BRIEF DESCRIPTION OF THE DRAWINGS
The following detailed description of the embodiments of the present invention can best be understood when read in conjunction with the following drawings, in which the same reference numerals are employed throughout for designating the same or similar features, and wherein the various features are not necessarily drawn to scale but rather are drawn as to best illustrate the pertinent features, wherein:
FIG. 1 schematically illustrates, in simplified cross-sectional view, a portion of a conventional magnetic recording, storage, and retrieval system comprised of a conventionally structured perpendicular magnetic recording medium and a single-pole magnetic transducer head;
FIG. 2 is a graph illustrating the variation of the net area (thus crystallinity) and 2θ position (in degrees) of the [110] peak of FeCo films as a function of the amount (in at. %) of Zr added to the FeCo films;
FIG. 3 is a graph illustrating the variation of surface roughness (in run) of FeCoZr films as a function of the amount (in at. %) of Zr added to the FeCo films, as well as the surface roughness of a FeCo film with about 13 at. % B added thereto;
FIG. 4 is a graph illustrating the variation of the experimentally measured and estimated saturation magnetizations Ms (in Teslas, T) of FeCoZr and CoZr films as a function of the amount (in at. %) of Zr added thereto;
FIG. 5 is a graph illustrating the variation of the edge corrosion (in %) of FeCoX and FeCoBX films (where X═Zr, Ru, Rh, Cr, or Pt) as a function of the amount (in at. %) of element X added thereto;
FIG. 6 is a graph illustrating the variation of the polarization resistance (thus corrosion resistance) of FeCoB films as a function of the amount of B (in at. %) added thereto and the variation of the polarization resistance of FeCoBX films (where X═Cr or Ru) as a function of the amount (in at. %) of Cr or Ru added thereto; and
FIG. 7 schematically illustrates, in simplified cross-sectional view, a perpendicular magnetic recording medium structured according to the present invention.
DESCRIPTION OF THE INVENTION
The present invention is based upon recognition by the inventors that the previously described drawbacks and disadvantages of CoFe-based alloy materials utilized as SUL's in high performance, high areal recording density perpendicular magnetic recording media, can be eliminated, or at least substantially reduced, by appropriate selection and control of the amount of alloying element(s) added thereto.
Specifically, the present inventors have determined that amorphous FeCo-based SUL's may be prepared which have a significantly lower surface roughness than conventional crystalline FeCo-based SUL's, which low surface roughness is required for optimal growth of the at least one magnetically hard recording layer thereover and for minimizing the transducer head-to-media spacing (“HMS”) in high performance, high areal density perpendicular magnetic recording media such as described above with reference to FIG. 1. In addition, the present inventors have developed amorphous FeCo-based SUL materials with compositions selected to provide substantially increased resistance to corrosion (relative to differently composed FeCo-based SUL materials), thereby facilitating fabrication of further improved performance perpendicular magnetic media which are free of corrosion-induced degradation over time.
Briefly stated, the present inventors have determined that improved magnetically soft materials comprising FeCo-based alloys are obtained by appropriate selection of the alloy compositions as to provide:
- (a) an amorphous microstructure with a smooth surface;
- (b) high saturation magnetization Ms greater than about 1.6 T; and
- (c) maximum corrosion resistance relative to differently composed FeCo-based SUL materials (as determined via techniques described in detail below).
According to certain preferred embodiments of the present invention, the FeCo-based alloy is an FeCoZr or FeCoZrX alloy, where X is Ta, Nb, Cr, Ru, Rh, or Pt. Preferably, the FeCoZr or FeCoZrX alloy contains more than about 9 at. % Zr or more than about 9 at. % Zr; whereas, according to certain other preferred embodiments of the present invention, the FeCo-based alloy is an FeCoBY alloy, where Y is Cr, Ru, Pt, or Rh and the FeCoBY alloy contains more than about 13 at. % Cr, Ru, Pt, or Rh, or more than about 10 at. % Cr, Ru, Pt, or Rh.
