Hyperfine Interactions in the Ground State and 22-keV State ofSm149in Ferrimagnetic Compounds of Samarium

Abstract

Hyperfine interactions in Sm${\mathrm{Fe}}_2$, SmFe${\mathrm{O}}_3$, Sm metal, and particularly in samarium iron garnet, SmIG, have been studied at various temperatures using the Mössbauer effect in ${\mathrm{Sm}}^{149}$. Large magnetic hyperfine interactions are observed in SmIG even at room temperature, in contrast with the observed Sm sublattice magnetization. A value of (1.55±0.25)×${10}^6$ Oe was found for the magnetic field acting on the Sm nucleus in samarium iron garnet at 20°K. For Sm${\mathrm{Fe}}_2$ an upper limit of ${10}^6$ Oe was found for $H_{\mathrm{eff}}$ at 77°K. The experimental results support an assignment of a spin $\frac{5}{2}$ for the 22-keV level and yield a value of 1.26±0.04 for the ratio of the magnetic moments of the 22-keV state and the ground state. A small isomeric shift -0.9±0.3 mm/sec was found between Sm metal and ${\mathrm{Sm}}_2$ ${\mathrm{O}}_3$ absorbers at 300° and 80°K. Upper limits for $H_{\mathrm{eff}}$ in SmFe${\mathrm{O}}_3$ were found to be 4×${10}^5$ Oe 16°K, 1.9×${10}^5$ Oe at 80°K, and 1.5×${10}^5$ Oe at 300°K. The origin of the temperature dependence of the hyperfine fields acting on Sm nuclei in SmIG is discussed, including the effects of the molecular exchange field and the electrostatic crystalline field. Comparison is also made with the theory of magnetization of Sm sublattices. Exchange effects are found to be strong and dominant at high temperatures, whereas at low temperatures, crystalline-field effects are found to predominate. These results are in fair agreement with theoretical predictions if it is assumed that at low temperatures, $H_{\mathrm{eff}}$ is determined mainly by the splitting of the quarter ${\Gamma }^8$ ground state (which is the lowest of the two levels into which the ${}_{}{}^6{}_{}{}^{}H_{\frac{5}{2}}^{}{}_{}{}^{}$ ground state is split by a cubic electric field) by the exchange interaction.