JP2011227992A - Perpendicular magnetic recording medium and magnetic recording and reproducing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a perpendicular magnetic recording medium that enhances thermal fluctuation resistance while reducing a magnetic field required for recording, and that besides improves a signal-to-noise ratio.SOLUTION: A perpendicular magnetic recording medium has a first magnetic layer, a nonmagnetic intermediate layer and a second magnetic layer in this order on a substrate. The first magnetic layer and the second magnetic layer are ferromagnetically coupled, and have crystal particles and an amorphous grain boundary layer. The crystal particles of the first magnetic layer contain Co, Pt and Cr, and the crystal particles of the second magnetic layer contain Co but not contain Cr. For the first magnetic layer and the second magnetic layer, when values of saturation magnetization are Ms, Msand values of anisotropic magnetic fields are Hk, Hk, the following relations are satisfied: Ms<Msand Hk>Hk.

Description

  The present invention relates to a perpendicular magnetic recording medium and a magnetic recording / reproducing apparatus including the perpendicular magnetic recording medium.

  In order to increase the recording density of a hard disk drive (HDD), it is necessary to reduce the size of one bit on the magnetic recording medium. It is necessary to make the constituent magnetic particles fine. When the particle size is reduced, the energy to maintain the recorded magnetization direction is reduced in proportion to the volume, and when this is close to room temperature thermal energy, magnetization reversal occurs without recording with a magnetic head. (Thermal fluctuation phenomenon).

  In order to prevent this, there is a way to increase the energy (uniaxial magnetic anisotropy energy; Ku) required for magnetization reversal per unit volume, but the head magnetic field required for recording is basically proportional to Ku. Therefore, when Ku is increased, sufficient recording cannot be performed unless the head magnetic field is increased accordingly. However, the saturation magnetic flux of the magnetic head is almost close to the physical limit at present, and the magnetic pole tip of the head becomes smaller as 1 bit becomes smaller, so it becomes very difficult to improve the recording performance of the head. ing.

  In other words, when trying to make magnetic particles finer in order to increase the recording density, it has become difficult to record, and there is a need for a method for reducing the magnetic field required for recording while improving the resistance to thermal fluctuation. Yes.

  As one of the methods, in recent years, one columnar particle is divided into a hard magnetic (high Ku) region and a soft magnetic (low Ku) region in the upper and lower directions, and these are combined with an appropriate interaction. Proposed (composite media) (Non-Patent Document 1). In such a configuration, since the two regions are not completely exchange coupled, when the magnetic field is applied from the outside, the two regions are not reversed together, but the soft magnetic region starts to rotate first. After rotating to some extent, it reverses by applying a force to rotate the hard magnetic region by appropriate interaction. When the two regions are completely exchange-coupled, when an external magnetic field is applied, the magnetic field (coercive force) corresponding to the average Ku is reversed, but by weakening the coupling appropriately, a magnetic field smaller than this is reversed. It is reported that it is reversed by (coercivity). There have been a number of reports that actually produced this composite media and evaluated its properties (for example, Non-Patent Document 2), but all are based on the original proposal and one region is soft magnetic. .

  It is true that the coercivity can be lowered by moderately weakening the coupling between the two regions compared to the case of complete coupling (the degree of the reduction depends on the respective Ku and saturation magnetization Ms). The base is the average value of Ku (weighted average according to the layer thickness), and it can be said that the coercive force is roughly determined by the average value of Ku.

  Assuming that the head magnetic field is already full, if the coercive force (reversal magnetic field) is set to the maximum value within the recordable range, the average value of Ku will be almost determined. become.

  In the so-called composite media, the Ku of the soft magnetic region is almost 0, and the Ku of the hard magnetic region can be doubled when there is no soft magnetic region. In other words, the composite media is also a method for handling the hard magnetic layer having a higher Ku, but the average Ku is significantly lower than the Ku of the hard magnetic layer.

On the other hand, CoCrPt-oxide granular hard magnetic layer is the mainstream in current perpendicular HDD media, and Ku can be increased by reducing Cr, but SNRm (media signal to noise ratio) is increased by increasing Cr. Thus, the Cr composition is high and Ku is suppressed to about 4 × 10 ⁶ erg / cc. Further, materials that can obtain a very large Ku such as an FePt alloy have been studied. However, there is no prospect of obtaining a high SNRm like a CoCrPt recording layer, and it is substantially possible to increase the Ku of the hard magnetic layer. It's a difficult situation.

  In addition to the above, perpendicular magnetic recording media using two magnetic layers as magnetic recording layers have been proposed (Patent Document 1 and Patent Document 2). However, it has not yet been possible to reduce the magnetic field necessary for recording while improving the resistance to thermal fluctuations and to improve the signal-to-noise ratio.

JP 2003-168207 A JP 2006-48900 A

R. H. Victora et al., IEEE Transactions on Magnetics, Vol. 41, p. 537 J. P. Wang et al., IEEE Transactions on Magnetics, Vol. 41, p. 3181

  An object of the present invention is to provide a perpendicular magnetic recording medium capable of reducing the magnetic field required for recording while improving the resistance to thermal fluctuation and improving the signal-to-noise ratio.

A perpendicular magnetic recording medium according to an embodiment of the present invention includes a first magnetic layer and a second magnetic layer on a substrate, and the first magnetic layer and the second magnetic layer have uniaxial magnetic anisotropy. When constants are Ku ₁ and Ku ₂ , saturation magnetization is Ms ₁ and Ms ₂ , anisotropic magnetic field is Hk ₁ > Hk ₂ , and thicknesses are t ₁ and t ₂ , Ku ₁ and Ku ₂ are 3 × 10 ^6. erg / cc or more, Ms ₁ <Ms ₂ , Hk ₁ > Hk ₂ , and t ₁ > t ₂ .

  According to the present invention, it is possible to provide a perpendicular magnetic recording medium capable of reducing the magnetic field necessary for recording while improving the resistance to thermal fluctuation and improving the signal-to-noise ratio.

1 is a cross-sectional view of a perpendicular magnetic recording medium according to an embodiment of the present invention. 1 is a partially exploded perspective view of a magnetic recording / reproducing apparatus according to an embodiment of the present invention. 2 is a magnetization curve of a perpendicular magnetic recording medium in Example 1. FIG. 4 is a graph showing the tilt angle dependence of the residual coercivity of the perpendicular magnetic recording medium in Example 1. FIG. 4 is a diagram showing the sweep time dependence of the coercivity of the perpendicular magnetic recording medium in Example 1. The magnetization curve of the perpendicular magnetic recording medium in Comparative Example 1. 7 is a magnetization curve of a perpendicular magnetic recording medium in which the thickness of the nonmagnetic intermediate layer in Example 2 is 0 nm. The magnetization curve of the perpendicular magnetic recording medium which made the nonmagnetic intermediate | middle layer thickness in Example 2 2 nm. The figure which shows the intermediate layer thickness dependence of Hc and Hs about the perpendicular magnetic recording medium in Example 2. FIG. FIG. 7 is a diagram showing the dependence of Hc and Hs on the SiO ₂ composition of the perpendicular magnetic recording medium in Example 3. FIG. 10 is a diagram showing the dependence of Hc and Hs on the second magnetic layer thickness for the perpendicular magnetic recording medium in Example 4;

  The magnitude of the reversal magnetic field expected from Ku in the magnetic layer is given by 2 Ku / Ms and is called an anisotropic magnetic field (Hk). The point of the composite media is that the soft magnetic region is reversed first, and in terms of ease of reversal, it is preferable that Ku approaches 0, Hk approaches 0, but Ku is not necessarily close to 0, but Ms. In contrast, if Hk is small, inversion occurs first.

  Therefore, we basically use the current CoCrPt-oxide granular hard magnetic layer as the first magnetic layer, and use a “hard” magnetic layer with high Ms and low Hk instead of the soft magnetic layer as the second magnetic layer. Figured out.

