Ferromagnets with Random Anisotropy: Longitudinal Spin Resonance and Movable Anisotropy

Random anisotropy in ferromagnets causes small reorientations of the spins, so that they are not precisely collincar. As a consequence, an anisotropy torque develops for rotations about any axis. This requires the introduction of both a new macroscopic spin variable (to represent the macroscopic orientation about the magnetization) and an anisotropy triad. The macroscopic dynamics, including the dissipative motion of the anisotropy triad, is obtained, and a number of applications are considered. Most striking is the prediction of a longitudinal spin resonance mode.