Temperature-dependent magnetic properties of bismuth substituted terbium-iron garnets

The crystallographic structure and magnetic properties of bismuth substituted terbium-iron garnets have been studied using X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and Mossbauer spectroscopy. The crystal structures were found to be a cubic garnet structure with space group Ia3d. The lattice constants increase linearly with increasing bismuth concentration. The field-cooled magnetization curves measured at various external fields. The compensation temperature (T-comp) were determined at 260,170, and 120 K, for x = 0.0, 0.5, and 1.0, respectively. Bisubstituted samples have the negative value with large coercivity below T-comp. In order to study the change of the detailed local structure on bismuth substituted samples, Mossbauer spectra were measured at various temperatures from 4.2 K to Neel temperature. Above T-comp, the spectra for the samples consist of two sextets, and the isomer shifts at room temperature of (16a) and (24d) sites are 0.26 and 0.04 mm/s. Below T-comp, the spectra for the samples consist of three sublattice structures (16a), (16a'), and (24d). In octahedral (16a') site, electric quadrupole splitting decreases up to -0.2 mm/s with decreasing temperature from T-N to 4.2 K. Therefore, we insist that the negative magnetization is related to the local anisotropy of 16a' site by the strong covalent interaction between bismuth and iron.