Patent Application
20120147718
1. Field of the Invention
This invention relates generally to patterned perpendicular magnetic recording media, such as disks for use in magnetic recording hard disk drives, and more particularly to patterned disks with data islands having improved magnetic recording properties.
2. Description of the Related Art
Magnetic recording hard disk drives with patterned magnetic recording media have been proposed to increase data density. In conventional continuous magnetic recording media, the magnetic recording layer is a continuous layer over the entire surface of the disk. In patterned media, also called bit-patterned media (BPM), the magnetic recording layer on the disk is patterned into small isolated data islands arranged in concentric data tracks. While BPM disks may be longitudinal magnetic recording disks, wherein the magnetization directions are parallel to or in the plane of the recording layer, perpendicular magnetic recording disks, wherein the magnetization directions are perpendicular to or out-of-the-plane of the recording layer, will likely be the choice for BPM because of the increased data density potential of perpendicular media. To produce the magnetic isolation of the patterned data islands, the magnetic moment of the spaces between the islands are destroyed or substantially reduced to render these spaces essentially nonmagnetic. Alternatively, the media may be fabricated so that there is no magnetic material in the spaces between the islands.
Nanoimprint lithography (NIL) has been proposed to form the desired pattern of islands on BPM disks. NIL is based on deforming an imprint resist layer by a master template or mold having the desired nano-scale pattern. The master template is made by a high-resolution lithography tool, such as an electron-beam tool. The substrate to be patterned may be a disk blank formed of an etchable material, like quartz, glass or silicon, or a disk blank with the magnetic recording layer, and any required underlayers, formed on it as continuous layers. Then the substrate is spin-coated with the imprint resist, such as a thermoplastic polymer, like poly-methylmethacrylate (PMMA). The polymer is then heated above its glass transition temperature. At that temperature, the thermoplastic resist becomes viscous and the nano-scale pattern is reproduced on the imprint resist by imprinting from the template at a relatively high pressure. Once the polymer is cooled, the template is removed from the imprint resist leaving an inverse nano-scale pattern of recesses and spaces on the imprint resist. As an alternative to thermal curing of a thermoplastic polymer, a polymer curable by ultraviolet (UV) light, such as MonoMat available from Molecular Imprints, Inc., can be used as the imprint resist. The patterned imprint resist layer is then used as an etch mask to form the desired pattern of islands in the underlying substrate.
The islands in BPM need to be sufficiently small and of sufficient magnetic quality to support high bit areal densities (e.g., 500 Gb/in2 and beyond). For example, to achieve a bit areal density of 1 Tb/in2, the data islands will have diameters approximately 15 to 20 nm with the nonmagnetic spaces between the islands having widths of about 10 to 15 nm. It is thus important that as the size of the islands decreases, the thermal stability of the islands is maintained.
Another critical issue for the development of BPM is that the switching field distribution (SFD) (i.e., the island-to-island variation of the coercive field) needs to be narrow enough to insure exact addressability of individual islands without overwriting adjacent islands. Ideally the SFD width would be zero, meaning that all the bits would switch at the same write field strength. The SFD has many origins, such as variations in the size, shape and spacing of the patterned islands, the intrinsic magnetic anisotropy distribution of the magnetic material used, and dipolar interactions between adjacent islands. Additionally, it has been found that the SFD broadens (that is, the bit-to-bit variation in the coercive field increases) as the size of the magnetic islands is reduced, which limits the achievable bit areal density of BPM.
Exchange-spring media, also called exchange-coupled composite (ECC) media, are known for perpendicular magnetic recording. An ECC perpendicular recording material is a composite of two or more ferromagnetically exchange-coupled magnetic layers with substantially different anisotropy fields (Hk). (The effective anisotropy field Hk of a ferromagnetic layer with uniaxial magnetic anisotropy Ku is essentially the magnetic field that needs to be applied along the hard axis to align the magnetization completely into the external field direction.) Magnetic simulation of this composite medium shows that in the presence of a uniform write field the magnetization of the lower-Hk layer will rotate first and assist in the reversal of the magnetization of the higher-Hk layer. This behavior is sometimes called the “exchange-spring” behavior. Various types of ECC media are described by R. H. Victora et al., “Composite Media for Perpendicular Magnetic Recording”, IEEE Trans MAG 41 (2), 537-542, February 2005; and J. P. Wang et al., “Composite media (dynamic tilted media) for magnetic recording”, Appl. Phys. Lett. 86 (14) Art. No. 142504, Apr. 4, 2005. Pending application Ser. Nos. 11/751,823 and 12/412,403, both assigned to the same assignee as this application, describe various types of perpendicular BPM with data islands formed of ECC material.
What is needed is a patterned perpendicular magnetic recording medium that has islands of ECC material with high thermal stability and a narrow SFD.
