Magnetic recording medium and magnetic recording medium driving apparatus

A magnetic recording medium and a magnetic recording medium driving apparatus are disclosed. By providing a plurality of intermediate layers made of a CoCr alloy of which saturation magnetic flux densities are controlled within a predetermined range, the magnetic recording medium, and the magnetic recording medium driving apparatus, realize a high S/Nm and thermal stability.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a magnetic recording medium and a magnetic recording medium driving apparatus, and more particularly, to a thermally stable high density magnetic recording medium that makes low noise and a magnetic recording medium driving apparatus emptying same.

2. Description of the Related Art

Developments in information technology require an increase in recording density of magnetic recording devices used for computer peripheral storage systems. One of the important properties required of magnetic recording media is a high S/Nm ratio (a signal to media noise ratio).

It is known that the pulse width Pw50 of a readback waveform of a general horizontal magnetic recording medium is related to a coercive force Hc, a remanent magnetic flux density Br, and a magnetic layer thickness t as follows:
a∝(t×Br/Hc)1/2, and
Pw50=(2×(a+d)2+(a/2)2)1/2,
where d is a magnetic spacing.

The pulse width at 50% amplitude Pw50 is desired to be as small as possible in order to increase the resolution of a readback signal. Accordingly, it is desirable to design a magnetic recording medium having a smaller magnetic layer thickness t and a larger coercive force Hc.

To achieve a reduction in media noise, the size of magnetic grains can be reduced and intergranular magnetic interactions can be weakened. Some methods that make magnetic grains small by adding, for example, tantalum Ta, niobium Nb, boron B, or phosphorus P, to a CoCr-based alloy are proposed.

It is further known that the addition of platinum Pt to a CoCr-based alloy, for example, increases the coercive force Hc of a magnetic layer.

Furthermore, adding Cr to the magnetic layer of a CoCr-based alloy is reported to be effective in reducing intergranular magnetic interactions in the magnetic layer. However, it is known that, if a large quantity of nonmagnetic material is added to a Co-based alloy, the in-plane orientation in the direction of the easy axis of magnetization, the hcp-C axis, is weakened.

It is reported that providing a Co-based alloy intermediate layer having a stronger in-plane orientation between the magnetic layer and an underlayer solves the problem by enhancing the in-plane orientation (as reported by S. Ohkijima et al, Digests of IEEE-Inter-Mag., AB-03, 1997, for example).

The related art in the Japanese laid-open patent application 2000-251237 discloses that the coercive force is improved by adding metal such as Pt to a CoCr-based alloy formed as an intermediate layer.

The conventional magnetic recording medium, however, still has the following two problems.

The first problem relates to an intermediate layer. The intermediate layer exhibits properties of the in-plane orientation being weakened as the quantity of nonmagnetic material increases. Accordingly, the in-plane orientation is enhanced by providing an intermediate layer made of Co-based alloy with a smaller quantity of nonmagnetic material such as Cr, Ta, Nb, B, Mn, Re, and Pt.

If the quantity of nonmagnetic material is reduced, however, a saturation magnetic flux density Bs of the intermediate layer is increased and an interfacial exchange combination between the intermediate layer and the magnetic layer is strengthened. Moreover, the magnetic layer thickness t may be kept relatively thin. A magnetocrystalline anisotropy field Hk is reduced consequently. This decrease in the magnetocrystalline anisotropy field Hk makes the magnetic recording medium thermally unstable. The high saturation magnetic flux density Bs of the intermediate layer increases intergranular interactions among magnetic grains and media noise in the transitional region.

The second problem relates to the magnetic layer. As mentioned previously, one effective approach to the reduction of the media noise is to decrease the size of the magnetic grains in the magnetic layer. However, the reduction of the magnetic grain size results in another problem, the recording destruction due to thermal instability, since the volume of magnetization per bit is also reduced. The thermal instability is controlled by increasing the quantity of Pt in the magnetic layer because the magnetocrystalline anisotropy field Hk is also increased. However, the increase in the Pt density increases the intergranular interaction, and consequently increases the media noise.

The ratio of an isolated pulse signal to a media noise (Siso/Nm) is decreased to a desired level by increasing Cr density. If Pt density is increased instead of the Cr density to increase the magnetocrystalline anisotropy field Hk, the Siso/Nm cannot be decreased enough.

FIG. 1is a graph of the coercive force Hc of the magnetic layer of CoβCrαPt8B3(α=20−25 at %, β=100−(8+3+α)) as a function of the Cr density of the magnetic layer.FIG. 1shows that the coercive force Hc of the magnetic layer depends on the Cr density in the magnetic layer.

As the Cr density increases, the media noise is reduced. But one may notice fromFIG. 1that the coercive force Hc decreases. The decrease in the coercive force Hc is caused by the decrease in the magnetocrystalline anisotropy field Hk. This means that the magnetic recording medium becomes thermally unstable. According to the conventional technique, it is difficult to manufacture magnetic recording medium with low noise and high thermal stability.

By the way, the recording magnetic layer of a magnetic recording medium such as a hard disk (HD) is made up with CoCrPt alloy. The recording magnetic layer is formed on an intermediate layer made up with CoCr alloy including additives such as Pt. The intermediate layer increases the magnetic anisotropy of the recording magnetic layer.

The following documents describe the related art.

If the magnetic anisotropy of the recording magnetic layer is improved, it is possible to prevent the magnetization of the recording magnetic layer from being degraded for a long time period. The magnetic recording medium can withstand the thermal instability. However, if the magnetic anisotropy is increased, a strong magnetic field is required for writing information on the magnetic recording medium. A writing head needs to support such a strong magnetic field.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to provide a novel and useful magnetic recording medium in which one or more of the problems described above are eliminated.

Another and more specific object of the present invention is, first, to provide a magnetic recording medium characterized by an intermediate layer provided between an underlayer and a magnetic layer which realizes both low noise properties and thermal stability, and secondly, to provide a magnetic recording medium characterized by a magnetic layer which realizes both low noise and thermal stability.

Yet another object of the present invention is to provide a magnetic recording medium and a magnetic recording medium driving apparatus that can prevent magnetization from being degraded for a long time period without excessively increasing the magnetic anisotropy.

In order to achieve at least one of the above objects, a magnetic recording medium according to the present invention, includes: a substrate; an underlayer on the substrate; a first intermediate layer on the underlayer; a second intermediate layer on the first intermediate layer; a third intermediate layer on the second intermediate layer; and a recording layer on the third intermediate layer, wherein the first intermediate layer is made of a cobalt based alloy containing chromium equal to or greater than about 7 at % and equal to or smaller than about 28 at %; and the second intermediate layer is made of a cobalt based alloy containing chromium equal to or greater than about 29 at %; the third intermediate layer is made of a cobalt based alloy containing chromium equal to or greater than about 29 at %; and the recording layer includes a cobalt based alloy.

