The present invention is generally related to the field of magnetic disk recording, and more specifically to perpendicular magnetic recording media having a single domain exchange-coupled laminated soft magnetic underlayer.
Perpendicular magnetic recording is a form of magnetic recording in which bits of information are stored in a direction that is perpendicular to the plane of the recording media, which is typically a rotating disk forming part of a disk drive. To obtain this orientation of the bit magnetization, the anisotropy constant of the magnetic recording layer is configured such that its “easy” magnetic axis is perpendicular to the plane of the media. The magnetization establishing each bit is imparted by a write head. The layer in which the bits are formed is typically a magnetically “hard” recording layer. (“Hardness” and “softness” in this context refers to the ability for producing saturation in a magnetic layer with increasing external magnetic fields. A soft layer can produce magnetic saturation significantly faster than a hard layer.) In order to provide an appropriate closed loop for the field created by the write head when writing each bit, the hard recording layer is usually formed atop a “soft” magnetic underlayer (“SUL”). While the role of the recording layer is to carry the individual bits of recorded data, the role of the SUL is to guide the magnetic write field flux perpendicularly through the recording layer and then through the SUL in a return path to the write head. Thus, radial magnetic orientation of an SUL is needed for improving magnetic flux return efficiency. The radial orientation of the SUL depends on relative values of the anisotropy constant field of the SUL material used, and the applied radial field strength during film deposition. In order to induce radial magnetic anisotropy, radial field strength is typically maintained greater than the anisotropy constant field of the SUL material. Furthermore, uniform radial magnetic field distribution along the radial direction during film formation is needed.
As formed, a soft magnetic film is generally comprised of multiple groups of dipoles. Within each group, those dipoles couple together in a preferential direction. Groups of dipoles with a common or net preferred direction are referred to as magnetic domains. Such multiple domains may arise from magnetic charge accumulation at the inner and outer edges of the disk. These magnetic charges create a demagnetizing field, which in turn generates edge domains and 180° domains when a demagnetizing field is greater than the coercivity of the SUL. It is well known that multiple magnetic domains within such media (e.g., within the SUL) lead to media noise. For example, recorded data is detected as transitions from a region having one magnetic orientation to a region having an opposite magnetic orientation. Boundaries between magnetic domains in the SUL include out-of-plane magnetization components. These magnitudes are significantly high compared to the signal of the magnetic recording layer. Thus, they are read as erroneous data, called spike noise. Thus, it is desirable to provide a magnetic recording media structure with minimal magnetic domains, ideally 1.
One such example of a multi-tiered SUL teaches a laminated structure including as-deposited ferromagnetic and anti-ferromagnetic materials. See U.S. Pat. No. 6,723,457, incorporated herein by reference. Unidirectional uncompensated spins of the anti-ferromagnetic materials are induced along the magnetization direction of SUL during film deposition or a post magnetic field annealing process. The single domain state of the SUL is achieved by an exchange coupling with the anti-ferromagnetic pinning layer and it is also independent of stray magnetic fields. However, anti-ferromagnetic material (e.g., IrMn, PtMn, etc.) with reasonable blocking temperature at which exchange bias field strength (Hex), defined as a horizontal loop shift of the magnetization-magnetic field (M-H) loop, becomes zero has historically been quite expensive, and tends to exhibit poor corrosion resistance because of the cost of Mn. Corrosion resistance has also proven difficult to obtain in such structures. Additionally, a magnetic field annealing process at 250-300° C. above a blocking temperature of anti-ferromagnetic material used is needed for better alignment of uncompensated spins on the surface of the anti-ferromagnetic material, which further reduces media noise.
