm or less, data signals can be recorded and reproduced by using a highly sensitive magnetic head such as a TMR head.

  Next, a method for measuring the thickness of each layer of the magnetic recording medium of this embodiment will be described.

  The present inventors have previously described a method for measuring a layer thickness in a structure having layers with different elemental compositions, which does not involve artificial judgment and does not have inaccuracies such as a value changing depending on conditions. No. (Patent Document 5). The thickness of each layer of the magnetic recording medium of the present embodiment is defined as the thickness measured by the following method (hereinafter referred to as “the layer thickness measurement method of the present embodiment”) based on the layer thickness measurement method.

  In the layer thickness measuring method of this embodiment, first, a carbon layer having a thickness of about 50 to 100 nm is formed on the surface of the magnetic layer of the magnetic recording medium to be measured by sputtering, and further a thickness of about 50 to 100 nm is formed thereon. A Pt—Pd layer is formed by sputtering. Next, a cross section of a sample including a carbon layer, a Pt—Pd layer, a magnetic layer, and an undercoat layer was produced using a focused ion beam (FIB) apparatus, and the obtained cross section was subjected to YAG using a scanning electron microscope (SEM). Observation is performed using a (Yttrium Aluminum Garnet) detector, and an acceleration voltage is set to 7 kV to obtain a backscattered electron (BSE) image of the cross section. Next, the image data is digitized to obtain image luminance data in the thickness direction, and a luminance curve is created from the image luminance data.

  In the above digitization, the obtained cross-sectional image (for example, FIG. 1) is divided into a predetermined number in the X-axis direction (the thickness direction of each layer) and the Y-axis direction (the surface direction of each layer), and each divided coordinate point Are converted into a predetermined number of gradation values. Specifically, when a cross-sectional image is obtained as a photograph, the photograph image is converted into digital data by reading it with a scanner. For example, data of 256 gradations (0 to 255) is obtained by processing the luminance with 8 bits. can get. In the case of this example, the luminance data is obtained by being divided into 2560 in the Y-axis direction. Even when a cross-sectional image is obtained via a photoelectric conversion element such as a CCD, it is converted into digital data, and digital data of each coordinate point of the cross-sectional image is obtained.

  Next, the luminance values (for example, 2560) are averaged in the Y coordinate direction for each X coordinate of the obtained two-dimensional data of luminance, and the Y-axis direction in the coating film thickness direction as shown in FIG. 1B An averaged luminance curve can be obtained.

  Finally, the luminance curve is differentiated to create a differential curve, the boundary of each layer is determined from the peak position of the differential curve, and the average thickness t of the magnetic layer is first determined from the distance between the peaks.

  Further, the present inventors have further studied and found that there is a correlation between the shape of the luminance curve and the electromagnetic conversion characteristics and running durability of the magnetic recording medium. This will be described with reference to FIG.

  FIG. 1A is a cross-sectional photograph of a magnetic recording medium as an example of the present embodiment taken with a scanning electron microscope (SEM), and FIG. 1B is a diagram showing a luminance curve of the cross section of the magnetic recording medium shown in FIG. 1A. FIG. 1C is a diagram showing a differential curve of the luminance curve shown in FIG. 1B. In this embodiment, the distance between the peak P1 and the peak P2 of the differential curve in FIG. 1C is defined as the average thickness t of the magnetic layer. In this embodiment, when the average luminance value of the undercoat layer is 0 and the maximum luminance value of the magnetic layer is 100, the luminance value 70 on the surface side opposite to the undercoat layer side of the magnetic layer is determined. A region having an average thickness L up to 30 is defined as a surface mixed layer, and a region having an average thickness M from 70 to 30 on the undercoat layer side of the magnetic layer is defined as an interface mixed layer. Thereby, the average thickness L of the surface mixed layer and the average thickness M of the interface mixed layer can be clearly measured.

  The region excluding the region overlapping the surface mixed layer and the interface mixed layer from the region of thickness t called the magnetic layer is a substantially pure magnetic layer region, whereas the region of the surface mixed layer Is considered to be a region where a large amount of lubricant, resin, filler, voids and the like are unevenly distributed with respect to the components inside the magnetic layer, and the region of the interface mixed layer is considered to be a region where the undercoat layer component has been mixed.

  According to the study by the present inventors, the average thickness L of the surface mixed layer is correlated with the running durability and the S / N ratio of the electromagnetic conversion characteristics, and the average thickness M of the interface mixed layer is the S of the electromagnetic conversion characteristics. It was found that there is a correlation with the / N ratio.

  The surface mixed layer is a mixed layer formed on the outer surface of the magnetic layer, and is considered to be a region in which many lubricants, resins, fillers, voids, and the like are unevenly distributed with respect to the components of the magnetic layer region. It is considered that the presence of such a material improves running durability and improves practical characteristics without greatly degrading the electromagnetic conversion characteristics of the magnetic recording medium.

  The average thickness L of the surface mixed layer is set in a range of 4 nm ≦ L ≦ 8 nm, and L is more preferably in a range of 7 nm ≦ L ≦ 8 nm. In conventional magnetic recording media, the average thickness of the surface mixed layer is reduced and the spacing value is reduced, thereby reducing the spacing loss during recording and reproduction and suppressing the deterioration of the electromagnetic conversion characteristics. It was. On the other hand, in the magnetic recording medium of this embodiment, the running durability is improved by setting the average thickness L of the surface mixed layer to a certain extent. Further, in the magnetic recording medium of the present embodiment, even when the average thickness L of the surface mixed layer is set to be somewhat thick, since ε-iron oxide is used as the magnetic particles, the electromagnetic conversion characteristics can be maintained high, Even if the spacing loss during recording and reproduction increases, it is possible to suppress a decrease in electromagnetic conversion characteristics.

  On the other hand, if the value of L is less than 4 nm, the running durability deteriorates because there is little uneven distribution of lubricant, resin, filler, voids, etc. on the surface of the magnetic layer, and if the value of L exceeds 8 nm, the magnetic head Even if ε-iron oxide is used as the magnetic particles, the spacing loss during recording and reproduction tends to increase and the electromagnetic conversion characteristics tend to deteriorate.

