Magnetic and electric hyperfine interactions in the rare-earth indium compounds R2In studied by 111Cd perturbed angular correlations -: art. no. 094404

The magnetic and electric hyperfine interactions of the probe nucleus Cd-111 on the In site of the ferromagnetic rare-earth (R) indium compounds R2In (R=Pr, Nd, Sm, Gd, Dy, Ho, Er, and Tm) have been investigated by perturbed angular correlation (PAC) spectroscopy. In the paramagnetic phase, the axially symmetric quadrupole interaction (QI) decreases by more than a factor of 20 from R=Pr to R=Er indicating a significant influence of the 4f electrons on the charge distribution at the In site. In the ferromagnetic phase, the spin and temperature dependence of the magnetic hyperfine field B-hf and its orientation relative to the c axis of hexagonal R2In have been determined by measuring the combined magnetic and electric hyperfine interaction as a function of temperature for all R constituents. The sign of Bhf, determined by applying an external field of 4 T, is positive for light and negative for heavy R2In. The comparison of the Cd-111 hyperfine fields in R2In and in R metals suggests that the indirect 4f-4f coupling is mediated by intra-atomic 4f-5d exchange and 5d-5d interaction of neighboring R atoms rather than by 4f exchange with the s-conduction electrons. The spin dependence of the saturation value of B-hf(0) and of the Curie temperature in R2In reflect a substantial difference of the f-d exchange parameter F between the light ( LR=Pr, Nd, Sm) and the heavy (HR=Gd, Tb, R,In compounds: Gamma(LR)/Gamma(HR)=1.5(l). The deviation of the magnetic hyperfine field from its saturation value at low temperatures can be explained by the excitation of spin waves. In Gd2In, the low temperature decrease of B-hf follows Bloch's T-3/2 relation. In the other R2In compounds with nonzero angular momentum of the R constituent, the temperature dependence of B-hf is best described by the modified power law T-3/2 exp(-Delta/k(B)T) which suggests the existence of an energy gap of about Delta/k(B) approximate to 15-20 K in the spin wave spectrum of these anisotropic ferromagnets. With the exception of Pr2In and Nd2In, where the hyperfine field vanishes discontinuously at the Curie temperature, the magnetic phase transitions of R2In are of second order. A critical increase of the linewidth of the magnetic interaction close to the phase transition indicates a spatial variation of the exchange interaction with a spread of the Curie temperature of about 2-5 K. The temperature-induced changes of the orientation of the 4f spins relative to the c axis of R2In have been deduced from the angle between B-hf and the symmetry axis of the electric field gradient.