Abstract:
A method includes: constructing a multilayer structure including a first layer of Pt, a first layer of A1 phase FePt on the first layer of Pt, and a second layer of Pt on the layer of FePt, and annealing the multilayer structure to convert the A1 phase FePt to L1 0  phase FePt.

Description:
BACKGROUND 
       [0001]    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/in 2  will require materials with magnetic anisotropies greater than conventional media materials. There are known bulk permanent magnetic materials having crystalline phases with magnetocrystalline anisotropy that theoretically can hold densities greater than 1 Tbit/in 2 . 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 fine, nanocrystalline, exchange decoupled or partially exchange decoupled grains while maintaining thermal stability. 
         [0002]    L 1   0  phase FePt binary alloys have magnetocrystalline anisotropy as high as 7×10 7  erg/cc, which is well suitable for future magnetic recording media to achieve density over 1 Tb/in 2 . However, FePt is typically deposited as the face centered cubic (fcc) phase (i.e., the A 1  phase) and subsequent annealing is needed to transform (i.e., chemically order) the material into the high anisotropy L 1   0  phase. 
         [0003]    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 be desirable to produce FePt media with a small grain size and with magnetic characteristics that are compatible with current writer technology. 
       SUMMARY 
       [0004]    In a first aspect, the invention provides a method including: constructing a multilayer structure including a first layer of Pt, a first layer of A 1  phase FePt on the first layer of Pt, and a second layer of Pt on the layer of FePt, and annealing the multilayer structure to convert the A 1  phase FePt to L 1   0  phase FePt. 
         [0005]    In another aspect, the invention provides a method including: constructing a multilayer structure including a first layer of Fe, a first layer of A 1  phase FePt on the first layer of Fe, and a second layer of Fe on the layer of FePt, and annealing the multilayer structure to convert the A 1  phase FePt to L 1   0  phase FePt. 
         [0006]    In another aspect, the invention provides a method including: constructing a multilayer structure including a first layer of Pt, a first layer of A 1  phase FePt on the first layer of Pt, and a first layer of Fe on the layer of FePt, and annealing the multilayer structure to produce a graded FePt composite structure with an anisotropy that decreases in a direction from a bottom of the multilayer structure to a top of the multilayer structure. 
         [0007]    In another aspect, the invention provides a method including: constructing a multilayer structure including a first layer of Fe, a first layer of A 1  phase FePt on the first layer of Fe, and a first layer of Pt on the layer of FePt, and annealing the multilayer structure to produce a graded FePt composite structure with an anisotropy that increases in a direction from a bottom of the multilayer structure to a top of the multilayer structure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a side elevation view of a material stack used in a first aspect of the invention. 
           [0009]      FIG. 2  is a graph of a typical hysteresis loop for materials that can be used in the material stack of  FIG. 1 . 
           [0010]      FIG. 3  is a side elevation view of another material stack used in another aspect of the invention. 
           [0011]      FIG. 4  is a graph of a typical hysteresis loop for materials that can be used in the material stack of  FIG. 3 . 
           [0012]      FIG. 5  is a schematic representation of a granular media. 
           [0013]      FIG. 6  is a schematic representation of a process for producing exchange coupled composite media. 
           [0014]      FIG. 7  is a schematic representation of a process for producing graded anisotropy media. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    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 A 1  phase FePt layer, and anneals the multilayer structure to convert the A 1  phase FePt to L 1   0  phase FePt at a relatively low anneal temperature. 
         [0016]      FIG. 1  is a side elevation view of a material stack  10  used in a first aspect of the invention. A multilayer structure  12  is formed on a substrate  14 . The multilayer structure includes a first layer  16  (e.g., a seed layer) of Pt, a first layer  18  of A 1  phase FePt on the first layer of Pt, and a second layer  20  (e.g., a cap layer) of Pt on the layer of FePt. The substrate can be, for example, glass, aluminum or its alloys, etc. 
         [0017]    The layers can be formed by physical vapor deposition methods, such as magnetron sputtering, pulsed laser deposition, or ion beam deposition. 
         [0018]    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. 
         [0019]    The multilayer structure is annealed to convert the A 1  phase FePt to L 1   0  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 A 1  phase FePt to L 1   0  phase FePt will depend on the layer thicknesses. 
         [0020]      FIG. 2  is a graph of magnetization versus applied magnetic field (i.e., a hysteresis loop) for a multilayer structure as shown in  FIG. 1  in which the FePt layer comprises Fe 62 Pt 38 . During annealing, there will be some diffusion between the layers, which will depend on annealing temperature used. 
         [0021]      FIG. 2  shows a coercivity of about 9130 Oe for the multilayer structure. The hysteresis loop was measured by monitoring the magnetic moment change while the applied field is sweeping from −18000 Oe to +18000 Oe and then swept backwards using a vibrating sample magnetometer (VSM) at room temperature. The magnetic field was applied with its direction parallel to the film plane, which is called an in-plane measurement, and the coercivity is then called the in-plane coercivity. 
