Introduction

Thermomagnetic recording involves the laser-assisted nucleation and growth of reverse-magnetized domains under the influence of external and/or internal magnetic fields. The nature of nucleation, the speed and uniformity of growth, and the local pinning of domain boundaries due to structural inhomogeneities of the material are among the factors that determine the final shape and size of recorded domains. Since it is important to control domain size, while avoiding nonuniformities and jagged boundaries, and since it is important to create stable domains, a knowledge of magnetization-reversal dynamics, including the effects of the nanostructure within the “amorphous” material, is desirable.

The observed magnetization reversal in RE–TM films is a nucleation and growth process. During bulk reversal (at temperatures not too close to the Curie point Tc) the nucleation sites are mostly reproducible. Rather than being driven by thermal fluctuations, nucleation is believed to be rooted in the nonuniform spatial distribution of the structural and magnetic properties of the media. The measured hysteresis loops generally have high squareness, which indicates that the coercivity of wall motion is less than the nucleation coercivity. In other words, once the nuclei are created at a certain applied field, they continue to grow without further hindrance. The recording and erasure processes rely on the creation and annihilation of reverse-magnetized domains. High densities are achieved when the recorded domains are small and uniform, have smooth boundaries, and are precisely positioned.