ESR of antiferromagnetic cluster

Electron spin resonance in an antiferromagnetic cluster is studied by a direct numerical method to obtain the dynamical susceptibility chi (omegaw) expressed by the Kubo formula. Temperature dependence of the dipolar broadening is investigated in the isotropic Heisenberg model. The width decreases monotonically as the temperature decreases in the case of high frequency. In the case of low frequency, however it is found to show non-monotonic dependence. The frequency-and temperature-dependence of the resonant field of Ising-like Heisenberg model is studied, and it is compared with the conventional antiferromagnetic resonance (AFMR). It is found that the paramagnetic resonance (PMR) coexists with the antiferromagnetic resonance in the wide range of the temperature when the field is applied parallel to the easy axis. A branch of the conventional AFMR is found to split into double peaks. In the case of the field perpendicular to the easy axis, PMR is very broad and disappears at low temperature.