Frequency Dependence of the EPR Linewidth in MnF2 Near the Critical Point

Abstract

EPR linewidth measurements at 24.43 GHz have been made near the critical point T_N of MnF₂. Comparison with similar measurements at 9 and 35 GHz shows that the EPR linewidth near T_N depends on the Larmor frequency, its magnitude at a given temperature being smaller at higher frequencies. This is the first observation of this effect recently predicted by Kawasaki. It is suggested that the observed frequency dependence near T_N is due to breakdown of the usual exchange narrowing condition.