Coercivity mechanisms in magneto-optical recording media

The sources for the nucleation coercivity and wall motion coercivity in magneto‐optical recording thin films are investigated based on the Connection Machine simulations. It was assumed that the thin films consist of nanoscale patches which are magnetic structure other than columnar structure or crystal grains. The postulated inhomogeneities have not been observed directly, but we believe that magnetic structure must exist in view of the fact that the media suitable for practical applications do not have columnar structure and crystal grains, yet they exhibit coercivity phenomena which can only be caused by nanoscale inhomogeneities. The nucleation and wall motion processes were simulated in the patchy films with different types of inhomogeneities and with the average patch size ranging from 60 to 200 A. The simulation results show that the nucleation coercivity depends sensitively on the patch size characterizing the fluctuation of the anisotropy constant, but weakly on the exchange at the patch borders and the fluctuation of easy‐axis orientation. To account for the observed nucleation coercivity in magneto‐optical thin films the average patch size should be on the order of 100 A. In contrast, the wall motion coercivity is mostly caused by the fluctuation in the exchange stiffness constant and the patch‐to‐patch variations of the easy‐axis orientation. It was found that the domain wall, when encountering a high anisotropy region, first encircles it and then reverses its magnetization due to the domain wall force. This suggests a scenario other than thermal fluctuation by which a wall can go over a local energy barrier.