Magnetic Resonance and Magnetostatic Modes in Europium-Sulfide

An experimental study of the resonance magnetic properties. carried out on a thin, nearly square platelet of the Heisenberg. ferromagnet EuS (T(,c) = 16.5 K) at 9.2 GHz and between 60 and. 4.2 K is reported. This is the first observation and analysis of the. magnetostatic (MS) modes in a Heisenberg ferromagnet. About. 20 well-resolved MS modes were observed at 4.2 K with the field. H parallel to the plane of a thin disc. The experimental values of(' ). the resonance fields H(,r) of the various MS modes are used to test. the Damon-Eshbach theory modified to include the demagnetizing. fields in a finite disc. Except for the (1,1) and (1,3) modes which. appear as surface MS modes, all the other modes (1,n(,z)) with n(,z) =. 5,7,...,41 appear as volume MS modes. The experimental and. calculated values of H(,r)(1,n(,z)) agree within (+OR-) 3%. However, for n(,z) >. 31, there is increasing deviation between theory and experiment. leading to an absorption edge whose field position is about 8%. lower than the predicted value. This is not fully understood. The. linewidth of the modes varies approximately as 1/n(,z). For H, at an(' ). angle (theta) out of the plane of the disc H(,r) of the first several modes. varies as H(,r)((theta)) = H(,r)(0)/cos (theta) for (theta) < 75(DEGREES). An explanation for this. result is given. Above T(,c), the linewidth (DELTA)H(,p) and resonance field H(,o) of the EPR line were studied parallel and perpendicular to the symmetry axis. The symmetry axis is normal to the disc. The temperature-dependence of the observed H(,o(PARLL)) and H(,o(PERP)) is consistent with the Huber-Seehra theory of anisotropic magnets. The angular variation of the ratio (DELTA)H(,p)/H(,o) is not understood completely. The expressions are developed to relate shape-independent (internal) spin -spin relaxation time T(,2)('i) to the measured linewidths (DELTA)H(,p(PARLL)) and (DELTA)H(,p(PERP)). These expressions yield essentially the same T(,2)('i) even though there is considerable anisotropy between (DELTA)H(,p(PARLL)) and (DELTA)H(,p(PERP)). For the precritical region, 4(pi)(chi) < 1, (chi)(' 3/4) dependence of 1/T(,2)('i) predicted by the theories and observed in other ferromagnets is not observed in EuS. For 4(pi)(chi) > 1, 1/T(,2)('i) varies as (chi)('-1), following the predicted conventional behavior. These variations of 1/T(,2)('i) are understood to result from the dominant role of the dipole-dipole interaction on the spin-spin relaxation in EuS.