By providing a plurality of intermediate layers made of a CoCr alloy of which saturation magnetic flux densities are controlled within a predetermined range, the magnetic recording medium realizes a high S/Nm and thermal stability at the same time.

In this specification, an intermediate layer is defined as a 0.5-5.0 nm thick layer formed between the recording layer and the underlayer, for the purpose of improving the in-plane orientation and crystallization. Because the coercive force Hc of the intermediate layer, if formed alone on a Cr based alloy underlayer, becomes 2000 (×1/4π kA/m) or less, the intermediate layer is not suitable as the recording layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The first through third embodiments of the present invention are now described by reference to the figures. The first and second embodiments relate to a magnetic recording medium having a laminated intermediate layer between an underlayer and a magnetic layer. The third embodiment relates to a magnetic recording medium having a magnetic layer including Ag.

FIG. 2is a schematic illustration of the lamination structure of a magnetic recording medium10relative to the first embodiment of the present invention. A substrate11shown inFIG. 2is, for example, a non-magnetic Al substrate which is coated with a NiP film by electroless plating and texture processing. The magnetic recording medium10includes, from the bottom in order, an underlayer12made of Cr, an underlayer13made of CrMo, the first intermediate layer14made of Co (Cr, Pt, Ta, B), the second intermediate layer15made of CoCr, a magnetic layer16made of CoCrPtB, and a carbon-based overcoat17, laminated on the substrate11.

The abovementioned expression Co (Cr, Pt, Ta, B) means that the first intermediate layer14is made of Co alloy including at least one of Cr, Pt, Ta, and B.

FIG. 3is a schematic illustration of a lamination structure of a magnetic recording medium, as an example for comparison, which is not an embodiment of the present invention. A magnetic recording medium100includes, from the bottom in order, an underlayer102made of a Cr alloy, an underlayer103made of a CrMo alloy, an intermediate layer104made of CoCrTa, a magnetic layer105made of CoCrPtB, and a carbon-based overcoat107laminated on a substrate101which is formed similarly to the substrate11of the magnetic recording medium10.

The two magnetic recording media described above differ in that the magnetic recording medium10relative to the embodiment of the present invention includes the two intermediate layers14and15, whereas the magnetic recording medium100shown as a comparative example includes the only one intermediate layer104.

A description of the manufacturing process of the magnetic recording medium10shown inFIG. 2is now given for example. The magnetic recording medium10is manufactured by laminating each layer in order using the sputtering method. The air in a sputtering chamber is exhausted below 9.0×10−5Pa before sputtering the underlayers12and13, and the substrate11is heated up to about 210° C. After the heating, Ar gas is injected to maintain the pressure in the chamber at about 0.67 Pa. A 5 nm-thick Cr layer as the first underlayer12, a 2 nm-thick CrMo layer as the second underlayer13, a 2 nm-thick CoCrTa layer as the first intermediate layer14, a 3 nm-thick CoCr layer as the second intermediate layer15, a 15 nm CoCrPtB layer as the magnetic layer16, and a 5 nm-thick carbon-based layer as the overcoat17are sputtered in order on the NiP—Al substrate in order to manufacture the magnetic recording medium10.

The magnetic recording medium100described as an example for comparison is manufactured in the same manner except for making one intermediate layer104instead of two.

The magnetic recording medium10will be further described as the preferred embodiment in comparison with the magnetic recording medium100. It is assumed, in this description of the magnetic recording medium10, that the first intermediate layer14is made of Co80Cr18Ta2(hereinafter, subscript figures mean percentage of composition) and the second intermediate layer15is made of Co58Cr42, unless otherwise specified. The single intermediate layer104of the magnetic recording medium100is made of Co80Cr18Ta2.

FIG. 4is a graph of a coercive force Hc of the magnetic layer16of the magnetic recording medium10as a function of a thickness of the intermediate layer. It is obvious fromFIG. 4that the coercive force Hc of the magnetic layer16depends on the intermediate layer thickness.

In this measurement of the coercive force Hc of the magnetic recording medium10, the thickness of the first intermediate layer (Co80Cr18Ta2) is an independent variable, whereas the thickness of the second intermediate layer (Co58Cr42) is fixed at 3 nm. To measure the coercive force Hc of the magnetic layer105of the magnetic recording medium100, the thickness of the single intermediate layer104(Co80Cr18Ta2) is changed.

For both the magnetic recording media10and100, the product (hereinafter, referred to as “tBr”) of the magnetic layer thickness t and a remanent magnetic flux density Br is controlled to be 5.0 nTm.

As shown inFIG. 4, the coercive force Hc of the magnetic recording medium100having the single intermediate layer104rapidly decreases as the thickness of the single intermediate layer104increases beyond 1 nm.

However, the coercive force Hc of the magnetic recording medium10having the two intermediate layers14and15is kept at a high level even if the thickness of the first intermediate layer (Co80Cr18Ta2) is more than 1 nm.

The saturation magnetic flux densities Bs of the 15 nm-thick first intermediate layer (Co80Cr18Ta2) and the 15 nm-thick second intermediate layer (Co58Cr42) measured by a Vibrating Sample Magnetometer (VSM) at room temperature are 0.6 T and about 0 T, respectively.

FIG. 5is a graph of a magnetocrystalline anisotropy field Hk of the magnetic layer16of the magnetic recording medium10as a function of a thickness of the first intermediate layer. It is clear fromFIG. 5that the magnetocrystalline anisotropy field Hk depends on the thickness of the first intermediate layer.

The magnetocrystalline anisotropy field Hk is calculated based on a Dynamic-Hc value measured by the DC-Erase method. The magnetocrystalline anisotropy field Hk is obviously improved as the thickness of the first intermediate layer14(Co80Cr18Ta2) of the magnetic recording medium10is increased beyond 1 nm.

FIG. 6is a graph of the age-based decay of 350 kFCI reproduction signal of both the magnetic recording medium10and the magnetic recording medium100.FIG. 6shows that the age-based decay of the magnetic recording medium10is less than that of the magnetic recording medium100since the magnetic recording medium10is more thermally stable than the magnetic recording medium100.

The magnetic recording medium10according to the embodiment is thermally stable enough for practical use even if the tBr of the magnetic layer16is less than 7.0 nTm. This thermal stability is achieved since a plurality of intermediate layers is provided and the magnetocrystalline anisotropy field Hk is consequently increased.

