JP4525496B2 - Disk substrate for perpendicular magnetic recording medium and perpendicular magnetic recording medium - Google Patents

Description

  The present invention relates to a disk substrate for a perpendicular magnetic recording medium and a perpendicular magnetic recording medium using the same, and is particularly suitable as a perpendicular magnetic recording medium mounted on a hard disk device.

In recent years, hard disk drives are often used as storage devices for computers, digital home appliances, etc. As a technology for realizing high-density recording of magnetic recording media mounted on them, perpendicular magnetic recording is used instead of the conventional longitudinal magnetic recording method. The method is getting attention.
In particular, as shown in Patent Document 1, a soft magnetic underlayer called a soft magnetic backing layer that is easy to pass a magnetic flux generated from a magnetic head and has a high saturation magnetic flux density Bs is provided below a magnetic recording layer that plays a role of recording information. Double-layer perpendicular magnetic recording media with a magnetic film increase the magnetic field strength and magnetic field gradient generated by the magnetic head, improve recording resolution and increase the magnetic flux leakage from the medium, enabling high-density recording It is known that it is suitable as a perpendicular magnetic recording medium.
As this soft magnetic backing layer, a Ni—Fe alloy film, a Fe—Si—Al alloy film having a film thickness of about 200 nm to 500 nm formed by a sputtering method, an amorphous alloy film mainly composed of Co, or the like is generally used. Although being used, it is not preferable to form these relatively thick films by sputtering from the viewpoint of production cost and mass productivity.

In order to solve such problems, it has been proposed to use a soft magnetic film formed by an electroless plating method as a soft magnetic backing layer. For example, Patent Document 2 proposes a Co-B film, and Patent Document 3 proposes a Ni-Fe-P film, which suggests the possibility of use as a soft magnetic underlayer. Non-Patent Document 1 proposes a CoNiFeP film.
Here, it is known that when a soft magnetic underlayer forms a magnetic domain structure and a magnetization transition region called a domain wall occurs, noise called spike noise generated from the domain wall deteriorates the performance as a perpendicular magnetic recording medium. Yes. Therefore, it is necessary for the soft magnetic underlayer to suppress the formation of domain walls. Therefore, it is described in Non-Patent Document 2 that in the NiFeP plating film described above, it is necessary to suppress the domain wall formation by forming a MnIr alloy thin film on the plating film by a sputtering method in order to suppress spike noise. .

  Further, in Patent Document 6, “in the VSM measurement, the strength of the measurement applied magnetic field when Bs is obtained is defined as Hs (see FIG. 10). This Hs is an index for determining the easy magnetization direction. The smaller Hs, the easier the axis of magnetization is in that direction, and the ratio (isotropic degree) in directions different by 90 ° indicates the magnetic orientation (isotropicity) of the entire film. The closer the value is to 1.0, it can be considered magnetically isotropic.The soft magnetic film formed by the conventional electroless plating method has an orientation, so that when measured by the VSM, the radial direction is obtained. The same Bs value is obtained in the circumferential direction, but there is a difference in the measured applied magnetic field from which this Bs value can be obtained, so that the underlying film formed on the disk-shaped nonmagnetic substrate is shown in FIG. Cut out the whole board as in Hs was obtained, and the degree of isotropicity was determined according to the following formula: Isotropic = Hs (circumferential direction) / Hs (radial direction) ”, and Hs in the circumferential direction (hereinafter“ The domain wall formation is suppressed when the ratio (isotropic degree) of Hs in the radial direction (hereinafter referred to as “Hsr”) is within 1.0 ± 0.2, which causes the substrate to be in parallel magnetic field. It is said that this is achieved by performing plating while rotating in parallel with respect to.

On the other hand, in a magnetic recording medium for a hard disk using a longitudinal magnetic recording method that is currently in practical use, the film thickness is about 11 wt% (19 at%) formed by electroless plating on an Al alloy substrate. Is a non-magnetic substrate having a non-magnetic Ni—P plating film of about 8 μm to 15 μm. This non-magnetic Ni—P plating film mainly fills in a defect such as a depression existing in the Al alloy substrate and plays a role for obtaining a smooth surface by polishing the surface of the plating film. Furthermore, it is used to maintain the surface hardness required for a hard disk substrate. That is, it is necessary to maintain a certain degree of surface hardness so that the substrate is not damaged when the magnetic head collides with the magnetic recording medium during the operation of the hard disk device.
Here, since such a non-magnetic Ni—P plating film becomes a ferromagnetic material by performing a heat treatment at a temperature of about 300 ° C. or higher, it is used as a soft magnetic backing layer of a perpendicular magnetic recording medium. It has also been proposed. For example, Patent Document 4 proposes forming a Ni—P film having soft magnetism by applying heat treatment to a nonmagnetic Ni—P plating film at 300 ° C. or more and using it as a soft magnetic backing layer. Similarly, in Patent Document 5, a nonmagnetic Ni—P plating film is laminated with a soft magnetic Ni—P film obtained by heat treatment at a temperature of 250 ° C. to 500 ° C. and a sendust film formed by sputtering. Thus, it has been proposed that the Ni-P film works as an aid for exerting the function of the sendust film and an effective soft magnetic backing layer can be formed.

