Patent Application
20120307395
The present invention relates to a perpendicular magnetic recording medium, and particularly to a perpendicular magnetic medium which allows a large volume of information to be recorded having an inverted Hk structure, and to a magnetic storage device employing the same.
In order to increase the capacity of magnetic recording devices it is advantageous to enhance the performance of the perpendicular magnetic recording media by improving the writeability and the signal-to-noise ratio (SNR), while maintaining the thermal stability of magnetized information (magnetic recording bits). Currently, many perpendicular magnetic recording media which are commercially supplied have a layered structure comprising a granular recording layer which contains oxide and has a granular structure, and a ferromagnetic metal layer which does not contain oxide and does not have a clear granular structure, as disclosed in Japanese Unexamined Patent Application Publication Nos. 2001-23144, 2003-91808, and 2003-168207, for example. The ferromagnetic metal layer is formed from a material having a relatively low magnetic anisotropy and therefore has a small switching field and serves to improve writeability. The granular recording layer is formed from a material having high magnetic anisotropy and therefore serves to improve the thermal stability. In addition, the granular recording layer also serves to reduce the magnetic cluster size and to reduce noise. The granular recording layer is typically formed from a material in which nonmagnetic compounds, such as oxides and nitrides, are mixed with a CoCrPt alloy. In this layer, magnetic grains, which are mainly formed by Co atoms, are surrounded by non-magnetic Cr-oxide and/or other added oxides or nitrides. As a result, the magnetic cluster size is reduced, and therefore the noise is reduced. By combining the granular recording layer and the ferromagnetic metal layer in this way, it is possible to achieve high writeability and a high SNR while maintaining the thermal stability.
If the magnetic anisotropy of the granular recording layer has a gradation in such a way that the value becomes smaller toward the upper layer, the incoherent rotation mode of the magnetic moment is promoted, and this sometimes makes it possible to further improve the writeability.
Japanese Unexamined Patent Application Publication Nos. 2009-187597 and 2009-11060, for example, disclose recording media in which a lower granular recording layer formed on a substrate side is formed from a magnetic alloy having a relatively high magnetic anisotropy, and, in succession, an exchange coupling control layer is formed thereon, a granular recording layer having relatively low magnetic anisotropy is formed thereon, and a ferromagnetic metal layer formed from a non-granular material is formed thereon. If the granular recording layer has a gradation such that the magnetic anisotropy becomes smaller toward the upper layer, the incoherent rotation mode is promoted, and the writeability is improved. Furthermore, Japanese Unexamined Patent Application Publication No. 2009-59402 also discloses a similar kind of configuration. A granular recording layer having a low magnetic anisotropy is formed directly under the ferromagnetic metal layer in order to suppress the problem of the magnetic transition width enlarging. It is disclosed that as a result, the SNR and resolution may be improved. Furthermore, K. Tanahashi, H. Nakagawa, R. Arai, H. Kashiwase, H. Nemoto, “Dual Segregand Perpendicular Recording Media With Graded Properties, IEEE Trans. Magn. 45 (2009) 799, discloses a configuration in which the granular layer is formed by a two- or three-layer structure, and the magnetic anisotropy becomes smaller in stages toward the upper layer. It is disclosed that this configuration promotes an incoherent rotation mode, and therefore it is possible to reduce the film thickness of the ferromagnetic metal layer while maintaining writeability. The reduction of the film thickness of the ferromagnetic metal layer reduces the magnetic cluster size, and therefore the SNR and ATI (adjacent track interference) characteristics may be improved.
According to one embodiment, a perpendicular magnetic recording medium includes a first granular recording layer having a magnetic anisotropy Ku1, a second granular recording layer above the first granular recording layer having a magnetic anisotropy Ku2, and a third granular recording layer above the second granular recording layer having a magnetic anisotropy Ku3, where the respective magnetic anisotropies have a mathematical relationship wherein Ku3<Ku2>Ku1.
In another embodiment, a perpendicular magnetic recording medium includes a first granular recording layer with a first CoCrPt alloy in a first ratio X1, defined as a concentration of Pt divided by a concentration of Cr in the first CoCrPt alloy, a second granular recording layer above the first granular recording layer and having a second CoCrPt alloy in a second ratio X2, defined as a concentration of Pt divided by a concentration of Cr in the second CoCrPt alloy, and a third granular recording layer above the second granular recording layer and having a third CoCrPt alloy in a third ratio X3 defined as a concentration of Pt divided by a concentration of Cr in the third CoCrPt alloy, wherein the respective ratios adhere to a mathematical relationship X3<X2>X1.
In yet another embodiment, a perpendicular magnetic recording medium includes a first granular recording layer, a second granular recording layer above the first granular recording layer, an exchange coupling control layer above the second granular recording layer and a third granular recording layer above the exchange coupling control layer, wherein the exchange coupling control layer has a saturation magnetization of no greater than about 300 emu/cc.
Any of these embodiments may be implemented in a magnetic data storage system such as a disk drive system, which may include a magnetic head, a drive mechanism for passing a magnetic storage medium (e.g., hard disk) over the head, and a control unit electrically coupled to the head for controlling operation of the head.
Other aspects and advantages of the present invention will become apparent from the following detailed description, which, when taken in conjunction with the drawings, illustrate by way of example the principles of the invention.
The following description is made for the purpose of illustrating the general principles of the present invention and is not meant to limit the inventive concepts claimed herein. Further, particular features described herein can be used in combination with other described features in each of the various possible combinations and permutations.
Unless otherwise specifically defined herein, all terms are to be given their broadest possible interpretation including meanings implied from the specification as well as meanings understood by those skilled in the art and/or as defined in dictionaries, treatises, etc.
It must also be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless otherwise specified.