  In the present invention, even in a range larger than the thickness range of the first mixed layer in Patent Document 6, electromagnetic conversion characteristics can be reduced by using ε-iron oxide having an average particle size of 15 nm or less for the magnetic particles. In other words, the present invention has been found to improve running durability.

  The method for controlling the value of L within the above range is not particularly limited, but preferably, the following method is exemplified.

  (A) A method of transferring the lubricant onto the outer surface of the magnetic layer and applying it by sliding the belt-like coating cloth impregnated with the lubricant while running on the outer surface of the magnetic layer.

  More specifically, as described in Japanese Patent Application Laid-Open No. 2014-157644 (Patent Document 7), a belt-shaped coating cloth impregnated with a lubricant is slid on the outer surface of the magnetic layer while running. When the lubricant is transferred to the outer surface of the magnetic layer and applied, the amount of lubricant on the outer surface of the magnetic layer increases and L increases. In this way, the size of L can be finely controlled by transferring and applying the lubricant to the outer surface of the magnetic layer. On the other hand, in the conventional application of the lubricant, since the lubricant is directly applied (top coat) to the outer surface of the magnetic layer, the size of L cannot be controlled finely.

  In the magnetic recording medium of the present embodiment, since the spherical ε-iron oxide particles having an average particle diameter of 15 nm or less are used as the magnetic particles, the magnetic layer has a dense structure and is included in the magnetic layer and the undercoat layer. It is difficult to move the lubricant to the outer surface of the magnetic layer, but by transferring the lubricant to the outer surface of the magnetic layer as described above, the lubricant can be reliably supplied to the outer surface of the magnetic layer, The size of L can be easily controlled.

  (B) A method of directly controlling the thickness of the surface mixed layer by forming the magnetic layer and polishing the surface with a rotor, blade, wrapping tape, polishing wheel, or the like.

  After the magnetic layer is formed, polishing with a rotor, blade, wrapping tape, polishing wheel, etc. removes any material present on the surface of the magnetic layer, such as lubricants and fillers, and at the same time reduces the height of the protrusions on the surface of the magnetic layer. L can be reduced.

  (C) A method of setting the calendering conditions of the magnetic layer to a linear pressure of 196 to 294 kN / m and a temperature of 70 to 120 ° C.

  More specifically, when the linear pressure or temperature of the calendar condition is increased, the protrusion on the surface of the magnetic layer, which is one of the elements constituting L, is reduced, so that L is reduced. Moreover, since the space | gap in a surface mixing layer can also be made small by calendering, L can be controlled.

  (D) A method of controlling the amount of the binder on the surface of the magnetic layer according to the amount of binder resin added, the method of kneading the magnetic particles, the degree of surface treatment of the magnetic particles, and the like.

  More specifically, by increasing the amount of binder resin added, more binder resin that does not adsorb on the surface of the magnetic particles will be present on the surface of the magnetic layer, and L will increase. Also, the surface treatment of the magnetic particles can reduce the adsorption of the binder resin to the surface of the magnetic particles, and as a result, there are many binder resins on the surface of the magnetic layer that are not adsorbed on the surface of the magnetic particles. As a result, L increases. However, when the dispersion state of the paint changes depending on the kneading method and the amount of binder resin adsorbed on the surface of the magnetic particles increases, the amount of binder on the surface of the magnetic layer decreases and L decreases.

  (E) A method of controlling the amount of filler and the amount of voids on the surface of the magnetic layer by the amount of filler added, the timing of addition, and the surface treatment.

  More specifically, when the amount of filler added increases, the number of fillers present on the surface of the magnetic layer increases as the amount of filler in the magnetic layer increases, so that L increases. In addition, as the filler is added later in the paint manufacturing process, the added filler is more difficult to disperse and is present more on the surface of the magnetic layer, so that L increases. Further, the amount of the binder resin adsorbed on the surface of the magnetic particles is reduced by the surface treatment of the filler, and as a result, a large amount of the binder resin that does not adsorb on the surface of the magnetic particles is present on the surface of the magnetic layer. Become.

  In this embodiment, a magnetic recording medium in which the value of L is controlled within the above range can be manufactured by using the above methods alone, and preferably using several of these methods together. In particular, it is preferable to combine the transfer application of the lubricant (a) and the polishing treatment of the magnetic layer (b).

  On the other hand, the interfacial mixed layer is a mixed layer formed between the magnetic layer and the undercoat layer, and ideally there is no interfacial mixed layer. However, in reality, when the magnetic layer is formed on the undercoat layer, in the case of simultaneous multilayer coating in which the magnetic layer is formed on the undercoat layer before it is dried, In the sequential multi-layer coating in which mixing with the magnetic paint occurs and the undercoat layer is dried and then the magnetic layer is formed thereon, the surface roughness of the undercoat layer and the magnetic paint in the gaps in the undercoat layer An interfacial mixed layer having a certain thickness is formed due to intrusion or disturbance of the interface due to dissolution of the surface of the undercoat layer when the magnetic paint is applied.

  The average thickness t of the magnetic layer and the average thickness M of the interface mixed layer preferably satisfy the relationship of 0.1 ≦ M / t ≦ 0.45, and 0.1 ≦ M / t ≦ 0. It is more preferable to satisfy the relationship of 40. M is preferably as small as possible, and ideally 0 is most preferable. However, there is actually a technical limit, and the value of M / t is about 0.1. On the other hand, if the value of M / t exceeds 0.45, the average thickness M of the interfacial mixed layer becomes too large with respect to the average thickness t of the magnetic layer, and the number of magnetic powders effective for recording and reproduction decreases. Thus, the S / N ratio tends to decrease.

  The method for controlling the value of M / t within the above range is not particularly limited, but preferably, the following method is exemplified.