  Reversal starts earlier by lowering the Hk of the second magnetic layer, and it can be expected that the reversal magnetic field (coercive force to magnetic field necessary for head recording) is reduced by promoting reversal of the first magnetic layer (two Unlike theoretical calculations that assume that each region has one magnetic moment, it is actually a collection of a large number of magnetic moments. Therefore, the directions of the moments in the region are not uniform, even when they are completely exchange-coupled. , Magnetization rotation starts from a portion far from the boundary of the low Hk region). Further, by providing a nonmagnetic intermediate layer between the two magnetic layers to weaken the interaction between the two magnetic layers, the magnetic field required for recording can be further reduced.

In addition, if the Ku of the second magnetic layer is somewhat higher, the thermal fluctuation resistance can be increased by an increase in the average Ku, and if the Ku is the same or lower, the thermal fluctuation resistance is reduced to the same degree. However, if Ku is on the order of the sixth power, it is considered that the decrease in thermal fluctuation resistance can be suppressed by reducing the layer thickness or adjusting the Ku of the first magnetic layer (as described above). When the first magnetic layer is the current hard magnetic layer and the soft magnetic layer is used as the second magnetic layer, the average Ku is greatly reduced). If Ku is 3 × 10 ⁶ erg / cc or more in both the first and second magnetic layers, high thermal fluctuation resistance can be expected without sacrificing the degree of freedom such as layer thickness.

  Therefore, this method reduces the magnetic field required for recording (improves overwriting characteristics / OW) while maintaining the resistance to thermal fluctuation while using the CoCrPt-oxide granular hard magnetic layer with good SNRm, or recording. It is considered that the thermal fluctuation resistance can be improved while maintaining the magnetic field necessary for the operation.

According to the present invention, the perpendicular magnetic recording layer has a two-layer structure of a first magnetic layer / second magnetic layer, and Ku ₁ and Ku ₂ are both 3 × 10 ⁶ erg / cc or more, Ms ₁ <Ms ₂ , Hk ₁ > By setting Hk ₂ and t ₁ > t ₂ , it is possible to provide a magnetic recording medium with improved medium noise, resistance to thermal fluctuation, and overwrite characteristics, and a magnetic recording / reproducing apparatus with improved surface recording density.

  Hereinafter, embodiments of the present invention will be described in more detail.

  FIG. 1 is a sectional view of a perpendicular magnetic recording medium according to an embodiment of the present invention. This perpendicular magnetic recording medium comprises a substrate 1, a soft magnetic backing layer 2 including a lower soft magnetic layer 2a, a nonmagnetic intermediate layer 2b, and an upper soft magnetic layer 2c, a seed layer 3, a nonmagnetic underlayer 4, and a first magnetic recording medium. The magnetic layer 5, the nonmagnetic intermediate layer 6, the second magnetic layer 7, and the protective layer 8 are stacked. Hereinafter, suitable materials will be described.

<Board>
As the substrate, for example, a glass substrate, an Al alloy substrate, a ceramic substrate, a carbon substrate, an Si single crystal substrate having an oxidized surface, or the like can be used. Examples of the material of the glass substrate include amorphous glass and crystallized glass. As the amorphous glass, for example, general-purpose soda lime glass and aluminosilicate glass can be used. Further, as the crystallized glass, for example, lithium-based crystallized glass can be used. As the ceramic substrate, for example, a sintered body mainly composed of general-purpose aluminum oxide, aluminum nitride, silicon nitride, or the like, or a fiber reinforced material thereof can be used. Alternatively, a substrate in which a NiP layer is formed on the surface of the above-described metal or nonmetal substrate using a plating method or a sputtering method can also be used.

<Soft magnetic backing layer>
In the present invention, a so-called perpendicular double-layer medium having a perpendicular magnetic recording layer on the soft magnetic backing layer is formed by providing a soft magnetic backing layer having a high magnetic permeability. In this perpendicular double-layer medium, the soft magnetic backing layer is a magnetic head for circulating a recording magnetic field from a magnetic head for magnetizing the perpendicular magnetic recording layer, for example, a single magnetic pole head, to the magnetic head side in the horizontal direction. It plays a part of the function and can play a role of improving the recording and reproducing efficiency by applying a steep and sufficient perpendicular magnetic field to the magnetic recording layer.

  For the soft magnetic underlayer, for example, a material containing Fe, Ni, and Co can be used. Examples of such materials include FeCo alloys such as FeCo and FeCoV, FeNi alloys such as FeNi, FeNiMo, FeNiCr, and FeNiSi, FeAl alloys, FeSi alloys such as FeAl, FeAlSi, FeAlSiCr, FeAlSiTiRu, and FeAlO. Examples thereof include FeTa, FeTaC, and FeTaN, and FeZr alloys such as FeZrN. Further, a material having a fine crystal structure such as FeAlO, FeMgO, FeTaN, FeZrN, or the like containing 60 atomic% or more of Fe or a granular structure in which fine crystal particles are dispersed in a matrix can be used.

  As another material of the soft magnetic backing layer, a Co alloy containing Co and at least one of Zr, Hf, Nb, Ta, Ti, and Y can be used. Co is preferably contained at 80 atomic% or more. When such Co alloy is formed by sputtering, an amorphous layer is likely to be formed, and amorphous soft magnetic materials do not have magnetocrystalline anisotropy, crystal defects, and grain boundaries, and thus have excellent soft magnetism. Show. Examples of such amorphous soft magnetic materials include alloys containing cobalt as a main component and zirconium as a subcomponent, such as CoZr alloys such as CoZr, CoZrNb, and CoZrTa. B can be further added to the above materials for the purpose of facilitating the formation of an amorphous state.

  When an amorphous material is used for the soft magnetic backing layer, it has almost no direct effect on the crystal orientation of the metal layer formed on it, as with an amorphous substrate. There are no major changes in the structure or crystallinity of the magnetic recording layer, and basically the same magnetic characteristics and recording / reproducing characteristics can be expected. If the third element is of a different level, such as a CoZr alloy, the differences in saturation magnetization (Ms), coercive force (Hc), magnetic permeability (μ), etc. are small. Recording / reproduction characteristics can be obtained.

  As shown in FIG. 1, the soft magnetic backing layer has a three-layer structure including a lower soft magnetic layer, an antiferromagnetic coupling layer, and an upper soft magnetic layer, and the lower soft magnetic layer and the upper soft magnetic layer are You may make it ferromagnetically couple.

<Seed layer>
In the perpendicular magnetic recording medium of the present invention, a seed layer may be provided between the soft magnetic underlayer and the nonmagnetic underlayer. By providing the seed layer, the crystal grain size and crystal orientation of the magnetic recording layer can be improved through the nonmagnetic underlayer. If the nonmagnetic underlayer can be thinned by these improvements, the distance between the magnetic head and the soft magnetic underlayer can be shortened to improve the recording / reproducing characteristics. Regarding the magnetism of the seed layer, if it can have soft magnetic characteristics, it can function as a backing layer, which is preferable because the distance from the magnetic head can be further reduced.

  The thickness of the seed layer is preferably 0.1 to 20 nm, more preferably 0.2 to 10 nm. If the average layer thickness is 1 atomic layer or less, even if it can be formed completely uniformly, it will not be a completely continuous layer, but it will improve the crystal grain size and crystal orientation even if it has an island-like structure. Can be expected. On the other hand, if the seed layer is a soft magnetic material having good characteristics, the maximum value is not limited from the viewpoint of spacing, but if there is no magnetism, the spacing is increased.

  As a material for the seed layer, there is an advantage that the crystal orientation of hcp or fcc is easy to improve, but even when bcc metal is used, the crystal grain size of the underlayer is made finer due to the difference in crystal structure with the underlayer. Can be expected. A seed layer is not essential, but if provided, a suitable material can include at least one selected from the group consisting of Pd, Pt, Ni, Ta, Ti, and alloys thereof, for example. In order to further improve the characteristics, these materials may be mixed, another element may be mixed, or they may be laminated.

<Nonmagnetic underlayer>
For example, Ru can be used as the nonmagnetic underlayer. Ru is the same hcp as the main component Co of the recording layer, and is preferable in that the lattice mismatch with Co is not too large, the grain size is small, and columnar growth is easy.