Referring now to FIG. 2, which is a graph illustrating the variation of the net area (thus crystallinity) and 2θ position (in degrees) of the [110] peak of FeCo films as a function of the amount (in at. %) of Zr added to the FeCo films, it is observed that addition of Zr to the FeCo films results in expansion of the crystal lattice, with a shift in the [110] peak (as measured by the 2θ position in degrees) to lower angles, and a loss of crystallinity. In particular, when the amount of Zr added to the CoFe films exceeds from about 6 to about 9 at. %, the films are essentially amorphous. (As defined herein and employed in the appended claims, the expression “amorphous” refers to materials having no long-range order as defined according to conventional principles of crystallography, and may include materials containing nanocrystals. However, while broad peak(s) may be exhibited in X-ray diffraction spectra of the material, sharp peak(s) resulting from crystalline structure is (are) not exhibited in the X-ray diffraction spectra).
Adverting to FIG. 3, shown therein is a graph illustrating the variation of surface roughness (in nm) of FeCoZr films as a function of the amount (in at. %) of Zr added to the FeCo films, as well as the surface roughness of a FeCo film with about 13 at. % B added thereto. As is evident from the graph, the surface roughness of the FeCoZr films decreases with increasing amount of Zr atoms added thereto, with low surface roughness achieved when the Zr content is at least about 6 at. %, with even lower surface roughness achieved when the Zr content is at or above 9 at. %. In addition, FIG. 3 indicates that FeCoB films containing 13 at. % B also exhibit very low surface roughness less than about 0.4 nm. (As defined herein and employed in the appended claims, the expression “smooth surface” refers to CoFe-based alloy materials, e.g., FeCoZr, with surface roughness, measured in nm, which is at least 50% less than that of CoFe).
With reference to the graph of FIG. 4, illustrated therein is the variation of experimentally measured and estimated saturation magnetizations Ms (in Teslas, T) of FeCoZr and CoZr films as a function of the amount (in at. %) of Zr added thereto. According to the results shown therein, addition of Zr atoms to FeCoZr and CoZr films reduces Ms by about 0.06/atom, and the Ms values of the FeCoZr films are consistently about 0.4 T to about 0.5 T larger than the Ms values of CoZr films, indicating greater utility of the FeCoZr films as SUL's in perpendicular magnetic recording media by virtue of their high Ms values (e.g., >˜1.6 T for FeCoZr films containing >˜9 at. % Zr).
Referring to FIG. 5, shown therein is a graph illustrating the variation of the “edge corrosion” (in %) of FeCoX and FeCoBX films (where X═Zr, Ru, Rh, Cr, or Pt) as a function of the amount (in at. %) of element X added thereto. The expression “edge corrosion” refers to the formation of corrosion-induced defects in perpendicular magnetic recording media when the media are exposed to a vapor of 0.5N HCl for 24 hrs. in an enclosed chamber. Perpendicular media having metal constituent layers which are prone to corrosion are vulnerable to formation of this type of defects. Specifically, when the HCl vapor attacks the edges of the media, the metal layers are corroded. Other, non-corroded layers of the media relieve any stress in the media, and gas bubbles are formed in the corroded areas due to hydrogen gas evolution caused by the corrosion process. The bubbles eventually burst when excessive pressure builds up, resulting in a unique morphology of the corroded areas at the media edges. After exposure to HCl vapors, the edge corrosion defects are identified by means of an optical microscope scanned 360° around the edge of the media, and the percent coverage of the defects over the entire circumference is measured.
According to FIG. 5, it is evident that edge corrosion of FeCoX amorphous films or layers is reduced when X═Zr and substantially eliminated when 9 at. % Zr is contained therein, thereby providing significantly enhanced corrosion resistance vis-a-vis differently composed FeCo-based SUL materials. In addition, the data of FIG. 5 reveal that edge corrosion of FeCoBX amorphous films or layers is also reduced when X═Cr and substantially eliminated when ˜10 at. % Cr is contained in therein, again demonstrating the enhanced corrosion resistance of FeCo-based SUL materials according to the present invention.
In view of the foregoing, it is seen that addition of from about 6 to about 9 at. % Zr to FeCo reduces the surface roughness of the layers from about 0.9 nm to a smooth surface having a significantly lower roughness nm while simultaneously improving the corrosion resistance and incurring an acceptable reduction in Ms from about 2.4 T to a still high value of about 1.8 T. By contrast, currently available FeCoBCr and CoZr-based SUL materials are susceptible to corrosion, and have similar surface roughness as the FeCoZr materials of the present invention, but a substantially lower Ms value of about 1.2 T.
