FIELD OF THE INVENTION
The invention relates to magnetic thin film media with laminated magnetic layers and more particularly to magnetic properties and selection of materials used for the plurality of thin films in such media.
BACKGROUND OF THE INVENTION
A typical prior art head and disk from a magnetic disk drive 10 are illustrated in block form in FIG. 1. In operation the magnetic transducer 20 is supported by the suspension 13 as it flies above the disk 16. The magnetic transducer 20, usually called a “head” or “slider,” is composed of elements that perform the task of writing magnetic transitions (the write head 23) and reading the magnetic transitions (the read head 12). The electrical signals to and from the read and write heads 12, 23 travel along conductive paths (leads) 14 which are attached to or embedded in the suspension 13. The magnetic transducer 20 is positioned over points at varying radial distances from the center of the disk 16 to read and write circular tracks (not shown). The disk 16 is attached to a spindle 18 that is driven by a spindle motor 24 to rotate the disk 16. The disk 16 comprises a substrate 26 on which a plurality of thin films 21 are deposited. The thin films 21 include ferromagnetic material in which the write head 23 records the magnetic transitions in which information is encoded.
The conventional disk 16 includes substrate 26 of glass or AlMg with an electroless coating of Ni3P that has been highly polished. The thin films on the disk typically include a chromium or chromium alloy underlayer and at least one ferromagnetic layer based on various alloys of cobalt. For example, a commonly used magnetic alloy is CoPtCr. Additional elements such as tantalum and boron are often used in the magnetic alloy. A protective overcoat layer is used to improve wearability and corrosion resistance. Various seed layers, multiple underlayers and laminated magnetic films have all been described in the prior art. The laminated magnetic films have included multiple ferromagnetic layers that are separated by nonmagnetic spacer layers and more recently antiferromagnetic coupling has been proposed. It is known that substantially improved SNR can be achieved by the use of a laminated magnetic layer structure in which two magnetic layers are substantially decoupled. The reduced media noise is believed due to the reduced exchange coupling between the magnetic layers. The use of lamination for noise reduction has been extensively studied to find favorable spacer layer materials which include Cr, CrV, Mo and Ru, and spacer thicknesses from a few angstroms upward that result in the best decoupling of the magnetic layers and the lowest media noise.
Published US patent application 2005/0019609 by Kai Tang (Jan. 27, 2005) describes an embodiment of the invention which includes at least two laminated ferromagnetic layers with differing magnetic anisotropy. The independent magnetic layer farther away from the recording head is selected to have a lower magnetic anisotropy to allow magnetic switching of the multiple magnetic layers to occur at approximately the same head write current even though the recording head field is reduced with increased distance from the head. The improved switching yields improved magnetic recording performance. Laminated magnetic media according to the described invention can have a single peak in the normalized DC erase noise vs. head write current plot indicating that the magnetic transitions in the non-slave magnetic layers are written at the same head write current. As a result the magnetic pulse width (PW50) is reduced, overwrite (OW) is improved and media signal-to-noise ratio (S0NR) is improved.
Published US patent application 2002/0098390 by H. V. Do, et al. (Jul. 25, 2002) describes a laminated medium for horizontal magnetic recording that includes an antiferromagnetically (AF)-coupled magnetic layer structure and a conventional single magnetic layer. The AF-coupled magnetic layer structure has a net remanent magnetization-thickness product (Mrt) which is the difference in the Mrt values of its two ferromagnetic films. The type of ferromagnetic material and the thickness values of the ferromagnetic films are chosen so that the net moment in zero applied field will be low, but nonzero. The Mrt for the media is given by the sum of the Mrt of the upper magnetic layer and the Mrt of the AF-coupled layer stack.
Published US applications 2003/0148143 by Kanabe, et al. (Aug. 7, 2003) and 2003/0104253 by Osawa, et al. (Jun. 5, 2003) describe various magnetic media. However, they do not disclose the use of a laminated magnetic media where the top two magnetic layers are substantially decoupled from one another.
The convention for alloy composition used in this application gives the atomic percentage (at. %) of an element as a subscript; for example, CoCr10 is 10 atomic percent Cr with balance being Co and CoPt11Cr20B7 is 11 atomic percent Pt, 20 atomic percent Cr and 7 atomic percent boron with the balance being Co.
