1. Field of the Invention
This invention relates generally to perpendicular magnetic recording media, such as perpendicular magnetic recording disks for use in magnetic recording hard disk drives, and more particularly to a perpendicular magnetic recording medium with laminated magnetic layers.
2. Description of the Related Art
Horizontal or longitudinal magnetic recording media, wherein the written or recorded bits are oriented generally parallel to the surfaces of the disk substrate and the planar recording layer, have been the conventional media used in magnetic recording hard disk drives. Perpendicular magnetic recording media, wherein the recorded bits are stored in the recording layer in a generally perpendicular or out-of-plane orientation (i.e., other than parallel to the surfaces of the disk substrate and the recording layer), provides a promising path toward ultra-high recording densities in magnetic recording hard disk drives. A common type of perpendicular magnetic recording system is one that uses a “dual-layer” medium. This type of system is shown in
The SUL serves as a flux return path for the field from the write pole to the return pole of the recording head. In
One type of conventional material for the RL is a granular polycrystalline ferromagnetic cobalt (Co) alloy, such as a CoPtCr alloy. The ferromagnetic grains of this material have a hexagonal-close-packed (hcp) crystalline structure and out-of-plane or perpendicular magnetic anisotropy as a result of the c-axis of the hcp crystalline structure being induced to grow generally perpendicular to the plane of the layer during deposition. To induce this epitaxial growth of the hcp RL, the EBL onto which the RL is formed is also typically an hcp material.
Both horizontal and perpendicular magnetic recording media that use recording layers of granular polycrystalline ferromagnetic Co alloys exhibit increasing intrinsic media noise with increasing linear recording density. Media noise arises from irregularities in the recorded magnetic transitions and results in random shifts of the readback signal peaks. High media noise leads to a high bit error rate (BER). Thus to obtain higher areal recording densities it is necessary to decrease the intrinsic media noise, i.e., increase the signal-to-noise ratio (SNR), of the recording media. The media SNR is to first order proportional to N1/2, where N is the number of magnetic grains per unit area in the media. Accordingly, increases in SNR can be accomplished by increasing N. The granular cobalt alloys in the RL structure should thus have a well-isolated fine-grain structure to reduce intergranular exchange-coupling, which is responsible for high intrinsic media noise. Enhancement of grain segregation in the cobalt alloy RL can be achieved by the addition of segregants, such as oxides of Si, Ta, Ti, Nb, Cr, V, and B. These segregants tend to precipitate to the grain boundaries, and together with the elements of the cobalt alloy, form nonmagnetic intergranular material. The addition of SiO2 to a CoPtCr granular alloy by sputter deposition from a CoPtCr—SiO2 composite target is described by H. Uwazumi, et al., “CoPtCr—SiO2 Granular Media for High-Density Perpendicular Recording”, IEEE Transactions on Magnetics, Vol. 39, No. 4, July 2003, pp. 1914-1918. The addition of Ta2O5 to a CoPt granular alloy is described by T. Chiba et al., “Structure and magnetic properties of Co—Pt—Ta2O5 film for perpendicular magnetic recording media”, Journal of Magnetism and Magnetic Materials, Vol. 287, February 2005, pp. 167-171.
Perpendicular magnetic recording media with RLs containing oxides or other segregants for improved SNR are subject to thermal decay. As the magnetic grains become smaller to achieve ultrahigh recording density they become more susceptible to magnetic decay, i.e., magnetized regions spontaneously lose their magnetization, resulting in loss of data. This is attributed to thermal activation of small magnetic grains (the superparamagnetic effect). The thermal stability of a magnetic grain is to a large extent determined by KuV, where Ku is the magnetic anisotropy constant of the magnetic recording layer and V is the volume of the magnetic grain. Thus a RL with a high Ku is important for thermal stability, although the Ku can not be so high as to prevent writing on the RL.
In horizontal recording media, the complete absence of intergranular exchange-coupling provides the best SNR. However, in perpendicular recording media the best SNR is achieved at some intermediate level of intergranular exchange-coupling in the RL. Also, intergranular exchange-coupling improves the thermal stability of the magnetization states in the media grains. Thus in perpendicular recording media, some level of intergranular exchange-coupling is advantageous. One approach for increasing the intergranular exchange-coupling is by adding a continuous ferromagnetic exchange-coupling layer (ECL), also called a “capping” layer, on top of the underlying oxide-containing granular Co alloy magnetic layer (MAG) to provide effective intergranular exchange-coupling among the segregated grains of the MAG. The ECL is typically a CoCr—, CoPtCr—, or CoPtCrB-based ferromagnetic alloy with no oxides or other segregants, while the MAG layer is a Co—, CoPt—, or CoPtCr-based alloy with an oxide or other segregants. This type of structure is described by Choe et al., “Perpendicular Recording CoPtCrO Composite Media With Performance Enhancement Capping Layer”, IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005, pp. 3172-3174.
While an RL formed of a MAG with an ECL allows for a tuning of the effective intergranular exchange-coupling in the underlying MAG, and results in improved recording performance, the overall intrinsic media noise level and thus the SNR improvement is limited by the media grain structure of the MAG.
What is needed is a perpendicular magnetic recording medium with an RL structure that takes advantage of effective intergranular exchange-coupling provided by an ECL, but wherein the SNR improvement is not limited by the media grain structure of the MAG.
