Perpendicular magnetic recording media with magnetic anisotropy gradient and local exchange coupling

Abstract

A perpendicular magnetic recording medium adapted for high recording density and high data recording rate comprises a non-magnetic substrate having at least one surface with a layer stack formed thereon, the layer stack including a perpendicular recording layer containing a plurality of columnar-shaped magnetic grains extending perpendicularly to the substrate surface for a length, with a first end distal the surface and a second end proximal the surface, wherein each of the magnetic grains has: (1) a gradient of perpendicular magnetic coercivity H.sub.k extending along its length between the first end and second ends; and (2) predetermined local exchange coupling strengths along the length.

Claims

1. A stack, comprising: a substrate having at least one surface; and a layer stack on the surface of said substrate, the layer stack including a perpendicular recording layer comprising a plurality of columnar-shaped magnetic grains extending perpendicularly between opposing surfaces of the recording layer, each columnar-shaped magnetic grain comprising a plurality of sub-layers including: a first sub-layer comprising a first magnetic material and having a first magnetic anisotropy field H.sub.k1; and a second sub-layer disposed on the first sub-layer, the second sub-layer comprising a second magnetic material and having a second magnetic anisotropy field H.sub.k2, wherein H.sub.k1>H.sub.k2, and the second material differs from the first material in one or both of composition and dopant concentration and wherein at least one of the first and second materials comprises Fe.

2. The stack of claim 1, wherein the second material comprises CoPt.

3. The stack of claim 1, wherein at least one of the first and second materials comprises Pt.

4. The stack of claim 1, wherein each of the sub-layers has a perpendicular magnetic anisotropy field that is greater than at least about 1000 Oe.

5. The stack of claim 1, further comprising: a third sub-layer disposed on the second sub-layer, the third sub-layer comprising a third magnetic material and having a third magnetic anisotropy field H.sub.k3, wherein H.sub.k1>H.sub.k2>H.sub.k3, and the third material differs from the first and second materials in one or both of composition and dopant concentration.

6. The stack of claim 5, wherein H.sub.U is between 8,000 and 20,000 Oe and H.sub.k3 is between 1,000 and 9,000 Oe.

7. The stack of claim 1, further comprising a non-magnetic, paramagnetic, or superparamagnetic spacer layer disposed at an interface between at least one pair of adjacent sub-layers for setting local exchange coupling between the adjacent sub-layers at a preselected strength.

8. The stack of claim 1, wherein the first sub-layer has a first thickness and the second sub-layer has a second thickness and the sum of the first and second thicknesses is less than local exchange coupling distances of the first and second magnetic materials.

9. A stack, comprising: a substrate having at least one surface; and a layer stack on the surface of said substrate, the layer stack including a perpendicular recording layer comprising a plurality of columnar-shaped magnetic grains extending perpendicularly between opposing surfaces of the recording layer, each columnar-shaped magnetic grain comprising a plurality of sub-layers including: a first sub-layer comprising a first magnetic material and having a first magnetic anisotropy field H.sub.k1; and a second sub-layer disposed on the first sub-layer, the second sub-layer comprising a second magnetic material and having a second magnetic anisotropy field H.sub.k2, wherein H.sub.k1>H.sub.k2 and wherein at least one of the first and second materials comprises Fe.

10. The stack of claim 9, wherein the first material and the second material have different compositions.

11. The stack of claim 10, wherein the second material comprises CoPt.

12. The stack of claim 9, wherein the second magnetic material differs from the first magnetic material in dopant concentration.

13. The stack of claim 9, wherein at least one of the first and second materials comprises Pt.

14. The stack of claim 9, wherein each of the sub-layers has a perpendicular magnetic anisotropy field that is greater than at least about 1000 Oe.

15. The stack of claim 9, wherein each of the sub-layers comprises a cobalt-based alloy.

16. An apparatus, comprising: a columnar-shaped magnetic grain comprising: a first magnetic layer comprising a first magnetic material and having a first magnetic anisotropy field, H.sub.k1; a first spacer layer adjacent to the first magnetic layer; a second magnetic layer adjacent to the first spacer layer, the second magnetic layer comprising a second magnetic material and having a second magnetic anisotropy field, H.sub.k2; a second spacer layer adjacent to the second magnetic layer; and a third magnetic layer adjacent to the second spacer layer, the third magnetic layer comprising a third magnetic material and having a third magnetic anisotropy field H.sub.k3, wherein each of the first, second, and third magnetic materials differs from the other magnetic materials in one or both of composition and dopant concentration, and H.sub.k1>H.sub.k2 >H.sub.k3, wherein each of the magnetic lavers has a perpendicular magnetic anisotropy field that is greater than at least about 1000 Oe.

