Shape dependence of the EPR linewidth, resonance field and spin-spin relaxation rate of EuS near Tc

Abstract

The effect of sample shape on the measured EPR linewidth Delta H and resonance field Hr has been investigated at 9.2 GHz by experiments on EuS for temperatures from Tc ( approximately=16.55K) to about 60K. The data were taken on a thin, square disc of EuS for two orientations of the applied field H: (i) H|| the normal to the disc and (ii) H ⊥ to the normal. The observed values of Hr and Delta H for the two cases are different at most temperatures signifying the effect of the demagnetising fields. Using independent measurements of the magnetisations M and the uniform susceptibility chi, it is shown that the temperature dependence of Hr|| and Hr ⊥ is described well by the Huber-Seehra theory (1976) down to about 19K. The observed deviations from the theory below 19K are believed to be due to the non-linear behaviour of M versus H. Expressions are developed to relate the shape-independent internal spin relaxation time T12 to the measured linewidths (Delta H)|| and (Delta H)⊥. For the precritical region, 4 pi chi 1, 1/(T12) varies as chi-1, following the predicted conventional behaviour. These variations of 1/(T12) are understood to result from the dominant role of the dipole-dipole interaction in the spin relaxation in EuS.