According to one general embodiment, a perpendicular magnetic recording medium includes a first granular recording layer having a magnetic anisotropy Ku1, a second granular recording layer above the first granular recording layer having a magnetic anisotropy Ku2, and a third granular recording layer above the second granular recording layer having a magnetic anisotropy Ku3, where the respective magnetic anisotropies have a mathematical relationship wherein Ku3<Ku2>Ku1.
In another general embodiment, a perpendicular magnetic recording medium includes a first granular recording layer with a first CoCrPt alloy in a first ratio X1, defined as a concentration of Pt divided by a concentration of Cr in the first CoCrPt alloy, a second granular recording layer above the first granular recording layer and having a second CoCrPt alloy in a second ratio X2, defined as a concentration of Pt divided by a concentration of Cr in the second CoCrPt alloy, and a third granular recording layer above the second granular recording layer and having a third CoCrPt alloy in a third ratio X3 defined as a concentration of Pt divided by a concentration of Cr in the third CoCrPt alloy, wherein the respective ratios adhere to a mathematical relationship X3<X2>X1.
In yet another general embodiment, a perpendicular magnetic recording medium includes a first granular recording layer, a second granular recording layer above the first granular recording layer, an exchange coupling control layer above the second granular recording layer and a third granular recording layer above the exchange coupling control layer, wherein the exchange coupling control layer has a saturation magnetization of no greater than about 300 emu/cc.
To enhance the recording density of a perpendicular magnetic recording medium, high linear density recording patterns may be clearly written and retained in the medium. Accordingly, improvement of the resolution of the recording patterns is important. One method for achieving this involves reducing the magnetic cluster size, according to one embodiment. This is because the magnetic transition width is narrowed if the magnetic cluster size is reduced, and therefore high linear density recording patterns become possible. For example, it is possible to reduce the magnetic cluster size by increasing the oxide content of a CoCrPt alloy in a granular recording layer and physically enlarging the magnetic grain boundary width, in one approach. However, if the amount of oxide is excessive, oxide penetrates to the interior of the magnetic grains (magnetic grain core) which, as a result, may cause a deterioration in the magnetic anisotropy of the magnetic grains and an increase in the switching field distribution, so the signal-to-noise ratio (SNR) deteriorates. This means that there is a limit to improving the resolution by changes to the alloy composition and the oxide content.
Another effective and promising method for improving the resolution of the magnetic recording media involves using a thinner recording layer. If the recording layer thickness is reduced, the effective field intensity and the field gradient from the magnetic head received by the recording layer increases, and the magnetic transition width may be reduced, in one approach. Consequently, a high linear recording density pattern becomes clear and the resolution is improved. However, if the recording layer thickness is reduced (thinned) via simple scaling, the thermal stability deteriorates because the magnetic volume decreases. The relationship between the thermal stability and the film thickness in the recording layer is that of a trade-off.
According to one embodiment, a perpendicular magnetic recording medium in which the recording layer film thickness is reduced while maintaining thermal stability is possible, and a high resolution and a high signal-to-noise ratio (SNR) is realized. In another embodiment, a method for producing the perpendicular magnetic recording medium described above is provided, along with a magnetic recording device employing the perpendicular magnetic recording medium described above.
The granular recording layer 16 is formed from a magnetic alloy in which the main components are Co, Cr and Pt and includes an oxide, in one approach. Specifically, the magnetic alloy may include a Co—Cr—Pt alloy, Co—Cr—Pt—B alloy, Co—Cr—Pt—Mo alloy, Co—Cr—Pt—Nb alloy, Co—Cr—Pt—Ta alloy, Co—Cr—Pt—Ni alloy, Co—Cr—Pt—Ru alloy, etc., and also one or more from among oxides of Si, Ti, Ta, Nb, B; W, and Cr, etc. A first granular recording layer 16-1, a second granular recording layer 16-2, and a third granular recording layer 16-3 are formed in succession from the substrate side. The first granular recording layer is formed, at least partially to promote thermal stability and segregation among magnetic grains in the second granular recording layer. The second granular recording layer is the main recording layer and is formed, at least partially, to improve thermal stability, reduce the magnetic cluster size, and reduce noise. The third granular recording layer is formed, at least partially, to effectively transmit the switching torque from the ferromagnetic metal layer 17 to the second granular recording layer, promoting incoherent rotation of magnetization, and improving writeability, in various embodiments.
This perpendicular magnetic recording medium is characterized in that the following relationships are satisfied among the magnetic anisotropy Ku1 of the first granular recording layer, the magnetic anisotropy Ku2 of the second granular recording layer, and the magnetic anisotropy Ku3 of the third granular recording layer.
Ku2>Ku1, Ku2>Ku3 Expression 1
The reason why Ku2 is greatest, in one embodiment, can be explained as follows. Two adjacent layers generally undergo exchange interaction through the magnetic moment present at the interface of these two adjacent layers. By way of exchange interaction, a layer having high magnetic anisotropy (Ku) suppresses thermal agitation in the magnetic moment present in an adjacent layer having low magnetic anisotropy Ku. The thermal stability of the film as a whole is improved the greater the number of layers in contact with the layer having the highest Ku. As a result, it is possible to reduce the film thickness of the granular recording layer while maintaining thermal stability. As can be understood from the above, positioning the layer having the highest Ku in the center of the granular layer helps to reduce the film thickness of the granular recording layer.