FIG. 7is a graph of the isolated pulse signal to media noise ratio (Siso/Nm) of the magnetic recording medium10, as a function of the thickness of the first intermediate layer14. It is obvious fromFIG. 7that the Siso/Nm depends on the first layer thickness. If the thickness of the first intermediate layer is set at 1-5 nm, the Siso/Nm exceeds a desired value, 25 dB, for example.

FIG. 8is a graph showing the functional relationship between Siso/Nm and the thickness of the second intermediate layer15of the magnetic recording medium10. The thickness of the first intermediate layer14is assumed to be 1 nm. The functional relationship between Siso/Nm and the thickness of the intermediate layer104of the magnetic recording medium100showed inFIG. 3, which is not an embodiment of the present invention, is showed inFIG. 8for comparison.

One may understand, by reference toFIG. 8, that Siso/Nm decreases as the thickness of the second intermediate layer15increases and, when the thickness of the second intermediate layer15is 1-5 nm, Siso/Nm exceeds 25 dB, which is desirable.

FIG. 9is a graph of the ratio S/Nm of a reproduction signal to media noise of the magnetic recording medium10, measured by recording and reproducing data at a speed of 200 kFCI, as a function of a saturation magnetic flux density Bs. The saturation magnetic flux density Bs can be controlled by changing composition of the first intermediate layer14.

FIG. 9shows that, in the case wherein a signal of 200 kFCI is recorded to and reproduced from the magnetic recording medium10, S/Nm depends on the saturation magnetic flux Bs of the first intermediate layer14. The saturation magnetic flux density Bs of the first intermediate layer14was measured with a VSM method at room temperature using a sample in which each intermediate layer material formed a 15 nm-thick single layer on the Cr underlayer.

FIG. 9further shows that the best S/Nm is obtained at a saturation magnetic flux density Bs of about 0.6 T. If an S/Nm of 21 dB or more is desired, the saturation magnetic flux density Bs of the first intermediate layer needs to be controlled between 0.4 T and 0.9 T.

FIG. 10is a graph of the saturation magnetic flux density Bs as a function of a Cr density in the first intermediate layer14.FIG. 10shows that, in the case that the first intermediate layer14is made of a CoCr-based dual alloy, if the saturation magnetic flux density Bs is desired to be maintained within a range of 0.4-0.9 T, the Cr density can be controlled within a range of 7-28 at %.

In this embodiment of the magnetic recording medium10, the second intermediate layer15over the first intermediate layer14weakens the ferromagnetic interaction (exchange interaction) between the first intermediate layer14and the magnetic layer16. Accordingly, the saturation magnetic flux density Bs of the second intermediate layer is desired to be lower than that of the first intermediate layer. As a result, the second intermediate layer having a saturation magnetic flux density Bs of 0-0.4 T is required to manufacture the magnetic recording medium10which satisfies some desirable conditions of magnetic recording media, i.e., a good in-plane orientation, a high magnetocrystalline anisotropy field Hk, a high thermal stability, and a high S/Nm.

In the case that the second intermediate layer is made of a CoCr-based dual alloy, for example, it is known by reference toFIG. 10that the Cr density is preferably within a range of 29-45 at % to obtain the saturation magnetic flux density Bs of 0-0.4 T.

This first embodiment of the present invention is an example having two intermediate layers14and15, but a magnetic recording medium having three or more intermediate layers is also a variation or a modification of the present invention. It is desired that intermediate layers laid on the first intermediate layer have less saturation magnetic flux density Bs than the first intermediate layer does. A magnetic recording medium having three or more intermediate layers which has a good in-plane orientation, a high magnetocrystalline anisotropy field Hk, a high thermal stability, and a high S/Nm can be manufactured by setting the saturation magnetic flux density Bs of the second intermediate layer or each additional intermediate layer formed after the second intermediate layer at 0-0.4 T.

FIG. 11is a graph of the isolated pulse signal to media noise ratio (Siso/Nm) of the magnetic recording medium10, as a function of the Cr density in the magnetic layer16. It is obvious fromFIG. 11that the Siso/Nm of the magnetic recording medium10depends on the Cr density of the magnetic layer16.

FIG. 11shows that the media noise of the magnetic recording medium10is reduced as a Cr density in the magnetic layer16is increased. A desired Siso/Nm of 25 dB, for example, is obtained on condition that the Cr density of the magnetic layer16is 17 at % or more.

FIG. 12is a graph of a saturation magnetic flux density Bs of the magnetic layer16as a function of a Cr density in the magnetic layer16.FIG. 12shows an obvious dependency of the saturation magnetic flux density Bs on the Cr density. The saturation magnetic flux density Bs drops as the Cr density in the magnetic layer16increases.FIG. 12shows that the Cr density is preferably below 27 at % to obtain a practical saturation magnetic flux density Bs of 0.25 T or more.

Accordingly, the preferable range of the Cr density in the magnetic layer16is 17≦Cr≦27 at % and more preferably, 20≦Cr≦26 at %.

FIG. 13is a graph of a coercive force Hc of the magnetic layer16as a function of a Pt density in the magnetic field. The magnetic layer16is made of CoαCr24PtβB6, where α=(100−(24+6+β)) (at %).FIG. 13shows an obvious dependency of the coercive force Hc of the magnetic layer16on the Pt density. The coercive force Hc increases linearly as the Pt density increases up to 12 at %.

FIG. 13shows that, if the coercive force of Hc=2000 (×4/π kA/m) is desired, the Pt density is preferred to be 6 at % or more. On the other hand, the phase diagram (not shown) of a Co—Pt dual alloy indicates that the Pt density of 20 at % or less is required in order to obtain the εCo (hcp) phase. Therefore, the Pt density of the magnetic layer16is preferably within a range of 6≦Pt≦20 at % to obtain a high coercive force Hc of a ferromagnetic phase.

FIG. 14is a graph of the coercive force Hc of the magnetic layer16as a function of boron (B) density in the magnetic layer16.FIG. 14shows that, if the B density is approximately 1≦B≦7 at %, the coercive force Hc of the magnetic layer16exceeds 2000 (×4/π kA/m).

As a related matter, it is preferable to reduce the misfit between the lattice of the magnetic layer having a relatively high Pt density (6≦Pt≦20 at %) and the lattice of the underlayer made of Cr. To achieve this object, it is advisable to add 1 at % or more of at least one of molybdenum Mo, tantalum Ta, titanium Ti, tungsten W, and vanadium V, to the underlayer, of which the principal element is Cr, of the magnetic recording medium10relative to this embodiment. This additional ingredient makes the in-plane orientation stronger.