SUMMARY OF THE INVENTION
An embodiment of the invention is a laminated magnetic recording medium comprising two magnetic layers that are substantially decoupled. The upper and lower magnetic layers are separated by a nonmagnetic spacer. The upper magnetic layer (nearest the air-bearing surface) is preferably a cobalt alloy having from 12 to 16 at. % platinum (Pt), from 11 to 20 at. % chromium (Cr), from 6 to 14 at. % boron (B) and from 0.5 to 2 at. % tantalum (Ta). The addition of Ta to the upper magnetic layer provides the advantages of improving media SNR with good thermal stability. The lower magnetic layer can be a cobalt alloy having from 11 to 16 at. % platinum (Pt), from 15 to 25 at. % chromium (Cr), from 3 to 14 at. % boron (B) and from 0 to 2 at. % tantalum (Ta). The lower magnetic layer may also be a dual magnetic layer which is comprised of two magnetic sublayers with different compositions for improved recording properties, such as higher SNR, better OW, narrower PW50, higher resolution and better thermal stability. The laminated structure can be used in an embodiment which has a slave magnetic layer separated from the lower magnetic layer by an AFC spacer.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a symbolic illustration of the prior art showing the relationships between the head and associated components in a disk drive.
FIG. 2 is an illustration of a prior art layer structure for a magnetic thin film disk with which the magnetic layer stack of the invention can be used.
FIG. 3A is an illustration of a laminated magnetic layer stack comprising an upper magnetic layer and a lower magnetic layer for a magnetic thin film disk according to the prior art.
FIG. 3B is an illustration of a laminated magnetic layer stack comprising an upper magnetic layer and a lower magnetic layer, and the lower magnetic layer being antiferromagnetically coupled to an AFC slave magnetic layer through an AFC spacer layer according to the prior art.
FIG. 3C is an illustration of a laminated magnetic layer stack comprising an upper magnetic layer and a lower magnetic layer, and the lower magnetic layer being a dual magnetic layer according to the prior art.
FIG. 3D is an illustration of a laminated magnetic layer stack comprising an upper magnetic layer and a lower magnetic layer, and the lower magnetic layer being a dual magnetic layer and the lower magnetic layer being antiferromagnetically coupled to an AFC slave magnetic layer through an AFC spacer layer according to the prior art.
FIG. 4A is an illustration of a laminated magnetic layer stack without tantalum in the upper magnetic layer.
FIG. 4B is an illustration of a laminated magnetic layer stack with tantalum in the upper magnetic layer according to the invention.
FIG. 4C is an illustration of a laminated magnetic layer stack with the lower magnetic layer comprising first and second sublayers and without tantalum in the upper magnetic layer.
FIG. 4D is an illustration of a laminated magnetic layer stack with the lower magnetic layer comprising first and second sublayers and with tantalum in the upper magnetic layer according to the invention.
FIG. 5 is a graph of the S0NR of magnetic films according to the invention versus a prior art example.
DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENTS
FIG. 2 illustrates a prior art layer structure 21 of a thin film magnetic disk 16 in which the layer stack according to the invention can be used. The layers under the underlayer(s) 33 may be any of several combinations of seed layers 32 and pre-seed layers 31 as noted in more detail below. Useful pre-seed layers include, but are not limited to, amorphous or nanocrystalline CrTi, CrTiAl or CrTiY. Seed layers are crystalline and are typically used on nonmetallic substrates, but the invention can also be used with metallic substrates such as NiP-coated AlMg. Conventionally NiP-coated AlMg substrates are used with an underlayer structure 33 of Cr, Cr alloy or multiple Cr and Cr alloy layers which are sputter deposited directly onto the NiP. The invention is also not dependent on any particular underlayer being used, but CrTi is used in the preferred embodiment.
The layer structure shown in FIG. 2 can be used with a variety of magnetic layer stacks 34. For example, a laminated magnetic layer structure can be used as illustrated in FIG. 3A. In this structure there is an upper magnetic layer 36, a spacer layer 37, a lower magnetic layer 38 and an onset layer 39. The spacer layer 37 material and thickness are selected according to the prior art to substantially decouple the upper and lower magnetic layers. The preferred method for determining the thickness of the spacer layer is an empirical one in which tests are performed with varying thicknesses to determine the change in S0NR. For laminated media the S0NR will change in a gradual manner in a range of thicknesses before dropping sharply at a certain lower thickness. The spacer thickness is selected to be in the range where high S0NR is achieved. A typical thickness of the spacer layer is about 8 angstroms. The onset layer 39 which is included in the preferred embodiment is described in the prior art. The onset layer material used with the invention is preferably nonmagnetic or weakly ferromagnetic. A preferred material is CoCr having from 18 to 32 at. % Cr.