As described above, the nonmagnetic Ni—P plating film has already been used for a nonmagnetic substrate for a hard disk, and a production method for mass production and a surface smoothing technique by polishing are well known. . Therefore, if this can be heated to form a soft magnetic backing layer and can be used as a substrate for a perpendicular magnetic recording medium, it is very promising from the viewpoint of production cost, and the above-mentioned nonmagnetic Ni—P The plating contains 10 to 13 wt% (17.4 to 22.1 at%) of P, but focusing on the fact that if P is 6 wt% (10.8 at%) or less, it becomes a ferromagnetic thin film exhibiting soft magnetism, As described in Non-Patent Document 3, it has also been proposed to use a Ni—P alloy film containing a relatively low concentration of P of 6 wt% or less as a soft magnetic backing layer. Such a soft magnetic Ni—P plating film does not form a domain wall without using a special process.
Japanese Patent Publication No.58-91 Japanese Patent Laid-Open No. 5-1384 JP 7-66034 A Japanese Patent Laid-Open No. 1-285022 JP-A-10-228620 JP 2004-335068 A 9th Joint MMM / Intermag Conference, EP-12, P.259 (2004) Journal of Japan Society of Applied Magnetics, Vol.28, No.3, P.289-294 (2004) The 9th Joint MMM / Intermag Conference, GD-13, P.368 (2004)

As described above, the soft magnetic backing layer needs to suppress the formation of the domain wall in order to prevent the occurrence of spike noise. Patent Document 4 discloses a method of forming a MnIr alloy thin film by sputtering, and Patent Document 6 discloses. Although methods for plating in a magnetic field have been proposed, all of them are not desirable from the viewpoint of productivity because the number of processes increases and the facilities become complicated. In addition, the soft magnetic Ni—P plating film having a P concentration of 6 wt% or less does not form a domain wall and does not generate spike noise, but Bs is as small as 0.2 T to 0.5 T as compared with a CoNiP film, a CoNiFeP film, or the like. However, it is necessary to increase the thickness of the soft magnetic underlayer for forming the soft magnetic underlayer, which increases the noise caused by the soft magnetic underlayer and deteriorates the recording / reproducing characteristics.
As described above, when the conventional technology is used, it has been difficult to realize a soft magnetic backing layer of a perpendicular magnetic recording medium capable of high-density recording with low production cost and mass productivity.

  In view of the above points, the present invention provides a disk substrate for a perpendicular magnetic recording medium in which a soft magnetic underlayer is formed on a nonmagnetic substrate, and the roughness of the surface on which the soft magnetic underlayer is to be formed falls within an appropriate range. It is an object of the present invention to provide a disk substrate for a perpendicular magnetic recording medium capable of suppressing the formation of a domain wall without using special equipment by controlling, and a perpendicular magnetic recording medium using the same.

In order to achieve the above object, the disk substrate for perpendicular magnetic recording media of the present invention has a surface roughness Ra of a nonmagnetic substrate of 5 nm to 20 nm, and the soft magnetic underlayer has a surface roughness of nonmagnetic. A soft magnetic film having a saturation magnetic flux density of 0.4 T or more and 1.5 T or less is formed on a substrate to a thickness of 0.15 μm or more and 10 μm or less.
In the disk substrate for the perpendicular magnetic recording medium of the present invention, the soft magnetic underlayer includes a lower layer made of a soft magnetic film having a saturation magnetic flux density of 0.2 T or more and 0.5 T or less and a coercive force of 10 Oe or more and 30 Oe or less, It consists of an upper layer made of a soft magnetic film having a saturation magnetic flux density of 0.4T or more and 1.5T or less, and the thickness of the lower layer is made 0.5 μm or more and 10 μm or less and the surface roughness Ra is made 0.7 nm or more and 10 nm or less. The upper layer is preferably formed to a thickness of 0.1 μm or more and 10 μm or less on the lower layer having the surface roughness.