17. The apparatus of claim 16, wherein at least one of the first, second, and third magnetic materials comprises Pt.

18. The apparatus of claim 16, wherein at least one of the first magnetic layer, the second magnetic layer, and the third magnetic layer comprises at least one of a Co alloy, alternating layers of a Co alloy and a Pt alloy, or alternating layers of a Co alloy and a Pd alloy.

19. The apparatus of claim 16, wherein H.sub.k1is between 8,000 and 20,000 Oe and H.sub.k3 is between 1,000 and 9,000 Oe.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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 various features are not necessarily drawn to scale but rather are drawn as to best illustrate the pertinent features and the same reference numerals are employed throughout for designating similar features, wherein:

(2) FIG. 1 schematically illustrates, in simplified cross-sectional view, a portion of a magnetic recording, storage, and retrieval system according to the conventional art, comprised of a perpendicular magnetic recording medium and a single pole transducer head;

(3) FIG. 2 schematically illustrates the magnetism reversal mechanism of the magnetically hard perpendicular magnetic recording layer of the conventional perpendicular medium of FIG. 1;

(4) FIG. 3 schematically shows the quasi-incoherent (buckling) magnetization reversal process of a magnetically hard perpendicular recording layer of a perpendicular medium according to the invention, comprised of a stack of 4 sub-layers;

(5) FIG. 4 schematically illustrates a perpendicular recording layer according to embodiments of the invention wherein a non-magnetic spacer layer positioned at the interface between adjacent sub-layers is utilized for tailoring the local exchange coupling strength between the sub-layers;

(6) FIG. 5 schematically illustrates a perpendicular recording layer according to embodiments of the invention wherein layers of soft magnetic material are positioned between adjacent sub-layers of magnetically hard material for tailoring the local exchange coupling strength between the sub-layers; and

(7) FIGS. 6(A)-6(B), respectively, graphically show the magnetization distributions at the written transitions in the case of large and small deviation angles of the easy axis of magnetic media.

DETAILED DESCRIPTION OF THE INVENTION

(8) The present invention is based upon recognition by the inventors that perpendicular magnetic recording media fabricated with a main recording layer comprised of columnar-shaped magnetic grains with specifically designed gradients of magnetic anisotropy, i.e., gradients of perpendicular magnetic coercivity (H.sub.k), and with local exchange coupling strengths which provide good writability and signal-to-noise ratio (SNR) at ultra-high recording densities (i.e., >250 Gbits/in.sup.2) and high data recording rates (i.e., >2,000 Mbits/sec.) without significant sacrifice in thermal stability of the media. Further, it has been determined that all significant performance parameters of the media can be controllably optimized via appropriate selection of the H.sub.k gradient and local exchange coupling strength(s).

(9) Briefly stated, perpendicular media and systems fabricated according to the principles of the invention are structurally similar to media 21 and system 20 shown in FIG. 1, except that the magnetically hard perpendicular magnetic recording layer 6 is replaced with recording layer 6, which, as indicated above, comprises a plurality of columnar-shaped magnetic grains with specifically designed gradients of magnetic anisotropy, i.e., gradients of perpendicular magnetic coercivity (H.sub.k), and with preselected local exchange coupling strengths.

(10) The principles of the present invention will now be described with reference to FIGS. 2 and 3, wherein FIG. 2 illustrates the magnetism reversal mechanisms of the magnetically hard perpendicular magnetic recording layer 6 of a conventional perpendicular medium (such as media 21) including a uniform magnetic grain of thickness and FIG. 3 shows the progressive, quasi-incoherent magnetization reversal process (such as a buckling process) of a magnetically hard perpendicular recording layer 6 of a perpendicular medium according to the invention. (In each of these figures, the direction of magnetization within a grain or sub-layer is indicated by the arrow, and optional interlayer 5 of FIG. 1 is omitted for clarity).