FIG. 3B illustrates a second example where a laminated magnetic layer structure is used with an AFC slave magnetic layer 42 separated from the lower magnetic layer 38 by an AFC spacer layer 41. The AFC slave layer 42 which is included in an embodiment is described in the prior art. The AFC slave magnetic layer 42 material used with the invention is preferably ferromagnetic. A preferred material for the AFC slave layer 42 is CoCr having from 6 to 27 at. % Cr. A preferred material for the AFC spacer layer 41 is Ru. In both FIGS. 3A and 3B, the lower magnetic layer can be a dual magnetic layer which is comprised of two magnetic sublayers, illustrated as 38A and 38B in FIGS. 3C and 3D respectively, with different compositions for improved recording properties, such as higher SNR, better OW, narrower PW50, higher resolution and better thermal stability.
Reduction of grain size and decoupling of the grains are helpful for increasing medium signal-to-noise ratio. However, these microstructural changes can degrade medium thermal stability. To alleviate the degradation, Ta is added in the upper magnetic layer 36 in the embodiments illustrated in FIGS. 3A, 3B, 3C and 3D. Ta pushes Cr to the grain boundaries from inside the grains. With such Cr enrichment at the grain boundaries, the grains are well decoupled without significant increase in B content. Increasing B could result in significant increase of thickness of amorphous grain boundaries and refinement of grain size. As a result of adding Ta, medium signal-to-noise ratio can be improved without sacrificing thermal stability.
FIG. 4A illustrates a disk, including a laminated magnetic layer stack 34. The magnetic layer nearest to the surface of the disk, the upper magnetic layer 36, is selected according to the prior art for laminated media. In a particular embodiment described below CoPt13Cr15B8 is used for the upper magnetic layer. The preferred spacer layer 37 is ruthenium. The lower magnetic layer 38 is CoPt13Cr20B5Ta1. In the sample embodiment the AFC spacer layer 41 is ruthenium, the AFC slave layer 42 is CoCr10, the underlayer 33 is CrTi20, the seed layer 32 is RuAl50 with a B2 structure and the preseed layer 31 is amorphous or nanocrystalline CrTi50.
FIG. 4B also illustrates a disk, including a laminated magnetic layer stack 34. The magnetic layer nearest to the surface of the disk, the upper magnetic layer 36, is selected to include Ta. In this embodiment CoPt14Cr14B10Ta1 is used for the upper magnetic layer 36. The preferred spacer layer 37 is ruthenium. The lower magnetic layer 38 is CoPt12Cr20B5Ta1. In the sample embodiment the AFC spacer layer 41 is ruthenium, the AFC slave layer 42 is CoCr10, the underlayer 33 is CrTi20, the seed layer 32 is RuAl50 with a B2 structure and the preseed layer 31 is amorphous or nanocrystalline CrTi50.
FIG. 5 indicates by using a Ta alloy as upper magnetic layer 36 shown in FIG. 4B, media S0NR can be improved over the prior art shown in FIG. 4A. Such SNR improvement results in an improved error rate from −5.2 to −5.4 order. Good media thermal stability is also maintained as indicated by an SNR decay rate of 0.92% per decade for a disk with a Ta alloy as the upper magnetic layer versus a 0.97% per decade decay rate for the prior art with a CoPtCrB alloy as the upper magnetic layer.
FIG. 4C illustrates a disk, including a laminated magnetic layer stack 34. The magnetic layer nearest to the surface of the disk, the upper magnetic layer 36, is selected according to the prior art for laminated media. In a particular embodiment described below CoPt13Cr15B8 is used for the upper magnetic layer 36. The preferred spacer layer 37 is ruthenium. The lower magnetic layer 38 is comprised of upper magnetic sublayer 38A, whose composition is CoPt13Cr11B15, and lower magnetic sublayer 38B, whose composition is CoPt13Cr25B6. In the sample embodiment the onset layer 39 is CoCr22, the underlayer 33 is CrTi22, the seed layer 32 is RuAl50 with a B2 structure and the preseed layer 31 is amorphous or nanocrystalline CrTi50.