The reason why the magnetic anisotropy Ku1 of the first granular recording layer is smaller than the magnetic anisotropy Ku2 of the second granular recording layer can be explained as follows. When it is only improved thermal stability which is being considered, a greater Ku1 results in a greater improvement. However, the section at the bottom-most layer of the granular recording layer is distant from the write head, and therefore the head field intensity sharply decreases. This means that if Ku1 is too great, the magnetic moment of the first granular recording layer cannot be switched by the head field, and hence the writeability deteriorates considerably. On the other hand, if Ku1 is set at less than Ku2, there is no large deterioration in the writeability. This is because, in this case, if the magnetic moment of the second granular recording layer switches (rotates), the magnetic moment of the first granular recording layer can also switch (rotate). It is therefore useful for Ku1 to be in a range of values which are less than Ku2 and to be appropriately set so as to match the head with which it is used. On the other hand, the saturation magnetization (magnetization saturation moment) (Ms) of the first granular recording layer preferably is not excessively large from a point of view of restricting noise. Regions where the grain boundary is unclear are present in the initial growth region of the granular recording layer in contact with the interlayer 15. In the regions where the grain boundary is unclear, the magnetic grains are magnetically coupled, and therefore the magnetic cluster size increases if Ms is large. Noise increases as a result. Accordingly, the Ms of the first granular recording layer is preferably smaller than the Ms of the second granular recording layer which is the main recording layer. This condition is also consistent with Expression 1, above. This is because if the Ms is reduced with a CoCrPt alloy, Ku is also reduced at the same time.
The reason why Ku3 is to be smaller than Ku2, in some embodiments, may be explained as follows. The ferromagnetic metal layer is generally formed from a material having far lower magnetic anisotropy than the granular recording layer in order to maintain the writeability. If a layer having a large Ku is used as the third granular recording layer, the difference in Ku of the ferromagnetic metal layer and the granular recording layer becomes excessive, and the switching torque from the ferromagnetic metal layer is no longer effectively transmitted to the granular recording layer below. As a result, the incoherent rotation mode of switching is suppressed, and therefore the writeability deteriorates. Accordingly, Ku3 is to be set smaller than Ku2 in order to achieve adequate writeability, in some embodiments.
It is understood from the above that if Ku in each granular layer is set in such a way that Expression 1 is satisfied, it is possible to improve the trade-off relationship between improved thermal stability and reduced film thickness in the recording layer with keeping thermal stability. With most conventional perpendicular magnetic recording media in which the granular recording layer has a three-layer structure, the magnetic anisotropy has a relationship where Ku1>Ku2>Ku3. In this case, there is one layer which is adjacent to the layer having the highest Ku, and therefore the effect of improving thermal stability is not great. Consequently, if the film thickness of the granular recording layer is reduced, thermal stability deteriorates and adequate performance cannot be achieved.
According to one embodiment, a perpendicular magnetic recording medium includes a first granular recording layer having a magnetic anisotropy (Ku1), a second granular recording layer above the first granular recording layer, the second granular recording layer having a magnetic anisotropy (Ku2), and a third granular recording layer above the second granular recording layer, the third granular recording layer having a magnetic anisotropy (Ku3), wherein Ku3<Ku2l >Ku1.
In one embodiment, the perpendicular magnetic recording medium may be characterized by having ratios such that Ku3<Ku1.
In another embodiment, the perpendicular magnetic recording medium may further include an interlayer (including the first and second interlayers 14, 15) below the first granular recording layer 16-1, a soft magnetic underlayer 12 below the interlayer, and a ferromagnetic metal layer 17 above the third granular recording layer. According to a further embodiment, the interlayer may include a first interlayer 14 positioned above the soft magnetic underlayer and a second interlayer 15 positioned above the first interlayer.
In one approach, the ferromagnetic metal layer 17 may be positioned directly on the third granular recording layer 16-3. The ferromagnetic metal layer may be essentially free of oxides (of any type known in the art, within reasonable limitations) and the first granular recording layer, the second granular recording layer, and the third granular recording layer may each comprise an oxide, of any type known in the art and in concentrations as described herein according to various embodiments.
In addition to the above described embodiments, it is possible to adjust Ku in a CoCrPt alloy by adjusting a concentration ratio of Cr and Pt in a CoCrPt alloy.
X=(Pt concentration)/(Cr concentration) Expression 2
Specifically, as defined in Expression 2 above, X is a ratio of concentrations of Pt versus Cr in a CoCrPt alloy. It was discovered that the value of Ku and X are directly proportional. Accordingly, Expression 1, which represents one feature of a perpendicular magnetic recording medium, may be expressed in the following manner using the Pt and Cr concentration ratio X1 in the first granular recording layer, the Pt and Cr concentration ratio X2 in the second granular recording layer, and the Pt and Cr concentration ratio X3 in the third granular recording layer, as follows.
X2>X1; X2>X3 Expression 3
In addition, the first granular recording layer may be formed to magnetically separate among the magnetic grains, and therefore the Cr content thereof is preferably at least about 18 at. %. Cr atoms included in the granular recording layer are oxidized to form Cr oxides, the magnetic grains are segregated, and the magnetic coupling between the magnetic grains splits. However, if the Cr concentration is about 18 at. % or less, the magnetic coupling between magnetic grains becomes too great, and therefore noise increases. The second granular recording layer preferably has magnetic anisotropy of around 5.0×106 erg/cc or greater, although this depends on the combination with the head. To this end, X2 may be preferably set to around 1.0 or greater, in one approach. If X2 is below about 1.0, adequate thermal stability may not be achieved, and the magnetic cluster size also increases, causing increased noise. The film thickness of the second granular recording layer may be preferably set at between approximately 2.5 nm and about 5.5 nm in one approach, although this depends on the Ku of the alloy forming this layer and the combination with the head. If it is less than about 2.5 nm in thickness is used, adequate thermal stability may not be achieved, while if it is greater than about 5.5 nm in one approach, the writeability may deteriorate.
When the field intensity of the head which is used in the magnetic recording system is small and the writeability is inadequate, an exchange interaction control layer, having a thickness of about 1 nm in one approach, may be inserted between the second granular recording layer and the third granular recording layer. Of course, any thickness may be used as would be understood by one of skill in the art, such as about 0.75 nm, 1.25 nm, 1.5 nm, 2 nm, etc. This layer makes it possible to promote the incoherent rotation mode and to improve the writeability as a result, by providing suitable exchange interactions between the second granular recording layer and the third granular recording layer. The saturation magnetization of the exchange coupling control layer is preferably no greater than about 250 emu/cc, about 300 emu/cc, about 350 emu/cc, about 400 emu/cc, etc. If the saturation magnetization is greater than about 300 emu/cc, the exchange interactions between the second granular recording layer and the third granular recording layer may be too strong, and the effect of improving the writeability may not be provided.