  Further, by increasing the Ar gas pressure during film formation, the particle size can be further refined, the dispersion of the particle size can be improved, and the separation between the particles can be promoted. In this case, although the crystal orientation tends to deteriorate, it can be compensated for by combining with Ru having a low gas pressure that facilitates enhancing the crystal orientation, if necessary. It is preferable to set the first half to a low gas pressure and the second half to a high gas pressure, and the same effect can be expected if the gas pressure in the second half is relatively higher than the gas pressure in the first half, and may be 10 Pa or higher. Further, the layer pressure ratio is preferably thicker in the low gas pressure layer if priority is given to crystal orientation, and thicker in the high gas pressure layer if priority is given to the refinement of grain size.

  The separation between particles can be further promoted by adding an oxide. As the oxide, at least one selected from the group consisting of silicon oxide, chromium oxide, and titanium oxide is particularly preferable.

  The thickness of the nonmagnetic underlayer is preferably 2 to 50 nm, more preferably 4 to 30 nm. Not only Ru but if the underlayer is too thin, it will not be a sufficient continuous film and it will be difficult to improve the crystallinity, and it will be difficult to improve the fine structure of the magnetic recording layer formed thereon. Increasing the thickness makes it easier to increase the crystallinity and the coercive force of the magnetic recording layer on top of it, but increasing the thickness too much results in a decrease in recording capability and recording resolution by the magnetic head due to increased spacing. .

  Up to this point, Ru has been mainly described. However, even if fcc metal is used for the nonmagnetic underlayer, the Co-based recording layer is made to have hcp (00.1) orientation by adopting (111) orientation. Therefore, in consideration of lattice mismatch with Co, for example, Rh, Pd, Pt or the like can be used. Further, an alloy made of at least one selected from the group consisting of Ru, Rh, Pd, and Pt and at least one selected from the group consisting of Co and Cr can also be used. Furthermore, for example, at least one selected from the group consisting of B, Ta, Mo, Nb, Hf, Ir, Cu, Nd, Zr, W, and Nd can be added.

<Perpendicular magnetic recording layer>
The first magnetic layer used in the present invention is, for example, a ferromagnetic layer, and preferably has a saturation magnetization Ms of 200 ≦ Ms <700 emu / cc.

  As the first magnetic layer used in the present invention, for example, a CoPt-based alloy can be used. The ratio of Co to Pt in the CoPt alloy is preferably 2: 1 to 9: 1 from the viewpoint of obtaining high uniaxial magnetocrystalline anisotropy Ku. The CoPt-based alloy preferably further contains Cr.

  The first magnetic layer preferably further contains oxygen. Oxygen can be added as an oxide. As the oxide, at least one selected from the group consisting of silicon oxide, chromium oxide, and titanium oxide is particularly preferable. By such an oxide, the first magnetic layer has a so-called granular structure including magnetic crystal grains containing Co and a grain boundary phase containing an amorphous oxide surrounding the first magnetic layer.

  The magnetic crystal grains preferably have a columnar structure penetrating the perpendicular magnetic recording layer vertically. By forming such a fine structure, the crystal orientation and crystallinity of the magnetic crystal grains in the perpendicular magnetic recording layer are improved, and as a result, the reproduction signal output / noise ratio (S / N ratio) suitable for high-density recording is achieved. ) Can be obtained.

  The content of the oxide for obtaining such a fine structure is preferably 3 mol% to 20 mol% with respect to the total amount of Co, Cr, and Pt. More preferably, it is 5 mol% to 18 mol%. The above range is preferable as the oxide content in the perpendicular magnetic recording layer. When the layer is formed, an amorphous grain boundary layer having weak or little magnetism is formed around the magnetic crystal grains. This is because it can be isolated and miniaturized.

  In the first magnetic layer, when the content of the oxide exceeds 20 mol%, the oxide remains in the magnetic crystal grains, and the orientation and crystallinity of the magnetic crystal grains are impaired. Oxides are deposited on the upper and lower sides, and as a result, there is a tendency that the columnar structure in which the magnetic crystal grains penetrate the vertical magnetic recording layer vertically is not formed. In addition, when the oxide content is less than 3 mol%, separation and refinement of the magnetic crystal particles are insufficient, resulting in an increase in noise during recording and reproduction, and a signal / noise ratio suitable for high-density recording ( (S / N ratio) tends not to be obtained.

  The first magnetic layer preferably has a Cr content of 2 atomic% to 30 atomic%. When the Cr content is in the above range, the uniaxial crystal magnetic anisotropy constant Ku of the magnetic crystal grains is not lowered too much, and high magnetization is maintained. As a result, recording / reproduction characteristics suitable for high-density recording and sufficient heat Fluctuation characteristics tend to be obtained. When the Cr content exceeds 30 atomic%, Ku of the magnetic crystal particles is reduced, so that the thermal fluctuation characteristics are deteriorated. Also, the magnetization is reduced and the reproduction signal output is lowered, resulting in poor recording / reproduction characteristics. Tend.

  The first magnetic layer preferably has a Pt content of 10 atomic% to 25 atomic%. The Pt content is in the above range because Ku required for the perpendicular magnetic recording layer is obtained, and the crystallinity and orientation of the magnetic crystal grains are good. As a result, thermal fluctuation characteristics suitable for high-density recording, recording This is because reproduction characteristics are obtained, which is preferable.

  When the Pt content exceeds 25 atomic%, a layer having an fcc structure is formed in the magnetic crystal grains, and the crystallinity and orientation tend to be impaired. Further, when the Pt content is less than 10 atomic%, there is a tendency that Ku for obtaining thermal fluctuation characteristics suitable for high density recording cannot be obtained.

  The first magnetic layer is composed of B, Ta, Mo, Cu, Nd, W, Nb, Sm, Tb, Ru, and Re as additional subcomponents in addition to the main components such as Co, Cr, Pt, and oxide. One or more selected elements may be included. By including the above elements, it is possible to promote miniaturization of magnetic crystal grains or improve crystallinity and orientation, and to obtain recording / reproducing characteristics and thermal fluctuation characteristics suitable for higher density recording.

  The total content of the subcomponents is preferably 8 atomic% or less. If the content exceeds 8 atomic%, a phase other than the hcp phase is formed in the magnetic crystal particles, so that the crystallinity and orientation of the magnetic crystal particles are disturbed. As a result, recording / reproduction characteristics suitable for high-density recording, thermal fluctuations There is a tendency that characteristics cannot be obtained.

  In addition to the above alloy, the first magnetic layer is selected from the group consisting of other CoPt alloys, CoCr alloys, CoPtCr alloys, CoPtO, CoPtCrO, CoPtSi, CoPtCrSi, and Pt, Pd, Rh, and Ru. It is possible to use a multilayered structure of Co and an alloy containing at least one kind as a main component and CoCr / PtCr, CoB / PdB, CoO / RhO to which Cr, B and O are added. In any case, it is preferable that the perpendicular magnetic recording layer has Co as a main component because Co has an uniaxial crystal magnetic anisotropy with an hcp structure and easily obtains a high coercive force.

  Unlike the conventional composite media, the second magnetic layer used in the present invention is a hard magnetic layer, and the saturation magnetization Ms preferably satisfies 700 ≦ Ms ≦ 1422 emu / cc. Since the average Ms of the practical CoCrPt-oxide granular hard magnetic layer is less than 700 emu / cc, it can be said that generally higher Ms can be obtained in the second magnetic layer at 700 emu / cc or more. Further, when an ideal continuous film of pure Co can be formed, Ms = 1422 emu / cc which is the same as that of the bulk can be expected. However, when a nonmagnetic grain boundary layer is formed, it is proportional to the packing density of crystal grains. Thus, the average Ms of the film becomes small. For example, even if pure Co has a packing density of 50%, Ms = 711 emu / cc, so the average film thickness Ms varies depending on the film forming conditions and the structure of the first magnetic layer.

