Electron paramagnetic resonance in weakly anisotropic Heisenberg magnets with a symmetric anisotropy

Abstract

Motivated by the recent results on new oxides, we revise the theory of the electron paramagnetic resonance (EPR) in weakly anisotropic Heisenberg magnets with a symmetric (or dipole-dipole) anisotropy. In the framework of Mori's formalism, we calculate the EPR absorption and obtain a self-consistent perturbative expression for ${\ensuremath{\chi}}^{\mathrm{xx}}(\ensuremath{\omega})$ at finite temperatures. We show that generally the EPR signal is a mixture of the absorption and the dispersion. In the case where the self-energy can be estimated within the RPA decoupling scheme, expressions for the line shape, the linewidth, and the signal position are also obtained. In particular, we show that the position of the EPR signal is defined by three contributions: the anisotropy of the static susceptibility, the spin dynamics, and the asymmetric part of the line shape.