  (A) In the case where the undercoat layer and the magnetic layer are formed by simultaneous multilayer coating, the rheological properties (flow characteristics) of the undercoat paint and the magnetic paint are matched as much as possible.

  By simultaneously applying the multilayer coating by matching the rheological properties of the undercoat paint and the magnetic paint as much as possible, the mixing at the interface between the undercoat paint and the magnetic paint is reduced, the fluctuation of the magnetic layer thickness is reduced, and M is reduced.

  (B) In the case where the undercoat layer and the magnetic layer are formed by simultaneous multi-layer coating, a method using a non-pulsating pump for supplying each paint to the coating machine.

  By supplying the undercoat paint and the magnetic paint without pulsation, fluctuations in the coating thickness of each paint can be suppressed, and M is reduced.

  (C) A method of preventing deviation of the high-frequency vibration component from occurring in the conveyance speed of the nonmagnetic support when applying the undercoat layer and the magnetic layer.

  By reducing the deviation of the high-frequency vibration component in the conveyance speed of the non-magnetic support, so-called non-magnetic support flicker is eliminated, variation in coating thickness can be suppressed, and M is reduced.

  (D) When the undercoat layer and the magnetic layer are formed by sequential multilayer coating, the undercoat layer is formed and dried, and then the undercoat layer is calendered and smoothed. A method of preventing the surface of the undercoat layer from being dissolved when the magnetic layer is applied by heat curing or radiation curing and crosslinking curing.

  By this method, the mixing at the interface between the undercoat layer and the magnetic layer is reduced, the variation in the thickness of the magnetic layer is reduced, and M is reduced.

  (E) A method in which a resin layer is formed on the surface of the undercoat layer and crosslinked and cured so that the magnetic coating does not penetrate into the undercoat layer when the magnetic layer is applied.

  By this method, mixing at the interface between the magnetic layer and the resin layer is reduced, fluctuations in the thickness of the magnetic layer are reduced, and M is reduced.

  In the present embodiment, a magnetic recording medium in which the value of M / t is controlled within the above range can be manufactured by using the above methods alone, and preferably using several of these methods together.

  Next, the magnetic recording medium of this embodiment will be described with reference to the drawings. FIG. 2 is a schematic cross-sectional view showing an example of the magnetic recording medium of the present embodiment.

  In FIG. 2, the magnetic recording medium 10 of the present embodiment includes a nonmagnetic support 11, an undercoat layer 12 formed on one main surface (here, the upper surface) of the nonmagnetic support 11, and an undercoat layer. 12 is a magnetic tape having a magnetic layer 13 formed on the main surface (here, the upper surface) opposite to the nonmagnetic support 11 side. A back coat layer 14 is formed on the main surface (here, the lower surface) on the side where the undercoat layer 12 is not formed.

<Magnetic layer>
The magnetic layer 13 includes magnetic particles and a binder. As the magnetic particles, ε-iron oxide particles having an average particle diameter of 15 nm or less are used.

The ε-iron oxide particles are preferably formed of a single phase represented by the general composition formula ε-Fe ₂ O ₃ . This is because the coercivity of the magnetic layer decreases when α-iron oxide or γ-iron oxide is mixed. However, as long as the coercive force of the magnetic layer does not decrease, α-iron oxide or γ-iron oxide may be included as impurities.

  In this embodiment, ε-iron oxide can be distinguished from other γ-iron oxide and α-iron oxide by analyzing their crystal structures by X-ray diffraction.

The coercive force of the ε-iron oxide particles is preferably 3000 oersted [Oe] or more. Thereby, the coercive force in the thickness direction of the magnetic layer can be 3000 oersted [Oe] or more. In addition, when the ε-iron oxide particles represented by the general composition formula ε-Fe ₂ O ₃ contain impurities, the coercive force of the ε-iron oxide particles decreases, so it is preferable that no impurities are contained. However, in the ε-iron oxide particles, a part of Fe site in the crystal is substituted with a trivalent metal element such as aluminum (Al), gallium (Ga), rhodium (Rh), indium (In) or the like. Thus, the coercive force can be controlled. For this reason, the ε-iron oxide particles may contain a metal element other than iron as an impurity as long as the coercive force can maintain 3000 oersted [Oe] or more.

  As described above, the average particle diameter of the ε-iron oxide particles contained in the magnetic layer is preferably set to 8 nm or more and 15 nm or less. When the average particle diameter of the ε-iron oxide particles exceeds 15 nm, the noise of the magnetic recording medium increases particularly in short wavelength recording, so that there is a tendency that high electromagnetic conversion characteristics cannot be obtained.

  In the present embodiment, the average particle diameter of the magnetic particles contained in the magnetic layer is determined by using a scanning electron microscope (SEM) “S-4800” manufactured by Hitachi, Ltd. on the surface of the magnetic layer, acceleration voltage: 2 kV, and magnification: 10000 times (10k times), Observation condition: From the photograph taken with U-LA100, using 100 magnetic particles in one field of view, it is determined as follows.

  When the particles are needle-shaped, the average major axis diameter of 100 particles, when the particles are plate-shaped, the average maximum plate diameter of 100 particles, the ratio of the major axis length to the minor axis length of the particles In the case of a spherical or ellipsoidal shape having a diameter of 1 to 3.5, the average maximum diameter of 100 particles is calculated and determined.

  As the binder contained in the magnetic layer 13, a conventionally known thermoplastic resin, thermosetting resin, or the like can be used. Specific examples of the thermoplastic resin include vinyl chloride resin, vinyl chloride-vinyl acetate copolymer resin, vinyl chloride-vinyl alcohol copolymer resin, vinyl chloride-vinyl acetate-vinyl alcohol copolymer resin, vinyl chloride. -Vinyl acetate-maleic anhydride copolymer resin, vinyl chloride-hydroxyl group-containing alkyl acrylate copolymer resin, polyester polyurethane resin and the like. Specific examples of the thermosetting resin include phenol resins, epoxy resins, polyurethane resins, urea resins, melamine resins, alkyd resins, and the like.