(11) More specifically, FIG. 2 shows the coherent magnetization reversal (rotation) process within a conventional (i.e., uniform) columnar-shaped magnetic grain which is effected by means of an externally applied magnetic field from a write head spaced at a distance d from the upper end of the magnetic grain (in the following the head field gradient is assumed to be Karlqvist type), where the initial magnetization direction prior to application of the external magnetic field is indicated at T=t.sub.o and the final magnetization direction after application of the external magnetic field is indicated at T=t.sub.1; whereas FIG. 3 shows the quasi-incoherent magnetization reversal process (such as a buckling process) of a magnetic grain according to the invention, comprised of 4 moderately exchange coupled, vertically stacked sub-layers layers M.sub.1, M.sub.2, M.sub.3, and M.sub.4 with respective thicknesses .sub.1, .sub.2, .sub.3, and .sub.4, and where the perpendicular magnetic coercivity H.sub.k progressively decreases from sub-layer M.sub.1 to sub-layer M.sub.4 (SUL 5 is omitted from the figure for clarity). As shown, the magnetization direction of each of the sub-layers M.sub.1 to M.sub.4 is the same at T=t.sub.o (i.e., the initial magnetization direction before application of an external magnetic field from the write head), and magnetization reversal (rotation) occurs quasi-incoherently in progressive stages illustrated at T=t.sub.1, T=t.sub.2, and T=t.sub.3. Complete reversal of the initial magnetization direction is indicated at T=t.sub.4.

(12) In the above, H.sub.k1>H.sub.k2>H.sub.k3>H.sub.k4, the topmost sub-layer M.sub.1 has the highest switching field, and it is assumed that the permeability of the underlying SUL is infinite. Assuming that no interlayer (e.g., such as layer 5 of FIG. 1) is present, according to the incoherent magnetization reversal process of the invention, the magnetization direction of sub-layers M.sub.1, M.sub.2, M.sub.3, and M.sub.4 occurs sequentially, as illustrated. The magnetization reversal process is initiated at the bottom-most sub-layer M.sub.4 of lowest perpendicular magnetic coercivity H.sub.k4, and proceeds upwardly in sequence from sub-layer M.sub.4 to the overlying sub-layers M.sub.3 and M.sub.2 of progressively lower perpendicular magnetic coercivities H.sub.k3 and H.sub.k2, and is ultimately controlled by the topmost sub-layer M.sub.1 of greatest perpendicular magnetic coercivity H.sub.k1. More particularly, re-orientation or reversal of the magnetization direction of the entire grain occurs when the magnetization direction of the topmost sub-layer M.sub.1 is completely reversed (as at T=t.sub.4 in the illustrated case).

(13) Stated differently, when the magnetic grains are comprised of sub-layers with a coercivity gradient, application of the external writing field from the head causes the sub-layer with the smallest perpendicular magnetic coercivity H.sub.k, i.e., the lowermost sub-layer of the stack, to switch or reverse its magnetization direction. This substantially simultaneously induces a quasi-incoherent rotation process in the overlying sub-layers of higher magnetic coercivity. The magnetization reversal process in each grain is essentially an incoherent rotation process, i.e., a type of induced quasi-buckling or curling process, which is generated in the lowermost sub-layer with high magnetic moment and relatively lower intrinsic coercivity, via tailored exchange interactions. By contrast, coherent magnetization reversal in conventional magnetic grains requires a larger switching field and poor media writability, resulting in difficulty in obtaining high density recording with good writability and thermal stability.

(14) It is also noted that, with the materials conventionally utilized for fabricating high performance magnetic recording media, the intrinsic exchange coupling within the media is usually too strong to allow for any incoherent magnetization reversal as required by the invention. Therefore, according to the inventive methodology, multi-step incoherent magnetization reversal within the grains is facilitated by suitably tailoring the perpendicular magnetic coercivity of the various sub-layers to obtain a desired gradient of H.sub.k and the local exchange coupling strengths between adjacent sub-layers. In this regard, it is noted that the strength of the exchange coupling between adjacent sub-layers and the thickness of each sub-layer play important roles in dictating the overall magnetization reversal/re-orientation process. For example, if the exchange coupling strength is too small, the overall magnetization reversal/re-orientation process can become a quasi-fanning process which does not afford good thermal stability. On the other hand, if the thickness of the lower-most sub-layer is too little, triggering of the magnetization reversal process would not be strong enough to induce incoherent magnetization reversal in the overlying layers if the exchange coupling strength is too high. Tailoring of the local exchange coupling strength between adjacent sub-layers is therefore necessary in order to achieve maximum local magnetization reversal torque, and for significantly reducing the overall switching field of each grain.

(15) According to the invention, tailoring of the local exchange coupling strength between adjacent sub-layers to achieve a desired coupling strength is accomplished by utilizing one or more of the following approaches: (1) positioning a non-magnetic, paramagnetic, or superparamagnetic spacer layer SL of selected thickness at the interface between adjacent sub-layers, as schematically shown in FIG. 4; (2) forming the adjacent sub-layers in direct contact; and (3) positioning a magnetic layer of selected thickness between adjacent magnetically hard sub-layers, as schematically shown in FIG. 5 for a grain structure comprised of n stacked sub-layers of magnetically hard material with intervening magnetic layers.

