Absorption of Microwaves by Random-Anisotropy Magnets

Abstract: Microscopic model of the interaction of spins with a microwave field in a random-anisotropy magnet has been developed. Numerical results show that microwave absorption occurs in a broad range of frequencies due to the distribution of Imry-Ma domains on sizes and effective anisotropy. That distribution is also responsible for the weak dependence of the absorption on the damping. At a fixed frequency of the ac-field the spatial pattern of the amplitude of spin oscillations exhibits maxima corresponding to the oscillations of resonant magnetic domains. The dependence of the peak absorption frequency on the ratio of the magnitude of per-spin random anisotropy to the strength of the ferromagnetic exchange agrees with the scaling derived from the Imry-Ma argument. The effect noticeably increases in low-dimensional systems, suggesting that materials comprised of microscopic amorphous leaves and wires can be promising candidates for enhanced microwave absorption.