  Since the second magnetic layer requires high Ms and high Ku, it is preferable to use Co as a main component, and it is not preferable to use Cr, which tends to greatly reduce Ms. When Pt is added to Co, the decrease in Ms is almost proportional to the composition of Pt, whereas Ku can be increased, so a CoPt alloy may be used. The content of Pt is preferably 0 atomic% to 25 atomic%. Since the second magnetic layer may be pure Co, the Pt content may be 0 atomic%. On the other hand, when the Pt content exceeds 25 atomic%, a layer having an fcc structure is formed in the magnetic crystal grains, and the crystallinity and orientation of the hcp-CoPt alloy tend to be impaired. Pd exhibits the same action as Pt. One or more elements selected from B, Ta, Mo, Cu, Nd, W, Nb, Sm, Tb, Ru, and Re can be included as subcomponents. By including the above elements, it is possible to promote the miniaturization of the magnetic crystal grains or improve the crystallinity and orientation.

The uniaxial magnetic anisotropy constants Ku ₁ and Ku ₂ of the first magnetic layer and the second magnetic layer are both 3 × 10 ⁶ erg / cc or more.

  The thickness of the first magnetic layer is preferably 3 to 40 nm, more preferably 5 to 20 nm. Within this range, the magnetic recording / reproducing apparatus suitable for higher recording density can be operated. If the thickness of the first magnetic layer is less than 3 nm, the noise component tends to be higher because the crystal orientation is low, the segregation is insufficient and the reproduction output is too low, and the thickness of the first magnetic layer is 40 nm. Beyond that, the playback output tends to be too high and distort the waveform. The second magnetic layer preferably has a thickness of 1 to 10 nm.

  The coercive force of the perpendicular magnetic recording layer is preferably 237000 A / m (3000 Oe) or more. When the coercive force is less than 237000 A / m (3000 Oe), the thermal fluctuation resistance tends to be inferior.

  The perpendicular squareness ratio of the perpendicular magnetic recording layer is preferably 0.8 or more. When the vertical squareness ratio is less than 0.8, the thermal fluctuation resistance tends to be inferior.

<Protective layer>
The protective layer can prevent corrosion of the perpendicular magnetic recording layer and also prevent damage to the medium surface when the magnetic head comes into contact with the medium. Examples of the material include those containing C, SiO ₂ and ZrO ₂ . The thickness of the protective layer is preferably 1 to 10 nm. Thereby, the distance between the head and the medium can be reduced, which is suitable for high-density recording.

  A lubricating layer (not shown) can be provided on the protective layer. As the lubricant used in the lubricating layer, conventionally known materials such as perfluoropolyether, fluorinated alcohol, and fluorinated carboxylic acid can be used.

<Magnetic recording / reproducing device>
FIG. 2 is a partially exploded perspective view showing an example of the magnetic recording / reproducing apparatus according to the present invention.

  As shown in the figure, a hard disk drive (hereinafter referred to as HDD) as a disk device includes a rectangular box-shaped case 110 having an upper surface opened, and is screwed to the case by a plurality of screws to close the upper end opening of the case (not shown). And a top cover.

  In the case 110, a magnetic disk 112 as a recording medium, a spindle motor 113 for supporting and rotating the magnetic disk 112, and a magnetic head such as a single pole type magnetic recording head for recording and reproducing information on the magnetic disk. 133, a head actuator 114 that movably supports the magnetic head 133 with respect to the magnetic disk 112 according to the present invention, and a voice coil motor (hereinafter referred to as VCM) 116 that rotates and positions the head actuator around a pivot 124. When the magnetic head moves to the outermost periphery of the magnetic disk, a ramp load mechanism 118 that holds the magnetic head 133 at a position separated from the magnetic disk 112, and when an impact or the like acts on the HDD, the head actuator is held at the retracted position. Inertia latch mechanism 120, And a flexible printed circuit board unit electronic parts such as the preamplifier is mounted (hereinafter, referred to as FPC unit) 117 is housed.

  A printed circuit board (not shown) that controls the operation of the spindle motor 113, the VCM 116, and the magnetic head via the FPC unit 117 is screwed to the outer surface of the case 110, and is positioned to face the bottom wall of the case. Yes.

  The magnetic disk 112 is formed with a diameter of 65 mm (2.5 inches), for example, and has a magnetic recording layer. The magnetic disk 112 is fitted to a hub (not shown) of the spindle motor 113 and is clamped by a clamp spring 121. The magnetic disk 112 is rotationally driven at a predetermined speed by a spindle motor 113 as a drive unit.

  The magnetic head 133 is a so-called composite head formed on a substantially rectangular slider (not shown), a write head having a single magnetic pole structure, a read head using a GMR film or a TMR film, and an MR (magnetic) for recording and reproduction. Resistance) head, and is fixed to a gimbal portion formed at the tip of the suspension 132 together with the slider.

  Examples of the present invention will be described below.

[Example 1]
<Preparation of perpendicular magnetic recording medium>
As a non-magnetic substrate, a disk-shaped glass substrate that had been washed (Ohara, Inc., outer diameter: 2.5 inches) was prepared. The glass substrate is accommodated in a film forming chamber of a magnetron sputtering apparatus (C-3010 manufactured by Canon Anelva Co., Ltd.), and the inside of the film forming chamber is evacuated until the ultimate vacuum is 2 × 10 ⁻⁵ Pa or less. Unless otherwise specified, magnetron sputtering was performed in an Ar atmosphere at a gas pressure of about 0.6 Pa as follows.

  On the nonmagnetic substrate, a CoZrNb alloy having a thickness of 30 nm, a Ru having a thickness of 0.7 nm, and a CoZrNb alloy having a thickness of 30 nm were sequentially formed as a soft magnetic backing layer. These two CoZrNb layers are antiferromagnetically coupled by Ru provided therebetween. Next, a Pd seed layer having a thickness of 6 nm was formed on the CoZrNb layer. Subsequently, after forming a Ru layer having a thickness of 10 nm, the Ar gas pressure was increased to 6 Pa, and then a Ru layer having a thickness of 10 nm was further laminated to form a nonmagnetic underlayer having a total thickness of 20 nm. Thereafter, a perpendicular magnetic recording layer was formed by sequentially laminating a first magnetic layer, a nonmagnetic intermediate layer, and a second magnetic layer.

The first magnetic layer was formed by sputtering in a 6 Pa Ar atmosphere using a (Co-16 atomic% Pt-10 atomic% Cr) -8 mol% SiO ₂ composite target, and the thickness was 15 nm. .

  As the nonmagnetic intermediate layer, Pt having a thickness of 1 nm was formed.

The second magnetic layer was formed by simultaneously sputtering different targets of Co and SiO ₂ to form Co-30 vol% SiO ₂ (design value) having a thickness of 3 nm. Here, cosputtering was performed because it was easy to change the composition. However, using a composite target is not concerned about the timing of discharge, and from the viewpoints of composition uniformity, reproducibility, and particles. It is considered preferable.

  Subsequently, a C protective layer having a thickness of 6 nm was laminated. After laminating up to the protective layer as described above, it is taken out from the film forming chamber, and a lubricating layer made of perfluoropolyether having a thickness of 1.5 nm is formed on the protective layer by a dipping method to obtain a perpendicular magnetic recording medium. It was. The obtained perpendicular magnetic recording medium has the configuration shown in FIG. 1 except that the lubricating layer is not shown.

<Section TEM (transmission electron microscope) measurement>
In order to investigate the fine structure of the obtained perpendicular magnetic recording medium, the cross-sectional TEM was used for observation. In the cross-sectional TEM, the particle size is difficult to evaluate due to the overlapping of particles in the transmission direction of the electron beam, but the particle size of the Ru underlayer appears to be about 8 to 10 nm, and the first magnetic layer is an amorphous grain boundary layer made of oxide. It was possible to observe the formation of crystal grains and the resulting reduction in crystal grains. The nonmagnetic intermediate layer and the second magnetic layer above it are thin and the boundary is difficult to distinguish, but it is observed that the column is epitaxially grown as one columnar particle from the Ru underlayer to the second magnetic layer. did it.

<Measurement of recording and playback characteristics>
The recording / reproduction characteristics were evaluated using a read / write analyzer and a spin stand (manufactured by GUZIK, USA).

