Perpendicular magnetic recording, wherein the recorded bits are stored in the generally planar recording layer in a generally perpendicular or out-of-plane orientation (i.e., other than parallel to the surface of the recording layer), is a promising path toward ultra-high recording densities in magnetic recording systems, such as hard disk drives. The perpendicular magnetic recording layer is typically a continuous layer on the disk substrate, like in conventional perpendicular magnetic recording disk drives. However, magnetic recording disk drives with patterned perpendicular magnetic recording layers have been proposed to increase data density. In patterned media the perpendicular magnetic recording layer on the disk is patterned into small isolated data islands arranged in concentric data tracks. To produce the magnetic isolation of the patterned data islands, the magnetic moment of the spaces or regions between the data islands is destroyed or substantially reduced to render these regions essentially nonmagnetic. Alternatively, the media may be fabricated so that there is no magnetic material in the regions between the data islands.
A problem associated with continuous perpendicular magnetic recording media is the thermal instability of the recorded magnetization patterns. In continuous perpendicular magnetic recording layers, the magnetic material (or media) for the recording layer on the disk is chosen to have sufficient coercivity such that the magnetized data bits are written precisely and retain their magnetization state until written over by new data bits. As the areal data density (the number of bits that can be recorded on a unit surface area of the disk) increases, the magnetic grains that make up the data bits can be so small that they can be demagnetized simply from thermal instability or agitation within the magnetized bit (the so-called “superparamagnetic” effect). To avoid thermal instabilities of the stored magnetization, media with high magneto-crystalline anisotropy (KU) may be required. However, increasing KU in recording media also increases the switching field, H0, which is proportional to the ratio KU/MS, where MS is the saturation magnetization (the magnetic moment per unit volume). The switching field H0 is the field required to reverse the magnetization direction at short time scales on the order of 1 ns relevant for the data rates achieved in modern hard disk drives. For most magnetic media H0 is greater but of similar magnitude than the coercivity or coercive field HC of the material measured at time scales of 1 s that are easily accessible in magnetometry experiments. H0 cannot exceed the write field capability of the recording head, which currently is limited to about 15 kOe for perpendicular recording.
One approach to addressing this problem is thermally-assisted recording (TAR) using a magnetic recording disk like that described in U.S. Pat. No. 6,834,026 B2, assigned to the same assignee as this application. This disk has a bilayer medium of a high-coercivity, high-anisotropy ferromagnetic material like FePt as the storage or recording layer and a material like FeRh or Fe(RhX) (where X is Ir, Pt, Ru, Re or Os) as a “transition” layer that exhibits a transition or switch from antiferromagnetic to ferromagnetic (AF-F) at a transition temperature less than the Curie temperature of the high-coercivity, high-anisotropy material of the recording layer. The recording layer and the transition layer are ferromagnetically exchange-coupled when the transition layer is in its ferromagnetic state. To write data the bilayer medium is heated above the transition temperature of the transition layer with a separate heat source, such as a laser or electrically resistive heater. When the transition layer becomes ferromagnetic, the total magnetization of the bilayer is increased, and consequently the switching field required to reverse a magnetized bit is decreased without lowering the anisotropy of the recording layer. The magnetic bit pattern is recorded in both the recording layer and the transition layer. When the media is cooled to below the transition temperature of the transition layer, the transition layer becomes antiferromagnetic and the bit pattern remains in the high-anisotropy recording layer.
However, the FeRh or Fe(RhX) transition layer required for this type of TAR must be grown at high temperatures, i.e., greater than 500° C., and is difficult to deposit on the substrate in a manner that assures reliable and repeatable magnetic properties.
A problem associated with patterned perpendicular media is broadening of the switching field distribution (SFD). During the writing of an individual data island, the dipolar interaction of fields from neighboring data islands causes a relatively wide distribution of the switching field, i.e., the write field required to switch the magnetization of the data island from one state to the other state. The SFD broadens (that is, the bit-to-bit variation in the switching field increases) as the size of the data islands is reduced, which limits the achievable data density of patterned perpendicular media.
What is needed is improved perpendicular magnetic recording media, usable for either continuous or patterned media, that takes advantage of heating the recording layer to address the problems of thermal instability and SFD.