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 TN of MnF2. Comparison with similar measurements at 9 and 35 GHz shows that the EPR linewidth near TN 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 TN is due to breakdown of the usual exchange narrowing condition.