There is no restriction as to the material of the exchange coupling control layer provided that the saturation magnetization thereof is no greater than about 300 emu/cc and provided that it is inserted for the purpose of controlling the exchange coupling of the third granular recording layer and the second granular recording layer; however, it may preferably be a granular material including a Co—Cr—Pt alloy, Co—Cr—Pt—B alloy, Co—Cr—Pt—Mo alloy, Co—Cr—Pt—Nb alloy, Co—Cr—Pt—Ta alloy, Co—Cr—Pt—Ni alloy, Co—Cr—Pt—Ru alloy, etc., and also one or more from among oxides of Si, Ti, Ta, Nb, B, W, and Cr, etc. Furthermore, the Cr concentration may more preferably be about 25 at. % or greater, about 30 at. % or greater, about 35 at. % or greater, etc., in order to keep the saturation magnetization of the exchange coupling control layer at no greater than about 300 emu/cc.
According to another embodiment, a perpendicular magnetic recording medium may include a first granular recording layer comprising a first CoCrPt alloy in a first ratio (X1, defined by a concentration of Pt divided by a concentration of Cr in the first CoCrPt alloy), a second granular recording layer above the first granular recording layer, the second granular recording layer comprising a second CoCrPt alloy in a second ratio (X2, defined by a concentration of Pt divided by a concentration of Cr in the second CoCrPt alloy) and a third granular recording layer above the second granular recording layer, the second granular recording layer comprising a third CoCrPt alloy in a third ratio (X3, defined by a concentration of Pt divided by a concentration of Cr in the third CoCrPt alloy), wherein X3<X2>X1.
In another embodiment, the perpendicular magnetic recording medium may be characterized by having ratios such that X3<X1.
According to another embodiment, the Cr concentration in the first granular recording layer may be between about 18 at. % and about 30 at. %, between about 21 at. % and about 27 at. %, between about 20 at. % and about 25 at. %, etc.
In still another embodiment, the perpendicular magnetic recording medium may further include an interlayer below the first granular recording layer, a soft magnetic underlayer below the interlayer, and a ferromagnetic metal layer above the third granular recording layer. In a further embodiment, the interlayer may include a first interlayer above the soft magnetic underlayer and a second interlayer above the first interlayer:
In another embodiment, a perpendicular magnetic recording medium includes a first granular recording layer, a second granular recording layer above the first granular recording layer, an exchange coupling control layer above the second granular recording layer and a third granular recording layer above the exchange coupling control layer, where the exchange coupling control layer has a saturation magnetization of no greater than about 300 emu/cc.
In one approach, the perpendicular magnetic recording medium may have a Cr concentration in the exchange coupling control layer of at least about 25 at. %, at least about 30 at. %, in a range from about 20 at. % to about 50 at. %, etc.
In another embodiment, a perpendicular magnetic recording medium as described above in any embodiment may have a first granular recording layer with a magnetic anisotropy (Ku1), a second granular recording layer with a magnetic anisotropy (Ku2), and a third granular recording layer with a magnetic anisotropy (Ku3). Furthermore, each layer's magnetic anisotropy may be comparatively described by the relationship were Ku3<Ku2>Ku1, and further still, in some arrangements by a relationship where Ku3<Ku1.
A preferred embodiment of the perpendicular magnetic recording medium is described below with respect to elements other than the granular recording layer. Various kinds of substrates may be used for the substrate 10, such as a glass substrate, an aluminum alloy substrate, a plastic substrate, a silicon substrate, etc.
There is no particular restriction as to the material of the adhesion layer 11 provided that it adheres well to the substrate 10 and has excellent planarity, but it may preferably include a material comprising at least two materials selected from: Ni, Co, Al, Ti, Cr, Zr, Ta and Nb. Specifically, it is possible to use TiAl, NiTa, TiCr, AlCr, NiTaZr, CoNbZr, TiAlCr, NiAlTi and CoAlTi, among others. The film thickness is preferably in a range from about 2 nm to about 40 nm. If it is less than about 2 nm, the effect as an adhesion layer may be impaired, and there is no improvement in performance as an adhesion layer even if it is formed to greater than about 40 nm, so the returns are diminished and since this would reduce productivity, it is undesirable.
The soft magnetic underlayer 12 serves to control the expansion of the magnetic field produced by the magnetic head and to magnetize the recording layer 16 effectively. There is no particular restriction as to the material of the soft magnetic underlayer provided that the saturation flux density (Bs) thereof is at least about 1 Tesla or greater, uniaxial anisotropy is imparted in the radial direction of the disk substrate, the coercive force measured in the head travel direction is no greater than about 2.4 kA/m, and the surface planarity is excellent, in one approach.
Specifically, the characteristics mentioned above may be readily obtained by using an amorphous alloy in which Co or Fe is the main component and Ta, Nb, Zr, B, Cr, etc., are added thereto. The optimum film thickness value varies according to a distance from the soft magnetic underlayer 12 to the recording layer 16 and the material, and also the magnetic head with which the medium is used, but a range of about 20 nm to about 100 nm may preferably be used, in one approach. Furthermore, it is also possible to use an alloy having an fcc structure for part of the soft magnetic underlayer 12. This is formed for the purpose of controlling the crystal orientation of the seed layer 13 formed above the soft magnetic underlayer 12, and specifically it is possible to use a material in which Ta, Nb, W, B, V, etc., are added to CoFe, in various approaches. The film thickness may preferably be in a range from about 1 nm to about 10 nm, in one approach. The seed layer 13 serves to control the crystal orientation of the interlayer 14 and the grain size, in one approach. For this it is possible to use a metal having an fcc structure or an amorphous material. Specific materials having an fcc structure include Ni, Cu, Pd, Pt, etc., and favorable crystal orientation is achieved by adding one or more elements selected from Cr, W, V, Mo, Ta and Nb, among others.