  The content of the binder in the magnetic layer 13 is preferably 7 to 50 parts by mass and more preferably 10 to 35 parts by mass with respect to 100 parts by mass of the magnetic particles.

  Moreover, it is preferable to use together with the said binder the thermosetting crosslinking agent which couple | bonds with the functional group etc. which are contained in a binder, and forms a crosslinked structure. Specific examples of the crosslinking agent include isocyanate compounds such as tolylene diisocyanate, hexamethylene diisocyanate and isophorone diisocyanate; reaction products of isocyanate compounds and compounds having a plurality of hydroxyl groups such as trimethylolpropane; isocyanate compounds And various polyisocyanates such as condensation products. The content of the crosslinking agent is preferably 10 to 50 parts by mass with respect to 100 parts by mass of the binder.

  The magnetic layer 13 may further contain additives such as an abrasive, a lubricant, and a dispersant as long as the magnetic layer 13 includes the above-described magnetic particles and a binder. In particular, abrasives and lubricants are preferably used from the viewpoint of durability.

  Specific examples of the abrasive include α-alumina, β-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, corundum, artificial diamond, silicon nitride, silicon carbide, titanium carbide, Examples thereof include titanium oxide, silicon dioxide, and boron nitride. Among these, an abrasive having a Mohs hardness of 6 or more is more preferable. These may be used alone or in combination. The average particle diameter of the abrasive is preferably 10 to 200 nm, although it depends on the type of abrasive used. The content of the abrasive is preferably 5 to 20 parts by mass and more preferably 8 to 18 parts by mass with respect to 100 parts by mass of the magnetic particles.

  Examples of the lubricant include fatty acids, fatty acid esters, and fatty acid amides. The fatty acid may be any of linear, branched and cis / trans isomers, but is preferably a linear type having excellent lubricating performance. Specific examples of such fatty acids include lauric acid, myristic acid, stearic acid, palmitic acid, behenic acid, oleic acid, linoleic acid, and the like. Specific examples of the fatty acid ester include n-butyl oleate, hexyl oleate, n-octyl oleate, 2-ethylhexyl oleate, oleyl oleate, n-butyl laurate, heptyl laurate, and myristic. Examples include n-butyl acid, n-butoxyethyl oleate, trimethylolpropane trioleate, n-butyl stearate, s-butyl stearate, isoamyl stearate, and butyl cellosolve stearate. Specific examples of the fatty acid amide include palmitic acid amide and stearic acid amide. These lubricants may be used alone or in combination.

  Among these, it is preferable to use a fatty acid ester and a fatty acid amide in combination. In particular, 0.2 to 3 parts by mass of a fatty acid ester and 0.5 to 5 parts by mass of a fatty acid amide are used with respect to 100 parts by mass of the solid content of magnetic particles and abrasives in the magnetic layer 13. Is preferred. If the content of the fatty acid ester is less than 0.2 parts by mass, the effect of reducing the friction coefficient is small, and if it exceeds 3.0 parts by mass, side effects such as sticking of the magnetic layer 13 to the head may occur. It is. Further, if the content of the fatty acid amide is less than 0.5 parts by mass, the effect of preventing seizure caused by mutual contact between the magnetic head and the magnetic layer 13 becomes small, and exceeds 5 parts by mass. This is because the fatty acid amide may bleed out.

  The magnetic layer 13 may contain carbon black for the purpose of improving conductivity and surface lubricity. Specific examples of such carbon black include acetylene black, furnace black, and thermal black. The average particle size of carbon black is preferably 0.01 to 0.1 μm. When the average particle diameter is 0.01 μm or more, the magnetic layer 13 in which carbon black is well dispersed can be formed. On the other hand, when the average particle diameter is 0.1 μm or less, the magnetic layer 13 having excellent surface smoothness can be formed. Moreover, you may use 2 or more types of carbon black from which an average particle diameter differs as needed. The content of the carbon black is preferably 0.2 to 5 parts by mass and more preferably 0.5 to 4 parts by mass with respect to 100 parts by mass of the magnetic particles.

  The surface roughness of the magnetic layer 13 is preferably less than 2.0 nm as the centerline average roughness Ra defined by Japanese Industrial Standard (JIS) B0601. As the surface smoothness of the magnetic layer 13 is improved, a higher output is obtained. However, if the surface of the magnetic layer 13 is too smooth, the friction coefficient is increased and the running stability is lowered. For this reason, it is preferable that Ra is 1.0 nm or more.

<Undercoat layer>
Under the magnetic layer 13, it is preferable to provide an undercoat layer 12 having a function of retaining a lubricant and a buffering function of external stress (for example, pressure applied by a magnetic head). In addition, since the strength of the magnetic recording medium 10 is increased by providing the undercoat layer 12, calendering can be performed when the magnetic recording medium 10 is formed, and the filling property of the magnetic layer 13 can be improved. The undercoat layer 12 contains nonmagnetic powder, a binder, and a lubricant.

  Examples of the nonmagnetic powder contained in the undercoat layer 12 include carbon black, titanium oxide, iron oxide, and aluminum oxide. Usually, carbon black is used alone, or carbon black, titanium oxide, and iron oxide are used. In addition, other nonmagnetic powders such as aluminum oxide are mixed and used. In order to form a smooth undercoat layer 12 by forming a coating film with little thickness unevenness, it is preferable to use a nonmagnetic powder having a sharp particle size distribution. The average particle size of the nonmagnetic powder is preferably 10 to 1000 nm, for example, and preferably 10 to 500 nm, from the viewpoint of uniformity of the undercoat layer 12, surface smoothness, ensuring rigidity, and ensuring conductivity. Is more preferable.