Here, the surface roughness Ra of the lower layer is preferably 1 nm or more and 5 nm or less.
Furthermore, the perpendicular magnetic recording medium of the present invention has at least a magnetic recording layer formed on the above-described disk substrate for the perpendicular magnetic recording medium of the present invention, and the soft magnetic underlayer of this disk substrate is a soft magnetic layer for the magnetic recording layer. It is characterized by being used as at least a part of the backing layer.
As a result of intensive investigations to solve the problems of the present invention, the inventors have controlled the surface roughness of the disk-shaped nonmagnetic substrate so that Ra is 1.5 nm to 20 nm, preferably 2 nm to 10 nm. Therefore, an MnIr alloy thin film or an apparatus for plating in a magnetic field is not required, and a soft magnetic film having a saturation magnetic flux density of 0.4 T or more and 1.5 T or less is excellent but a domain wall is easily formed. Even if it is formed as a formation, a ratio Hsc / Hsr of Hsc (the intensity of the applied magnetic field when the circumferential saturation magnetic flux density is obtained) and Hsr (the intensity of the applied magnetic field when the saturation magnetic flux density in the radial direction is obtained). (Isotropic degree of Patent Document 6) can be made within 1.0 ± 0.2, succeeding in suppressing the formation of domain walls and preventing the occurrence of spike noise.

Further, as the soft magnetic underlayer, a soft magnetic underlayer having a saturation magnetic flux density of 0.2 T to 0.5 T, a coercive force of 10 Oe to 30 Oe, and a surface roughness Ra of 0.5 nm to 20 nm, preferably 1 nm to 5 nm. On the lower layer made of a soft magnetic film that is inferior in magnetic properties but hard to form a domain wall, it consists of a soft magnetic film that has excellent soft magnetic properties with a saturation magnetic flux density of 0.4 T or more and 1.5 T or less, but is easily formed with a domain wall. It has also been found that by forming the upper layer, Hsc / Hsr can be within 1.0 ± 0.2, and the formation of domain walls can be suppressed and the occurrence of spike noise can be prevented.
According to these means, an expensive sputtering apparatus is not added, and a special plating apparatus is not required as in the case of plating in a magnetic field, so that it is excellent in mass productivity. In this case, as the nonmagnetic substrate, an Al alloy or glass is generally used, but other metals, ceramics or plastics may be used.

  When controlling the surface roughness of the non-magnetic substrate, it is difficult to suppress the domain wall formation if the surface roughness Ra is less than 1.5 nm, and the domain wall formation is suppressed even if the surface roughness Ra exceeds 20 nm. However, the surface of the film formed on such a rough substrate is naturally rough, and it takes a long time to perform mirror polishing to Ra ≦ 0.5 nm required for a magnetic recording medium. Is low. This is because the hard disk drive has a structure in which the magnetic head floats only a few nanometers above the magnetic recording medium and seeks at high speed, and the surface of the magnetic recording medium is required to be an ultra-flat surface. The film thickness of the soft magnetic underlayer to be formed is preferably 0.15 μm to 10 μm. When the thickness is less than 0.15 μm, a sufficient recording / reproducing output cannot be obtained when a perpendicular magnetic recording medium is used. Even when the thickness exceeds 10 μm, the function as a perpendicular magnetic recording medium is not impaired, but it functions sufficiently with a film thickness of 1/50 or less, which is not desirable in terms of productivity.

  When the soft magnetic underlayer has a two-layer structure consisting of an upper layer and a lower layer, the saturation magnetic flux density is 0.4 T or more and 1.5 T or less, the coercive force is 1 Oe or more and 10 Oe or less, and the soft magnetic property is excellent, but the domain wall is formed. Compared with the case where an easy soft magnetic film is formed as a single layer, the surface roughness on which the soft magnetic underlayer necessary for suppressing the domain wall formation and preventing the occurrence of spike noise is set to Ra 0.7 nm (lower layer) The surface roughness of the soft magnetic underlayer can be shortened, and the productivity can be increased. The minimum film thickness of the lower layer capable of preventing the occurrence of spike noise is 0.5 μm, and the minimum film thickness of the upper layer capable of obtaining a sufficient recording / reproducing output when used as a perpendicular magnetic recording medium is 0.1 μm. Even if the thickness of both layers exceeds 10 μm, the function as a perpendicular magnetic recording medium is not impaired, but it is not desirable in terms of productivity. In addition, there is an advantage that noise caused by the soft magnetic underlayer can be reduced as compared with the case where a soft magnetic film which is easy to form a domain wall on a nonmagnetic substrate but has excellent soft magnetic characteristics is formed as a single layer.

Although the present invention is effective regardless of the method of forming the soft magnetic underlayer, a perpendicular magnetic recording medium can be manufactured at a very low cost when applied to an electroless plating method excellent in mass productivity.
In addition, this invention does not exclude that another layer intervenes between each layer.

According to the disk substrate for perpendicular magnetic recording medium of the present invention, by controlling the roughness of the surface on which the soft magnetic underlayer is to be formed within an appropriate range, there is no need to use special equipment such as an apparatus for plating in a magnetic field. The formation of the domain wall can be suppressed and the occurrence of spike noise can be prevented.
Therefore, according to the perpendicular magnetic recording medium of the present invention, at least a magnetic recording layer is formed on the disk substrate for the perpendicular magnetic recording medium of the present invention, and the soft magnetic underlayer of the disk substrate is at least part of the soft magnetic backing layer. By using as, it is excellent in mass productivity and can be made very inexpensive.