(16) It should be noted that despite apparent differences of approaches (1)-(3), the underlying physics is equivalent, because the fundamental magnetic properties of the magnetic material are associated with the dimensionality of the material per se. For quasi one-dimensional and two-dimensional thin-film magnetic materials, the weak spin effects will lead to reductions in the anisotropy, magnetic moment, and local exchange coupling strength. Approach (3) has an advantage in that continuity of the microstructure of the magnetic grains is more easily maintained. Finally, manipulation of the sub-layer thicknesses allows obtaining of desirable local magnetic properties for achieving optimal recording performance.

(17) Tailoring of the magnetic anisotropies, i.e., the perpendicular magnetic coercivities H.sub.k, of each of the sub-layers is accomplished, in known fashion, as by appropriate selection of the magnetic alloys and their processing conditions; and each of the sub-layers and spacer layers are sequentially epitaxially deposited (by conventional methodologies, including sputtering techniques) so as to replicate the crystal structure and cross-sectional dimensions of the underlying grains (i.e., grain sizes) and form a magnetically hard perpendicular recording layer comprised of columnar-shaped magnetic grains extending perpendicularly to a substrate for a desired length. Granular perpendicular magnetic recording layers embodying the principles of the present invention may be formed by means of reactive sputtering techniques, as known in the art and described above.

(18) Advantageously, when the magnetization reversal process is incoherent according to the invention, the read/write head spacing is reduced, as compared with the head-media spacing (HMS) with conventional coherent magnetization reversal. More specifically, in the incoherent case (FIG. 3), the HMS is given by (d+.sub.1/2), which is much smaller than the d spacing in the coherent case (FIG. 2), which is given by (d+.sub.2). For instance, if d=6 nm and =20 nm in the conventional, coherent reversal case, and .sub.1=5 nm in the incoherent reversal case, the effective HMS would be 8.5 nm in the incoherent case and 16 nm in the coherent case. As a consequence, the SNR's of the inventive coercivity gradient grains and conventional, uniform grains will be dramatically different. For example, it is conservatively estimated that use of a 3 sub-layer perpendicular magnetic recording layer with coercivity gradient according to the invention would provide at least a 1-3 db increase in SNR (facilitating a corresponding increase in recording density) by virtue of the dramatic decrease in HMS afforded by the invention.

(19) It should be noted that the head field magnetic gradient should be less than the gradient of magnetic coercivity of the various sub-layers constituting the magnetic grains, which requirement places several constraints on media design practice, resulting in significant reduction of the effective head-media spacing (HMS), and thus providing a very substantial improvement in recording performance. In addition, it should be recognized that coercivity gradient perpendicular media fabricated according to the invention can also advantageously exhibit substantially reduced easy axis distributions by virtue of the presence of several sub-layers within a single columnar-shaped magnetic grain, leading to a reduction of the media switching distribution and an increase in the media nucleation field. In this regard, the number of sub-layers within a grain is not limited to the illustrative embodiments described below which comprise 2, 3, or 4 sub-layers. Rather, the greater the number of sub-layers within a grain, the smaller the deviation angle of the easy axis. As a consequence, the resultant magnetization becomes sharper and/or more symmetric at the written transition locations. For example, FIGS. 6(A)-6(B), respectively, graphically show the magnetization distributions at the written transitions in the case of large and small deviation angles of the easy axis, wherein it is evident that the resultant magnetization becomes sharper and/or more symmetric at the written transition locations when the deviation angles of the easy axis are smaller.

(20) Additional advantages of the inventive media include reduced grain size distributions and the ability to fabricate granular media with ultra-small grain sizes via reactive oxidation/sputtering processing.