This invention relates to bit-patterned media (BPM) wherein the recording layer (RL) in the discrete magnetic islands is an exchange-coupled composite (ECC) structure with a high-Hk chemically-ordered FePt alloy lower layer, a lower-Hk Co/X laminate or multilayer (ML) upper layer with perpendicular magnetic anisotropy, wherein X is Pt, Pd or Ni, and an optional nonmagnetic separation layer or coupling layer (CL) between the FePt layer and the ML. The hard (high_Hk) FePt layer is preferably the chemically-ordered equiatomic binary alloy FePt based on the L10 phase, but may also be a pseudo-binary alloy based on the FePt L10 phase, e.g., (Fe(y)Pt(100−y))-X, where y is between about 45 and 55 atomic percent and the element X may be Ni, Au, Cu, Pd or Ag and is present in the range of between about 0% to about 20% atomic percent. The FePt alloy layer is sputter deposited onto a seed layer structure, like a CrRu/Pt bilayer, while the disk substrate is maintained at an elevated temperature to assure the high anisotropy field Hk is achieved. The high-temperature deposition, together with the CrRu/Pt seed layer structure, provides a very smooth surface for subsequent deposition of the ML (and optional CL). The ML is formed on the FePt layer (or on the optional CL) and comprises a series of Co/X bilayers, wherein X is Pt, Pd or Ni. The number of bilayers and the relative thicknesses of the Co and X layers are selected to achieve the desired magnetic properties, including the value of the anisotropy field Hk. The separate Co/X ML has by itself a very narrow switching field distribution (SFD), more narrow than the SFD for the FePt layer, so that the SFD of the composite RL has a narrow SFD. The ECC RL provides a strong readback signal due to the well defined perpendicular anisotropy of both the hard (high-Hk) FePt layer and the soft (lower-Hk) Co/X ML.
The ECC RL is used in the discrete data islands of perpendicular BPM disks that may have a soft magnetic underlayer (SUL) below the data islands to act as a flux return path for the magnetic write field, and an exchange break layer (EBL) between the SUL and the data islands to break the magnetic exchange coupling between the RL and the SUL. The ECC RL may also be used in the discrete data islands of perpendicular BPM disks in thermally-assisted recording (TAR) disk drives. In a TAR disk drive, a heat sink layer may be located below the data islands.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.
The patterned-media magnetic recording disk 200 includes a hard or rigid disk substrate and discrete data islands 30 of magnetizable material on the substrate. The data islands 30 are arranged in radially-spaced circular tracks 118, with only a few islands 30 and representative tracks 118 near the inner and outer diameters of disk 200 being shown in
BPM disks like that shown in
The hard disk substrate 201 may be any commercially available glass substrate, but may also be a conventional aluminum alloy with a NiP surface coating, or an alternative substrate, such as silicon, canasite or silicon-carbide. An adhesion layer or onset layer (OL) for the growth of the SUL may be an AlTi alloy or a similar material with a thickness of about 2-10 nm is deposited on substrate surface 202.
The SUL may be formed of magnetically permeable materials such as alloys of CoNiFe, FeCoB, CoCuFe, NiFe, FeAlSi, FeTaN, FeN, FeTaC, CoTaZr, CoFeTaZr, CoFeB, and CoZrNb. The SUL may also be a laminated or multilayered SUL formed of multiple soft magnetic films separated by nonmagnetic films, such as electrically conductive films of Al or CoCr. The SUL may also be a laminated or multilayered SUL formed of multiple soft magnetic films separated by interlayer films that mediate an antiferromagnetic coupling, such as Ru, Ir, or Cr or alloys thereof. The SUL may have a thickness in the range of about 5 to 50 nm.
An exchange-break layer (EBL) is typically located on top of the SUL. It acts to break the magnetic exchange coupling between the magnetically permeable films of the SUL and the RL and also serves to facilitate epitaxial growth of the RL. The EBL may not be necessary, but if used it can be a nonmagnetic titanium (Ti) layer; a non-electrically-conducting material such as Si, Ge and SiGe alloys; a metal such as Cr, Ru, W, Zr, Nb, Mo, V, Ta and Al; a metal alloy such as NiW, NiTa, CrTi and NiP; an amorphous carbon such as CNx, CHx and C; or oxides, nitrides or carbides of an element selected from the group consisting of Si, Al, Zr, Ti, and B. The EBL may have a thickness in the range of about 1 to 40 nm.
The disk of
In the patterned perpendicular media of this invention the RL in the discrete magnetic islands is an exchange-coupled composite (ECC) structure with a high-Hk chemically-ordered FePt alloy lower layer, a lower-Hk Co/X laminate or multilayer (ML) upper layer with perpendicular magnetic anisotropy, wherein X is Pt, Pd or Ni, and an optional nonmagnetic separation layer or coupling layer (CL) between the FePt layer and the ML.
The hard (high-Hk) layer 250 in the ECC structure is preferably the chemically-ordered equiatomic binary alloy FePt based on the L10 phase. Chemically-ordered alloys of FePt (and FePd) ordered in L10 are known for their high magneto-crystalline anisotropy and magnetization, properties that are desirable for high-density magnetic recording materials. The chemically-ordered FePt alloy, in its bulk form, is known as a face-centered tetragonal (FCT) L10-ordered phase material (also called a CuAu material). The c-axis of the L10 phase is the easy axis of magnetization and is oriented perpendicular to the disk substrate. The chemically-ordered FePt alloy layer 250 may also be a pseudo-binary alloy based on the FePt L10 phase, e.g., (Fe(y)Pt(100−y))-X, where y is between about 45 and 55 atomic percent and the element X may be Ni, Au, Cu, Pd or Ag and is present in the range of between about 0% to about 20% atomic percent. While the pseudo-binary alloy in general has similarly high anisotropy as the binary alloy FePt, it allows additional control over the magnetic and structural properties of the RL.