Further, the magnetic recording medium10relative to this embodiment, including a textured substrate11laminated with two underlayers12and13, the first Cr-based underlayer and the second CrMo-based underlayer, has a higher orientation ratio (O.R.), the ratio of saturation squareness along the disk circumferential direction to that along the radial direction, as well as strong in-plane orientation.FIG. 15shows the improvement in the orientation ratio.

FIG. 15shows how, in the case of the magnetic recording medium10according to the embodiment wherein two underlayers are formed by Cr80Mo20(4 nm) and Cr, respectively, the orientation ratio (O.R.) depends on the total thickness of underlayers, compared to the case wherein a single underlayer is formed by Cr and the case wherein a single underlayer is formed by Cr80Mo20. The graph shows that, in the case of two underlayers of Cr80Mo20/Cr, respectively, the orientation ratio (O.R.) is improved in comparison with the case of one underlayer formed by Cr80Mo20. One can confirm by measuring the coercive force Hc in the direction perpendicular to the film and a direction in the plane of the film, that if the bottom underlayer is formed by Cr and the top underlayer is formed by a CrMo alloy, the magnetic recording medium has better in-plane orientation

The O.R. is considered to be caused by the difference in the distortion of underlayer lattice between in the circumferential direction and in the radial direction. Actually, since the difference in the underlayer lattice distortion between in the circumferential direction and in the radial direction of a Cr-based alloy is higher than that of a CrMo-based alloy, the O.R. of the Cr-based alloy is higher. The magnetic recording medium10relative to this embodiment includes two underlayers, the Cr underlayer12on the substrate and the CrMo underlayer13on the Cr underlayer, in order to increase both O.R. and in-plane orientation by using the pure Cr underlayer and the CrMo underlayer, respectively. It is confirmed that the S/Nm depends on the temperature of the sputtering chamber, and by conducting the sputtering at the temperature of 170-300° C., a high S/Nm is obtained.

The second embodiment of the present invention will be described next. This second embodiment is a magnetic recording medium having a plurality of intermediate layers related to the first embodiment. In the first embodiment, the second intermediate layer is made of CoCr. The second intermediate layer of this embodiment is made of CoCr—M. This “M” represents at least one element selected out of Mn, Re, and Mo. The structure of the magnetic recording medium according to this embodiment is similar to that of the magnetic recording medium according to the first embodiment showed inFIG. 2. Accordingly, in the description of the magnetic recording medium30according to this embodiment, the portion that is equivalent to the portion of the magnetic recording medium inFIG. 2is referred to by the same numerals and its description will be omitted. What is different will be mainly described below.

FIG. 16is a schematic illustration of the lamination structure of a magnetic recording medium30according to the second embodiment. The magnetic recording medium30includes an Al substrate11coated with an electroless plated NiP film, its surface being textured by texture processing. The magnetic recording medium30further includes a Cr underlayer12, a CrMo underlayer13, a CoCrTa first intermediate layer14, a CoCrMn second intermediate layer35, a CoCrPtB magnetic layer16, and a carbon-based overcoat17, all laminated on the substrate11in that order.

The magnetic recording medium30according to this embodiment is manufactured by laminating each layer in order using the sputtering method. Before laminating the Cr underlayer12, for example, the air in the sputtering chamber is exhausted below 1.0×10−5Pa, and the substrate11is heated up to about 210° C. After the heating, Ar gas is injected to maintain the pressure in the sputtering chamber at about 0.67 Pa. Then, the Cr underlayer (5 nm), the CrMo underlayer (2 nm), the CoCrTa intermediate layer (2 nm), the CoCrMn intermediate layer (2 nm), the CoCrPtB magnetic layer (15 nm), and the carbon-based protective layer (5 nm) are formed in that order on the nonmagnetic NiP—Al substrate.

FIG. 17shows the dependency of isolated pulse signal to media noise ratio (Siso/Nm) on the thickness of the first intermediate layer14, which is made of CoCr18Ta2, in the case wherein the second intermediate layer35is a 2 nm thick CoCr25Mn5layer.FIG. 17shows that, when the CoCrTa first intermediate layer14is 1-5 nm thick, Siso/Nm exceeds 25 dB, which is preferable.

InFIG. 17, if the thickness of CoCr18Ta2is zero, that is, the first intermediate layer14does not exist, Siso/Nm is considerably reduced. In this case, the structure is the same as the conventional one in which only the second intermediate layer35is formed.

FIG. 18shows, in contrast toFIG. 17, the dependency of signal to total noise ratio (S/Nt) on the thickness of the CoCr25Mn5second intermediate layer, in the case such that the thickness of the CoCr18Ta2first intermediate layer14is fixed at 1 nm.FIG. 18shows, when the CoCr25Mn5second intermediate layer35is 1-5 nm thick, S/Nt is improved and becomes 17.0 dB or more, which is desirable.

In addition, if the thickness of the CoCr25Mn5second intermediate layer is zero, that is, there is no second intermediate layer, S/Nt is lowered considerably. In this case, only the first intermediate layer is provided in the magnetic recording medium.

FIG. 19shows the relationship between signal to media noise ratio S/Nm and saturation magnetic flux density Bs of the first intermediate layer14, in the case wherein the composition of the first intermediate layer14according to this embodiment is changed and a signal of 200 kFCI is recorded/reproduced. The relationship showed inFIG. 19is substantially identical to that showed inFIG. 9in the case of the first embodiment. In addition, in this embodiment, the saturation magnetic flux density Bs of each intermediate layer was measured by a VSM method at room temperature using a sample comprising a 15 nm thick layer of each intermediate layer material on a Cr underlayer. One can be sure that, when the saturation magnetic flux density Bs of the material forming the first intermediate layer14is 0.6 T, the S/Nm hits the maximum value. Since S/Nm of 21 dB or more is desired, the saturation magnetic flux density Bs of the magnetic material forming the first intermediate layer14is preferred to be 0.4-0.9 T.

For example, one can be sure by reference toFIG. 20that, in the case such that a Co—Cr dual alloy is used for the first intermediate layer14, Bs falls within the range of 0.4-0.9 T when the Cr density is 7-28 at %.FIG. 20is substantially identical toFIG. 10according to the first embodiment.

