The Physics of Thermomagnetic Recording in Thin Films of Rare Earth-Transition Metal Alloys

The goal of this research is to better understand the dynamic phenomena occurring during the writing and erasure processes. A domain wall motion model is proposed. The relationship of the wall velocity to the effective pressure on the wall and the mobility is given. Computer simulation is used to directly calculate the transient position of the wall. In general, the motion of the domain wall is governed by the wall mobility. The domain wall mobility for amorphous alloys of rare earth-transition metal thin films has been measured at high temperatures by means of the dynamic bubble collapse method for films in the (Gd,Tb)(Fe,Co) system. The domain wall velocity is linearly proportional to the applied field if the field is significantly larger than the coercivity. The mobility increases slightly with temperature. Increasing the ratio of Gadolinium to Terbium significantly raises the value of the mobility. Local fluctuations of the magnetic properties in amorphous alloys, specifically the random anisotropy, cause irregularity in the domain shape and writing noise. A model for coercivity based on the pinning of domain walls by the randomly fluctuating anisotropy is proposed. This model is particularly suitable for materials containing non S-state rare earth ions like terbium. The new coercive force model is used to predict the domain shape during the growth process. Wall motion near the magnetization and angular momentum compensation temperatures is modeled. When a domain wall gets close to the compensation temperatures the structure of the wall and its dynamic properties are changed. In alloys having angular momentum compensation temperatures above the ambient temperature domain erasure can be achieved without an applied field. In some cases erasure is still possible even with a field applied in a direction favoring the write process. Anomalous wall dynamics, which occur when the angular momentum is compensated, are believed to be responsible for the erase process in single layer direct overwrite magneto-optical recording media. (Abstract shortened with permission of author.).