JP2019508834A - Magnetic recording medium and method of manufacturing the same - Google Patents

Abstract

An object of the present invention is to provide a magnetic recording medium which realizes further thinning of a lubricating layer without impairing the reliability of the magnetic recording medium such as corrosion resistance and durability, and which is more adapted to the requirement of wear resistance. A protective layer containing carbon is formed on a laminate including a substrate and a metal film layer formed on the substrate, and (2) on the protective layer. A method of manufacturing a magnetic recording medium is provided, comprising the steps of: forming a lubricating layer; and (3) fluorine plasma treating the lubricating layer. [Selected figure] Figure 1

Description

  The present invention relates to a method of manufacturing a magnetic recording medium. Typically, the present invention relates to a method of manufacturing a magnetic recording medium used in a hard disk magnetic recording device.

  In order to improve the recording density of the HDD (hard disk drive), at the same time as improving the magnetic recording layer, the distance between the magnetic head for reading and writing information and the magnetic recording layer should be as small as possible There is a need. For this reason, various techniques such as FOD (flying on demand) have been used, such as a technique for reducing the flying height of the magnetic head and a technique for reducing the effective flying height by projecting the element portion of the magnetic head. . In particular, thinning of the protective layer formed on the magnetic recording layer and thinning of the lubricating layer formed on the protective layer are performed.

  At present, the lubricating layer is required to have good bonding with the protective layer from the viewpoint of wear resistance as well as the demand for thinning.

  As a method of improving the bondability, there has been proposed a technique in which a disk having a lubricating layer formed thereon is exposed to plasma of an inert gas such as nitrogen (see Patent Documents 1 and 2).

JP, 2010-55680, A U.S. Pat. No. 4,960,609 WO 2004/083295

  From the viewpoint of wear resistance, there is a need for magnetic recording media having even better cohesion between the lubricating layer and the protective layer.

  One of the objects of the present invention is to provide a magnetic recording medium satisfying the requirements for wear resistance without impairing the reliability of the magnetic recording medium such as corrosion resistance and durability.

One example of means for solving the problems of the present invention is
(1) forming a protective layer containing carbon on a laminate including a substrate and a metal film layer formed on the substrate;
(2) forming a lubricating layer on the protective layer;
(3) fluorine plasma treatment of the lubricating layer;
And a method of manufacturing a magnetic recording medium.

  Preferably, the lubricating layer has a thickness of 0.1 to 1.8 nm. Preferably, the lubricating layer is formed of a lubricant selected from the group consisting of perfluoropolyethers (PFPE), hydrocarbons, and cyclic phosphazene compounds. Preferably, the processing time of the fluorine plasma processing is 5 seconds to 5 minutes. Preferably, the step of nitriding the protective layer is further included between the step (1) and the step (2).

Another example of the means for solving the problems of the present invention is
A protective layer containing carbon is formed on a laminate including a substrate and a metal film layer formed on the substrate,
A lubricating layer is formed on the protective layer,
A magnetic recording medium, wherein a reaction product of a reaction product of the lubricating layer and the protective layer and having a C—F bond is formed at an interface of the lubricating layer and the protective layer.

  Preferably, the lubricating layer has a thickness of 0.1 to 1.8 nm. Preferably, the protective layer is nitrided.

  According to the present invention, a magnetic recording medium with improved lubricity and durability can be provided. Specifically, by performing the fluorine plasma treatment after the formation of the lubricating layer, the lubricity and durability of the protective layer provided with the lubricating layer can be enhanced.

  Further features of the present invention will become apparent from the following description of the exemplary embodiments, with reference to the attached drawings.

FIG. 1 shows an F1s spectrum by ESCA of a disk without a lubricant layer (protective layer surface). FIG. 2 shows a C1s spectrum by ESCA of a disk without a lubricant layer (protective layer surface). FIG. 3 shows an F1s spectrum by ESCA of a disk with a lubricant layer. FIG. 4 shows the C1s spectrum by ESCA of a disc with a lubricant layer. FIG. 5 shows a comparison of surface energy with and without plasma treatment. FIG. 6 shows a drug test for the presence or absence of plasma treatment. FIG. 7 shows a POD test with or without plasma treatment.