  For information recording / reproduction, for perpendicular recording, which has a single pole type (sealed pole type) recording element formed so that the tip of the auxiliary magnetic pole extends close to the main magnetic pole and a giant magnetoresistive (GMR) reproducing element. The composite type head was used. Although a shielded pole type recording element is used here, a conventional single pole type recording element in which the auxiliary magnetic pole is separated from the main magnetic pole may be used. Further, although CoFeNi is used as the material of the recording magnetic pole, materials such as CoFe, CoFeN, NbFeNi, FeTaZr, and FeTaN may be used. Further, an additional element may be added with these magnetic materials as a main component.

  When the overwrite and reproduction signal output / medium noise ratio were measured for the obtained perpendicular magnetic recording medium, good values of -44.3 dB and 22.1 dB were obtained, respectively.

  Here, “overwrite” means “60 kFCI reproduction signal output / original 60 kFCI reproduction signal output when recording at a linear recording density of 450 kFCI after recording at a linear recording density of 60 kFCI”. This is an index of how cleanly the original signal is disappeared / written (hereinafter referred to as OW). Further, the reproduction signal output / medium noise ratio is [amplitude at linear recording density 75 kFCI / mean square value of medium noise when recording at linear recording density 900 kFCI] (hereinafter referred to as SNRm).

<Measurement of magnetization curve>
In order to investigate the magnetic properties of the obtained perpendicular magnetic recording medium, a polar Kerr effect evaluation device (manufactured by Neoarc), VSM (vibrating sample magnetometer; manufactured by Riken Denshi Co., Ltd.), and vector VSM (manufactured by ADE Technologies) were used. The magnetization curve was measured.

  The polar Kerr effect evaluation apparatus can measure the magnetization curve of the perpendicular magnetic recording medium itself (with a soft magnetic underlayer) but cannot measure the saturation magnetization Ms. When the VSM has a soft magnetic backing layer, the VSM and the perpendicular magnetic recording layer are measured together, and the VSM cannot always be evaluated well. Therefore, when measuring Ms or the like with VSM, a soft magnetic underlayer is not formed, but instead a sample is used in which a nearly magnetic NiTa layer capable of obtaining a substantially similar magnetization curve is formed.

  Unless otherwise specified, all magnetization curves are measurement results in the direction perpendicular to the film surface, and the standard sweep time when measuring the major loop is 45 seconds for the polar Kerr effect evaluation device, 3 minutes for the VSM, vector VSM was 40 minutes.

  FIG. 3 shows the magnetization curve of the perpendicular magnetic recording medium according to the first embodiment. The solid line is the measurement result, and the square plot and the triangle plot are the calculation results (the calculation will be described later). The perpendicular squareness ratio is as high as about 1 to 3 kOe, and the saturation magnetic field Hs (the magnetic field that completes the reversal of all particles) is as small as 10 kOe or less. As can be seen from FIG.

  The residual magnetization curve of the perpendicular magnetic recording medium according to Example 1 was measured while changing the angle of the applied external magnetic field from the perpendicular direction, and the result of plotting the residual coercivity (Hcr) against the angle was shown in FIG. Shown in The triangular plot is the measurement result, and the other measurement results will be described later. For the sake of comparison with other measurement results, it is standardized with a value at 0 °, but it can be seen that above about 70 °, Hcr is larger than that at 0 °. In the calculation result of the originally proposed composite media (Non-patent Document 1), since the Hcr continues to increase as the angle increases from 0 °, it seems that a tendency close to that appears. It is done.

FIG. 5 shows the sweep time dependence of the coercive force Hc of the perpendicular magnetic recording medium according to the first embodiment. The triangular plot is the measurement result, and the other measurement results will be described later. From this result, fitting using the so-called Sharrock's equation (MP Sharrock, IEEE Transactions on Magnetics, Vol. 35, p. 4414) was performed, the coercive force H _{0 at} 1 ns close to the magnetization reversal speed by the head, and the thermal fluctuation resistance. VKu / kT (v: activation volume (average value of minimum unit of magnetization reversal), Ku: uniaxial magnetic anisotropy constant, k: Boltzmann constant, T: absolute temperature) The values H ₀ = 8.4 kOe, vKu / kT = 112 were obtained. Since the lower limit of vKu / kT is taken as a standard from the information magnetization retention period in the HDD, it can be seen that the thermal fluctuation resistance of this perpendicular magnetic recording medium is sufficiently high.

[Comparative Example 1]
A perpendicular magnetic recording medium was manufactured in the same manner as in Example 1 except that the nonmagnetic intermediate layer and the second magnetic layer were not formed.

  The recording / reproduction characteristics of the obtained perpendicular magnetic recording medium were measured in the same manner as in Example 1. As a result, OW and SNRm were −24.0 dB and 20.5 dB, respectively. OW is a value that cannot be said to be sufficiently recorded. In such a case, considering that the SNRm tends to be lower than when sufficiently recorded, the original potential of Comparative Example 1 is obtained. Is not necessarily inferior to the first embodiment. However, in other words, it can be considered that there is little decrease in SNRm due to the addition of the nonmagnetic intermediate layer and the second magnetic layer in Example 1, and Example 1 including improvement of OW is better. It can be seen that a high SNRm is obtained.

  The result of measuring the magnetization curve of the perpendicular magnetic recording medium according to Comparative Example 1 is shown in FIG. The solid line is the measurement result, and the square plot and the triangle plot are the calculation results (the calculation will be described later). The vertical squareness ratio is preferably about 1 and Hn is larger than that of Example 1, but Hc is 7 kOe or more and Hs is about 13 kOe, which is expected to be too large for sufficient recording with the head used this time. For this reason, OW is thought to have deteriorated.

  The angular dependence of the residual coercive force Hcr of the perpendicular magnetic recording medium according to Comparative Example 1 is indicated by a rhombus plot (broken line) in FIG. It can be seen that Hcr at 85 ° is approximately 1, and the curve is convex downward and symmetrical about 45 °. In the magnetization reversal model (Stoner-Wohlfarth model) of single domain particles with uniaxial magnetic anisotropy, the relative value of Hcr at 45 ° is 0.5, so it seems that there is some non-ideal part. In general, the magnetic particles in the magnetic recording layer are considered to have a single magnetic domain.

  On the other hand, the standardized Hcr of Example 1 shows a tendency to become larger than that of Comparative Example 1 on the higher angle side as described above, but originally proposed composite media (Non-Patent Document 1). In the calculation result of the above, considering that the Hcr of 0 ° is a minimum and reaches nearly 3.5 times of 0 ° at 80 °, the angular dependence of Example 1 is similar to that of Comparative Example 1 close to the SW model. It is thought that.

  Actually, when the angle dependency of Hcr is greatly different, an obvious change should appear at the recording bit end in the track width direction in which the magnetic field from the head is likely to spread. However, in Example 1 and Comparative Example 1, there is a difference in OW. There was no change in recording track width greater than expected. In addition, a shielded pole type recording element with a relatively large in-plane magnetic field is thought to easily affect the recording resolution in the linear direction, but there was no obvious change here. That is, it can be interpreted that the angle dependence of the first embodiment can be regarded as close to that of the first comparative example in terms of recording / reproducing characteristics, and this is based on the current head designed based on the magnetization reversal of the SW model. Is rather preferred in that it can be used without significant changes.

The dependence of the coercive force Hc of the perpendicular magnetic recording medium according to Comparative Example 1 on the sweep time is indicated by a rhombus plot (broken line) in FIG. When the same fitting as in Example 1 was performed, H ₀ = 11.5 kOe and vKu / kT = 108 were obtained, and the thermal fluctuation resistance was almost the same as in Example 1. The notation "vKu" is generally used here and is used here as it is. However, in actuality, it is more accurate to call the magnetization reversal energy barrier ΔE, and v and Ku are premised on the SW model. Has been broken down. When the result of Example 1 is decomposed into v and Ku, Ku is decreased because v is considered to have increased. However, since the magnetization reversal of the composite media is different from the SW model, it is decomposed into v and Ku. Therefore, it is not appropriate to compare with Comparative Example 1. In composite media, it has been reported that the energy barrier ratio against the reversal magnetic field is improved (Non-Patent Document 1), and the ease of recording is improved while maintaining thermal fluctuation (Non-Patent Document 2). In Comparative Example 1, H ₀ is too high and OW is insufficient, whereas Example 1 can achieve a significant reduction in H ₀ at substantially the same vKu / kT, and good OW is obtained.