Hereinafter, embodiments of the present invention will be described.
<Embodiment of Disk Substrate for Perpendicular Magnetic Recording Medium>
FIG. 1 is a schematic sectional view showing Embodiment 1 of a disk substrate for a perpendicular magnetic recording medium according to the present invention. As shown in FIG. 1, the disk substrate 10 for perpendicular magnetic recording medium according to Embodiment 1 of the present invention has a saturation magnetic flux density of 0.4 T or more and 1.5 T or less on a disk-shaped nonmagnetic substrate 1. A soft magnetic underlayer 2 made of a soft magnetic film that has excellent magnetic characteristics but is easy to form a domain wall is formed. Although not shown, the soft magnetic underlayer 2 can be similarly provided on the other surface side of the nonmagnetic substrate 1.
As the non-magnetic substrate 1, a non-magnetic Ni—P alloy plating applied to an Al—Mg alloy plate used for a conventional hard disk substrate, glass, or a similar non-magnetic material can be used.

By setting the surface roughness Ra of the nonmagnetic substrate 1 to 1.5 nm or more and 20 nm or less, preferably 2 nm or more and 10 nm or less, the domain wall formation of the soft magnetic underlayer 2 is suppressed.
As the soft magnetic underlayer 2, a soft magnetic alloy film containing one or more of Ni, Co, and Fe is used.
FIG. 2 is a schematic sectional view showing Embodiment 2 of the disk substrate for perpendicular magnetic recording media according to the present invention. As shown in FIG. 2, the disk substrate 10 for perpendicular magnetic recording medium according to the second embodiment of the present invention has a saturation magnetic flux density of 0.2 T to 0.5 T and a coercive force on a disk-like nonmagnetic substrate 1. 10 Oe or more and 30 Oe or less, the soft magnetic property is inferior but the domain wall is difficult to form, and the lower layer 2a is composed of a soft magnetic film, the saturation magnetic flux density is 0.4 T or more and 1.5 T or less, the soft magnetic property is excellent, but the domain wall is A soft magnetic underlayer 2 comprising an upper layer 2b made of an easily formed soft magnetic film is formed. Although not shown, the soft magnetic underlayer 2 composed of the lower layer 2 a and the upper layer 2 b can be similarly provided on the other surface side of the nonmagnetic substrate 1.

As the upper layer 2b of the nonmagnetic substrate 1 and the soft magnetic underlayer 2, those having the same materials and compositions as those of the nonmagnetic substrate 1 and the soft magnetic underlayer 2 of the first embodiment can be used.
As the lower layer 2 a of the soft magnetic underlayer 2, a Ni—P alloy film containing about 1 to 10 at% P or a Ni—B alloy film containing the same amount of B can be used.
The surface roughness Ra of the lower layer 2a is 0.7 nm or more and 10 nm or less, preferably 1 nm or more and 5 nm or less, thereby suppressing the domain wall formation of the upper layer 2b.
As a method of forming the soft magnetic underlayer 2 as described above on the nonmagnetic substrate 1, it is preferable to use an electroless plating method because of excellent mass productivity.
The film thickness of the soft magnetic underlayer 2 will be described. In the structure in which the soft magnetic underlayer 2 shown in FIG. 2 needs to be 0.15 μm or more, and in the structure in which the soft magnetic underlayer 2 of FIG. 2 is composed of the lower layer 2a and the upper layer 2b, the lower layer 2a does not form a domain wall that generates spike noise. Therefore, the upper layer 2b needs to be 0.1 μm or more in order to obtain a sufficient reproduction output when a perpendicular magnetic recording medium is used. Moreover, although the upper limit of the film thickness of the soft magnetic underlayer 2, the lower layer 2a, and the upper layer 2b is not particularly defined in terms of characteristics, all of them are preferably 10 μm or less from the viewpoint of manufacturing cost.

