Recently, the recording density of hard disk driver is increasing rapidly and around year 2000,areal recording density is expected to approach up to 10Gbit/$inch^2$. In order to achieve higher recording densities, it is necessary to reduce the grain size of recording medium. As the grains size decreases, magnetic aftereffects due to thermal fluctuation may play significant role on stability of written bits. The purpose of this study is to investigate thermal stability of written bits as a function of time in perpendicular magnetic recording medium at the recording density of 10Gbits/$inch^2$ region. Micromagnetic and Monte Carlo method were employed as the simulation tool. In part 1, we have simulated hysteresis loops with varying exchange coupling constants($C^*$) between grains. In part 2, we have studied the thermal stability of di-bit by varying the exchange coupling constants($C^*$), the exposed temperature(T) and the film thickness(t).
When C* was zero, the magnetization reversal by thermal fluctuation was done randomly or individual basis near the center of the bits. In the case of $C^*$=0.04, the magnetization reversal was done collectively near the center of the bits representing domain wall motion behaviour. However, when $C^*$=0.08, the reversal started at the transition region but number of reversed grains did not increase with time indicating a good thermal stability at room temperature. As the exposed temperature increases from 300K to 350K, magnetization reversal proceeds more rapidly. However, when the exchange coupling constant is higher, magnetization reversal becomes very much reduced even at 350K.
In order to investigate film thickness effect, the film of three different thicknesses, namely t=200Å, t=300Å and 400Å were studied. As the thickness increases from 200Å to 300Å, the magnetization reversal decreased substantially.
In the present study, it was found that the thermal stability of the perpendicular magnetic recording media at 10Gbits/$inch^2$ areal density was sustained when the $K_uV/kT$ was greater than approximately 80 in the case of no exchange coupled grains. This is a considerably larger value than the usually quoted superparamagnetic limit, $K_uV/kT$=25. But in the case of the medium of weak exchange coupled grains, the thermal stability has kept when $K_uV/kT$ is 70.