One example of the mode for carrying out the present invention is
(1) forming a protective layer containing carbon on a laminate including a substrate and a metal film layer formed on the substrate;
(2) forming a lubricating layer on the protective layer;
(3) fluorine plasma treatment of the lubricating layer;
And a method of manufacturing a magnetic recording medium.

<Substrate>
Preferably, the substrate is nonmagnetic and can be formed using any material conventionally used in the manufacture of magnetic recording media. For example, the substrate can be manufactured using materials such as aluminum alloy plated with Ni-P, glass, ceramic, plastic, silicon and the like.

<Metal film layer>
In the present application, the metal film layer can be produced by a general configuration and method. The metal film layer formed on the substrate includes at least a magnetic recording layer. The metal film layer may optionally further include a layer, such as a nonmagnetic underlayer, a soft magnetic layer, a seed layer, and an intermediate layer, between the magnetic recording layer and the substrate.

  For example, the magnetic recording layer can be formed using a ferromagnetic material of an alloy containing Co and Pt. The easy magnetization axis of the ferromagnetic material needs to be oriented in the direction in which the magnetic recording is performed. In order to perform perpendicular magnetic recording, the magnetization easy axis of the material of the magnetic recording layer, for example, the c axis of the hexagonal close-packed (hcp) structure, is oriented perpendicularly to the recording medium surface, ie, the main plane of the substrate. It is necessary. The magnetic recording layer can be formed using, for example, an alloy material such as CoPt, CoCrPt, CoCrPtB, or CoCrPtTa. The thickness of the magnetic recording layer is not particularly limited. However, from the viewpoint of improvement in productivity and recording density, the magnetic recording layer preferably has a thickness of 30 nm or less, more preferably 15 nm or less.

  Optionally, a nonmagnetic underlayer may be provided. The nonmagnetic underlayer can be formed using a nonmagnetic material containing Ti or Cr such as a CrTi alloy.

  Optionally, a soft magnetic layer may be provided. The soft magnetic layer is formed using a crystalline material such as FeTaC or Sendust (FeSiAl) alloy; a microcrystalline material such as FeTaC, CoFeNi or CoNiP; or an amorphous material containing a Co alloy such as CoZrNd, CoZrNb or CoTaZr can do. The soft magnetic layer has a function of concentrating the perpendicular magnetic field generated by the magnetic head on the magnetic recording layer in the perpendicular magnetic recording medium. The thickness of the soft magnetic layer may vary in optimum value depending on the structure and characteristics of the magnetic head used for recording. Preferably, the thickness of the soft magnetic layer is approximately 10 nm or more and 500 nm or less in consideration of the productivity.

  Optionally, a seed layer may be provided. The seed layer is a permalloy-based material such as NiFeAl, NiFeSi, NiFeNb, NiFeB, NiFeNbB, NiFeMo, NiFeCr, etc .; a material obtained by further adding Co to a permalloy-based material such as CoNiFe, CoNiFeSi, CoNiFeB, CoNiFeNb, etc .; It can be formed using a Co-based alloy such as CoB, CoSi, CoNi, or CoFe. The seed layer desirably has a thickness sufficient to control the crystal structure of the magnetic recording layer. Preferably, the seed layer has a thickness of 3 to 50 nm.

  Optionally, an intermediate layer may be provided. The intermediate layer can be formed using Ru or an alloy containing Ru as a main component. Preferably, the intermediate layer has a thickness of 0.1 to 20 nm. By setting the thickness within such a range, it becomes possible to impart the characteristics necessary for high density recording to the magnetic recording layer without deteriorating the magnetic characteristics and electromagnetic conversion characteristics of the magnetic recording layer.

  The formation of the nonmagnetic underlayer, soft magnetic layer, seed layer, intermediate layer and magnetic recording layer is known in the art such as sputtering (including DC magnetron sputtering, RF magnetron sputtering, etc.), vacuum evaporation, etc. It can be implemented using any method.

<Protective layer>
The formation method of a protective layer is demonstrated below.
The protective layer is a protective layer containing carbon, and may be, for example, DLC, which is mainly used carbon generally used. More preferably, carbon is a main component and is 85 atomic% or more. In this range, further adhesion and durability between the protective layer and the lubricating layer can be satisfied. The protective layer can be formed by a sputtering method, a CVD method, an IBD method, an FCVA method, or the like.