On the other hand, in order to use the disk substrate 10 having these configurations for a perpendicular magnetic recording medium, the soft magnetic underlayer 2 has a surface roughness Ra of 0.5 nm or less and a small surface waviness Wa of 0.5 nm or less. It is necessary to make the flying height of the magnetic head for recording and reproducing information about 10 nm or less. Here, the surface roughness Ra indicates the centerline surface roughness of a three-dimensional image when the surface shape of a 5 μm square region is measured using an atomic force microscope AFM, and the minute surface waviness Wa is Using an optical surface shape measuring machine manufactured by Zygo, undulation measured on a 1 mm square region through a filter having a long wavelength of 500 μm and a short wavelength of 50 μm is shown.
In order to realize such a surface shape, it is necessary to smooth the surface of the soft magnetic underlayer 2 by polishing using loose abrasive grains. As described above, the polishing process can utilize substantially the same technology as in the case of the conventional non-magnetic Ni-P layer. For example, by using a double-side polishing machine with a polishing pad made of urethane foam, It can be performed by polishing while supplying a suspension of colloidal silica as an abrasive.
<Embodiment of Perpendicular Magnetic Recording Medium>
FIG. 3 is a schematic cross-sectional view showing an embodiment of the perpendicular magnetic recording medium according to the present invention. As shown in FIG. 3, the perpendicular magnetic recording medium of the embodiment of the present invention is formed on the disk substrate 10 for the perpendicular magnetic recording medium of Embodiment 1 or Embodiment 2 of the present invention shown in FIG. At least the nonmagnetic seed layer 20, the magnetic recording layer 30, and the protective layer 40 are sequentially formed. Although not shown, the nonmagnetic seed layer 20, the magnetic recording layer 30, and the protective layer 40 can be similarly provided on the other surface side of the disk substrate 10.

For the nonmagnetic seed layer 20, a material for preferably controlling the crystal orientation, crystal grain size, etc. of the magnetic recording layer 30 can be used without particular limitation. For example, if the magnetic recording layer 30 is a perpendicular magnetization film made of a CoCr alloy, a CoCr alloy, Ti, Ti alloy, Ru, or the like can be used as the nonmagnetic seed layer 20. Is a so-called laminated perpendicular magnetization film in which a Co-based alloy or the like is laminated with Pt or Pd or the like, Pt or Pd or the like can be used as the nonmagnetic seed layer 20. Further, providing a pre-seed layer or an intermediate layer above or below the nonmagnetic seed layer 20 does not hinder the effects of the present invention.
As the magnetic recording layer 30, any material that can be used for recording and reproduction as a perpendicular magnetic recording medium can be used. That is, a so-called perpendicular magnetization film such as the above-described CoCr alloy, a film in which a Co alloy or the like is laminated with Pt or Pd, or the like can be used.

As the protective layer 40, for example, a thin film mainly composed of carbon is used. In addition, after forming the protective layer 40, a liquid lubricant layer made of, for example, perfluoropolyether may be applied and formed.
The nonmagnetic seed layer 20, the magnetic recording layer 30, and the protective layer 40 can be formed by any thin film forming method such as sputtering, CVD, vacuum deposition, or plating.
A soft magnetic auxiliary layer having a product of a film thickness and a saturation magnetic flux density of 150 G · μm or more and a film thickness of 50 nm or less is applied by sputtering between the soft magnetic underlayer 2 and the nonmagnetic seed layer 20 of the disk substrate 10. be able to. In this case, both the soft magnetic auxiliary layer and the soft magnetic underlayer 2 function as a soft magnetic backing layer, thereby further improving the performance as a two-layer perpendicular medium and reducing random noise generated from the soft magnetic underlayer 2. Also effective. As the soft magnetic auxiliary layer, the product of the film thickness and the saturation magnetic flux density is preferably 150 G · μm or more in order to improve the performance as the soft magnetic backing layer. When the film thickness is 50 nm or more, a domain wall is easily formed in the soft magnetic auxiliary layer, and spike noise is generated and productivity is not preferable.

  Further, when the surface of the soft magnetic underlayer 2 of the disk substrate 10 is preferably subjected to texturing in the circumferential direction using loose abrasive grains, and then the layers are sequentially formed on the surface by sputtering. Further, it is more preferable because fine defects due to random scratches inevitably remaining on the surface of the soft magnetic underlayer 2 by the polishing process of the disk substrate 10 can be removed by the texturing process.

Examples of the present invention that are specific to the above-described embodiment will be described below.
[Example 1]
Example 1 corresponds to Example 1 described above.
(Production of substrate)
A disc-shaped lithium silicate crystallized glass substrate with a nominal diameter of 2.5 inches and polished to a surface roughness of Ra ≦ 0.5 nm and Wa ≦ 0.5 nm was prepared by changing the concentration from 0.01 wt% to 2 wt%. Etching was performed by immersing in an aqueous hydrogen halide solution, and Ra was changed from 0.5 nm to 20 nm to obtain a nonmagnetic substrate 1.
Thereafter, using the following plating bath (1) or (2), a CoNiP alloy film single layer having a film thickness of 1.0 μm to 11 μm, a saturation magnetic flux density of 0.2 T to 1.5 T, and a coercive force of 1 Oe to 20 Oe. Alternatively, the soft magnetic underlayer 2 made of a single NiP alloy film was formed.