As described above, intermediate layers formed above the first intermediate layer14weaken the ferromagnetic interaction (exchange interaction) between the first intermediate layer14and the magnetic layer. That is the reason why Bs of the material of the intermediate layers above the first intermediate layer is preferred to be lower than Bs of the first intermediate layer. When Bs of the material of the intermediate layers above the first is 0-0.4 T, the magnetic recording medium has preferable in-plane orientation, high magnetocrystalline anisotropy field Hk, high thermal stability, and high signal to media noise.

By the way, the second intermediate layer35according to this embodiment further contains element M (Mn, Re, Mo). The present inventors have confirmed that Bs of the second intermediate layer35depends on (Y+Z) wherein “Y” (at %) means the density of Cr and “Z” (at %) means the density of “M”.

Therefore, by reference toFIG. 20, one may know that, if the preferable range of Bs is 0-0.4 T, the density of (Cr+Mn) is required to be about 29-45 at %. The present inventors further confirmed that, when Mn is replaced with Mo or Re, a similar effect is obtained.

Additionally, in order to reduce the misfit between the crystalline structure of the magnetic layer containing Pt at a relatively high density (6≦Pt≦20 at %) and the crystalline structure of the Cr underlayer, it is desirable that the Cr-based underlayer contains at least one of molybdenum, tantalum, titanium, tungsten, and vanadium, by 1 at % or more. One can thereby improve the in-plane orientation of the magnetic recording medium.

Furthermore, when two underlayers are formed on a textured substrate, the bottom one being made of Cr and the top one being made of CrMo, one can improve the orientation ratio (O.R.) as well as the in-plane orientation.

The dependency of O.R. on the total thickness of all the underlayers of the magnetic recording medium30according to this embodiment is substantially identical to that of the magnetic recording medium10according to the first embodiment, in which two underlayers are formed, showed inFIG. 15.

The third embodiment of the present invention will now be described.

The magnetic recording medium relative to the third embodiment includes at least one underlayer made of Cr or a Cr-based alloy on a non-magnetic substrate made of glass or aluminum, for example. It is desired that the magnetic recording medium relative to the second embodiment also includes an intermediate layer made of a Co-based alloy on the underlayer(s). A magnetic layer including Ag is included on the intermediate layer, if any.

The magnetic recording medium is made of a CoCrPtBAg-based alloy, for example, and a composition ratio of Co(100−a−b−c−d)CraPtbBcAgd(21≦a+c≦30 at %, 6≦b≦20 at %, 1≦d≦9 at %) is preferable.

Furthermore, the magnetic layer of this embodiment can be made of different magnetic materials, or can include at least two magnetic layers as a complex magnetic layer made of a combination of a CoCrPtB-based alloy and a CoCrPtAg-based alloy, for example.

It is desired that the underlayer be made of a Cr-based alloy including as an ingredient at least one of W, V, or Mo, and have a crystal structure of a body-centered cubic structure (bcc structure).

It is further desirable that the intermediate layer be made of a Co-based alloy including as an ingredient at least one of Cr, Ta, Mo, Mn, Re, and Ru, and have a crystal structure of a hexagonal close-packed structure (hcp structure). The intermediate layer is desired to be 0.5-3.0 nm thick.

The magnetic recording medium relative to the third embodiment realizes both low noise and improved magnetocrystalline anisotropy field.

FIG. 21is a schematic diagram of the lamination structure of a magnetic recording medium20relative to the third embodiment.

A substrate21is an aluminum substrate, coated by electroless plating of a NiP film, of which surface is treated mechanically or by laser texture along concentric circles.

The magnetic recording medium20of this embodiment can be manufactured through a thin film formation technique using sputtering equipment. More particularly, the DC magnetron sputtering technique is used in this embodiment. The air in a sputtering chamber is exhausted below 4.0×10−5Pa and Ar gas is injected into the sputtering chamber up to 0.67 Pa.

The substrate11is heated up to above 200° C. before forming a thin film in order to clean up the surface of the substrate11, control a crystal orientation, and reduce intergranular interaction in the magnetic layer25. It is preferable to control the temperature of the substrate between 200 and 270° C. because a NiP may crystallize at a substrate temperature over 270° C. In this embodiment, the temperature of the substrate11is, before forming the thin film, set at 220° C.

A Cr-based alloy having a larger lattice constant than does pure Cr is used to form an underlayer. Accordingly, a good in-plane orientation is obtained because of improved spacing matching between the magnetic layer25and the Cr-based alloy underlayer23. In this embodiment, a 5 nm-thick Cr layer is formed as the first underlayer22and a 2 nm-thick CrMo-based alloy layer is formed as the second underlayer23.

It is desired that the intermediate layer24be made of a Co-based alloy having an hcp structure. The Co-based alloy improves an in-plane crystal orientation of the magnetic layer25. As the thickness of the intermediate layer24increases, the resolution of the magnetic recording medium decreases. Preferred range of the thickness is 0.5-3.0 nm. In this embodiment, the intermediate layer24is a 1 nm-thick CoCrTa-based alloy.

The magnetic layer25includes at least one layer made of a CoCrPtBAg-based alloy. In this embodiment, the magnetic layer25is made of Co(100−x)Cr22Pt10B4Agx(x=1−11).

A coercive force Hc increases as the product tBr of a thickness t and a remanent magnetic flux density Br of the magnetic layer25increases, but too large a tBr results in a drop in a record/reproduction resolution and the coercive force Hc. A preferable range of tBr of the magnetic layer25is 2.0-10.0 nTm. In this embodiment, tBr of the magnetic layer is set at 6.0 nTm.

An overcoat26formed on the magnetic layer25is mainly made of carbon (C). The overcoat26of this embodiment is a 6 nm-thick carbon film.

FIG. 22is a graph of an magnetocrystalline anisotropy field Hk measured by the torque-loss method at room temperature as a function of Ag density of the magnetic layer25having a composition of Co(100−x)Cr22Pt10B4Agx(x=1−11).FIG. 22shows the dependency of the magnetocrystalline anisotropy field Hk on the Ag density of the magnetic layer25.

In case of the magnetic layer25of the magnetic recording medium20of this embodiment, even though the sum of a Cr density and a B density is 28 at % where media noise is low enough, the magnetocrystalline anisotropy field Hk is improved.

FIG. 23is a graph of the ratio (S/Nm) of a readout signal of a recording density 351 kFCI to media noise of the magnetic layer25having a composition of Co(100−x)Cr22Pt10B4Agx(x=1−11) as a function of Ag density.FIG. 23shows a dependency of the S/Nm on the Ag density of the magnetic layer25. The magnetic layer25of the magnetic recording medium20of this embodiment exhibits the properties of lower noise and higher S/Nm than a conventional magnetic layer made of a CoCrPtB-based alloy.