Furthermore, a similar action to that of the soft magnetic underlayer can be imparted by using a magnetic material which also has an fcc structure at the same time, and it is possible to reduce the distance between the magnetic head and the soft magnetic layer. Specifically, 10 at. % or less of Ta, W, Nb, Cr, B, etc., may be added to the NiFe or CoFe in the composition having the fcc structure, in one approach. An optimum film thickness value of the seed layer varies according to the material and film thickness of the interlayers and the recording layer, and the head, but a range of about 2 nm to about 10 nm may be used, in some approaches. If it is less than about 2 nm, the crystal orientation may deteriorate, which is undesirable. If it is greater than about 10 nm, the crystal grain size in the recording layer may increase, which is also undesirable. In addition, if the crystal grain size in the recording layer is to be reduced, it is possible to use an amorphous material. Specifically, such materials include Ta, TiAl, CrTi, NiTa, etc., and favorable crystal orientation may be achieved when the film thickness is in a range from about 1 nm to about 4 nm.
The first interlayer 14 is formed to enhance the crystal orientation of the recording layer 16 and also promotes the grain separation in the recording layer 16. Specifically, it is possible to use Ru or a Ru alloy forming an hcp structure in which an element selected from Cr, Ta, W, Mo, Nb, Co, etc., is added to Ru. Changing the formation conditions and changing the materials can accommodate both the crystal orientation and the grain separation in the methods disclosed herein. Specifically, one formation technique may include forming the layer under a possible low gas pressure at the start of film formation with the aim of enhancing the crystal orientation, and immediately before the end of film formation raising the gas pressure for the purposes of grain separation, or it is equally feasible to produce a two-stage structure or three-stage structure under conditions in which the gas pressure and materials are varied. Low gas pressure refers specifically to about 1 Pa or less. High gas pressure refers to a range between about 2 Pa and about 6 Pa, and when this range is adopted, the surface roughness of the Ru is increased and voids are formed at the grain boundary. The film thickness is preferably in a range from about 8 nm to about 20 nm, in one approach. If it is less than about 8 nm, the crystal orientation deteriorates, while if it is greater than about 20 nm, the distance between the magnetic head and the soft magnetic underlayer increases, and therefore writeability deteriorates.
The second interlayer 15 is formed to promote grain separation of the recording layer 16. Specifically, it is possible to use a Ru alloy in which an element selected from Ti, Ta, Nb, Al, Si, etc., is added to Ru, in one approach. It is also possible to use a Ru alloy in which some of the elements added are in oxide form, in one approach. In the ferromagnetic metal layer 17, at least one element selected from B, Ta, Ru, Ti, W, Mo, Nb, Ni, Mn is preferably added, in one approach, with CoCrPt as a main component. There is no particular restriction as to the respective compositions and film thicknesses, provided that they can be adjusted to suit the film thickness of the soft magnetic underlayer and the magnetic head performance, and they are in a range allowing the thermal stability to be maintained.
The overcoat layer 18 may preferably be formed using a film having carbon as a main component at a thickness of between about 2 nm and about 5 nm, in one approach, such as a carbon overcoat. Furthermore, the lubricant layer 19 preferably makes use of a lubricant such as perfluoroalkyl polyether. This makes it possible to obtain a highly reliable magnetic recording medium.
According to one embodiment, it is possible to maintain thermal stability in a perpendicular magnetic recording medium and to reduce the film thickness of the recording layer, and therefore the resolution and SNR can be improved. A perpendicular magnetic recording medium exhibiting high resolution and high SNR is essential for increasing recording density, and by using this kind of perpendicular magnetic recording medium it is possible to provide a compact and high-capacity magnetic recording device.
In one embodiment, a perpendicular magnetic recording medium as described above in any embodiment may have a first granular recording layer including a first CoCrPt alloy in a first ratio (X1) defined as a concentration of Pt divided by a concentration of Cr in the first CoCrPt alloy, a second granular recording layer including a second CoCrPt alloy in a second ratio (X2), defined as a concentration of Pt divided by a concentration of Cr in the second CoCrPt alloy and a third granular recording layer including a third CoCrPt alloy in a third ratio (X3), defined as a concentration of Pt divided by a concentration of Cr in the third CoCrPt alloy. Furthermore, in this embodiment, the respective ratios may be mathematically related such that X3<X2>X1, and further still, in some approaches, the respective ratios may be mathematically related such that X3<X1.
In another arrangement, the perpendicular magnetic recording medium as described above may exhibit a Cr concentration in the first granular recording layer between about 18 at. % and about 30 at. %, between about 21 at. % and about 27 at. %, between about 20 at. % and about 25 at. %, etc.
According to one embodiment, a magnetic data storage system may include at least one magnetic head, a perpendicular magnetic recording medium as described according to any embodiment herein, a drive mechanism for passing the perpendicular magnetic recording medium over the magnetic head(s) and a controller electrically coupled to the magnetic head(s) and capable of controlling operation of the magnetic head(s).