Plating bath (1)
Nickel sulfate hexahydrate 5-25 g / liter Cobalt sulfate heptahydrate 5-25 g / liter Sodium hypophosphite 25 g / liter Sodium citrate 60 g / liter pH 8.0 ± 0.1 (NaOH and H ₂ SO ₄ Adjusted by)
Liquid temperature 85 ± 1 ℃
Plating bath (2)
Nickel sulfate hexahydrate 25 g / liter Sodium hypophosphite 10-20 g / liter Sodium acetate 20 g / liter Glycine 10 g / liter pH 6.5 ± 0.1 (adjusted with NaOH and H ₂ SO ₄ )
Liquid temperature 90 ± 1 ℃
Further, the surface of the soft magnetic underlayer 2 was polished with colloidal silica having an average particle diameter of 30 nm and a foamed urethane polishing pad, washed and dried to produce a disk substrate 10 for a perpendicular magnetic recording medium shown in FIG. . The polishing amount by polishing was about 1.0 μm in terms of film thickness, and a soft magnetic underlayer 2 having a smooth surface with a film thickness of 0 to 10 μm was obtained.
(Substrate evaluation)
Regarding the disk substrate 10 for perpendicular magnetic recording medium thus manufactured, the surface roughness Ra of the nonmagnetic substrate 1 was measured using an atomic force microscope AFM, and the magnetization characteristics were measured using a vibrating sample magnetometer VSM.

FIG. 4 shows the relationship between the surface roughness Ra of the nonmagnetic substrate 1 and | 1-Hsc / Hsr |, which is an index of the difficulty of generating the domain wall. If Hsc / Hsr = within 1.0 ± 0.2, that is, | 1-Hsc / Hsr | ≦ 0.2, it is determined that the domain wall formation is suppressed.
When the saturation magnetic flux density is less than 0.4T, | 1-Hsc / Hsr | ≦ 0.2 regardless of the surface roughness. However, at 0.4T or more, | 1 when Ra is less than 1.5 nm. −Hsc / Hsr |> 0.2 and it is easy to form a domain wall. However, when Ra is 1.5 nm or more, | 1−Hsc / Hsr | ≦ 0.2 and a domain wall is difficult to be formed. When the saturation magnetic flux density is 0.4 T or more and Ra is 2.0 nm or more, | 1-Hsc / Hsr | ≦ 0.1, which is more preferable.
(Production of media)
Next, the produced perpendicular magnetic recording medium disk substrate 10 is introduced into a sputtering apparatus, heated using a lamp heater so that the substrate surface temperature becomes 250 ° C., and then Ti using a Ti target. 10 nm of the nonmagnetic seed layer 20 made of a film, and subsequently 30 nm of the magnetic recording layer 30 made of a CoCrPt alloy film using a Co ₇₀ Cr ₂₀ Pt ₁₀ target, and finally a protective layer made of a carbon film using a carbon target. After film-forming 40 nm, the film was taken out from the vacuum apparatus. All of these sputtering films were formed by DC magnetron sputtering under an Ar gas pressure of 5 mTorr. Thereafter, a liquid lubricant layer 2 nm made of perfluoropolyether was formed by a dipping method to obtain a perpendicular magnetic recording medium shown in FIG.
(Evaluation of media)
Next, the recording / reproducing characteristics of these perpendicular magnetic recording media were measured with a single-pole magnetic head for perpendicular magnetic recording media using a spin stand tester.

FIG. 5 shows the write current dependency of the magnetic head on the signal reproduction output at a recording density of 300 kFCI (Flux Change per Inch). Here, Bs · δ represented by the unit T · μm is a product of the saturation magnetic flux density and the film thickness of the soft magnetic underlayer 2. When the thickness of the soft magnetic underlayer 2 is 0, that is, when the soft magnetic underlayer 2 is not present, almost no reproduction output is obtained. When Bs · δ of the soft magnetic underlayer 2 is smaller than 0.4T × 0.15 μm = 0.06 T · μm, the reproduction output is relatively low, and the reproduction output does not saturate with respect to the write current. I understand.
In this way, when the reproduction output is saturated with respect to the write current, a large current value is required to obtain a high output, and in a region where the reproduction output is not saturated, reproduction is performed against fluctuations in the write current. Since the output changes greatly, it is not practically preferable. On the other hand, when Bs · δ of the soft magnetic underlayer 2 is 0.06 T · μm or more, a sufficient reproduction output is obtained and the reproduction output is saturated at a low current value. is there.
[Example 2]
Example 2 corresponds to Example 2 described above.
(Production of substrate)
An Al—Mg alloy disk having a nominal diameter of 3.5 inches was used, and the surface was cleaned by alkali cleaning and acid etching, and zincate (substitution zinc plating) was applied as an initial reaction layer of electroless Ni—P plating. Thereafter, a commercially available electroless Ni-P plating solution for hard disk substrates (Nimden HDX, manufactured by Uemura Kogyo Co., Ltd.) was adjusted to Ni concentration 6.0 ± 0.1 g / L, pH 4.5 ± 0.1, and liquid temperature 92 ± 1 ° C. A nonmagnetic underlayer made of a Ni—P alloy film having a thickness of 10 μm was formed by using a controlled plating bath, and a nonmagnetic substrate 1 was obtained. The average P concentration of this nonmagnetic Ni—P plating film was 12 wt% (20.5 at%).