  The particle shape of the nonmagnetic powder contained in the undercoat layer 12 may be any of a spherical shape, a plate shape, a needle shape, and a spindle shape. The average particle diameter of the acicular or spindle-shaped nonmagnetic powder is preferably 10 to 300 nm in terms of the average major axis diameter, and more preferably 5 to 200 nm in terms of the average minor axis diameter. The average particle size of the spherical nonmagnetic powder is preferably 5 to 200 nm, and more preferably 5 to 100 nm. The average particle diameter of the plate-like nonmagnetic powder is preferably 10 to 200 nm with the largest plate diameter. Furthermore, a nonmagnetic powder having a sharp particle size distribution is preferably used in order to form the undercoat layer 12 which is smooth and has little thickness unevenness.

  As the binder and lubricant contained in the undercoat layer 12, the same binder and lubricant as those used for the magnetic layer 13 described above can be used. The content of the binder is preferably 7 to 50 parts by mass, and more preferably 10 to 35 parts by mass with respect to 100 parts by mass of the nonmagnetic powder. Moreover, the content of the lubricant is preferably 2 to 6 parts by mass, more preferably 2.5 to 4 parts by mass with respect to 100 parts by mass of the nonmagnetic powder.

  The magnetic particles used for the magnetic layer 13 are ε-iron oxide particles, and the saturation magnetization of the ε-iron oxide particles is 1/2 to that of the conventional ferromagnetic hexagonal ferrite particles. Since it is as small as 1/3, when recording a servo signal having a long recording wavelength, the undercoat layer 12 contains magnetic particles. As the magnetic particles, for example, acicular metallic iron-based magnetic particles, plate-shaped hexagonal ferrite magnetic particles, granular iron nitride-based magnetic particles, and the like can be used.

  The thickness of the undercoat layer 12 is preferably 0.1 to 3 μm, more preferably 0.3 to 2 μm. By setting the thickness within this range, the retaining function of the lubricant and the buffering function of the external stress can be maintained without unnecessarily increasing the total thickness of the magnetic recording medium 10.

<Non-magnetic support>
As the nonmagnetic support 11, a conventionally used nonmagnetic support for a magnetic recording medium can be used. Specific examples include films made of polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyolefins, cellulose triacetate, polycarbonate, polyamide, polyimide, polyamideimide, polysulfone, and aramid.

  The thickness of the nonmagnetic support 11 varies depending on the application, but is preferably 1.5 to 11 μm, more preferably 2 to 7 μm. If the thickness of the nonmagnetic support 11 is 1.5 μm or more, the film formability is improved and high strength can be obtained. On the other hand, if the thickness of the nonmagnetic support 11 is 11 μm or less, the total thickness is not unnecessarily increased. For example, in the case of a magnetic tape, the recording capacity per roll can be increased.

  The Young's modulus in the longitudinal direction of the nonmagnetic support 11 is preferably 5.8 GPa or more, more preferably 7.1 GPa or more. If the Young's modulus in the longitudinal direction of the nonmagnetic support 11 is 5.8 GPa or more, the running performance can be improved. In the magnetic recording medium used for the helical scan method, the ratio (MD / TD) of Young's modulus (MD) in the longitudinal direction to Young's modulus (TD) in the width direction is preferably 0.6 to 0.8. More preferably, it is 0.65-0.75, More preferably, it is 0.7. As long as the ratio is within the above range, output variation (flatness) between the input side and the output side of the track of the magnetic head can be suppressed. In the magnetic recording medium used for the linear recording system, the ratio (MD / TD) of Young's modulus (MD) in the longitudinal direction to Young's modulus (TD) in the width direction is preferably 0.7 to 1.3.

<Back coat layer>
A back coat layer 14 is preferably provided on the main surface (here, the lower surface) opposite to the main surface on which the undercoat layer 12 of the nonmagnetic support 11 is formed for the purpose of improving running performance and the like. . The thickness of the backcoat layer 14 is preferably 0.2 to 0.8 μm, and more preferably 0.3 to 0.8 μm. If the thickness of the backcoat layer 14 is too thin, the effect of improving running performance is insufficient, and if it is too thick, the total thickness of the magnetic recording medium 10 is increased, and for example, the recording capacity per magnetic tape roll is reduced.

  The back coat layer 14 preferably contains, for example, carbon black such as acetylene black, furnace black, or thermal black. Usually, small particle size carbon black and large particle size carbon black having relatively different particle sizes are used in combination. The reason for using it in combination is that the effect of improving running performance is increased.

  The back coat layer 14 contains a binder, and the same binder as that used for the magnetic layer 13 and the undercoat layer 12 can be used as the binder. Among these, in order to reduce the friction coefficient and improve the running performance of the magnetic head, it is preferable to use a cellulose resin and a polyurethane resin in combination.

  The back coat layer 14 preferably further contains iron oxide, alumina or the like for the purpose of improving the strength.

  Next, a method for manufacturing the magnetic recording medium of this embodiment will be described. The method of manufacturing the magnetic recording medium of the present embodiment includes, for example, mixing each layer forming component and a solvent to prepare a magnetic layer forming paint, an undercoat layer forming paint, and a backcoat layer forming paint, respectively. Apply the undercoat layer-forming paint on one side of the magnetic support and dry it to form the undercoat layer, then apply the magnetic layer-forming paint on the undercoat layer and dry it. Then, a backcoat layer-forming coating material is applied to the other surface of the nonmagnetic support and dried to form a backcoat layer. Thereafter, the whole is calendered to obtain a magnetic recording medium.

  Also, instead of the sequential multi-layer coating method, after applying the undercoat layer-forming paint on one side of the nonmagnetic support, before the undercoat layer-forming paint is dried, on the undercoat layer-forming paint. It is also possible to employ a simultaneous multilayer coating method in which a magnetic layer forming coating is applied and dried.

  The coating method of each paint is not particularly limited, and for example, gravure coating, roll coating, blade coating, extrusion coating and the like can be used.

  In the magnetic layer described above, the method of setting the average thickness L of the surface mixed layer to 4 nm ≦ L ≦ 8 nm includes the methods (a) to (e) described above, and these methods are carried out independently. In addition, a plurality of these methods can be combined, and as described above, in particular, (a) the transfer coating of the lubricant and (b) the polishing treatment of the magnetic layer can be combined. preferable.

