Characterization of coercivity at a recording speed of granular media for thermally assisted recording

As a fundamental study of thermally assisted recording, remanence coercivity at recording speed and the energy barrier for magnetization switching for granular media were evaluated as a function of temperature T. We characterized those for CoPtCr-oxide granular media as a trial examination. Remanence coercivity was measured at measurement time t′ of 1 s (using an electromagnet) and 10^-5 s (pulse field); they were defined, respectively, as H_r and H r P for T of 300-473 K. Assuming that the value of t′ at a recording speed is 1 ns and the value of remanence coercivity at the recording speed, H_r(1 ns), was evaluated by fitting H_r and H r P to Sharrock's equation. Moreover, the values of the energy barrier for magnetization switching, δE, were estimated. The value of H_r(1 ns) at T 573 K, which is a typical temperature for thermally assisted recording, as estimated by extrapolation was about 6.2 kOe for the single medium and 5.5 kOe for the stacked medium. These values were about 1 order of magnitude larger than those of H_r at T = 573 K. Moreover, the reduction ratio of δE on increasing T showed good agreement with that of perpendicular uniaxial magnetic anisotropy, K_u. These results indicate that a large K_u reduction on increasing T was necessary to enhance the reduction of recording coercivity at elevated recording temperatures.