         [0022]      FIG. 3  is a side elevation view of a material stack  30  used in another aspect of the invention. A multilayer structure  32  is formed on a substrate  34 . The multilayer structure includes a first layer  36  (e.g., a seed layer) of Fe, a first layer  38  of A 1  phase FePt on the first layer of Fe, and a second layer  40  (e.g., a cap layer) of Fe on the layer of FePt. The multilayer structure is annealed to convert the A 1  phase FePt to L 1   0  phase FePt. The substrate can be, for example, glass, aluminum or its alloys, etc. 
         [0023]    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. 
         [0024]    The multilayer structure is annealed to convert the A 1  phase FePt to L 1   0  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 A 1  phase FePt to L 1   0  phase FePt will depend on the layer thicknesses. 
         [0025]      FIG. 4  is a graph of magnetization versus applied magnetic field for a multilayer structure of  FIG. 3  in which the FePt layer comprises Fe 36 Pt 64 .  FIG. 4  shows a coercivity of about 1800 Oe for the multilayer structure. 
         [0026]    In the examples of  FIGS. 1 and 3 , atom diffusion at the two interfaces, between the seed layer and the FePt layer, and between the cap layer and FePt layer, facilitates the chemical ordering process for the whole stack. 
         [0027]    The platinum seed and cap combination of  FIG. 1  is suitable for an iron rich FePt alloy middle layer, as diffusion will drive the whole film towards the stoichiometric composition of 50 to 50. In one example using platinum as the seed and cap layer, an in-plane coercivity value of 9130 Oe has been achieved after annealing at a temperature as low as 300° C. Similarly, the iron seed and cap combination of  FIG. 3  is effective for ordering a platinum rich FePt alloy middle layer. 
         [0028]    Media grain size is reduced as the annealing temperature decreases. Thus the use of a low annealing temperature would limit the media grain size. A successful low temperature phase transformation method ensures small grain size for high recording areal density. 
         [0029]    Since the inter-diffusion occurs in a direction perpendicular to the plane of the films, grain isolation materials such as SiO 2 , 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 be applied either by embedding them into the target material, or through co-sputtering from a separate target in the same chamber. 
         [0030]      FIG. 5  is a schematic drawing of a media  42  having an FePt layer  44  between Pt seed and cap layers  46 ,  48 . Grain isolation material  50  is shown to be positioned between grains  52  in the stack. 
         [0031]    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 (ECC) media. ECC media include one magnetically hard phase (in this case low temperature ordered L 1   0  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. 
         [0032]    To make ECC media, L 1   0  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 L 1   0  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. 
         [0033]      FIG. 6  is a schematic representation of a process for producing exchange coupled composite media. The process starts with a material stack  54  including a FePt layer  56  sandwiched in between two Pt layers  58 ,  60 , which is annealed to convert the FePt to the L 1   0  phase in later  62 . Next, the cap layer is removed, for example, by plasma etching (e.g., sputtering) or ion beam milling (e.g., ion bombardment). Then a magnetically softer layer  64  is deposited onto the ordered FePt hard layer. Optionally, a thin layer  66  of Pt, Pd, or some non-magnetic material such as silicon containing material, can be deposited on the ordered FePt hard layer prior to the deposition of the magnetically softer layer to control the exchange coupling between the soft or hard layer. The bottom Pt partially diffuses into FePt, any leftover of Pt would be minimal so it should not affect media property. 
         [0034]      FIG. 6  shows the use of an iron rich middle layer that is converted to a stoichiometric composition of 50 to 50. Such conversion can be implemented by annealing or by elevated temperature deposition, wherein the media stack is deposited at temperatures higher than room temperature. 
         [0035]    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. 
         [0036]      FIG. 7  is a schematic representation of a process for producing graded anisotropy media. The process starts with a material stack  70  including an iron rich FePt layer  72  sandwiched in between a Pt seed layer  74  and a Fe cap layer  76 . The stack is annealed to produce a graded layer  78  having a substantially stoichiometric composition of 50 to 50 near the bottom and an iron rich composition near the top. Such conversion can also be implemented by elevated temperature deposition, wherein the media stack is deposited at temperatures higher than room temperature. 
         [0037]    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. 
         [0038]    FePt media anisotropy strongly depends on the composition, and maximizes around Fe 50-55 Pt 50-45 . The anisotropy decreases when the material composition is off stoichiometry. For the purposes of this description, perfect L 1   0  FePt stoichiometry refers to Fe 50-55 Pt 50-45 . 
         [0039]    With the process of  FIG. 7 , it is possible to establish gradient anisotropy media with a composition gradient. This can be, for example, achieved by using Pt diffusion from the bottom and Fe diffusion from the top layer, and results in close to equal stoichemistry at the bottom (high anisotropy K) and an Fe rich composition on the top (low anisotropy K). A similar approach can be implemented by sandwiching iron deficient FePt film between a Fe seed layer and a Pt cap layer, with Fe diffusion from the bottom and Pt diffusion from the top. 
         [0040]    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 be 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.