For reference, the sweep time dependence of a medium in which the thickness of the first magnetic layer in Comparative Example 1 is reduced to 9 nm and Hc is close to that of Example 1 is shown in FIG. Although it is thought that there is also a decrease in Ku, mainly due to a decrease in v, the resistance to thermal fluctuation is greatly decreased and the dependence on the sweep time is increased, and extrapolated H ₀ is not so decreased from the case of 15 nm. I understand that. That is, it can be said that the same characteristics as in Example 1 cannot be obtained even if the magnetic characteristics when the sweep time is slow with only the first magnetic layer are aligned.

  Therefore, it has been found that the configuration of the perpendicular magnetic recording layer as in Example 1 has the effect that the OW and SNRm can be improved while maintaining the angle dependency and thermal fluctuation resistance of Hcr substantially.

[Example 2]
A perpendicular magnetic recording medium was manufactured in the same manner as in Example 1 except that the thickness of the nonmagnetic intermediate layer was changed from 0 to 2 nm.

  When the recording / reproduction characteristics of the perpendicular magnetic recording medium having a nonmagnetic intermediate layer thickness of 0 nm (not formed) were measured in the same manner as in Example 1, OW and SNRm were −40.3 dB and 22 respectively. 0.0 dB. Although OW is lower than that of Example 1, it is not at a level where recording is insufficient, and SNRm is almost the same as that of Example 1, so it can be said that the medium is good in terms of recording and reproduction characteristics.

  FIG. 7 shows the result of measuring the magnetization curve of a perpendicular magnetic recording medium having a nonmagnetic intermediate layer thickness of 0 nm (not formed). The solid line is the measurement result, and the square plot and the triangle plot are the calculation results (the calculation will be described later). Hc is about 6 kOe and Hs is about 12 kOe, both of which are smaller than Comparative Example 1. However, if the magnetic recording layer is the same as that of Comparative Example 1 and has only one magnetic recording layer (single layer), sufficient recording is possible with the head. Is expected to be too big to do. However, in practice, good OW is obtained, and it is expected that the magnetization reversal mechanism is different from that in the case of a single layer, and that the reversal magnetic field at the time of head recording is smaller than that of a single layer medium having the same Hc. Is done.

  The angular dependence of the residual coercivity Hcr of a perpendicular magnetic recording medium with a nonmagnetic intermediate layer thickness of 0 nm (not formed) is shown by a square plot in FIG. This angle dependence almost overlaps with that of Comparative Example 1, and the curve is close to that of the SW model. Since the second magnetic layer is formed directly on the first magnetic layer, and the particles of the first magnetic layer and the particles of the second magnetic layer are considered to be sufficiently exchange coupled, the second magnetic layer in the columnar particles It is unlikely that the reversal of only the first region will be completed first, and it is expected that the magnetization reversal is close to the SW model, and this is supported.

  Further, since the track width at the time of evaluating the recording / reproducing characteristics of this medium was almost the same as that in Example 1, the difference in angle dependency with Example 1 due to the presence or absence of the intermediate layer (the difference in normalized Hcr was 45 °). Can be interpreted as not a big difference. This is because the difference in track width between the first embodiment and the first comparative example described above is mainly caused by the difference in OW, and the angular dependence of the first embodiment is regarded as almost the same as that of the first comparative example. Supports the interpretation that Therefore, regarding the angle dependency of Hcr, it can be said that the same effect as that of the formed medium can be obtained even when the nonmagnetic intermediate layer is not formed.

The dependence of the coercive force Hc of the perpendicular magnetic recording medium with the nonmagnetic intermediate layer thickness of 0 nm (not formed) on the sweep time is shown by a square plot in FIG. When the same fitting as in Example 1 was performed, H ₀ = 9.1 kOe, vKu / kT = 109, and the thermal fluctuation resistance was almost the same as in Example 1 and Comparative Example 1. Compared with Example 1, H ₀ is as high as 0.7 kOe and OW is reduced as 4 dB. However, the improvement from Comparative Example 1 is obvious, and even when the nonmagnetic intermediate layer was not formed, It was found that the effect of improving OW and SNRm can be obtained while maintaining the thermal fluctuation resistance substantially.

  FIG. 8 shows the result of measuring the magnetization curve of a perpendicular magnetic recording medium having a nonmagnetic intermediate layer thickness of 2 nm. The solid line is the measurement result, and the square plot and the triangle plot are the calculation results. Since the magnetization curve has two steps, it is considered that the first magnetic layer and the second magnetic layer are inverted independently, and the interaction between the two magnetic layers is eliminated by the formation of 2 nm Pt.

<Simulation>
Here, in order to examine the magnetic characteristics of the second magnetic layer and the magnetization reversal mechanism when combined with the first magnetic layer, a simulation was performed using the commercially available software “LLG Micromagnetics Simulator (created by MR Scheinfein et al.)”. . The simulation is of course effective for studying the magnetization reversal mechanism, but the Ku of the second magnetic layer can also be measured directly. There are various difficulties, such as the possibility of changes in structure and magnetic properties when the layer thickness is increased for accuracy and ease of measurement, or when the measurement is performed without the first magnetic layer. But it seems to be an effective method.

  As main calculation parameters, the sample size was set to 192 × 192 × 20 nm, and it was divided by cubic cells of 4 nm / side. The in-plane size of one particle is based on a square of 8 nm / side, but by using a Volonoi cell with a dispersion of 2 nm, it has a structure in which polygonal particles are densely packed. For convenience of cell size, both the first magnetic layer and the second magnetic layer were 1 nm thicker than Example 1 and 16 nm and 4 nm, respectively. Considering thermal fluctuations, the temperature was set to 300K, and since it did not converge, the calculation was terminated after 1000 iterations. The time increment is set as large as possible within a range where a stable solution can be obtained, and the attenuation constant α is set to 1. The dispersion of the easy magnetization axis was 5 °, and Ms and Ku had a dispersion of 20%.

The measurement of the magnetic properties of the first magnetic layer is not difficult as described above. As a result of measurement using VSM and a torque magnetometer (manufactured by Toei Kogyo Co., Ltd.), Ms = 570 emu / cc, Ku = 4 It was 2 × 10 ⁶ erg / cc. Since there are variations due to samples in the actual medium, the calculation is roughly set to Ms ₁ = 600 emu / cc and Ku ₁ = 4 × 10 ⁶ erg / cc. The exchange stiffness constant in the particles was A = 1.0 μerg / cm. Using these parameters, the intergranular coupling Ex is used as a variable, and the result of matching with the first magnetic layer is a square plot (Ex = 0.05 μerg / cm) and a triangular plot (Ex = 0.1 μerg) in FIG. / Cm). Although fine adjustment of Ms and Ex is possible, here, Ex ₁ = 0.1 μerg / cm is roughly selected as the intergranular coupling of the first magnetic layer.

Further, since Ms of Example 1 was 650 emu / cc, which was higher than that of Comparative Example 1, it is clear that Ms of the second magnetic layer is higher than that of the first magnetic layer. As a result of reverse calculation from the average thickness of Ms _2, it was decided that Ms ₂ also had a width of approximately 800 to 900 emu / cc (average of layer thickness was 640 to 660 emu / cc).

In addition to the above calculation conditions, Ms of the second magnetic layer, Ku and Ex, and instead of providing the non-magnetic intermediate layer exchange coupling A ₁₂ of the first magnetic layer and the second magnetic layer as a variable, non-magnetic intermediate layer thickness For 0, 1 (Example 1), and 2 nm, the simulation was performed so that the calculated value was close to the measured magnetization curve and the angle dependency of Hcr.

As a result, when Ms ₂ = 800 emu / cc, Ku ₂ = 3 × 10 ⁶ erg / cc, Ex ₂ = 0.2 μerg / cm, and Ms ₂ = 900 emu / cc, Ku ₂ = 4 × 10 ⁶ erg / cc. When cc, Ex ₂ = 0.2 μerg / cm, the calculated value closest to the actual measurement was obtained comprehensively.