FIG. 4D also illustrates a disk, including a laminated magnetic layer stack 34. The magnetic layer nearest to the surface of the disk, the upper magnetic layer 36, is selected to include Ta. In this embodiment CoPt14Cr15B10Ta1 is used for the upper magnetic layer. The preferred spacer layer 37 is ruthenium. The lower magnetic layer 38 is comprised of upper magnetic sublayer 38A, whose composition is CoPt14Cr11B15, and lower magnetic sublayer 38B, whose composition is CoPt13Cr25B6. In the sample embodiment the onset layer 39 is CoCr22, the underlayer 33 is CrTi22, the seed layer 32 is RuAl50 with a B2 structure and the preseed layer 31 is amorphous or nanocrystalline CrTi50.
When using a quinary magnetic alloy (such as CoPt14Cr15B10Ta1) as the top layer as shown in FIG. 4D, the media exhibits similar SNR and an increased AC squeeze of the medium compared to prior art shown in FIG. 4C, leading to narrowing of MCW (magnetic core width) which allows higher track density and therefore higher areal recording density.
For the invention, the upper layer 36 (nearest the air-bearing surface) is preferably a cobalt alloy having from 12 to 16 at. % platinum (Pt), from 11 to 20 at. % chromium (Cr), from 6 to 14 at. % boron (B) and from 0.5 to 2 at. % tantalum (Ta). One embodiment of the upper magnetic layer 36 is CoPt14Cr14B10Ta1. A second embodiment of the upper magnetic layer 36 is CoPt14Cr15B10Ta1.
In a first example, an embodiment of the invention, the lower magnetic layer is preferably a cobalt alloy having from 11 to 16 at. % platinum (Pt), from 15 to 25 at. % chromium (Cr), from 3 to 14 at. % boron (B) and from 0 to 2 at. % tantalum (Ta). One particular embodiment of the lower magnetic layer 38 is CoPt13Cr20B5Ta1.
In a second example embodiment of the invention, the lower magnetic layer is comprised of two magnetic sublayers. The upper magnetic sublayer 38A is preferably a cobalt alloy having relatively lower chromium and higher boron content in relation to the lower sublayer. The upper magnetic sublayer is preferably a cobalt alloy having from 9-17 at. % platinum (Pt), 9-15 at. % chromium (Cr), and 11-17 at. % boron (B). Optionally from 1 to 4 at. % of copper can be added to upper sublayer to possibly improve the SNR. The additional copper, if used, will reduce the cobalt content. The preferred thickness of the upper sublayer 38A is from 40-100 angstroms. One embodiment of the upper magnetic sublayer 38A is CoPt14Cr11B15. The lower magnetic sublayer 38B is preferably a cobalt alloy having higher chromium and lower boron content than the upper magnetic sublayer. The lower sublayer is preferably a cobalt alloy having from 9-17 at. % platinum (Pt), 20-28 at. % chromium (Cr), and 4-9 at. % boron (B). Optionally from 1 to 2 at. % of tantalum can be added to the lower sublayer to possibly improve segregation of the grains. The additional tantalum, if used, may reduce the cobalt content. The preferred thickness of the lower sublayer 38B is from 60-110 angstroms. Preferably the ratio of the thickness of the upper sublayer divided by the thickness of the lower sublayer should be from 0.35 to 2.5. One embodiment of the lower magnetic sublayer 38B is CoPt13Cr25B6.
The compositions of the upper and lower sublayers are selected to have properties that are different from each other and which would make either one not useful if used alone. The different properties of the sublayers combine to provide improved recording performance according to the invention. The upper sublayer composition is selected to have higher coercivity (Hc), narrower PW50 and higher resolution. The composition of the lower sublayer is selected for higher SNR, higher thermal stability and better overwrite.
The thin film structures described above can be formed using standard sputtering techniques. The films are sequentially sputter deposited with each film being deposited on the previous film. The upper magnetic layer 36 in the composition ranges given can be deposited with or without using negative substrate bias, ranging from approximately 0 to −300 volts. The upper sublayer 38A and lower sublayers 38B of the lower magnetic layer 38 in the composition ranges given are deposited using negative substrate bias from approximately −100 to −400 volts. The use of bias for these particular composition ranges increases coercivity and AC squeeze of the media, improves the crystallographic structure and grain boundary segregation.
The atomic percentage compositions given above are given without regard for the small amounts of contamination that invariably exist in sputtered thin films as is well known to those skilled in the art.
The invention has been described with respect to particular embodiments, but other uses and applications for the ferromagnetic structure according to the invention will be apparent to those skilled in the art.
Patent Application
20070037015