Thereafter, Ni-P for changing the P concentration from 2 at% to 10 at% and changing the film thickness from 0 to 10 μm to form the lower layer 2a of the soft magnetic underlayer 2 using the plating bath (3) shown below. A plating film was formed. Next, they were subjected to a heat treatment at an appropriate temperature of 0 to 300 ° C. for 30 minutes to form a lower layer 2a shown in FIG. 2 having a saturation magnetic flux density of 0.15T to 0.6T and a coercive force of 5Oe to 40Oe. Got.
FIG. 6 shows the relationship between the coercive force of the substrate measured using the vibrating sample magnetometer VSM and | 1-Hsc / Hsr |, which is an index of the difficulty of generating the domain wall. It can be seen that if the coercive force is 10 Oe or more and the saturation magnetic flux density is 0.6 T or less, the domain wall is not formed.
Plating bath (3)
Nickel sulfate hexahydrate 25 g / liter Sodium hypophosphite 10-20 g / liter Sodium acetate 20 g / liter Glycine 10 g / liter pH 6.5 ± 0.1 (adjusted with NaOH and H 2 SO 4)
Liquid temperature 90 ± 1 ℃
Next, the surface of the lower layer 2a of the soft magnetic underlayer 2 is polished using a double-side polishing machine in which an alumina polishing liquid having an average particle diameter of 15 μm to 0.5 μm, colloidal silica having an average particle diameter of 30 nm, and a urethane foam polishing pad are attached The surface roughness Ra was changed from 0.5 nm to 20 nm.

Furthermore, the upper layer 2b made of a CoNiP alloy film having a film thickness of 1.5 to 10 μm and a saturation magnetic flux density of 0.2 T or more and 1.5 T or less was formed thereon using the plating bath (1) of Example 1. .
Then, the surface of the upper layer 2b was polished with colloidal silica having an average particle diameter of 30 nm and a foamed urethane polishing pad, washed and dried to produce a disk substrate 10 for perpendicular magnetic recording media shown in FIG. The polishing amount by polishing was about 1.0 to 1.5 μm in terms of film thickness.
(Substrate evaluation)
Regarding the disk substrate 10 for the perpendicular magnetic recording medium thus manufactured, the surface roughness Ra of the lower layer 2a of the soft magnetic underlayer 2 is measured using an atomic force microscope AFM, and the magnetization characteristics are measured using a vibrating sample magnetometer VSM. Was measured.

FIG. 7 shows the relationship between | 1-Hsc / Hsr |, which is the value of the difficulty of generating the domain wall of the upper layer 2b of the soft magnetic underlayer 2, and the film thickness of the lower layer 2a. As the lower layer 2a of the soft magnetic underlayer 2, a 0.2T, 20Oe Ni—P plating film polished to Ra = 1 nm was used.
The thicker the lower layer, the smaller the upper layer saturation magnetic flux density, the harder the domain wall is generated. If the upper layer saturation magnetic flux density is 0.4 T or more and 1.5 T or less, the lower layer saturation magnetic flux density is 0, which is the lower limit. Even if it is .2T, if the film thickness is 0.5 μm or more, it is difficult to generate a domain wall sufficiently (| 1−Hsc / Hsr | <0.2), and a soft magnetic layer is obtained.
FIG. 8 shows the relationship between the surface roughness Ra of the lower layer 2a of the soft magnetic underlayer 2 and | 1-Hsc / Hsr |, which is an index of difficulty in generating a domain wall. The thickness of the lower layer 2a of the soft magnetic underlayer 2 was 2 μm.

When Ra is 0.7 nm or more, Hsc / Hsr is within 1.0 ± 0.2, and it is judged that domain wall formation is suppressed. Further, when Ra is 1.0 nm or more, Hsc / Hsr is more preferably within 1.0 ± 0.1.
(Production of media)
Next, the perpendicular magnetic recording medium shown in FIG. 3 was produced in the same manner as in Example 1 using the produced perpendicular magnetic recording medium disk substrate 10.
(Evaluation of media)
In the same manner as in Example 1, the recording / reproducing characteristics of these perpendicular magnetic recording media were measured using a single-pole magnetic head for perpendicular magnetic recording media using a spin stand tester. FIG. 9 shows the write current dependency of the magnetic head on the signal reproduction output at a recording density of 300 kFCI (Flux Change per Inch).