Experimental Results
A glass substrate of thickness of about 0.8 nm and diameter about 65 nm was used as the substrate 10. Without heating the substrate, Ni-37.5Ta was formed to about 15 nm as the adhesion layer 11 under conditions of Ar gas pressure about 0.5 Pa, and the soft magnetic underlayer 12 was formed as two layers, namely a Co-28Fe-3Ta-5Zr alloy film of thickness about 20 nm, with the interposition of an Ru film of thickness about 0.4 nm under conditions of Ar gas pressure about 0.4 Pa. A Ni-14Fe-6W film of thickness about 7 nm was formed thereon as the seed layer 13. Ru of thickness about 4 nm was formed under conditions of Ar gas pressure about 0.5 Pa as the first interlayer 14, and also Ru of thickness about 5 nm was formed under conditions of Ar gas pressure about 3.3 Pa, and Ru of thickness about 5 nm was formed thereon under conditions of Ar gas pressure about 6.0 Pa. The second interlayer 15 was formed using an Ru-20Ti-10TiO2 target under conditions of Ar gas pressure 4 Pa. The first granular recording layer, second granular recording layer, and third granular recording layer were all formed in different chambers. The first granular recording layer was formed under conditions of Ar gas pressure about 4 Pa and substrate bias about 200V using a [Co-18Cr-10.5Pt]-4SiO2-2.5Co3O4 target, and the Pt concentration was raised to about 22.5 at. % in order to vary the Pt and Cr concentration ratio X1. The film thickness was fixed at about 4 nm. The second granular recording layer was formed under conditions of Ar gas pressure about 4 Pa and substrate bias about 300V using a [Co-18Cr-22.5Pt]-4Si02-2.5Co3O4 target, and the Pt concentration was reduced to about 10.5 at. % in order to vary the Pt and Cr concentration ratio X2. The film thickness was fixed at about 4 nm. The third granular recording layer was formed to a film thickness of about 4.5 nm under conditions of Ar gas [Co-26Cr-10.5Pt]-4SiO2—Co3O4 pressure in the case of the layer was lower than that pressure about 0.9 Pa using a target. The Ar gas third granular recording of the other granular recording layers, which made it possible to improve the planarity of the medium surface. The film thickness and sputtering conditions in the case of the third granular recording layer were constant. The ferromagnetic metal layer 17 was formed to a film thickness of about 4 nm using a Co-15Cr-14Pt-8B target. A DLC film of thickness about 2.5 nm was formed as the overcoat layer 18. Finally, a polyether-based material lubricant in which a perfluoroalkyl had been diluted with a fluorocarbon material was applied as the lubricant layer 19.
Table 1 shows the target compositions of the first granular recording layer, second granular recording layer, and third granular recording layer in the samples produced; X1, X2 and X3 at that time and the mean value thereof; resolution obtained by using read/write measurements; and KuV/KBT.
Because the mean values of X1, X2, and X3 changed, the magnetic anisotropy of the whole film also changed, the mean values of X1, X2 and X3 were made the same for all samples. The read/write characteristics of the medium were evaluated by using a spin stand. The evaluation made use of a magnetic head comprising a writing element of single pole type which had a track width of about 70 nm, and a reading element having a track width of about 60 nm which employed tunnel magnetoresistance; the conditions for the evaluation were: circumferential speed about 10 m/sec, skew angle about 0°, and magnetic spacing approximately 8 nm. KuV/KBT were measured by using a Kerr magnetometer. The field sweep rate (sweep rate) was varied at about 212 kA/(m·sec), about 106 kA/(m·sec), about 53 kA/(m·sec), about 27 kA/(m·sec) and about 13 kA/(m·sec). The Kerr loop was measured while a magnetic field was applied in the direction perpendicular to the film surface of the sample, and KuV/KBT was obtained from the damping process of the coercive force (coercivity).
The method for evaluating KuV/KBT is disclosed in the following document, for example, Quingzhi Peng and Hans J. Richter, “Field Sweep Rate Dependence of Media Dynamic Coercivity,” IEEE Trans. Magn., 40 (2004) 2446.
According to the results shown in Table 1, all of the samples have a constant film thickness, and therefore the resolution is largely unchanged, but KuV/KBT increases with X. In particular, when X1 is smaller than X2, KuV/KBT is clearly smaller, and the thermal stability deteriorates. Accordingly, when the mean values of X1, X2 and X3 are compared under the same film thickness conditions, it is clear that the thermal stability improves when X2 is set to be greater than X1.
Here, samples in which the granular recording layer was formed as a single-layer structure to about 13 nm on the interlayer 15 were prepared in order to investigate the relationship of X and the magnetic anisotropy (Ku). The magnetic anisotropy was evaluated by the magnetic field angle dependence of the magnetic torque measured by using a torque magnetometer. The saturation magnetization Ms was measured by a vibrating sample magnetometer. The magnetic anisotropy was determined as the magnetic anisotropy on the inside of the magnetic grains. Also, the saturation magnetization was determined as the saturation magnetization on the inside of the magnetic grains. Specifically, the value obtained by subtracting the volume of the oxide content from the total film volume was taken as the volume of the magnetic grains contained in the film, and the magnetic anisotropy on the inside of the magnetic grains was obtained by using the volume of the magnetic grains.
When the abovementioned measurements were also carried out for the alloy compositions shown in Table 2, the relationship between X and Ku on the inside of the magnetic grains was as shown in
From the above, the results in Table 1 show that when the mean values of Ku1, Ku2, and Ku3 are compared under the same film thickness conditions, the thermal stability is improved by setting Ku1 to be greater than Ku2 regardless of whether the mean values of Ku1, Ku2, Ku3 and the total film thickness are the same.