(Recording / reproducing mechanism of magnetic recording medium)
Next, an embodiment of a recording / reproducing mechanism for a magnetic recording medium of the present invention will be described.

  The recording / reproducing mechanism of the magnetic recording medium of the present embodiment includes the magnetic recording medium of the above-described embodiment of the present invention and a TMR head. As described above, since the magnetic layer of the magnetic recording medium contains ε-iron oxide having an average particle diameter of 15 nm or less as magnetic particles, the electromagnetic conversion characteristics can be improved, but it is combined with a more sensitive TMR head. Thus, even better electromagnetic conversion characteristics (SN characteristics) can be obtained.

  The TMR head is highly sensitive and can be combined with fine particles of ε-iron oxide to obtain a high S / N ratio. However, since the TMR head has high sensitivity, the spacing with the magnetic layer is reduced. Thermal asperity noise (cooling noise), which is generated because the TMR element is instantaneously cooled due to heat radiation due to contact between the TMR head and the magnetic layer protrusion, is likely to occur. Since the average thickness L of the surface mixed layer is set to 4 nm ≦ L ≦ 8 nm, thermal asperity noise can be reduced.

  EXAMPLES Hereinafter, although an Example demonstrates this invention, this invention is not limited to the following Example. In the following description, “parts” means “parts by mass”.

Example 1
[Preparation of magnetic paint]
A magnetic paint component (1) shown in Table 1 was mixed at high speed with a high speed stirring mixer to prepare a mixture. Next, the obtained mixture was dispersed in a sand mill for 20 minutes, then the magnetic coating component (2) shown in Table 2 was added and dispersed for 230 minutes, and then the magnetic coating component (3) shown in Table 3 was added. A dispersion was prepared. Next, the obtained dispersion and the magnetic paint component (4) shown in Table 4 were stirred using a dispaper and filtered through a filter to prepare a magnetic paint.

[Preparation of undercoat]
A kneaded product was prepared by kneading the undercoat paint component (1) shown in Table 5 with a batch kneader. Next, the obtained kneaded material and the undercoat paint component (2) shown in Table 6 were stirred using a dispaper to prepare a mixed solution. Next, after the obtained mixed liquid was dispersed with a sand mill for 100 minutes to prepare a dispersion, this dispersion and the undercoat paint component (3) shown in Table 7 were stirred using a dispaper, and this was filtered. Undercoat paint was prepared.

[Preparation of paint for back coat layer]
A mixed liquid in which the coating components for the back coat layer shown in Table 8 were mixed was dispersed by a sand mill for 50 minutes to prepare a dispersion. 15 parts of polyisocyanate was added to the obtained dispersion and stirred, and this was filtered through a filter to prepare a coating material for a backcoat layer.

[Production of magnetic tape for evaluation]
On the non-magnetic support (polyethylene naphthalate film, thickness: 5 μm), the primer coating is applied so that the thickness of the primer layer after the calendering process is 1.1 μm and dried at 100 ° C. An undercoat layer was formed. Next, on the undercoat layer, a coater tension of 4.5 N / inch was applied using a die coater so that the thickness of the magnetic layer after calendering the magnetic coating was 55 nm, at 100 ° C. A magnetic layer was formed by drying. Thereafter, vertical alignment treatment was performed while applying an alignment magnetic field having a strength of 450 kA / m in the vertical direction using an NS counter magnet.

  Next, the thickness after the calendering is 0.5 μm on the surface of the non-magnetic support on the side opposite to the surface on which the undercoat layer and the magnetic layer are formed. And then dried at 100 ° C. to form a backcoat layer.

  Thereafter, a raw roll having an undercoat layer and a magnetic layer formed on the upper surface side of the non-magnetic support and a back coat layer formed on the lower surface side is heated at a temperature of 100 ° C. with a calender device having seven metal rolls. Calendar treatment was performed at a linear pressure of 300 kg / cm.

  Next, the obtained raw fabric roll was cured at 60 ° C. for 48 hours to produce a magnetic sheet. This magnetic sheet was cut into a 1/2 inch width to produce a magnetic tape.

  Next, surface treatment was performed on the produced magnetic tape. In this surface treatment, the surface of the magnetic layer of the magnetic tape is treated by rotating a super steel rotor provided with spiral grooves and sliding the rotor on the magnetic tape.

  Specifically, using a super steel rotor with a diameter of 30 mm and a groove width of 2 mm, the super steel rotor speed is 3000 rpm, the magnetic tape running speed is 5 m / sec, the magnetic tape winding tension is 120 g, and the unwinding tension is The surface treatment of the magnetic tape was performed at 80 g and the winding angle of the magnetic tape around the super steel rotor was 120 °.

  Further, the magnetic tape subjected to the surface treatment was lubricated. As described above, this lubrication treatment is performed by transferring the lubricant onto the outer surface of the magnetic layer by sliding the belt-like coating cloth impregnated with the lubricant while running on the outer surface of the magnetic layer. It is processing.

  Specifically, Technos Co., Ltd. application cloth “TECHNO WIPER (Techno Wiper) CRN500” (product name) is used as the application cloth, and the lubricant “Unistar H-” manufactured by NOF Corporation is used as the lubricant. Using a mixed solution obtained by mixing 10 parts by mass of 208BRS "and 90 parts by mass of hexane as a viscosity adjusting solvent, the coated cloth was impregnated with the above mixed solution and lubricated as follows.

  When the lubrication treatment was performed, the traveling speed of the magnetic tape was 5 m / sec, the traveling tension was 1 N, the traveling speed of the coated cloth was 100 mm / min, and the coated cloth was moved in the direction opposite to the traveling direction of the magnetic tape. The winding angle of the magnetic tape around the sliding contact guide pin around which the coated cloth was wound was 30 °, and the diameter of the sliding contact guide pin was 25 mm. In this example, the above-described lubrication treatment was performed twice to produce the evaluation magnetic tape of Example 1.