(21) According to an illustrative, but non-limitative, embodiment of the invention, each of the columnar-shaped magnetic grains comprises two overlying sub-layers with different magnetic material composition. A first sub-layer at the first (upper) end of each of the magnetic grains is comprised of CoCrX.sub.1 first magnetic material, where X.sub.1 is at least one element selected from the group consisting of Ta, Pt, B, V, C, Nd, Cu, Zr, Fe, P, O, Si, and Ni, with a magnetic moment M.sub.r from about 200 to about 800 emu/cc, a relatively high perpendicular coercivity H.sub.k from about 8,000 to about 20,000 Oe, a thickness .sub.1 from about 6 to about 25 nm, and a grain size from about 4 to about 10 nm. A second sub-layer at the second (lower) end of each of the magnetic grains is comprised of CoX.sub.2 second magnetic material, where X.sub.2 is at least one element selected from the group consisting of C, B, Cr, Pt, O, Fe, Ta, Cu, Nd, Ni, and Ti, with a magnetic moment M.sub.r from about 400 to about 900 emu/cc, a relatively low perpendicular coercivity H.sub.k from about 1,000 to about 9,000 Oe, a thickness .sub.2 from about 3 to about 15 nm, and a crystal structure and grain size matching those of the first sub-layer. The total thickness .sub.1+.sub.2 of the first and second sub-layers is less than the exchange coupling distances of the magnetic materials, whereby domain walls are not present in the magnetic grains. According to this embodiment, a non-magnetic spacer layer is present at an interface between the first and second sub-layers for providing an interfacial coupling strength between the first and second sub-layers from about 10.sup.2 to about 10.sup.9 erg/cm, the spacer layer having a thickness up to about 5 nm and comprised of at least one non-magnetic element selected from the group consisting of Cr, Pt, Cu, Zr, V, C, Ru, Ta, and Si.

(22) In accordance with another illustrative, non-limitative embodiment according to the present invention, each of the magnetic grains comprises three overlying sub-layers with different magnetic material composition, wherein the relatively high perpendicular magnetic coercivity H.sub.k1 of a first sublayer at the first (upper) end is about 12,000 Oe, the relatively low perpendicular magnetic coercivity H.sub.k3 of a third sub-layer at the second (lower) end is about 3,000 Oe, and the perpendicular magnetic coercivity H.sub.k2 of a second sub-layer intermediate the first and third sub-layers is about 9,000 Oe. The thickness of each of the three sub-layers is about 6-8 nm.

(23) According to yet another illustrative, non-limitative embodiment of the present invention, each of the magnetic grains comprises four overlying sub-layers with different magnetic material compositions. The relatively high perpendicular magnetic coercivity H.sub.k1 of a first sub-layer at the first (upper) end of the columnar-shaped magnetic grains is about 12,000 Oe, and the relatively low perpendicular magnetic coercivity H.sub.k4 of a fourth sub-layer at the second (lower) end is about 3,000 Oe. The perpendicular magnetic coercivity H.sub.k2 of a second sub-layer adjacent the first sub-layer is about 9,000 Oe, and the perpendicular magnetic coercivity H.sub.k3 of a third sub-layer adjacent the second sub-layer is about 6,000 Oe. The thickness of each of the four sub-layers is about 5 nm.

(24) It is noted that, while magnetic materials with coercivity values less than about 500 Oe are typically (or normally) characterized as soft magnetic materials and magnetic materials with coercivity values greater than about 2,000 Oe are typically characterized as hard magnetic materials, all magnetic materials utilized in the present invention have a large intrinsic coercivity, i.e., >3,000 Oe, and thus would normally be characterized as hard magnetic materials. Notwithstanding this characterization, the difference or variation between the intrinsic coercivities and anisotropies of the component magnetic materials of media according to the present invention can be fairly large, depending upon the purpose or ultimate use of the media design and the recording head field gradient. The invention, therefore, is conceptually different from merely combining hard and soft magnetic materials to form a recording medium. Rather, according to the underlying principle of the present invention, tailoring of the gradient of intrinsic perpendicular magnetic coercivity/anisotropy, as well as the local exchange coupling strengths of the perpendicular media are utilized in conjunction with the recording head field strength to provide the media with maximum gain in SNR, thermal stability, and writability. Optimized media designs facilitated by the present invention afford the smallest actual effective head-media spacing (HMS), highest actual magnetic volume K.sub.V, and highest achievable writability at the effective volume K.sub.V.

(25) In summary, the present invention provides perpendicular magnetic recording media fabricated with a main recording layer comprised of columnar-shaped magnetic grains with specifically designed gradients of magnetic anisotropy, i.e., gradients of perpendicular magnetic coercivity (H.sub.k), and with local exchange coupling strength(s) which provide good writability and signal-to-noise ratio (SNR) at ultra-high recording densities (i.e., >250 Gbits/in.sup.2) and high data recording rates (i.e., >2,000 Mbits/sec.) without significant sacrifice in thermal stability of the media. In addition, when the magnetization reversal process is incoherent according to the invention, the read/write head spacing is reduced, as compared with the head-media spacing (HMS) with conventional coherent magnetization reversal, resulting in the improved SNR's, i.e., at least a 1-3 db increase in SNR facilitating a corresponding increase in recording density.

(26) 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.

(27) 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.