An example of the CVD method is shown below.
The protective layer can be formed by plasma CVD (Chemical Vapor Deposition) method using hydrocarbon gas as a source gas. In this method, it is preferable to make the source gas into a plasma state and generate active radicals and ions to form an amorphous carbon thin layer which is a protective layer. Preferably, amorphous carbon is diamond like carbon (DLC, Diamond Like Carbon) from the viewpoint of surface smoothness and hardness.

  The power supply for plasma generation may be implemented capacitively or inductively. For power supply, for example, DC power, HF power (frequency: several tens to several hundreds kHz), RF power (frequency: 13.56 MHz, 27.12 MHz, 40.68 MHz, etc.), microwave (frequency: 2.45 GHz) Etc. can be used.

  In order to generate plasma, for example, a parallel plate type device, a filament type device, an ECR plasma generator, a helicon wave plasma generator or the like can be used. Preferably, in the present application, a filament type plasma CVD apparatus is used.

  Preferably, a mixed gas containing lower saturated hydrocarbon gas and lower unsaturated hydrocarbon gas can be used as the raw material gas. In general, the deposition rate by the lower saturated hydrocarbon gas is relatively small, whereas the deposition rate by the lower unsaturated hydrocarbon gas is relatively large. The film forming rate can be controlled by adjusting the mixing ratio using the mixed gas of the two.

  The lower saturated hydrocarbon gas may be selected from the group consisting of methane, ethane, propane, butane and mixtures of two or more thereof. The lower unsaturated hydrocarbon gas may be selected from the group consisting of ethylene, propylene, butylene, butadiene and mixtures of two or more thereof. Among these, it is preferable to use ethane as the lower saturated hydrocarbon gas and ethylene as the lower unsaturated hydrocarbon gas from the viewpoint of good corrosion resistance.

  As long as the effects of the present invention are not impaired, the raw material gas may contain other hydrocarbon gas such as acetylene and benzene in a small amount of less than 10 mol%.

  The thickness of the protective layer is preferably 1.2 nm or more from the viewpoint of obtaining good corrosion resistance. The thickness of the protective layer may be thin as long as it satisfies the corrosion resistance from the viewpoint of reducing the magnetic spacing loss with the magnetic head and obtaining good magnetic recording characteristics, and for example, 2.5 nm or less is preferable.

  The surface of the obtained protective layer may be further subjected to a nitriding treatment. According to this process, it is possible to further improve the floating characteristics of the magnetic head by introducing an N (nitrogen) component that promotes the bondability with the lubricating layer on the surface of the protective layer.

  The introduction of the N (nitrogen) component can be performed, for example, by introducing nitrogen gas into a plasma source and subjecting the surface of the carbon layer to nitrogen plasma treatment.

<Lubrication layer>
Next, the lubricating layer will be described.
The lubricating layer can be formed, for example, by dip coating or the like. Typically, the lubricating layer is formed on the surface of the carbon protective layer by immersing the disc in a solution containing a solvent and a lubricant. However, when the lubricating layer is made thin, the sliding durability to the magnetic head is reduced. In addition, when the magnetic head contacts the disk surface, the lubricating layer is deformed, and the contacting portion becomes thin and the periphery thereof becomes thick. Such uneven thickness of the lubricating layer also reduces the sliding durability.

  The lubricant may be selected from the group consisting of perfluoropolyethers (PFPE), hydrocarbons, and cyclic phosphazene compounds. Typical examples of cyclic phosphazene compounds are cyclic phosphazene compounds described in Patent Document 3. As the lubricant, it is preferable to use perfluoropolyether (PFPE) provided by solvay, Moresco, Dupon, AGC or the like generally used. As the solvent, substances capable of dissolving a lubricant such as perfluorocarbon, hydrofluorocarbon, hydrofluoroether and the like can be used.

  From the viewpoint of good durability, the thickness of the lubricating layer is preferably 0.1 nm or more. From the viewpoint of reducing the magnetic spacing loss with the magnetic head and obtaining good magnetic recording characteristics, the thickness of the lubricating layer is preferably 1.8 nm or less. As described later, in order to more effectively produce the effect by the fluorine plasma treatment, it is more preferable that the lubricating layer have a thickness of 0.8 to 1.5 nm. Within the above range, it is considered that the action of fluorine plasma can extend to the protective layer and the surface of the protective layer can be fluorinated. Here, unless specifically limited in the present application, all the disclosed ranges can be combined including the values of both ends. For example, the range of “preferably 0.1 to 1.8 nm, more preferably 0.8 to 1.5 nm” is the value at both ends of the range and all the values in the middle, for example, “0.1 nm, 1 “8 nm, 0.1 to 1.5 nm, 0.8 to 1.8 nm” and the like.