(Example 2)
A magnetic tape for evaluation of Example 2 was produced in the same manner as in Example 1 except that the lubrication treatment for the cut magnetic tape was performed four times.

(Example 3)
Example 1 except that the average particle diameter of the ε-Fe ₂ O ₃ magnetic powder of the magnetic coating component (1) shown in Table 1 was changed to 12 nm and the lubricating treatment for the magnetic tape after cutting was performed three times. Similarly, the magnetic tape for evaluation of Example 3 was produced.

Example 4
Example 4 is the same as Example 1 except that the strength of the orientation magnetic field in the vertical orientation treatment after the formation of the magnetic layer is increased to 550 A / m, and the lubricating treatment for the cut magnetic tape is performed three times. A magnetic tape for evaluation was prepared.

(Example 5)
The magnetic tape for evaluation of Example 5 is the same as Example 1 except that the coercive force of the ε-Fe ₂ O ₃ magnetic powder of the magnetic coating component (1) shown in Table 1 is changed to 2800 [Oe]. Produced.

(Example 6)
Example 1 except that the produced raw roll was calendered at a temperature of 90 ° C. and a linear pressure of 300 kg / cm in a calender apparatus having a seven-stage metal roll, and the lubricating treatment for the magnetic tape after cutting was performed three times. In the same manner as described above, an evaluation magnetic tape of Example 6 was produced.

(Example 7)
A magnetic tape for evaluation of Example 7 was produced in the same manner as in Example 1 except that the surface treatment was not performed on the cut magnetic tape and the lubrication treatment was performed three times.

(Comparative Example 1)
Example 1 except that the average particle diameter of the ε-Fe ₂ O ₃ magnetic powder of the magnetic coating component (1) shown in Table 1 was changed to 16 nm and the lubricating treatment for the magnetic tape after cutting was performed three times. Similarly, an evaluation magnetic tape of Comparative Example 1 was produced.

(Comparative Example 2)
Comparative Example 2 was carried out in the same manner as in Example 1 except that the strength of the orientation magnetic field in the vertical orientation treatment after the formation of the magnetic layer was reduced to 350 A / m and the lubrication treatment for the cut magnetic tape was performed three times. A magnetic tape for evaluation was prepared.

(Comparative Example 3)
The produced raw roll is calendered at a temperature of 110 ° C. and a linear pressure of 300 kg / cm with a calender device having a seven-stage metal roll, and the lubricated treatment is performed once without performing surface treatment on the cut magnetic tape. A magnetic tape for evaluation of Comparative Example 3 was produced in the same manner as in Example 1 except that this was performed.

(Comparative Example 4)
The produced raw roll is calendered at a temperature of 90 ° C. and a linear pressure of 300 kg / cm with a calender apparatus having a seven-stage metal roll, and the surface treatment is not performed on the cut magnetic tape, and the lubricating treatment is performed three times. A magnetic tape for evaluation of Comparative Example 4 was produced in the same manner as in Example 1 except that this was done.

(Comparative Example 5)
A magnetic tape for evaluation of Comparative Example 5 was produced in the same manner as in Example 1 except that the surface treatment was not performed on the magnetic tape after cutting and the lubrication treatment was performed once.

  Next, the following characteristics were measured using the produced magnetic tape for evaluation.

<Length of magnetization in the width direction of the magnetic layer>
The magnetization length of the signal recorded in the magnetic layer of the produced magnetic tape for evaluation, and the magnetization length in the width direction of the magnetic layer was measured as follows. That is, as a magnetic force microscope, “Nano Scope III” (product name) manufactured by Digital Instruments was used, and the length of magnetization in the width direction of the magnetic layer on which the signal was recorded was measured by a frequency detection method. A probe having a cobalt alloy coat (tip radius of curvature: 25 to 40 nm, coercive force: about 400 [Oe], magnetic moment: about 1 × 10 ⁻¹³ emu) is used as the measurement probe, and the scanning range is 5 μm square, scanning The speed was 5 μm / sec.

<Rectangle ratio and coercive force of magnetic layer>
Using a vibrating sample magnetometer “VSM-P7 type” (product name) manufactured by Toei Kogyo Co., Ltd., a hysteresis curve of the magnetic tape for evaluation was obtained. The squareness ratio and coercive force in the thickness direction of the magnetic layer were determined from the hysteresis curve. Specifically, the evaluation magnetic tape is cut into a circular shape with a diameter of 8 mm to obtain a cut sample, and the cut samples are stacked by aligning the thickness direction of the magnetic tape with the application direction of the external magnetic field and stacking 20 samples. did. As a data plot mode from the vibrating sample magnetometer, the applied magnetic field was set to -16 kOe to 16 kOe, the time constant TC was set to 0.03 sec, the drawing step was set to 6 bits, and the wait time was set to 0.3 sec.

<Average thickness L of the surface mixed layer of the magnetic layer>
The average thickness L of the surface mixed layer of the magnetic layer was measured by the layer thickness measurement method of the present embodiment described above.

<Space of magnetic layer>
Spacing after the surface of the magnetic layer was washed with n-hexane was measured using TSA (Tape Spacing Analyzer) manufactured by Micro Physics.

Specifically, the pressure for pressing the magnetic layer against the glass plate with a hemisphere made of urethane was 0.5 atm (5.05 × 10 ⁴ N / m). In this state, white light is irradiated from the stroboscope to a certain area (240000 to 280000 μm ² ) on the magnetic layer side surface of the magnetic tape for evaluation through a glass plate, and the reflected light from the light is reflected to an IF filter (633 nm) and an IF filter By receiving the light through the CCD through (546 nm), an interference fringe image generated by the unevenness in this region was obtained.

  Next, this image is divided into 66000 points, and the distance from the glass plate of each point to the magnetic layer surface is obtained, and this is used as a histogram (frequency distribution curve), and further as a smooth curve by low-pass filter (LPF) processing. The distance from the glass plate at the peak position to the surface of the magnetic layer was defined as spacing.

