Materials with increased magnetic anisotropies are desirable for various applications such as, for example, applications in the data storage industry where there is a continuous need to increase storage densities. Data storage media that can hold densities approaching 1 Tbit/in2 will require materials with magnetic anisotropies greater than conventional media materials. There are known hulk permanent magnetic materials having crystalline phases with magnetocrystalline anisotropy that theoretically can hold densities greater than 1 Tbit/in2. For bulk permanent magnetic materials, special heat treatments are typically used to control the phase formation and microstructure to optimize the material properties. In order to incorporate these materials into a data storage media, the correct crystalline phase must be obtained within a microstructure of tine, nanocrystalline, exchange decoupled or partially exchange decoupled grains while maintaining thermal stability.
L10 phase FePt binary alloys have magnetocrystalline anisotropy as high as 7×107 erg/cc, which is well suitable for future magnetic recording media to achieve density over 1 Tb/in2. However, FePt is typically deposited as the face centered cubic (fcc) phase (i.e., the A1 phase) and subsequent annealing is needed to transform (i.e., chemically order) the material into the high anisotropy L10 phase.
This high temperature processing is likely to enhance grain growth. which is opposite to the small grain size requirement for high density recording. On the other hand, fully ordered FePt media generally have a coercivity over 4 Tesla, which is beyond current writer technology capabilities. It would he desirable to produce FePt media with a small grain size d with magnetic characteristics that c compatible with current writer technology.
Various embodiments are generally directed to a multilayer structure and method for making the same. In accordance with some embodiments, a multilayer structure comprises a first layer of Fe, a layer or A1 phase FePt on the first layer of Fe. and a second layer of Fe on the layer of FePt. The multilayer structure is annealed to convert the A1 phase FePt to L1o phase FePt.
In a first aspect, the invention provides a method of fabricating a data storage media that uses a multilayer structure including seed and cap layers of either Fe or Pt on opposite sides of an A1 phase FePt layer, and anneals the multilayer structure to convert the A1 phase FePt to L1o, phase FePt at a relatively low anneal temperature.
The layers can be formed by physical vapor deposition methods, such as magnetron sputtering, pulsed laser deposition, or ion beam deposition.
In one example, layer 16 can have a thickness ranging from about 0.5 nm to about 2 nm. Layer 18 can have a thickness ranging from about 2 nm to about 10 nm. Layer 20 can have a thickness ranging from about 0 nm to about 5 nm.
The multilayer structure is annealed to convert the A1 phase FePt to L1o phase FePt. In one example, the annealing step can be performed at about 300° C. for 4 hours. In other examples, the minimum annealing temperature can be in a range from about 200° C. to about 500° C. The annealing temperature needed to convert the A1 phase FePt to L1o phase FePt will depend on the layer thicknesses.
In one example, layer 36 can have a thickness ranging from about 0.5 nm to about 2 nm. Layer 38 can have a thickness ranging from about 2 nm to about 10 nm. Layer 40 can have a thickness ranging from about 0 nm to about 5 nm.
The multilayer structure is annealed to convert the A1 phase FePt to L1o phase FePt. In one example, the annealing step can be performed at about 300° C. for 4 hours. In other examples, the minimum annealing temperature can he in a range from about 200° C. to about 500° C. The annealing temperature needed to convert the A1 phase FePt to L1o phase FePt will depend on the layer thicknesses.
In the examples of
The platinum seed and cap combination of
Media grain size is reduced as the annealing temperature decreases. Thus the use of a low annealing temperature would limit the media grain site. A successful low temperature phase transformation method ensures small grain size for high recording areal density.
Since the inter-diffusion occurs in a direction perpendicular to the plane of the films, grain isolation materials such as SiO2, carbon, boron, or other oxide or nitride material can be applied to keep the FePt grain size within several nm. These grain isolation materials can he applied either by embedding them into the target material. or through co-sputtering from a separate target in the same chamber.
In another aspect, the invention provides data storage media with enhanced writability. One way to enhance writability is to form so-called Exchange-Coupled Composite (FCC) media. FCC media include one magnetically hard phase (in this case low temperature ordered L10 FePt) and one magnetically soft phase, which can be disordered FePt or other high magnetization materials such as FeNi, FeCo, etc. As used in this description. a magnetically hard material is a material that typically has a coercive force higher than 2000 Oe, and a magnetically soft material is a material that typically has a coercive force lower than 2000 Oe.
To make FCC media, L10 FePt is prepared at low temperature to keep the grain size small. as described above. Then the top Pt (or Fe) cap layer is removed and the soft phase material is deposited on the L10 FePt. An optional thin exchange coupling control layer of another material can be deposited before the soft phase material such that the exchange coupling control layer is positioned between the hard and soft layers to tune the interlayer exchange coupling strength. The exchange coupling control layer can be, for example, Pt, PtSi, Pd, or PdSi.
In another aspect, the invention provides graded anisotropy (GA) media, which has been proposed theoretically to address the writability issue. Inter-diffusion of iron and platinum atoms is used to produce a composition gradient as well as a chemical ordering gradient, which in turn provides an anisotropy gradient.
It is well known that a concentration gradient can help the diffusion of atoms. In the annealing process, thermal energy helps to create vacancies and enhance the mobility of atoms to allow for atomic reorganization. The gradient can be tuned by adjusting seed and cap thickness, as well as by adjusting the annealing temperature or time.
FePt media anisotropy strongly depends on the composition, and maximizes around Fe50-55Pt50-45. The anisotropy decreases when the material composition is off stoichiometry. For the purposes of this description, perfect L10 FePt stoichiometry refers to Fe50-55Pt50-45.
With the process of
While the invention has been described in terms of several examples, it will be apparent to those skilled in the art that various changes can he made to the disclosed examples, without departing from the scope of the invention as set forth in the following claims. The implementations described above and other implementations are within the scope of the following claims.
This application is a divisional of copending U.S. patent application Ser. No. 12/369,844 tiled on Feb. 12. 2009.
Number | Date | Country | |
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Parent | 12369844 | Feb 2009 | US |
Child | 13418167 | US |