Paramagnetic resonance linebroadening and spin-spin relaxation near magnetic critical points

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Abstract
Recent studies of electron paramagnetic resonance (EPR) linewidths near critical points in magnetic insulators are reviewed. The behavior of representative three‐dimensional antiferromagnets RbMnF3, MnF2, NiCl2, MnO and MnS and ferrogagnets EuO and CrBr3 is analyzed. The connection between the EPR linewidth and the relaxation rate of the fluctuations in the total magnetization in zero field is established. Symmetry arguments are used to confirm the hypothesis that the anomalies in the EPR linewidth arise from a coupling between the fluctuations in the total magnetization and the long wavelength fluctuations in the order parameter. This coupling is induced by anisotropic terms in the Hamiltonian. The dependence of the linewidth on temperature and on the direction and magnitude of the applied field is discussed. The contribution to the linewidth coming from the spin‐phonon interaction is also considered.

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