Referring now to FIG. 6, shown therein is a graph illustrating the variation of the “polarization resistance” of FeCoB films as a function of the amount of B (in at. %) added thereto and of FeCoBCr and FeCoBRu films as a function of the amount (in at. %) of Cr or Ru added thereto. According to the “polarization resistance” electrochemical-based technique, the FeCo-based film and Pt-coated Nb serve as anode (test electrode) and cathode, respectively, in a 0.1 N NaCl electrolyte. According to the “electrochemical impedance spectroscopy” (“EIS”) corrosion measurement technique, a constant potential difference is applied between the anode and cathode, e.g., up to 200 mV above the open circuit potential, and a small amplitude AC potential (e.g., 10 mV) is applied to the anode and cathode at frequencies ranging from low (mHz) to high (MHz) frequencies. The resultant AC impedance is measured, and the “polarization resistance” component of the test electrode is deduced using a simple electrical model. When a potential is applied between the FeCo test electrode and the Pt-coated Nb electrode, the test electrode is “polarized”, and the resultant current is proportional to the corrosion rate of the test electrode. That is, for a given applied voltage, if the resultant current is large, the corrosion rate of the test sample is large, and vice versa. Stated differently, when the corrosion current is large, the polarization resistance is low, and vice versa.
The data of FIG. 6 indicate that for FeCoBCr and FeCoBRu films or layers, polarization resistance, hence corrosion resistance, increases with the amount of Cr or Ru in the films or layers. More specifically, when the amount of Cr or Ru exceeds about 13 at. %, the FeCoBCr and FeCoBRu films or layers are essentially corrosion resistant, i.e., they exhibit substantially enhanced corrosion resistance vis-a-vis other, differently composed FeCo-based SUL materials, e.g., those indicated in the figure. On the other hand, addition of Zr to the FeCo-based films or layers did not substantially change the polarization resistance over a fairly wide range of variation of Zr content.
With reference to FIG. 7, schematically illustrated therein, in simplified cross-sectional view, is a portion of a magnetic recording medium 11 according to an illustrative, but non-limitative, embodiment of the present invention. More specifically, medium 11 according to the present invention generally resembles the conventional perpendicular medium 1 of FIG. 1, and comprises a series of thin film layers arranged in an overlying (i.e., stacked) sequence on a non-magnetic substrate 2 comprised of a non-magnetic material selected from the group consisting of: Al, Al-Mg alloys, other Al-based alloys, NiP-plated Al or Al-based alloys, glass, ceramics, glass-ceramics, polymeric materials, and composites or laminates of these materials.
The thickness of substrate 2 is not critical; however, in the case of magnetic recording media for use in hard disk applications, substrate 2 must be of a thickness sufficient to provide the necessary rigidity. Substrate 2 typically comprises Al or an Al-based alloy, e.g., an Al-Mg alloy, or glass or glass-ceramics, and, in the case of Al-based substrates, includes a plating layer, typically of NiP, on the surface of substrate 2 (not shown in the figure for illustrative simplicity). An optional adhesion layer 3, typically a less than about 100 Å thick layer of an amorphous metallic material or a fine-grained material, such as a metal or a metal alloy material, e.g., Ti, a Ti-based alloy, Ta, a Ta-based alloy, Cr, or a Cr-based alloy, may be formed over the surface of substrate surface 2 or the NiP plating layer thereon.
Overlying substrate 2 or optional adhesion layer 3 is a thin magnetically soft underlayer (SUL) 4′ which comprises a layer of a material from about 50 to about 300 nm thick formed of an FeCo-based alloy material as described in detail above, having a composition selected to provide: (1) an amorphous microstructure with a smooth surface in contact with an overlying non-magnetic interlayer 5; (2) high saturation magnetization Ms greater than about 1.6 T; and (3) enhanced corrosion resistance. According to certain preferred embodiments of the present invention, the FeCo-based alloy is an FeCoZr or FeCoZrX alloy, where X is Ta, Nb, Cr, Ru, Rh, or Pt and the FeCoZr or FeCoZrX alloy contains more than about 9 at. % Zr or more than about 6 at. % Zr; whereas, according to certain other preferred embodiments of the present invention, the FeCo-based alloy is an FeCoBY alloy, where Y is Cr, Ru, Pt, or Rh and the FeCoBY alloy contains more than about 13 at. % Cr, Ru, Pt, or Rh or more than about 10 at. % Cr, Ru, Pt, or Rh.