The perpendicular magnetic recording medium according to another exemplary embodiment (exemplary embodiment 2) was prepared using the same process as in exemplary embodiment 1, except for the granular recording layer 16. The first granular recording layer was formed to a film thickness of about 3 nm under conditions of Ar gas pressure about 4 Pa and substrate bias about 200V using a [Co-26Cr-18.5Pt]-4SiO2-2.5Co3O4 target. The second granular recording layer was formed under conditions of Ar gas pressure 4 Pa and substrate bias about 300V using a [Co-18Cr-22.5Pt]-4SiO2-2.5Co3O target, and the Pt concentration was reduced to about 10.5 at. % in order to vary the Pt and Cr concentration ratio X2. The film thickness was fixed at 4 nm. The third granular recording layer was formed under conditions of Ar gas pressure about 0.9 Pa using a [Co-26Cr-10.5Pt]-4SiO2—Co3O4 target, and the Pt concentration was raised to about 22.5 at. % in order to vary the Pt and Cr concentration ratio X3. The film thickness was fixed at 4 nm. The film thickness and sputtering conditions in the case of the first granular recording layer were constant, using the above samples, the magnetic characteristics and the read/write characteristics were investigated for when the mean values of X2 and X3 are constant and X2 and X3 are varied. The methods for evaluating the magnetic characteristics and the read/write characteristics of the medium are the same as in the previous exemplary embodiment.
Table 3 shows the target compositions of the first granular recording layer, the second granular recording layer, and the third granular recording layer in the samples produced; X1, X2 and X3 at that time and the mean value thereof; resolution and overwrite obtained by using read/write measurements. It should be noted that when the mean values of X1, X2, and X3 changed, the magnetic anisotropy of the whole film also changed, and therefore the target compositions were selected in such a way that the mean values of X1, X2, and X3 were the same for all the samples. According to the results shown in Table 3, in all of the samples the resolution and KuV/KBT are largely unchanged, but overwrite becomes smaller the greater X3. In particular, when X3 is greater than X2, overwrite deteriorates considerably, and hence the writeability deteriorates. When X3 is greater than X2, it is believed that the switching torque from the ferromagnetic metal layer 17 is no longer effectively transmitted to the granular recording layer below, and therefore the writeability deteriorates. X3 therefore should be smaller than X2 in order to achieve adequate writeability.
The perpendicular magnetic recording medium according to another exemplary embodiment (exemplary embodiment 3) was prepared using the same process as in exemplary embodiment 1, except for the granular recording layer 16. Three types of targets were prepared as the targets for the granular recording layer 16, namely [Co-26Cr-18.5Pt]-4SiO2-2.5Co3O4, [Co-10.5Cr-22.5Pt]-5SiO2-5TiO2-1.5Co3O4, and [Co-26Cr-10.5Pt]-4SiO2-1Co3O4, and as shown in Table 4, samples having different stacking orders were produced.
Among these, [Co-10.5Cr-22.5Pt]-5SiO2-5TiO2-1.5Co3O4, which was the target composition in which Ku is the greatest (greatest X) was formed under conditions of Ar gas pressure about 4 Pa and substrate bias about 300V, and the film thickness was varied between about 1.5 nm and about 6.5 nm. [Co-26Cr-18.5Pt]-4SiO2-2.5Co3O4, in which Ku was the second greatest (second greatest X), was formed under conditions of Ar gas pressure about 4 Pa and substrate bias about −200 V, and the film thickness was fixed at about 3 nm. [Co-26Cr-10.5Pt]-4SiO2-1Co3O4, which had the smallest Ku (smallest X) among the granular layers, was formed under conditions of Ar gas pressure about 0.9 Pa, and the film thickness was fixed at about 4.5 nm. Examples 4-1 and 4-2 relate to perpendicular magnetic recording media which have a structure that satisfies the abovementioned Expression 1 and exhibit the features of this embodiment. The comparative samples cover all the combinations having a stacked structure which does not conform to the abovementioned Expression 1 when the materials are fixed and only the stacking order is changed.
Using the above samples, the magnetic characteristics read/write characteristics were evaluated and the superiority of the samples having a structure which satisfies the abovementioned Expression 1 was confirmed. The methods for evaluating the magnetic characteristics and the read/write characteristics of the medium are the same as in exemplary embodiment 1.
The trend of thermal stability and resolution in Comparative Ex. 4-3, 4-4 is equivalent to that of Examples 4-1, 4-2, and at a glance Comparative Ex. 4-3, 4-4 show that adequate performance is achieved. However, as shown in
The results in
To summarize the above, the trend of thermal stability and resolution is improved in Examples 4-1, 4-2 in contrast to Comparative Ex. 4-1, 4-2, and the trend of thermal stability and writeability, and thermal stability and SNR is improved in contrast to Comparative Ex. 4-3, 4-4, and therefore it is clear that these media structures are suitable for increasing density.
A perpendicular magnetic recording medium according to another exemplary embodiment (exemplary embodiment 4) was prepared using the same process as in exemplary embodiment 1, except for the granular recording layer 16.
The first granular recording layer was formed to a film thickness of about 3 nm using several targets in which the Cr concentrations were varied to those shown in Table 5, under conditions of Ar gas pressure 4 Pa and substrate bias about 200V. The second granular recording layer was formed to a film thickness of about 2.5 nm under conditions of Ar gas pressure about 4 Pa and substrate bias about −300V, using a [Co-10.5Cr-22.5Pt]-5SiO2-5TiO2-1.5Co3O4 target. The third granular recording layer was formed to a film thickness of about 4.5 nm under conditions of Ar gas pressure about 0.9 Pa, using a [Co-26Cr-10.5Pt]-4SiO2-1Co3O4 target.
Using the above samples, changes in KuV/KBT and SNR with respect to changes in Cr concentration in the first granular recording layer were investigated. The methods for evaluating the magnetic characteristics and the read/write characteristics of the medium are the same as in exemplary embodiment 1.
On the other hand, it is clear from the results of KuV/KBT in
A perpendicular magnetic recording medium according to another exemplary embodiment (exemplary embodiment 5) was prepared using the same process as in exemplary embodiment 1, except for the granular recording layer 16. The first granular recording layer was formed under conditions of Ar gas pressure 4 Pa and substrate bias about 200V. The second granular recording layer was formed under conditions of Ar gas pressure about 4 Pa and substrate bias about 300V. The third granular recording layer was formed under conditions of Ar gas pressure about 0.9 Pa. The target compositions and film thicknesses used to form each of the granular recording layers are shown in Table 6.