A description of a magnetic recording device as an embodiment of the present invention will now be given with reference toFIGS. 18 and 19.FIG. 24is a sectional view showing the main part of a magnetic recording device120as an embodiment of the present invention, andFIG. 25is a plan view showing the main part of the magnetic recording device120.

As shown inFIG. 24andFIG. 25, the magnetic recording device is covered by a housing123. The magnetic recording device120includes, in the housing123, a motor124, a hub125, a plurality of magnetic recording media126, a plurality of recording/reproduction heads127, a plurality of suspensions128, a plurality of arms129, and an actuator unit121. The magnetic recording media126are fixed to the hub125rotated by the motor124. Each recording/reproduction head127is a combination type recording/reproduction head of a reproduction head such as an MR Head and a GMR Head, and a recording head such as an inductive head. Each recording/reproduction head127is mounted on the point of a corresponding arm129through a suspension128. The arm129is driven by the actuator unit121. A detailed description of the configuration of this magnetic recording device is omitted because the essential structure of this magnetic recording device is publicly known.

The above embodiment of the magnetic recording device120is characterized by the magnetic recording medium126. Each magnetic recording medium126is structured as showed in FIGS.2,16, or21. The quantity of the magnetic recording media126is not limited to three, and can be one, two, or more than three. The configuration of the magnetic recording device120is not limited to those showed inFIGS. 24 and 25. The application of the magnetic recording medium126is not limited to hard disks.

FIG. 26is a schematic diagram showing the internal structure of a hard disk drive (HDD)211, which is an embodiment of a magnetic recording medium driving apparatus, according to an embodiment of the present invention. The HDD211is stored in a box-shaped chassis212, for example. One or more magnetic disks213are provided in the chassis212. The magnetic disks213are fixed on a rotation axis of a spindle motor214. The spindle motor214can rotate the magnetic disk213at a rotative speed of 7,200 rpm or 10,000 rpm, for example. The chassis212is closed with a cover (not shown).

A head actuator216is mounted on a spindle215extending vertically. The head actuator216is made of a rigid actuator arm217extending horizontally from the spindle215, and an elastic suspension218attached at the point of the actuator arm217and extending forward from the actuator arm217. A floating head slider219provided at the point of the elastic suspension218is supported at one end by means of a so-called gimbal spring (not shown). A force is applied to the floating head slider219by the elastic suspension218so that the floating head slider219is pushed towards the surface of the magnetic disk213. The rotation of the magnetic disk213generates air flow near the surface of the magnetic disk213, and a floating force is applied to the floating head slider219by the air flow. Because the pushing force generated by the elastic suspension218and the floating force generated by the air flow are balanced, the floating head slider219keeps floating over the surface of the magnetic disk213with relatively high rigidity.

A magnetic head (not shown), which may be an electro-magnetic conversion element, is mounted on the floating head slider219. The electro-magnetic conversion element may be made up with a reading element and a writing element. The reading element may be a Great Magneto Resistance Effect (GMR) element or a Tunnel Junction Magneto Resistance Effect (TMR) element, for example, that reads information written in the magnetic disk213using the change in resistance of a spin bulb film and a tunnel junction film, respectively. The writing element is an inductive writing head, for example, that writes information on the magnetic disk213using the magnetic field generated by a thin-film coil pattern.

The head actuator216rotates around the spindle215, and as a result, the floating head slider219moves across the magnetic disk213in a radial direction. The electro-magnetic conversion element mounted on the floating head slider219can be positioned at a designated recording track on the magnetic disk213. The head actuator216may be actuated by an actuator221such as a voice coil motor (VCM). If multiple magnetic disks213are provided in the chassis212, two actuator arms217, that is, two floating head sliders219, may be provided between adjacent magnetic disks213.

FIG. 27is a cross section showing the structure of the magnetic disk213. The magnetic disk213is configured as an in-plane magnetic recording medium. The magnetic disk213includes a substrate231and a polycrystalline-structured film232formed on the substrate231. The substrate231may be configured by a disk-shaped Al body233, and NiP film234formed on the Al body233. The surface of the substrate231may be texture-structured. The substrate231may be made of glass, silicon, or ceramic, for example. Information is recorded in the polycrystalline-structured film232. The polycrystalline-structured film232is coated with a protective overcoat235such as a diamond-like carbon (DLC) film and a lubricating film236such as a per-fluoro polyether (PFPR) film.

The polycrystalline-structured film232includes a recording magnetic layer237. The recording magnetic layer237is made up with aggregation of adjacent magnetic grains. The recording magnetic layer237may be configured by alloys including at least Co, Cr, and Pt. The alloys may further include at least one of B and Ta.

In the following description, an assumption is made that the recording magnetic layer237is a CoCrPtB film. The Pt density in the recording magnetic layer237may be set at 6-20 [at %]. The saturation magnetic flux density Bs of the recording magnetic layer237may be set at 0.1-0.8 [T].

The polycrystalline-structured film232includes a third intermediate layer238, that is, a third alloy layer, which on the surface thereof the recording magnetic layer237is formed. The third intermediate layer238is made up with an aggregation of adjacent grains. The third intermediate layer238may be configured by alloys including at least Co, Cr, and Pt. The alloy may further include at least one of B and Ta. The third intermediate layer238can be made up with the same kind of materials with which the recording magnetic layer237is configured. In the following description, an assumption is made that the third intermediate layer238is a CoCrPtB film, for example. The third intermediate layer238includes non-magnetic material at a higher density than the recording magnetic layer237does. The third intermediate layer238includes Pt at a higher density than the recording magnetic layer237does. For example, the third intermediate layer238may include Cr at a density of 29 [at %] or more, or in one exemplary embodiment at a density of 29-30 [at %], and Pt at a density of 15 [at %] or more, or in one exemplary embodiment at a density of 15-17 [at %]. The saturation magnetic flux density Bs of the third intermediate layer238may be set at 0-0.1 [T]. The saturation magnetic flux density Bs of the third intermediate layer238is set at a lower value than that of the recording magnetic layer237. The thickness of the third intermediate layer238may be set at 0.5 nm-5.0 nm, for example.

The third intermediate layer238is formed on the surface of a second intermediate layer239, that is, a second alloy layer. The second intermediate layer239is made up with an aggregation of adjacent grains. The second intermediate layer239may be configured by alloys including at least Co and Cr. In the following description, an assumption is made that the second intermediate layer239is a CoCr film, for example. The second intermediate layer239includes Cr at a density of 29 [at %] or more. The saturation magnetic flux density Bs of the second intermediate layer239is set at 0-0.4 [T], for example. The thickness of the second intermediate layer239may be set at 0.5 nm-5.0 nm, for example.

