COERCIVITY OF MAGNETOOPTICAL MEDIA BY SPIN DYNAMICS

Spin dynamics computer simulations have been carried out to study the effect of pinning on domain-wall motion in TbFeCo-like media. These calculations were done on a 30×30×1 mesh, where the spin direction at each lattice site was calculated with the Landau-Lifshitz-Gilbert equation. The simulations were made in an IBM 3090 mainframe-personal computer environment where the result of the calculation is a movie that runs at three frames/second on an AT and shows graphically the domain-wall-defect interaction. The domain wall is caused to move in an external field toward a defect, and the maximum field that pins the domain wall was observed. The defects have finite length and zero magnetization, which correspond to voids or nonmagnetic second phase in the media. The simulation shows that small defects on the order of 100 Å in size can pin walls with pinning strength appropriate to the coercivity of magneto-optical media, i.e., local coercivities in the range 1-10 kOe. For sufficiently high fields a single wall may break up into two separate sections at the defect, and then join together beyond the defect to become a single wall again. For rectangular defects, the coercivity depends strongly and nearly linearly on defect length (parallel to the domain-wall surface) and only weakly on defect width for widths greater than about 50 Å (perpendicular to the wall surface).