Random anisotropy in amorphous ferromagnets

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Abstract
Random anisotropy is present in all amorphous magnetic materials, and depending on its strength, it can dramatically affect the magnetic behavior. We describe the effect of strong random anisotropy in materials such as amorphous Tb-Fe and Dy-Fe at low temperature and also examine the role which weak random anisotropy might play in even ideally homogeneous soft materials. Much of our analysis is based on the simple model proposed by Harris, Plischke and Zuckerman for a ferromagnet with random-axis uniaxial anisotropy. We describe computer simulation results for this model and then develop scale length arguments which allow us to describe fluctuations in the magnetization direction. For a perfectly isotropic distribution of anisotropy axis we find that the conventional ferromagnetic ground state is unstable. The new ground state has large frozen in fluctuations but probably has a considerable moment and is, therefore, not spin glass-like. This system does not support domain walls of the conventional type. For a system with both a macroscopic anisotropy axis and random anisotropy, we can have domain walls. We present a theory for the intrinsic coercivity which gives values of about 10−6 Oe for Fe-metalloids and 0.2 Oe for Gd-Co-Mo. This indicates that inhomogeneities of larger than atomic scale are limiting the behavior of present materials. A model illustrating aspects of magnetic resonance behavior is also described.