When the nonmagnetic intermediate layer thickness is 0, 1 (Example 1), and 2 nm, the calculation results when A ₁₂ is 1.0, 0.3, and 0 μerg / cm are shown in FIGS. As shown in FIG. In each figure, when the square plot is Ms = 800 emu / cc and Ku ₂ = 3 × 10 ⁶ erg / cc, the triangular plot is Ms = 900 emu / cc and Ku ₂ = 4 × 10 ⁶ erg / cc It is a calculation result of time, and it can be seen that there is almost overlap in every figure.

In each calculation result, only Ms ₂ is increased / decreased in increments of 100 emu / cc, only Ku ₂ is increased / decreased in increments of 1 × 10 ⁶ erg / cc, and only Ex ₂ is increased / decreased in increments of 0.1 μerg / cm. As a result, it was clearly far from the actual measurement value than the above two calculation results. On the other hand, when Ms and Ku are changed in proportion, there is a tendency that the magnetization curve does not change so much, so even in the middle of two calculation results (for example, Ms = 850 emu / cc, Ku ₂ = 3.5 × 10 ⁶ erg / cc) Similarly, it can be expected that results close to actual measurement are obtained. From the above results, it was estimated that the Ku of the second magnetic layer was 3 × 10 ⁶ to 4 × 10 ⁶ erg / cc, which was as high as the first magnetic layer.

Ku of bulk Co (hcp) is 4.53 × 10 ⁶ erg / cc, and cannot be fabricated in bulk. Therefore, the cubic crystal magnetic anisotropy constant of fcc-Co fabricated in a thin film is K ₁ = 0.5 × 10 ^{6 to} 0.7 × 10 ⁶ erg / cc, K ₂ is reported to be on the order of 1 × 10 ⁴ erg / cc (T. Suzuki et al., Appl. Phys. Lett. 64, p. 2736: JA Wolf et al., Appl. Phys. Lett. 65, p. 1057). Since the second magnetic layer is as thin as 3 nm, it is difficult to directly analyze the crystal structure by X-ray or electron diffraction, but the Co crystal structure of the second magnetic layer is hcp due to the size of Ku. It is reasonable to think. The actual composition and filling rate of the crystal grains of the second magnetic layer have not been investigated because the analysis is difficult. However, Ku has a lower granularity due to the low filling rate and the Pt intermediate layer. It is conceivable that Ku is increased at the interface with owing to interfacial magnetic anisotropy and alloying. In particular, there is a high possibility that Pt diffuses to form a CoPt alloy, and the same effect can be expected even when the intermediate layer is made of Pd.

As described above, Ku of the second magnetic layer is a large value, but Ms is larger than that of the first magnetic layer. Since the base of the magnitude of the reversal magnetic field is the anisotropic magnetic field Hk = 2Ku / Ms, when Hk is calculated, Hk ₁ = 13.3 kOe and Hk ₂ = 7.5 to 8.9 kOe. That is, it can be seen that the second magnetic layer has a high Ku but a high Ms, so that Hk is clearly smaller than that of the first magnetic layer.

Therefore, when the nonmagnetic intermediate layer is formed to weaken the coupling between the first magnetic layer and the second magnetic layer, it can be estimated that the magnetization reversal starts from the region of the second magnetic layer even within one columnar grain. Actually, when the state of magnetization reversal is observed by simulation, it is clear from FIG. 8 when A ₁₂ = 0 μerg / cm, but the second magnetic layer is reversed first, and after the completion, the first magnetic layer is When A ₁₂ = 0.3 μerg / cm, when viewed in one columnar particle, the region of the second magnetic layer is reversed first, and immediately after that, the region of the first magnetic layer is also reversed. It was.

Further, when A ₁₂ = 1.0 μerg / cm is observed, the entire columnar particle is reversed at once when it is surely reversed, but it can be observed that the region of the second magnetic layer is inclined first. It was. This is close to the reversal mechanism shown in Non-Patent Document 1, and in Non-Patent Document 1, half of it is a soft magnetic region with Ku = 0, and the calculation model and parameters are different. Seems to be different. Nevertheless, it is common in that the low Hk region starts reversal first, and in fact, as already mentioned, there is a reduction in Hc and a significant improvement in OW compared to the case of a single layer (Comparative Example 1). It has been realized. Therefore, from the viewpoint of the magnetization reversal mechanism, it can be said that the effect of the low Hk of the second magnetic layer can be obtained even without the nonmagnetic intermediate layer.

Here, Ku of the current mainstream CoCrPt—SiO ₂ recording layer is about 4 × 10 ⁶ erg / cc. However, future HDD density increases, recording layer microparticles, and thermal fluctuation resistance associated therewith are expected. Considering the improvement, it is considered that it is not preferable to greatly reduce Ku, although it is necessary to further increase Ku. Further, in the above embodiment, considering that Ku of the second magnetic layer is 3 × 10 ⁶ to 4 × 10 ⁶ erg / cc which is the same as or slightly lower than that of the first magnetic layer, Ku ₁ and Ku ₂ are 3 It is considered preferable to be × 10 ⁶ erg / cc or more. If Ku is not significantly different (for example, one digit), as described above, it is important to lower Hk by increasing Ms of the second magnetic layer, but if within that range, 4 × 10 ⁶ erg It is presumed that the same effect can be obtained even if it is more than / cc.

Regarding the thickness of the magnetic layer, if the thickness of the second magnetic layer is increased, the ratio of the low Hk region is increased, so that the Hk of the entire recording layer, and hence Hc, is reduced, and the track width is widened. Absent. Also, since Ms is large, increasing the layer thickness tends to affect the reproduction signal and medium noise. CoCr-based recording layers, including CoCrPt-SiO ₂ recording layers, have been shown to reduce medium noise by increasing the Cr composition to some extent, but Cr is likely to greatly reduce Ms, so It is not preferable to add them. In general, in order to obtain a high Ms, it is necessary to reduce the amount of additive, so that it is difficult to reduce medium noise. Therefore, it is considered that the thickness of the second magnetic layer is preferably thinner than that of the first magnetic layer.

From the above, when Ku ₁ and Ku ₂ are both 3 × 10 ⁶ erg / cc or more, Ms ₁ <Ms ₂ , Hk ₁ > Hk ₂ , and t ₁ > t ₂ , medium noise, thermal fluctuation resistance, and overwrite characteristics It can be said that the effect of improving is obtained.

In addition, the material that can achieve both high Ms and high Ku is preferably a 3d transition metal, Co, considering the extension of the conventional HDD media, and may contain Pt and Pd as described above. As described above, it is not preferable to add Cr. As described above, it is considered that the first magnetic layer is preferably CoCrPt—SiO _{2 which} has a high Ku and has a proven record in SNRm.

  Next, FIG. 9 shows the result of plotting Hc and Hs against the intermediate layer thickness for a perpendicular magnetic recording medium with the nonmagnetic intermediate layer thickness changed from 0 to 2 nm. Plots are also plotted for the case where the nonmagnetic intermediate layer is changed to Pd. The Pt intermediate layer is rhombus plot (Hc) and square plot (Hs), and the Pd intermediate layer is triangular plot (Hc) and round. This is indicated by a plot (Hs). Even if Pt is changed to Pd, almost the same result is obtained. Therefore, it is considered that a material obtained by mixing these may be used for the intermediate layer.

  The magnetization curve in FIG. 7 when the intermediate layer thickness is 0 nm, FIG. 3 at 1 nm, FIG. 8 when the interlayer coupling is broken at 2 nm, and the continuous change in FIG. Coupling has no antiferromagnetic part and is always ferromagnetic, and it can be estimated that the strength decreases as the interlayer thickness increases. From this figure, it can be seen that the layer thickness around 1 nm is moderately weak and the upper and lower layers are coupled, and at this time, Hc, particularly Hs, can be reduced most.