  Further, the optical constants (phase, reflectance) of the magnetic layer surface used for the above spacing calculation were measured using a reflection spectral film thickness meter “FE-3000” (product name) manufactured by Otsuka Electronics Co., Ltd. A value around 546 nm was used.

  Cleaning with n-hexane was performed by immersing the magnetic tape for evaluation in n-hexane and ultrasonically cleaning at room temperature for 30 minutes.

<Output characteristics>
An induction / GMR composite magnetic head having a write track width of 5 μm and a read track width of 2.3 μm is attached to a linear tape electromagnetic conversion characteristic measuring device manufactured by modifying an LTO drive, and a tape speed of 1.5 m / In seconds, a signal having a recording wavelength of 200 nm (G7 × 1.05 times linear recording density) was recorded on an evaluation magnetic tape for evaluation.

  This apparatus is a traveling system to which two magnetic heads can be attached, and two magnetic heads are attached. The magnetic head is mounted on a precision piezo stage (moving resolution: 10 nm) that can be moved in the track width direction. Signal recording is performed with the upstream magnetic head, and AC erasing is performed with the downstream magnetic head in one run. A signal having a magnetization width of 0.8 μm was recorded on the magnetic tape for evaluation by offsetting the magnetic head on the side and the magnetic head on the downstream side by 0.8 μm in the track width direction.

  Next, the evaluation magnetic tape was run again to reproduce the signal. The reproduced signal was amplified by a commercially available MR head read amplifier, and then a spectrum analyzer “N9020A” manufactured by Keysight Technology (formerly Agilent Technologies). (Product name) was used to measure the fundamental component output (S) of the signal and the integrated noise (N) up to twice that frequency.

<Friction coefficient>
In order to evaluate repeated running durability in an actual drive, the friction coefficient of the magnetic layer of the magnetic tape for evaluation was repeatedly measured up to 800 passes using a stainless steel round bar. When this coefficient of friction becomes large, it becomes easy to cause running failure due to tape sliding, and repeated running durability deteriorates.

Specifically, the magnetic layer side of the magnetic tape for evaluation is wrapped at 90 ° on a stainless steel round bar having a diameter of 6 mm, and a load of 63.36 g is applied to the tip of the magnetic tape for evaluation at 1200 mm / min. The load during sliding at the 11th pass and the 800th pass was detected by a load cell as a measured load, and the friction coefficient was calculated by the following formula.
Friction coefficient = In [measurement load (g) /63.36 (g)] / 0.5π

<Thermal asperity noise>
Using the linear tape electromagnetic conversion characteristic measuring apparatus prepared by measuring the output characteristics described above, a TMR head having a track width of 0.2 μm was attached, the tape speed was 1.5 m / sec, and the recording wavelength was 200 nm (G7 × 1.05). Double linear recording density) was recorded on a magnetic tape for evaluation, and the number of thermal asperity noises at a tape length of 500 m was measured.

  The above evaluation results are shown in Table 9 and Table 10.

  From Table 10, Examples 1 to 7 according to the present invention have good output characteristics, little change in the coefficient of friction after 800 passes, good durability, little thermal asperity noise, and good error characteristics. You can see that it is obtained. That is, it can be seen that the present invention can provide a magnetic recording medium excellent in electromagnetic conversion characteristics and running durability.

  On the other hand, in Comparative Example 1 in which the average particle diameter of the magnetic particles exceeded 15 nm, the output characteristics deteriorated because the particle noise increased. In Comparative Example 2 in which the squareness ratio was less than 0.65, the magnetization transition width was increased, and the output characteristics deteriorated due to interference between waveforms. In Comparative Example 3 in which the average thickness of the surface mixed layer of the magnetic layer was less than 7 nm, the change in the friction coefficient after 800 passes was large and the durability was inferior, the thermal asperity noise was large, and the error characteristics were also inferior. In Comparative Example 4 in which the average thickness of the surface mixed layer of the magnetic layer exceeded 8 nm, the output characteristics deteriorated because the TSA spacing was increased. In Comparative Example 5 in which the average thickness of the surface mixed layer of the magnetic layer was less than 7 nm, the thermal asperity noise was large and the error characteristics were inferior.

  The magnetic recording medium of the present invention can be used as a magnetic recording medium excellent in electromagnetic conversion characteristics and running durability.

10 Magnetic recording media (magnetic tape)
11 Nonmagnetic support 12 Undercoat layer 13 Magnetic layer 14 Backcoat layer

Claims (8)

A magnetic recording medium comprising a nonmagnetic support and a magnetic layer containing magnetic particles,
The magnetization length of the signal recorded in the magnetic layer, and the magnetization length in the width direction of the magnetic layer is 1 μm or less,
The magnetic particles are composed of ε-iron oxide having an average particle diameter of 15 nm or less,
The squareness ratio in the thickness direction of the magnetic layer is 0.65 or more,
4. A magnetic recording medium, wherein the average thickness of the surface mixed layer formed on the surface of the magnetic layer opposite to the non-magnetic support side is 4 nm ≦ L ≦ 8 nm, where L is the average thickness.   The magnetic recording medium according to claim 1, wherein an average thickness L of the surface mixed layer is 7 nm ≦ L ≦ 8 nm.   The magnetic recording medium according to claim 1, wherein the ε-iron oxide is composed of spherical particles.   The magnetic recording medium according to claim 1, which is reproduced by a TMR head.   5. The magnetic recording medium according to claim 1, wherein a coercive force in a thickness direction of the magnetic layer is 3000 oersted [Oe] or more.   2. The spacing value is 5 nm or more and 12 nm or less when the spacing of the surface of the magnetic layer after the surface of the magnetic layer is washed with n-hexane is measured with TSA (Tape Spacing Analyzer). The magnetic recording medium of any one of -5.   The magnetic recording medium according to claim 1, wherein an average thickness t of the magnetic layer is 30 nm or more and 200 nm or less.   A magnetic recording medium recording / reproducing mechanism comprising the magnetic recording medium according to claim 1 and a TMR head.