The lubricating layer can be applied to a thickness of about 1 nm by dip coating or spin coating. The film thickness of the lubricating layer was measured by a Fourier transform infrared spectrophotometer (FT-IR). The measurement region is approximately 970 to 1500 cm ⁻¹ , and was calculated from the absorption intensity of C—F bond.

<Fluorine plasma treatment>
Next, fluorine plasma treatment will be described.
After forming the lubricating layer, the surface is subjected to fluorine plasma treatment. A fluorine plasma is a plasma formed by applying a high voltage to a fluorine gas or a gas containing fluoride, and contains fluorine ions or fluorine radicals. When the fluorine plasma is applied to the lubricating layer or the protective layer, the surface is fluorinated. Thus, irradiation with fluorine plasma is referred to as fluorine plasma treatment. The fluorine plasma treatment is a process for treating the surface of the lubricating layer, and it is not intended to laminate an entirely new compound or the like on the lubricating layer surface.

The fluoride gas is typically a fluorocarbon gas such as CF ₄ , C ₂ F ₆ , C ₃ F ₈ and the like. Preferably, the fluorocarbon gas is CF ₄ or C ₂ F ₆ . As the fluorine gas has a smaller molecular weight, the result is that the action of plasma extends to the surface of the protective film. A mixture of fluorine gas or fluoride gas and an inert gas such as nitrogen or argon may be used.

  As the plasma generation device, direct current arc discharge, high frequency plasma by inductive coupling type discharge, microwave discharge or the like can be used.

An example of processing conditions in the case of using high frequency plasma is described below.
Gas pressure: 1 to 100 Pa
・ High frequency: about 2.45 GHz ・ High frequency voltage: 1 kV to 3 kV

  The treatment time of the fluorine plasma treatment in this case is preferably 5 seconds to 5 minutes, more preferably 30 seconds to 2 minutes. It is preferable that the treatment time is 30 seconds or more because the surface is treated more uniformly. It is preferable that the treatment time is 2 minutes or less because the influence of surface damage by plasma is less.

  The high frequency voltage of the fluorine plasma treatment is preferably 1 kV to 3 kV, more preferably 1.5 kV to 2.5 kV. It is preferable that the high frequency voltage is 1 kV or more because the surface is treated more uniformly. It is preferable that the high frequency voltage is 3 kV or less because the influence of surface damage by plasma is less.

Production of Magnetic Recording Medium (1) Providing a Substrate Having a Metal Film Layer First, using an annular Ni-P-plated aluminum disc with an outer diameter of 95 mm, an inner diameter of 25 mm and a thickness of 1.27 mm, A substrate having a metal film layer was prepared by the following procedure.

  That is, first, Ni-P plating having a thickness of 12 μm was applied to the surface of the aluminum disk to prepare a nonmagnetic substrate. The obtained nonmagnetic substrate was subjected to smoothing and cleaning.

Next, a metal film layer (nonmagnetic underlayer, soft magnetic layer, seed layer, intermediate layer, first magnetic layer, and the like) sequentially including a plurality of metal layers on a cleaned nonmagnetic substrate using DC magnetron sputtering. The coupling control layer, the second magnetic layer and the third magnetic layer were formed. In particular,
A Cr ₅₀ Ti ₅₀ layer is laminated on a nonmagnetic substrate to form a nonmagnetic underlayer having a thickness of 6.0 nm.
A CoZrNb layer is laminated on a nonmagnetic underlayer to form a soft magnetic layer having a thickness of 20 nm.
A CoNiFe layer is laminated on the soft magnetic layer to form a seed layer having a thickness of 8.0 nm.
Depositing a Ru layer on the seed layer to form an intermediate layer with a thickness of 10 nm,
A CoCrPt—SiO ₂ layer is laminated on the intermediate layer to form a first magnetic layer having a thickness of 10 nm.
A Ru layer is stacked on the first magnetic layer to form a coupling control layer having a thickness of 0.2 nm,
A CoCrPt-SiO ₂ layer is laminated on the coupling control layer to form a second magnetic layer having a thickness of 3.0 nm, and a CoCrPt-B layer is laminated on the second magnetic layer, A third magnetic layer having a thickness of 6.0 nm was formed.