The second intermediate layer239is formed on the surface of a first intermediate layer241, that is, a first alloy layer. The first intermediate layer241is made up with an aggregation of adjacent grains. The first intermediate layer241may be configured by alloys including at least Co and Cr. The first intermediate layer241may further include at least one of Pt, B, and Ta. In the following description, an assumption is made that the first intermediate layer241is a CoCrTa film, for example. The first intermediate layer241includes non-magnetic material at a lower density than the density at which the recording magnetic layer237does. Similarly, the first intermediate layer241may include non-magnetic material at a lower density than the density at which the second intermediate layer239does. That is, the first intermediate layer241includes Cr of 7-28 [at %] density. The saturation magnetic flux density Bs of the first intermediate layer241is set at 0.4-0.9 [T], for example. The thickness of the first intermediate layer241is set at 0.5 nm-5.0 nm, for example.

The first intermediate layer241is formed on the surface of an underlayer242. The underlayer242is configured by a second underlayer242aand a first under layer242b. The second underlayer242ais made up with a Cr-based alloy layer (second non-magnetic underlayer) and is formed on the surface of the first underlayer242b. The first underlayer242bis made up with a Cr layer (first non-magnetic underlayer). The second non-magnetic underlayer242amay include atoms of at least one selected from a group of Mo, Ta, Ti, W, and V at a density of 1 [at %] or more. The consistency of crystal lattice between the underlayer242and the recording magnetic layer237can be improved by adding the above atoms. In the following description, an assumption is made that the second non-magnetic underlayer242ais a CrMo film. The thickness of the second non-magnetic underlayer242amay be set at about 2.0 nm, for example. The first non-magnetic underlayer242bis assumed to be a Cr film. The thickness of the first non-magnetic underlayer242bmay be set at about 4.0 nm, for example.

The underlayer242may be formed by a single layer made up with Cr-based alloy. The underlayer242may include atoms of at least one selected from a group of Mo, Ta, Ti, W, and V, at a density of 1 [at %] or more, for example. The underlayer242may be configured by multiple alloy layers including Cr. The alloy layers may include atoms of at least one selected from a group of Mo, Ta, Ti, W, and V, at a density of 1 [at %] or more, for example. The alloy layer on the surface of which the first intermediate layer241is formed preferably includes Cr at a greater density than the density at which an alloy layer formed on the surface of the substrate231includes.

The third intermediate layer238can be made up with the same kind of material as the recording magnetic layer237. As a result, the third intermediate layer238can be regarded as a part of the recording magnetic layer237. This results in the thickness of the recording magnetic layer237being increased in effect. The increase in the thickness of the recording magnetic layer237reduces thermal instability, and prevents the magnetization of the recording magnetic layer237from being reduced for a long time period.

Additionally, the first intermediate layer241includes less non-magnetic material than the recording magnetic layer237does. As a result, the easy axis of magnetization (the direction in which the first intermediate layer241is easily magnetized) of the first intermediate layer241is made parallel to the surface of the substrate231. The first intermediate layer241can reduce the thermal instability.

On the other hand, the second intermediate layer239includes sufficiently greater amount of non-magnetic material compared with the first intermediate layer241. The saturation magnetic flux density Bs of the second intermediate layer239remains low. As a result, the ferromagnetic interaction between the first intermediate layer241and the recording magnetic layer237is reduced by the second intermediate layer239. It is possible to substantially inhibit or prevent the magnetic anisotropy of the recording magnetic layer237from being reduced.

A method of manufacturing the magnetic disk213is described below. The disk-shaped substrate231is prepared. The surface of the substrate231is textured. A NiP film234is formed on the surface of the substrate231. The NiP film234may be formed by electroless plating, for example. The substrate231is set in a spattering apparatus, for example. A polycrystalline structured film232is formed on the surface of the NiP film234. The forming of the polycrystalline structured film232is described in detail below. Then, a protective film235of thickness of about 5 nm is formed on the surface of the polycrystalline structured film232. For example, CVD method (Chemical Vapor Deposition method) is used for the forming of the protective film235. A lubricating film236of thickness of about 1.0 nm is painted on the surface of the protective film235. The substrate231is dipped into solution including per-fluoro-polyether, for example, for painting the lubricating film236.

The polycrystalline structured film232is formed in the sputtering apparatus using DC magnetron sputtering method, for example. Before the polycrystalline structured film232is formed, the air in the chamber of the sputtering apparatus is removed to a pressure of 1.0×10−5[Pa], and Ar gas is introduced into the chamber. The pressure in the chamber is maintained at about 0.67 [Pa]. The substrate231is heated up to 160-300° C. In the following description, the substrate231is preferably heated to about 240° C., for example.

As shown inFIG. 28, the first non-magnetic underlayer242bis formed on the surface of the substrate231. A Cr target is used for the forming of the first non-magnetic underlayer242b. Cr atoms deposit and form grains on the surface of the substrate231. While the substrate231is kept at a high temperature, the 4.0 nm thick first non-magnetic underlayer242bis formed.

As shown inFIG. 29, the second non-magnetic underlayer242ais formed on the surface of the first non-magnetic underlayer242b. A CrMo target is used for the forming of the second non-magnetic underlayer242a. The CrMo target includes Mo of a predetermined density. The CrMo target preferably includes Mo of a density of 1 [at %] or more, for example. Cr atoms and Mo atoms deposit on the surface of the Cr film. As a result, the second non-magnetic underlayer242aof a thickness of 2.0 nm, for example, is formed on the surface of the first non-magnetic underlayer242b. The second non-magnetic underlayer242ais formed by epitaxial growth on the grains of the first non-magnetic underlayer242b.

In addition, as shown inFIG. 30, the first intermediate layer241is formed on the surface of the second non-magnetic underlayer242a. A CoCrTa target is used for the forming of the first intermediate layer241. The CoCrTa target preferably includes Cr of a density of 7-28 [at %]. As a result, the first intermediate layer241of a thickness of 2.0 nm is formed on the surface of the second non-magnetic underlayer242a. The first intermediate layer241is formed by the epitaxial growth on the grains of the second non-magnetic underlayer242a. The saturation magnetic flux density Bs of the first intermediate layer241is preferably 0.4-0.9 [T].

In addition, as shown inFIG. 31, the second intermediate layer239is formed on the surface of the first intermediate layer241. A CoCr target is used for the forming of the second intermediate layer239. The CoCr target preferably includes Cr of a density of 29 [at %] or more. As a result, the second intermediate layer239of a thickness of 1.5 nm is formed on the surface of the first intermediate layer241. The second intermediate layer239is formed by epitaxial growth on the grains of the first intermediate layer241. The saturation magnetic flux density Bs of the second intermediate layer239is preferably within a range of 0-0.4 [T].