This invention is a perpendicular magnetic recording layer (RL) structure with multiple granular ferromagnetic layers (MAGs) that are separated by ferromagnetic exchange-coupling layers (ECLs) as interlayers between the MAGs. The ECLs provide effective intergranular exchange-coupling in the MAGs. Each MAG is sufficiently thick to support independent recording states that are thermally stable, and does not rely on the overall RL thickness for thermal stability. For a structure with two independent MAGs, the number of grains per area is doubled, leading to an improvement in media signal-to-noise ratio (SNR). Each ECL has significant intralayer coupling of its grains. The material of the ECL may be a CoCr alloy, such as a CoCrPtB alloy, that preferably does not include any oxide or if it does, an amount substantially less than the amount of oxide in the MAG. The Cr and B in the ECL segregate to a much smaller extent than would an oxide, so that there are small segregation regions or sub-grains in the ECL that are exchange-coupled on a length scale smaller than the grain size. Due to the small length-scale of the segregation regions within the ECLs, as compared to the larger length-scale of the grains, the intralayer exchange coupling within the ECL has multiple weak spots for each individual MAG grain. Therefore, for each MAG grain, there exist a multitude of magnetic states corresponding to different transition positions in the ECL. These magnetic states are metastable and can be produced by a recording process, which in turn allows the RL structure to support a stable magnetization pattern with different magnetization states in adjacent MAGs. Thus, the magnetization states of the various MAGs may be fully correlated, but need not be fully correlated even though their granular structure is and all layers are ferromagnetically coupled together. This allows for a substantial reduction in media noise.
The RL structure may also include coupling layers (CLs) between the ECLs and adjacent MAGs to optimize the interlayer coupling strength. This will enable the correlated and non-correlated magnetization states of the MAGs to be equally likely to be populated, so that the reduced media noise can be achieved.
The invention is also a perpendicular magnetic recording system that includes the above-described medium and a magnetic recording write head.
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 prior art perpendicular magnetic recording medium wherein the RL includes an ECL on top of a single ferromagnetic layer (MAG) is depicted in schematic cross-section in
In this invention the RL structure comprises multiple granular ferromagnetic layers (MAGs) that are separated by exchange-coupling layers (ECLs) as interlayers between the MAGS to provide effective intergranular exchange-coupling in the MAGs. Due to the small length-scale of the segregation regions within the ECLs, as compared to the larger length-scale of the grains, the intralayer exchange coupling within the ECL has multiple weak spots for each individual media grain. Therefore, for each MAG grain, there exist a multitude of magnetic states corresponding to different transition positions in the ECL. These magnetic states are metastable and can be produced by a recording process, which in turn allows the RL structure to support a stable magnetization pattern with different magnetization states in adjacent MAGs. Thus, the magnetization states of the various MAGs may be fully correlated (as depicted in
The magnetization states of the bit transition in
In
The complete disk structure with the RL structure of
The RL structure of this invention may also include additional MAGs with additional ECLs between adjacent MAGs. For example, referring to
In this invention, it may be advantageous to introduce coupling layers (CLs) between the ECLs and adjacent MAGs to optimize the interlayer coupling strength, to either reduce or enhance it, so that the magnetization states in
Because each CL is below a MAG, it should be able to sustain the growth of the MAG, while mediating a weak ferromagnetic coupling between the adjacent ECLs or MAGs. Hexagonal-close-packed (hcp) materials for instance, which can mediate a weak ferromagnetic coupling and provide a good template for the growth of a MAG, are good candidates. Because the CL must enable an appropriate coupling strength, it should be either nonmagnetic or weakly ferromagnetic. Thus the CL may be formed of RuCo and RuCoCr alloys with sufficiently low Co content (< about 60 atomic percent), or CoCr and CoCrB alloys with high Cr and/or B content (Cr+B> about 30 atomic percent). Si-oxide or other oxides like oxides of Ta, Ti, Nb, Cr, V and B may be added to these alloys in an amount up to about 15 atomic percent. Depending on the choice of material for CL, and more particularly on the concentration of cobalt (Co) in the CL, the CL may have a thickness of less than 3.0 nm, and more preferably between about 0.2 nm and 1.5 nm, although in certain embodiments, the thickness of the CL may exceed 1.5 nm. Because Co is highly magnetic, a higher concentration of Co in the CL may be offset by thickening the CL to achieve an optimal interlayer exchange-coupling between the adjacent MAGs. The interlayer exchange-coupling between adjacent MAGs and ECLs may be optimized, in part, by adjusting the materials and thickness of the CLs and the ECLs. The CLs and the ECL together should provide a coupling strength that is small enough to not couple the adjacent MAGs rigidly together.
To achieve high performance perpendicular magnetic recording disks at ultra-high recording densities, e.g., greater than about 200 Gbits/in2, the RL should exhibit a coercivity Hc greater than about 4000 Oe and a nucleation field Hn greater (more negative) than about −1500 Oe. The nucleation field Hn is the reversing field, preferably in the second quadrant of the M-H hysteresis loop, at which the magnetization begins to drop from its saturation value (Ms). The more negative the nucleation field, the more stable the remanent magnetic state will be because a larger reversing field is required to alter the magnetization. Experimental structures like that depicted in
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.