The chemically-ordered FePt alloy layer 250 is sputter deposited onto the seed layer structure 240 while the disk substrate is maintained at an elevated temperature, above 300° C. and preferably above 500° C. The high-temperature deposition assures the high anisotropy field Hk can be achieved. The anisotropy field is preferably between about 30 and 150 kOe. The temperature of the disk substrate can be gradually decreased during the deposition, for example from a starting temperature of about 600° C. to a final temperature of about 300° C., to provide an FePt layer 250 with a graded anisotropy field, with the anisotropy field decreasing with increased thickness. The high-temperature deposition together with the CrRu/Pt seed layer structure 240, provide a very smooth surface for subsequent deposition of the CL 260 and ML 270. The upper surface of the FePt layer 250 should have a root-mean-square (RMS) surface roughness of less than 1 nm. As an alternative method for forming the high-Hk FePt layer 250, sequential alternating layers of Fe and Pt can be deposited by sputter depositing from separate Fe and Pt targets, using a shutter to alternately cover the Fe and Pt targets, followed by annealing the resulting structure at about 300° C. to 700° C. for about 1-30 min. Rapid thermal annealing (RTA), wherein the annealing time is very short (about 2 to 60 seconds) and the temperature is ramped up very quickly, may also be used.
The Co/X ML 270 preferably has between 2 and 10 Co/X bilayers. The Hk in the Co/X ML is highest for thin Co layers in a thickness range of 0.1-0.4 nm. Also, Co/Ni bilayers will generally provide a lower Hk than Co/Pd and Co/Pt bilayers. In one example, a Co/Pd multilayer of 5 Co(0.28 nm)/Pd(0.9 nm) bilayers will have a Hk of about kOe and a Co/Ni multilayer of 3 Co(0.2 nm)/Ni(0.6 nm) bilayers will have a Hk of about 5 kOe. The Co/X ML is deposited by sequentially sputter depositing the Co and X layers at room temperature or temperatures below 200° C. for the desired time to produce the desired thicknesses. The anisotropy field of the ML is preferably between about 1 and 40 kOe.
The ECC RL of this invention provides a narrow SFD. Chemically-ordered FePt, when deposited at lower temperatures (less than about 400° C.) to achieve a lower Hk, does not have a narrow SFD. The anisotropy field distribution becomes very broad with many grains being in-plane, while other grains are still partially L10 ordered. Therefore ECC structures based solely on FePt, such as a graded Hk FePt layer or separate FePt layers with different values of Hk, both of which require a part of the FePt ECC structure to be deposited at a lower temperature, are not desirable.
In the RL of this invention the separate Co/X ML, which can be deposited at room temperature, is used as the soft layer and has a very narrow SFD by itself.
Perpendicular magnetic recording disks with BPM have been proposed primarily for use in conventional magnetic recording, wherein an inductive write head alone writes data to the islands. However, perpendicular BPM disks have also been proposed for use in heat-assisted recording, also called thermally-assisted recording (TAR). In a TAR system, an optical waveguide with a near-field transducer (NFT) directs heat from a radiation source, such as a laser, to heat localized regions of the magnetic recording layer on the disk. The radiation heats the magnetic material locally to near or above its Curie temperature to lower the coercivity enough for writing to occur by the inductive write head. The ECC RL of this invention is also applicable to perpendicular BPM disks for TAR disk drives.
When write-current is directed through coil 56, the write pole 52 directs magnetic flux to the data islands 30, as represented by arrow 80 directed to one of the data islands 30. The dashed line 17 with arrows shows the flux return path back to the return pole 54. The NFT 74 directs near-field radiation, as represented by wavy arrow 82, to the data islands 31 as the TAR disk 10′ moves in the direction 23 relative to the slider. The electric charge oscillations in the NFT heat the data islands 30 at the same time the data islands are exposed to the write field from the write pole 52. This raises the temperature of the magnetic recording material in the data islands to near or above its Curie temperature to thereby lower the coercivity of the material and enable the magnetization of the data island to be switched by the write field. When the ECC RL according to this invention is used in the data islands in a TAR disk drive, the anisotropy field of the FePt layer is preferably between about 30 and 150 kOe and the anisotropy field of the ML is preferably between about 1 and 40 kOe and less than the anisotropy field of the FePt layer. Also, it may be desirable to alloy the Co with Ni in the ML in the data islands, e.g., CoNi/X (X═Pt or Pt) bilayers. This will allow tuning the Curie temperature of the soft ML to optimize performance in a TAR disk drive.
While the present invention has been particularly shown and described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention. Accordingly, the disclosed invention is to be considered merely as illustrative and limited in scope only as specified in the appended claims.