5TiO2—1.5Co3O
—3TiO2—2T
O
—1.5Co3O4
indicates data missing or illegible when filed
Table 6 includes the comparative samples used in exemplary embodiment 4 as a comparison. Using the above samples, the magnetic characteristics and read/write characteristics were investigated for when the oxide concentration is varied and for when additional elements are added to the CoCrPt alloy. As shown in Table 6, when additional elements are added to the CoCrPt alloy and when the oxide concentration is varied, it is clear that good resolution, overwrite and SNR are demonstrated while the thermal stability is maintained.
A glass substrate of thickness about 0.8 mm and diameter about 65 mm was used as the substrate 610, without heating the substrate, Ni-37.5Ta was formed to about 15 nm as the adhesion layer 611 under conditions of Ar gas pressure about 0.5 Pa, and the soft magnetic underlayer 612 was formed as two layers, namely a Co-28Fe-3Ta-5Zr alloy film of thickness about 20 nm, with the interposition of an Ru film of thickness about 0.4 nm under conditions of Ar gas pressure about 0.4 Pa. A Ni-14Fe-6W film of thickness about 7 nm was formed thereon as the seed layer 613. Ru of thickness 4 nm was formed under conditions of Ar gas pressure about 0.5 Pa as the first interlayer 614, and also Ru of thickness about 5 nm was formed under conditions of Ar gas pressure about 3.3 Pa, and Ru of thickness about 5 nm was formed thereon under conditions of Ar gas pressure about 6.0 Pa. The second interlayer 615 was formed using a Ru-20Ti-10TiO2 target under conditions of Ar gas pressure about 4 Pa. The first granular recording layer 616-1, second granular recording layer 616-2, and third granular recording layer 616-4 were all formed in different chambers. The first granular recording layer was formed to a film thickness of about 3 nm under conditions of Ar gas pressure about 4 Pa and substrate bias about 200V using a [Co-26Cr-18.5Pt]-4SiO2-2.5Co3O4 target. The second granular recording layer was formed to a thickness of about 2.5 nm under conditions of Ar gas pressure about 4 Pa and substrate bias −300V using a [Co-10.5Cr-22.5Pt]-5SiO2-5SiO2-1.5Co3O4 target. The third granular recording layer was formed to a film thickness of about 4.5 nm under conditions of Ar gas pressure about 0.9 Pa using a [Co-26Cr-10.5Pt]-4SiO2-1Co3O4 target. The exchange coupling control layer 616-3 shown in Table 7 was formed between the second granular recording layer and the third granular recording layer for the purpose of controlling the exchange interactions of these layers. The exchange coupling control layer was formed to a thickness of 1 nm under conditions of Ar gas pressure about 4 Pa and substrate bias about 200 V.
In one approach, a soft magnetic underlayer 612 may be positioned below the first granular recording layer 616-1, a first interlayer 614 above the soft magnetic underlayer 612, a second interlayer 615 above the first interlayer 614, and a ferromagnetic metal layer 617 directly on or above the third granular recording layer 616-4. The ferromagnetic metal layer may be essentially free of oxides (of any type known in the art, within reasonable limitations) and the first granular recording layer, the second granular recording layer, and the third granular recording layer may each comprise an oxide, of any type known in the art and in concentrations as described herein according to various embodiments.
The read/write performance for the above samples was investigated using a head having a low field intensity. The medium overwrite characteristics were evaluated by using a spin stand. The evaluation made use of a head having a narrower track width and a lower field intensity than the head used in the exemplary embodiments above. Specifically, the evaluation made use of a magnetic head comprising a writing element of single pole type which had a track width of about 60 nm, and a reading element having a track width of about 55 nm which employed tunnel magnetoresistance; the conditions for the evaluation were: circumferential speed about 10 m/sec, skew angle about 0°, and magnetic spacing approximately 8.5 nm. The evaluation results are shown in Table 7.
When the abovementioned head having a narrow track width was employed, the overwrite value of the medium produced in exemplary embodiment 4, Example 4-1, was no more than about 30 dB. As a general rule, in order to incorporate a perpendicular magnetic recording medium into a magnetic recording device, the overwrite characteristics are preferably at least about 30 dB. There is therefore a possibility that the medium produced in Exemplary Embodiment 4 will not perform adequately when it is used in combination with the abovementioned head having a low field intensity. On the other hand, the overwrite value improved with the perpendicular magnetic recording medium produced in this exemplary embodiment, which includes the exchange coupling control layer formed from a material having saturation magnetization of no more than about 300 emu/cc, and therefore the writeability is improved. This is believed to be because the presence of the exchange coupling control layer promotes the incoherent rotation mode. On the other hand, when an exchange coupling control layer formed from a material having saturation magnetization of greater than about 300 emu/cc is used, the writeability does not improve or deteriorates. This is believed to be because when the saturation magnetization of the exchange coupling control layer becomes excessively large, the exchange interactions between the second granular recording layer and the third granular recording layer become too strong, so the incoherent rotation mode is suppressed. It is clear from the above results that the magnitude of the exchange interactions between the second granular recording layer and the third granular recording layer should be suitably adjusted to match the head with which the medium is combined, and if the head field intensity is relatively small with respect to the switching field of the medium, an exchange coupling control layer formed from a material having saturation magnetization of no more than 300 emu/cc should be introduced.
A magnetic recording device according to another embodiment is shown schematically in
The relationship of the magnetic head 102 and the magnetic recording medium 100 is shown in
According to one embodiment, a magnetic data storage system may include at least one magnetic head, a perpendicular magnetic recording medium as described in any embodiment herein, a drive mechanism for passing the perpendicular magnetic recording medium over the magnetic head(s) and a controller electrically coupled to the magnetic head(s) and capable of controlling operation of the magnetic head(s).
While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of an embodiment of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.