To address the shortcomings of as-deposited ferromagnetic/anti-ferromagnetic laminates, SUL structures with evenly balanced anti-parallel ferromagnetic layers are currently being used in mass production. A synthetic anti-parallel (SAP) SUL laminate consists of two individual ferromagnetic layers sandwiching an Ru layer of a correct thickness (on the order of 6-8 {acute over (Å)}). In remanence, these layers remain magnetically anti-parallel, which results in perfect cancellation of magnetization. This configuration becomes magnetostatically stable, resulting in decreasing magnetic charges along the edge of a disk. Thereby it significantly reduces the number of magnetic domains compared to single ferromagnetic SUL. However, magnetic domains still remain. In SUL structures with evenly balanced anti-parallel ferromagnetic layers, magnetic switching priority depends on Zeeman energy (the energy of interaction between an atomic or molecular magnetic moment and an applied magnetic field) on each film, which for the purposes hereof may be given by Mr×He×V at a zero external field, where Mr is the remanent magnetization, Hc is the coercivity, and V is the volume of each layer of the SUL. Under the assumption of the same value of Hc on each soft ferromagnetic layer, Zeeman energy is proportional to film thickness (t) when one SUL material is used but it is proportional to Mrt (the product of Mr and the film thickness t) of each layer when different SUL materials are used. Soft ferromagnetic layers typically have low coercivity of less than 10 Oe and a high squareness ratio, with Ms≅Mr, where Ms is the saturation magnetization. Generally, the layer with lower Mst in synthetic anti-parallel (SAP) SULs has a higher magnetic switching priority than the layer with higher Mst assuming each has the same coercivity. Thus, such balanced SAP SULs cannot maintain a single domain state following removal of the magnetic field used in manufacturing due to the identical magnetic switching priority of the bottom and top ferromagnetic layers. Furthermore, low exchange coupling strength is observed when amorphous Co-based alloys are used. In order to improve exchange coupling strength, using CoFe-based alloys with high saturation magnetization is preferred but it deteriorates corrosion resistance.
In order to provide magnetic switching priority, SAP SULs with un-balanced Zeeman energy have been discussed in B. R. Acharya, et al., Anti-Parallel Coupled Soft Under Layers for High-Density Perpendicular Recording, IEEE Transactions on Magnetics, Vol. 40, No. 4, 2383, July 2004, incorporated herein by reference. Typically, a structure using two soft ferromagnetic layers sandwiching a Ru layer uses a thinner top ferromagnetic layer compared to the bottom ferromagnetic layer on top of NiP-plated substrate or adhesion layer when one soft magnetic material is used. The thinner top layer also provides higher exchange bias field strength for improving adjacent track erasure (ATE). A similar concept was used for a tri-layer SUL in U.S. Pat. No. 7,241,516, incorporated herein by reference, with first ferromagnetic layer thickness greater than second and third ferromagnetic layer thicknesses. The first and second soft ferromagnetic layers separated by a thin amorphous Ta layer ferromagnetically couple each other. The second and third ferromagnetic layers separated by a thin Ru layer anti-parallel couple. However, the first ferromagnetic layer with relatively thicker film thickness typically exhibits low coercivity of less than 10 Oe, which depends on both the kinds of soft magnetic materials and process conditions to be used. Magnetic domain states are controlled by coercivity in the first thicker ferromagnetic layer and stray field strength. Media with un-balanced SAP SULs demonstrate weak stray field robustness and magnetic domains are easily induced when external stray fields are higher than the Hc value in the first thicker ferromagnetic layer. Efforts have been made to develop pinning layers with anti-ferromagnetic layers, as discussed in U.S. Pat. No. 6,723,457, but such efforts have been unsatisfactory to date.
Another SUL structure with three soft ferromagnetic layers and two Ru coupling layers to induce anti-parallel exchange coupling between ferromagnetic layers is discussed in U.S. Pat. No. 7,166,375, incorporated herein by reference. This tri-layer SUL structure consisting of two soft magnetic materials of CoZr4Nb7 and FeAl9Si5 can increase exchange coupling strength of Hex and saturation field (Hs), defined as a field needed to saturate magnetization up to 95%, due to the contribution of two Ru layers. However, it does not consider magnetization cancellation: The product of saturation flux density (4 Ms) and film thickness (t) on each layer in the tri-layer SUL has the same value of 40 T nm, where the T unit means Tesla (1 T=10 kiloGauss (kG)). Degree of magnetic cancellation between ferromagnetic layers and magnetic domain configuration can not be controlled in this tri-layer SUL due to lack of magnetic switching priority.
Accordingly, there has not been a satisfactory solution to the problems of noise resulting from multi domain SUL, ATE, robustness of stray fields, and corrosion resistance. Therefore, there is needed in the art an arrangement, and process for making same, which yields a magnetic recording disk with single domain, high exchange coupling strength, and corrosion resistance, while still providing other desirable media attributes such as manufacturability and operational performance and longevity.
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