The increasing demands for higher areal recording density impose increasingly greater demands on thin film magnetic recording media in terms of coercivity (Hc), remanent coercivity (Hcr), magnetic remanance (Mr), which is the magnetic moment per unit volume of ferromagnetic material, coercivity squareness (S*), signal-to-medium noise ratio (SMNR), and thermal stability of the media. These parameters are important to the recording performance and depend primarily on the microstructure of the materials of the media. For example, as the SMNR is reduced by decreasing the grain size or reducing exchange coupling between grains, it has been observed that the thermal stability of the media decreases.
The requirements for high areal density, i.e., higher than 30 Gb/in2, impose increasingly greater requirements on magnetic recording media in terms of coercivity, remanent squareness, medium noise, track recording performance and thermal stability. It is difficult to produce a magnetic recording medium satisfying such demanding requirements, particularly a high-density magnetic rigid disk medium for longitudinal and perpendicular recording.
As the storage density of magnetic recording disks has increased, the product of Mr and the magnetic layer thickness t has decreased and Hcr of the magnetic layer has increased. This has led to a decrease in the ratio Mrt/Hcr. To achieve a reduction in Mrt, the thickness t of the magnetic layer has been reduced, but only to a limit because the magnetization in the layer becomes susceptible to thermal decay. This decay has been attributed to thermal activation of small magnetic grains (the super-paramagnetic 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 layer and V is the volume of the magnetic grain. As the magnetic layer thickness is decreased, V decreases. Thus, if the magnetic layer thickness is too thin, the stored magnetic information might no longer be stable at normal disk drive operating conditions.
The increase in Ku is limited to the point where the coercivity Hc, which is approximately equal to Ku/Mr, becomes too large to be written by a conventional recording head. On the other hand, a reduction in Mr of the magnetic layer for a fixed layer thickness is limited by the coercivity that can be written. Increasing V by increasing inter-granular exchange can also increase thermal stability. However, this approach could result in a reduction in the SMNR of the magnetic layer.
Some attempts have been made to solve the above-mentioned problem of thermal stability. For example, U.S. Pat. No. 5,462,796 (Teng) teaches a laminated longitudinal magnetic recording medium with Cr-containing non-magnetic layer between two magnetic layers. This medium exhibits a lower medium noise than that of a medium without the Cr-containing interlayer. However, when the medium Mrt is below 0.6 memu/cm2, the laminated medium has very poor thermal stability, which will be shown below. As recording density increases to about 30 Gb/in2, medium Mrt has been reduced to about 0.35 memu/cm2. Regular laminated medium can not be used in such low Mrt regime due to thermal stability issue.
In order to squeeze as much digital information as possible on a recording disc medium there is a need to find a film structure, which can benefit the low noise feature of laminated medium, but has acceptable thermal stability. Furthermore, in order to obtain high enough signal output, and reduce the medium noise of the medium with anti-ferromagnetic stabilization layers, further improvement of the medium is necessary.