Introduction

The process of magnetization reversal in thin magnetic films is of considerable importance in erasable optical data storage. The success of thermomagnetic recording and erasure depends on the reliable and repeatable reversal of magnetization in micron-sized areas within the storage medium. A major factor usually encountered in descriptions of the thermomagnetic write and erase processes is the coercivity of the magnetic material. Technically, the coercivity Hc is defined for a hysteresis loop as the value of the applied field at which the net magnetization becomes zero. Coercivity, however, is an ill-defined concept which may be useful in the phenomenology of bulk reversal, but its relevance to the phenomena occuring on the spatial and temporal scales of thermomagnetic recording must be seriously questioned. To begin with, there is the problem of distinguishing the nucleation coercivity from the coercivity of wall motion. Then there is the question of speed and uniformity of motion as the wall expands beyond the site of its origination. Finally one must address issues of stability and erasability, which are intimately related to coercivity, in a framework wide enough to allow the consideration of local instabilities and partial erasure. It is fair to say that the existing theories of coercivity are generally incapable of handling the problems associated with thermomagnetic recording and erasure. In our view, the natural vehicle for conducting theoretical investigations in this area is computer simulation based on the fundamental equations of micromagnetics, the basis for which was laid down in the preceding chapter.