  Although the magnetization reversal mechanism when the upper and lower layers are appropriately coupled has already been described, the mechanism in which the region of the second magnetic layer in the columnar particles is reversed first to promote the reversal of the region of the first magnetic layer, It is thought that it contributes to the reduction of Hc and Hs. Therefore, including the results described so far, better results can be obtained by providing a nonmagnetic intermediate layer with a layer thickness within a range in which ferromagnetic coupling can be obtained between the first magnetic layer and the second magnetic layer. It can be said that.

Further, in the simulation results described above, Ex ₁ = 0.1 μerg / cm, whereas Ex ₂ = 0.2 μerg / cm, the intergranular coupling was stronger in the second magnetic layer. It is considered that this intergranular coupling also helps the region of the second magnetic layer to be reversed first. This is an estimation from a simulation. For example, when a medium without a nonmagnetic intermediate layer having a relatively small magnetization curve is obtained, the inclination α in Hc is obtained (FIG. 7).
α = 4πdM / dH = 4π · 650 / 4500≈1.8
It is clearly larger than the standard α = 1 when there is no interparticle bonding, and it is clear that there is a certain degree of intergranular bonding as the whole medium (average). In order to secure the nucleation magnetic field Hn as large as possible by setting the vertical squareness ratio Rs to approximately 1, it is considered that the interparticle coupling that does not deteriorate the SNRm is preferable.

  Further, the high Ku of the second magnetic layer also contributes to an increase in Rs and Hn by increasing Hk without lowering Ku in the entire recording layer average. A composite media using a soft magnetic layer with Ku = 0 has been introduced as a method for producing a tilt media with the easy axis tilted from the vertical direction relatively easily (Non-Patent Document 1). In order to increase it, it is considered preferable to increase Ku so that the second magnetic layer has perpendicular magnetization.

[Example 3]
Perpendicular magnetic recording in the same manner as in the case where the nonmagnetic intermediate layer thickness was not formed in Example 1 and Example 2 except that the composition (design value) of SiO ₂ in the second magnetic layer was changed from 0 to 40 vol%. A medium was made.

FIG. 10 shows the SiO ₂ composition dependence of Hc and Hs. Square plots (Hc) and circle plots (Hs) are cases where there is a Pt nonmagnetic intermediate layer, and diamond plots (Hc) and triangle plots (Hs) are cases where there is no nonmagnetic intermediate layer. Although Hs with an intermediate layer tends to be slightly saturated, basically Hc and Hs tend to increase with increasing SiO ₂ composition. When SiO ₂ is 0 vol%, the value is almost the same regardless of whether or not the intermediate layer is present, but a difference occurs with the addition of SiO ₂ . It can be seen that when the Pt intermediate layer is provided and the SiO ₂ composition is about 30 vol%, a magnetization curve having a small difference between Hs and Hc and a large inclination can be obtained.

Further, the angle dependency of Hcr when SiO ₂ is 0 vol% is shown by a rhombus plot in FIG. It can be seen that Hcr is higher when the angle of the magnetic field is tilted than when SiO ₂ is present (square plot). However, as already mentioned, even when there is an intermediate layer with the largest Hcr (triangular plot), the recording / reproduction characteristics are not significantly different from the single layer (broken line), and are in the middle. Similar results are expected.

From the above results, the addition of SiO ₂ is not essential, but it is considered preferable because it provides an effect of increasing Hc and an effect of suppressing Hs when an intermediate layer is provided.

Similar results were obtained when Cr ₂ O ₃ or TiO ₂ was used instead of SiO ₂ .

[Example 4]
A perpendicular magnetic recording medium was manufactured in the same manner as in the case where the nonmagnetic intermediate layer thickness was not formed in Examples 1 and 2, except that the thickness of the second magnetic layer was changed to 0 to 5 nm.

FIG. 11 shows the dependence of Hc and Hs on the thickness of the second magnetic layer. Square plots (Hc) and circle plots (Hs) are cases where there is a Pt nonmagnetic intermediate layer, and diamond plots (Hc) and triangle plots (Hs) are cases where there is no nonmagnetic intermediate layer. Basically, as the layer thickness increases, Hc and Hs tend to decrease, but there is a difference due to the presence / absence of the intermediate layer (similar to SiO ₂ composition dependence). It can be seen that when the Pt intermediate layer is provided, a magnetization curve having a small gradient and a small difference between Hs and Hc can be obtained when the thickness of the second magnetic layer is around 3 nm.

  Although there is no significant difference even when the layer thickness is 5 nm, as already described, if the second magnetic layer is thickened, a tendency to increase medium noise is expected. Therefore, it is preferable that the thickness is at least thinner than the first magnetic layer.

[Example 5]
A magnetic recording / reproducing apparatus was manufactured by combining each perpendicular magnetic recording medium of the above example with a head similar to that used in the spin stand evaluation.

  When the recording / reproducing operation was performed and the bit error rate was evaluated, an improvement in the bit error rate corresponding to the above-described SNRm improvement was obtained. When an evaluation was also made on a ring type head for in-plane magnetic recording, the bit error rate was worse than that of a single pole type head. It was considered that the recording capability was lowered due to the difference in the shape of the recording element, and that the SNRm and the recording resolution were lowered, and it was found that the single pole type head is preferable.

  Although the effectiveness of the magnetic recording / reproducing apparatus has been shown here, the present invention is not limited to the magnetic disk apparatus but uses a tape medium or a drum-like medium because of the property of the magnetic recording medium in which the present invention is effective. In this case, the effect can be exerted in all the magnetic recording / reproducing apparatuses widely adopting the perpendicular magnetic recording system without depending on the shape of the magnetic recording medium.

  DESCRIPTION OF SYMBOLS 1 ... Substrate, 2 ... Soft magnetic backing layer, 3 ... Seed layer, 4 ... Nonmagnetic underlayer, 5 ... First magnetic layer, 6 ... Nonmagnetic intermediate layer, 7 ... Second magnetic layer, 8 ... Protective layer, 110 ... Case, 112 ... Magnetic disk, 113 ... Spindle motor, 114 ... Head actuator, 116 ... Voice coil motor, 117 ... FPC unit, 118 ... Ramp load mechanism, 120 ... Inertia latch mechanism, 121 ... Clamp spring, 124 ... Pivot, 132: Suspension, 133 ... Magnetic head.

Claims (9)

On the substrate, a first magnetic layer, a nonmagnetic intermediate layer, and a second magnetic layer are provided in this order, and the first magnetic layer and the second magnetic layer are ferromagnetically coupled to each other, and The magnetic layer and the second magnetic layer have crystal grains and an amorphous grain boundary layer, the crystal grains of the first magnetic layer contain Co, Pt, and Cr, and the crystal grains of the second magnetic layer contain Co. Containing Cr but not including Cr, when the saturation magnetization is Ms ₁ and Ms ₂ and the anisotropic magnetic field is Hk ₁ and Hk ₂ , Ms ₁ <Ms ₂ , Hk ₁ > A perpendicular magnetic recording medium having Hk ₂ .   2. The perpendicular magnetic recording medium according to claim 1, wherein the crystal grains of the second magnetic layer further contain at least one of Pt and Pd.   The perpendicular magnetic recording medium according to claim 1, wherein the nonmagnetic intermediate layer includes at least one of Pt and Pd.   2. The perpendicular magnetic recording medium according to claim 1, wherein the perpendicular squareness ratio is 0.8 to 1.   The perpendicular magnetic recording medium according to claim 1, wherein the inclination α of the coercive force Hc of the perpendicular magnetization curve is 1 or more.   The perpendicular magnetic recording medium according to claim 1, further comprising a soft magnetic backing layer containing Co and Zr between the substrate and the first magnetic layer.   The perpendicular magnetic recording medium according to claim 6, further comprising a nonmagnetic underlayer containing Ru between the soft magnetic underlayer and the first magnetic layer.   The seed layer including at least one selected from the group consisting of Pd, Pt, Ni, Ta, and Ti is provided between the soft magnetic underlayer and the nonmagnetic underlayer. The perpendicular magnetic recording medium described.   9. A magnetic recording / reproducing apparatus comprising the perpendicular magnetic recording medium according to claim 1 and a single-pole magnetic recording head.

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