  In the substrate, the magnetic recording layer has a four-layer structure of a first magnetic layer, a coupling control layer, a second magnetic layer, and a third magnetic layer.

(2) Formation of Protective Layer Next, a protective layer was formed on the obtained metal film layer using a plasma CVD method. A filament type plasma CVD apparatus was used. While supplying a predetermined current to the cathode filament to emit thermal electrons, hydrocarbon gas as a source gas was introduced into the apparatus to generate plasma of hydrocarbon gas.

A mixed gas of ethane (C ₂ H ₆ ) gas and ethylene (C ₂ H ₄ ) gas was used as a source gas. First, the ethane (C ₂ H ₆ ) gas flow rate is 45 sccm, the ethylene (C ₂ H ₄ ) gas flow rate is 15 sccm, the anode potential is +40 V, the bias potential is -60 V (that is, the ion acceleration potential difference is 100 V), and the substrate temperature is about The film formation time was adjusted to 180 ° C., and a 2.0 nm-thick protective layer (DLC layer) was formed on the metal film layer.

Furthermore, the surface of the protective layer was nitrided with a nitrogen (N ₂ ) gas flow rate of 50 sccm, a substrate temperature of about 180 ° C., and a processing time of 1.0 s. As used herein, “sccm” refers to the flow rate (in cm ³ ) per minute under standard conditions (1 atm / 0 ° C.).

(3) Formation of lubricant layer over the thus protecting layer formed by perfluoropolyether _{(HOCH 2 CH (OH) CH} 2 -OCH 2 CF 2 O- (CF 2 CF 2 O) n - (CF _{2 O) m -CF 2 CH 2} O-CH 2 CH (OH) CH 2 OH, the molecular weight of the liquid lubricant mainly containing 2000 to 4000) was applied by a dipping method, a lubricating layer having a thickness of 1.0nm Formed.

(4) Fluorine Plasma Treatment The lubricating layer thus formed was subjected to fluorine plasma treatment with microwave excitation plasma at 2.45 GHz using CF ₄ as a source gas.

Surface Spectrum Analysis The surface of the protective layer treated with fluorine plasma on the surface of the protective layer without a lubricating layer was analyzed by ESCA (Electron Spectroscopy for Chemical Analysis). Figures 1 and 3 show the spectra of binding energy from 680 eV to 700 eV, mainly showing the spectrum of F1s. Figures 2 and 4 show spectra of binding energy from 278 eV to 298 eV and show spectra such as C1s. Further, after forming the protective layer and the lubricating layer, the fluorine plasma treated surface was analyzed by ESCA. In addition, F1s shows the peak of 1s of F (fluorine), and C1s shows the peak of 1s of C (carbon).

When fluorine plasma treatment was performed on the protective layer in FIG. 1, a peak of F1s was newly observed. Moreover, in FIG. 2, the peak of C1s decreased and a peak showing C-F binding newly appeared. More specifically, (CH ₂ C * F ₂ ) and C ₆ F ₆ in FIG. 2 are fluorine compounds formed on the surface of the protective layer, and are considered to be peaks indicating that they have a C—F bond. . On the other hand, C1s indicates C-C bond and C-H bond. That is, it is considered that the C—H bond or the like of the protective layer is fluorinated to change to a C—F bond.

On the other hand, on the surface subjected to the fluorine plasma treatment after the formation of the lubricating layer, the F1s peak increased from FIG. This indicates that the surface is fluorinated by fluorine plasma treatment in addition to the fluorine originally derived from the fluorine-based lubricant. Then, as shown in FIG. 4, a peak was formed at the same position as the peak of the C—F bond formed on the protective layer. (CFHC * F ₂ ) and C ₆ F _{6 in} FIG. 4 are fluorine compounds formed on the surface of the protective film, and are considered to be peaks indicating that they have a C—F bond.

  From the above, it is confirmed that when fluorine plasma treatment is performed after forming the lubricating layer, C—F bonds and the like are formed on the protective layer and are fluorinated to form a fluorine compound. It is considered that this is because the surface of the protective layer is fluorinated through the lubricating layer, or partially because the surface of the protective layer is fluorinated, for example, at a thin portion of the lubricating layer.