Here, when the product Bs · δ (T · μm) of the saturation magnetic flux density and the film thickness of the upper layer 2b of the soft magnetic underlayer 2 is smaller than 0.4T × 0.1 μm = 0.04T · μm, the reproduction output is It is relatively low, and it can be seen that the reproduction output does not saturate with respect to the write current. In this way, when the reproduction output is saturated with respect to the write current, a large current value is required to obtain a high output, and in a region where the reproduction output is not saturated, reproduction is performed against fluctuations in the write current. Since the output changes greatly, it is not practically preferable.
On the other hand, when Bs · δ of the upper layer 2b of the soft magnetic underlayer 2 is 0.04 T · μm or more, a sufficient reproduction output is obtained and the reproduction output is saturated at a low current value. It can be seen that it is a medium.
As described above, according to the present invention, in the perpendicular magnetic recording medium in which at least the magnetic recording layer is formed on the substrate in which the soft magnetic underlayer is formed on the nonmagnetic substrate, the surface roughness of the nonmagnetic substrate. Ra is 1.5 nm to 20 nm, preferably 2.0 nm to 10 nm, the soft magnetic underlayer has a saturation magnetic flux density of 0.4 T to 1.5 T, and a film thickness of 0.15 μm to 10 μm, or The saturation magnetic flux density is 0.2 T or more and 0.5 T, the coercive force is 10 Oe or more and 30 Oe or less, the film thickness is 0.5 μm or more and 10 μm or less, and the surface roughness Ra is 0.7 nm or more and 10 nm or less, preferably 1.0 nm or more. By using a substrate having a soft magnetic underlayer composed of a lower layer having a thickness of 5 nm or less and an upper layer having a saturation magnetic flux density of 0.4 T or more and 1.5 T or less and a film thickness of 0.1 μm or more and 10 μm or less. Etc. to suppress the formation of domain wall does not require a device to be plated with MnIr alloy thin film or the like or a magnetic field in, it is possible to prevent the occurrence of spike noise.

1 is a schematic cross-sectional view showing Embodiment 1 of a disk substrate for a perpendicular magnetic recording medium according to the present invention. It is a cross-sectional schematic diagram which shows Embodiment 2 of the disk substrate for perpendicular magnetic recording media based on this invention. 1 is a schematic cross-sectional view showing an embodiment of a perpendicular magnetic recording medium according to the present invention. FIG. 6 is a diagram showing a relationship between | 1-Hsc / Hsr |, which is an index of difficulty in generating a domain wall of a soft magnetic underlayer, in the disk substrate for perpendicular magnetic recording media of Example 1 and the surface roughness Ra of a nonmagnetic substrate. . FIG. 3 is a diagram showing the write current dependence of the magnetic head of the signal reproduction output at a recording density of 300 kFCI (Flux Change per Inch) of the perpendicular magnetic recording medium of Example 1. FIG. 6 is a diagram showing a relationship between | 1-Hsc / Hsr |, which is an index of difficulty in generating a domain wall under a soft magnetic underlayer in a disk substrate for a perpendicular magnetic recording medium of Example 2, and coercivity. The figure which shows the relationship between | 1-Hsc / Hsr | which is the value of the difficulty of generating the domain wall of the upper layer of a soft-magnetic underlayer in the disk substrate for perpendicular magnetic recording media of Example 2, and the film thickness of the lower layer of a soft-magnetic underlayer It is. FIG. 6 shows the relationship between | 1-Hsc / Hsr |, which is an index of the difficulty of generating a magnetic wall of the soft magnetic underlayer, and the surface roughness Ra of the lower layer of the soft magnetic underlayer in the disk substrate for the perpendicular magnetic recording medium of Example 2. FIG. FIG. FIG. 10 is a diagram showing the write current dependence of the magnetic head of the signal reproduction output at a recording density of 300 kFCI (Flux Change per Inch) of the perpendicular magnetic recording medium of Example 2. It is a conceptual diagram which shows the magnetization characteristic of a soft magnetic film. It is a figure which shows the measurement procedure of the magnetization characteristic of the soft-magnetic film | membrane by VSM.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Nonmagnetic base | substrate 2 Soft magnetic underlayer 2a Lower layer 2b Upper layer 10 Disk substrate 20 for perpendicular magnetic recording media Nonmagnetic seed layer 30 Magnetic recording layer 40 Protective layer

Claims (2)

In a disk substrate for a perpendicular magnetic recording medium in which a soft magnetic underlayer is formed on a nonmagnetic substrate,
The nonmagnetic substrate has a surface roughness Ra of 5 nm to 20 nm, and the soft magnetic underlayer has a saturation magnetic flux density of 0.4 T or more and 1.5 T or less on the nonmagnetic substrate having the surface roughness. A disk substrate for a perpendicular magnetic recording medium, wherein a soft magnetic film is formed to a thickness of 0.15 μm to 10 μm. 2. At least a magnetic recording layer is formed on a disk substrate for a perpendicular magnetic recording medium according to claim 1, and the soft magnetic underlayer of the disk substrate is used as at least part of a soft magnetic backing layer for the magnetic recording layer. And a perpendicular magnetic recording medium.

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