Mechanisms of exchange anisotropy (invited)

Exchange anisotropy refers to a group of phenomena which appear in ferromagnetic-antiferromagnetic sandwiches, particularly to a field offset in the hysteresis loop. After a brief review of experiment and earlier theory, a new theory is described involving a random field at the interface which causes the antiferromagnet to break up into domains, whose size is inversely proportional to the exchange field offset. The theory is extended by considering the temperature dependence and also the topological properties of the domains, in particular nonzero winding numbers which increase the size of the domains and give them added stability. The metastability of such structures provides an explanation of the magnetic "training" effect observed in multiple cycles of the hysteresis loop.