Further, (CH ₂ C * F ₂ ) of the fluorine compound in FIG. 2 and (CFHC * F ₂ ) in FIG. 4 are different. A fluorine compound different from the fluorine compound generated on the surface of the protective layer by the fluorine plasma treatment is also formed. That is, it is considered that when fluorine plasma treatment is performed on the lubricating layer, reaction products of the lubricating layer and the protective layer are also formed. The reaction product has a C—F bond. When the lubricating layer is formed after fluorine plasma treatment on the protective layer, it is considered that a fluorine compound as shown by the peak of (CFHC * F ₂ ) is not formed. Therefore, it is believed that fluorine plasma treatment from above the lubricating layer results in the formation of reaction products of the lubricating layer and the protective layer. Such description is merely a consideration for understanding the present invention, and the present invention is not limited to the specific theory described above.

  Thus, the surface of the protective layer is fluorinated to form a fluorine compound, whereby the lubricity is improved, the adhesion between the protective layer and the lubricating layer is further improved, and the durability is considered to be improved.

Comparison of Surface Energy Also, with a contact angle meter, the surface energy of the surface after the formation of the lubricating layer was measured with and without the fluorine plasma treatment. The surface energy was measured separately for the nonpolar dispersion component and the polar component. As shown in FIG. 5, it can be seen that the polar component mainly decreases and the surface energy decreases. This indicates that the surface of the lubricating layer is inactivated by the fluorine plasma treatment. When the surface of the lubricating layer is inactive, it is difficult to adsorb water, gas and the like, so it is preferable for durability.

  Further, when the protective layer was subjected to fluorine plasma treatment to form a lubricating layer, the durability deteriorated. This corresponds to the undesirable situation because the bond between the carbon protective layer and the lubricating layer is deteriorated.

Drag test (altitude drag test)
The friction durability of the magnetic recording medium and the magnetic head was evaluated as follows. The magnetic recording medium was rotated, the magnetic head was brought into contact, the coefficient of friction was measured, and the number of cycles until the coefficient of friction increased was measured. The large number of cycles means that the wear resistance is high. As shown in FIG. 6, the wear resistance increased by four times or more by the fluorine plasma treatment.

Sliding durability evaluation (POD (Pin on Disk) test)
The following tests were conducted to evaluate the sliding durability of the surface of the magnetic recording medium.
An altic ball with a diameter of 2 mm was caused to slide on a magnetic recording medium sample at a load of 30 gf and a linear velocity of 25 cm / s, and the number of times of sliding until the protective layer broke was measured. As shown in FIG. 7, by the fluorine plasma treatment, the number of times of sliding increased by as many as 765 times and was 1.8 times, and was good.

  As is clear from the above results, the fluorine plasma treatment showed good sliding durability.

  Although the present invention has been described with reference to exemplary embodiments, it is understood that the present invention is not limited to the disclosed exemplary embodiments. The following claims are intended to cover all such modifications and equivalent structures and functions.

Claims (8)

(1) forming a protective layer containing carbon on a laminate including a substrate and a metal film layer formed on the substrate;
(2) forming a lubricating layer on the protective layer;
(3) fluorine plasma treatment of the lubricating layer;
A method of manufacturing a magnetic recording medium, comprising:   The method according to claim 1, wherein the lubricating layer has a thickness of 0.1 to 1.8 nm.   The manufacturing method according to claim 1, wherein the lubricating layer is formed of a lubricant selected from the group consisting of perfluoropolyether (PFPE), hydrocarbon, and cyclic phosphazene compound.   The manufacturing method of Claim 1 whose processing time of the said fluorine plasma processing is 5 second-5 minutes.   The manufacturing method according to claim 1, further comprising the step of nitriding the protective layer between the step (1) and the step (2). A protective layer containing carbon is formed on a laminate including a substrate and a metal film layer formed on the substrate,
A lubricating layer is formed on the protective layer,
A magnetic recording medium, wherein a reaction product of a reaction product of the lubricating layer and the protective layer and having a C-F bond is formed at an interface of the lubricating layer and the protective layer.   The magnetic recording medium according to claim 6, wherein the lubricating layer has a thickness of 0.1 to 1.8 nm.   The magnetic recording medium according to claim 6, wherein the protective layer is nitrided.

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