As before, an optional adhesion layer 3 may be included in the layer stack of medium 11 between the surface of substrate surface 2 and the SUL 4′, the adhesion layer 3 being less than about 200 Å thick and comprised of a metal or a metal alloy material such as Ti, a Ti-based alloy, Ta, a Ta-based alloy, Cr, or a Cr-based alloy.
Still referring to FIG. 7, the layer stack of medium 11 further comprises a non-magnetic interlayer stack 5 between SUL 4′ and at least one overlying perpendicular magnetic recording layer 6, which interlayer stack 5 is comprised of optional seed layer 5A, and interlayer 5B for facilitating a preferred perpendicular growth orientation of the overlying at least one perpendicular magnetic recording layer 6. Suitable non-magnetic materials for use as interlayer 5B adjacent the magnetically hard perpendicular recording layer 6 include hcp materials, such as Ta/Ru, TaX/RuY (where X═Ti or Ta and Y═Cr, Mo, W, B, Nb, Zr, Hf, or Re), Ru/CoCrZ (where CoCrZ is non-magnetic and Z=Pr, Ru, Ta, Nb, Zr, W, or Mo); suitable materials for use as optional seed layer 5A typically include an amorphous or fine-grained material, such as Ta, TaW, CrTa, Ti, TiN, TiW, or TiCr.
According to embodiments of the present invention, the at least one magnetically hard perpendicular magnetic recording layer(s) 6 is (are) typically comprised of (an) about 10 to about 25 nm thick layer(s) of Co-based alloy(s) including one or more elements selected from the group consisting of Cr, Fe, Ta, Ni, Mo, Pt, W, Cr, Ru, Ti, Si, O, V, Nb, Ge, B, and Pd. Preferably, the at least one perpendicular magnetic recording layer 6 comprises a fine-grained hcp Co-based alloy with a preferred c-axis perpendicular growth orientation; and the interlayer stack 5′ comprises a fine-grained hcp material with a preferred c-axis perpendicular growth orientation. In addition, the at least one perpendicular magnetic recording layer 6 is preferably comprised of at least partially isolated, uniformly sized and composed, magnetic particles or grains with c-axis growth orientation.
Finally, the layer stack of medium 11 includes a protective overcoat layer 7 above the at least one perpendicular magnetic recording layer 6 and a lubricant topcoat layer 8 over the protective overcoat layer 7. Preferably, the protective overcoat layer 7 comprises a carbon-based material, e.g., diamond-like carbon (“DLC”), and the lubricant topcoat layer 8 comprises a fluoropolymer material, e.g., a perfluoropolyether compound.
According to the invention, each of the layers 3, 4′, 5′, 6, 7, as well as the optional seed and adhesion layers (not shown in the figure for illustrative simplicity), may be deposited or otherwise formed by any suitable technique utilized for formation of thin film layers, e.g., any suitable physical vapor deposition (“PVD”) technique, including but not limited to, sputtering, vacuum evaporation, ion plating, cathodic arc deposition (“CAD”), etc., or by any combination of various PVD techniques. The lubricant topcoat layer 8 may be provided over the upper surface of the protective overcoat layer 7 in any convenient manner, e.g., as by dipping the thus-formed medium into a liquid bath containing a solution of the lubricant compound.
Thus, the present invention advantageously provides improved performance, high areal density, magnetic alloy-based perpendicular magnetic media and data/information recording, storage, and retrieval systems, which media include an improved, soft magnetic underlayers (SUL's) which afford improved performance characteristics by virtue of their smooth surfaces, very high Ms values, and enhanced corrosion resistance. The media of the present invention enjoy particular utility in high recording density systems for computer-related applications. In addition, the inventive media can be fabricated by means of conventional media manufacturing technologies, e.g., sputtering.
In the previous description, numerous specific details are set forth, such as specific materials, structures, processes, etc., in order to provide a better understanding of the present invention. However, the present invention can be practiced without resorting to the details specifically set forth. In other instances, well-known processing materials and techniques have not been described in detail in order not to unnecessarily obscure the present invention.
Only the preferred embodiments of the present invention and but a few examples of its versatility are shown and described in the present disclosure. It is to be understood that the present invention is capable of use in various other combinations and environments and is susceptible of changes and/or modifications within the scope of the inventive concept as expressed herein.
Patent Application
20080085427