In addition, as shown inFIG. 32, the third intermediate layer238is formed on the surface of the second intermediate layer239. A CoCrPtB target is used for forming the third intermediate layer238. The CoCrPtB target preferably includes Cr of a density of 29 [at %] or more and Pt of a density of 15 [at %] or more. The thickness of the third intermediate layer238formed on the surface of the second intermediate layer239is about 2.0 nm. The third intermediate layer238is formed by epitaxial growth on the grains of the second intermediate layer239. The saturation magnetic flux density Bs of the third intermediate layer238is preferably within a range of 0-0.1 [T].

In addition, as shown inFIG. 33, the recording magnetic layer237is formed on the third intermediate layer238. A CoCrPtB target used for forming the recording magnetic layer237includes Cr and Pt of smaller densities [at %] than the CoCrPtB target used for forming the third intermediate layer238. The CoCrPtB target preferably includes Pt of a density within 6-20 [at %]. The recording magnetic layer237formed on the surface of the third intermediate layer238is about 2.0 nm thick. The recording magnetic layer237is formed by the epitaxial growth on the grains of the third intermediate layer238. Cr diffuses into the recording magnetic layer237from the second intermediate layer239and the third intermediate layer238. The saturation magnetic flux density Bs of the recording magnetic layer237is preferably within a range of 0.1-0.8 [T].

The inventors measured the decrement of the signal output reproduced from the magnetic disk213. Three samples were manufactured using the above method.

A first sample had a 0.5 nm thick third intermediate layer238. A second sample had a 1.0 nm thick third intermediate layer238. A third sample had a 2.0 nm thick third intermediate layer238. The third intermediate layer238included 29 [at %] Cr, 17 [at %] Pt, and 2 [at %] B. Similarly, the inventors prepared a comparison sample. The comparison sample did not have the third intermediate layer.

The samples (including the comparison sample) had a 1.5 nm thick second intermediate layer239. The second intermediate layer239included 42 [at %] Cr. Information was recorded at a linear recording density of 400 [kFCI]. Reproduction output was measured at predetermined intervals.FIG. 34shows the S/Nt decrement [%] that was computed as described above. As shown inFIG. 34, the first, second, and third samples showed preferable S/Nt decrement compared with the comparison sample. It was observed that the third intermediate layer238considerably inhibits or prevents the magnetization of the recording magnetic layer237from being degraded.

The inventors measured the S/N ratio of an isolated wave of the magnetic disk213. Multiple pieces of a fourth sample were manufactured using the above method. 29 [at %] Cr, 17 [at %] Pt, and 2 [at %] B were added to the third intermediate layer238. The inventors prepared another comparison sample. The comparison sample did not have the third intermediate layer. The second intermediate layer239of the samples (including the comparison sample) was 1.5 nm thick. 42 [at %] Cr was added to the second intermediate layer239. Information was written using an isolated wave, and the S/N ratio was computed. As a result, and as shown inFIG. 35, it was observed that if the third intermediate layer238was 3.5 nm thick or less, the isolated wave S/N ratio of the fourth sample was better than that of the comparison sample. Especially, if the third intermediate layer238was set within a range of 0.5-2.0 nm, the isolated wave S/N ratio became especially preferable.

Next, the inventors measured the reproduction output resolution of the magnetic disk213. Multiple pieces of fifth samples were manufactured using the method described above. The fifth samples had third intermediate layers238of different thickness of 4.0 nm or less. 29 [at %] Cr, 17 [at %] Pt, and 2 [at %] B were added to the third intermediate layers238. The inventors prepared comparison samples in the same manner. The comparison samples did not have the third intermediate layer. The samples (including the comparison samples) had a 1.5 nm thick second intermediate layer239. 42 [at %] Cr was added to the second intermediate layer239. Information was recorded at a linear recording density of 385 [kFCI]. The reproduction output resolution was computed based on the written information. As a result, and as shown inFIG. 36, it was observed that, if the thickness of the third intermediate layer was within the range equal to or less than 4.0 nm, the reproduction output resolutions of the fifth samples were improved compared with those of the comparison samples. Especially, if the thickness of the third intermediate layer238was within the range of 2.0-3.5 nm, the reproduction output resolution was considerably improved.

The inventors also examined the relation between the reproduction output resolution and the saturation magnetic flux density Bs of the third intermediate layer238. Sixth samples and seventh samples were manufactured using the above method of manufacturing. 29 [at %] Cr was added to the third intermediate layer238of the sixth samples. 30 [at %] Cr was added to the third intermediate layer238of the seventh samples. The saturation magnetic flux density Bs of the third intermediate layer238of the sixth samples was different from that of the seventh samples. Information was recorded at a linear recording density of 385 [kFCI]. The reproduction output resolution was computed based on the written information. As a result, and as shown inFIG. 37, the smaller the saturation magnetic flux density Bs of the third intermediate layer238was set, the more the reproduction output resolution was improved.

The inventors also examined the relation between the saturation magnetic flux density Bs of the third intermediate layer238and the density [at %] of Pt. Eighth, ninth, and tenth samples were prepared. A single 50 nm thick CoCrPtB film was formed on the underlayer242. The CoCrPtB film of the eighth sample included Cr of a density of 28 [at %]. The CoCrPtB film of the ninth sample included Cr of a density of 29 [at %]. The CoCrPtB film of the eighth sample included Cr of a density of 30 [at %]. However, all samples included B of a density of 2 [at %]. Each sample included Pt of a different density within a range of 15-17 [at %].

The saturation magnetic flux density Bs was measured at a room temperature using a VSM (Vibrated Sample Magnetometer). As a result of the measurement, and as shown inFIG. 38, it was observed that, as the Cr density in the CoCrPtB film was increased, the saturation magnetic flux density Bs was reduced. On the other hand, it was also observed that, as the Pt density was increased, the saturation magnetic flux density Bs was reduced. Additionally, if the CoCrPtB film included 29 [at %] or more Cr and 15 [at %] or more Pt, the saturation magnetic flux density Bs of the CoCrPtB became 0.1 [T] or less.

This patent application is based on Japanese priority patent applications No. 2001-198538 filed on Jun. 29, 2001, No. 2002-121273 filed on Apr. 23, 2002, and No. 2003-131207 filed on May 9, 2003, the entire contents of which are hereby incorporated by reference.