Combined magnetic-dipole and electric-quadrupole hyperfine interactions in rare-earth orthoferrite ceramics.

Abstract

Perturbed-angular-correlation (PAC) spectroscopy was used to measure combined nuclear-magnetic-dipole and nuclear-electric-quadrupole interactions in rare-earth orthoferrite (REO) ceramics, R${\mathrm{FeO}}_{3}$ (R=La,Pr,Nd,...,Lu). The rare-earth orthoferrites are canted antiferromagnets that have orthorhombically distorted perovskite structures. With the $^{111}\ensuremath{\rightarrow}^{111}$Cd probe, the PAC measurements were made over a temperature range from laboratory temperature through the antiferromagnetic-to-paramagnetic transitions (740--620 K) and at or near 800 K. In the heavier REO's, the $^{111}\ensuremath{\rightarrow}^{111}$Cd probe substitutes primarily into the rare-earth sites; and in the lighter REO's, it can substitute into both the Fe and the rare-earth sites. At the rare-earth sites, the probe undergoes a high-frequency electric-quadrupole interaction that is shifted in energy by a weak magnetic-dipole interaction.The direction of the associated magnetic hyperfine field is nearly perpendicular to the principal z axis of the electric-field-gradient (EFG) tensor. The magnitude of this field is small, and it may be produced by transferred spin density. In the heavier REO's, the rare-earth-site EFG's are nearly axially symmetric; and, as the rare-earth atomic number decreases, the EFG asymmetry increases. At the Fe sites, the probe undergoes a strong magnetic-dipole interaction that is shifted in energy by a low-frequency electric-quadrupole interaction, which involves very asymmetric EFG's. For ${\mathrm{PrFeO}}_{3}$, as an example, the strong magnetic-dipole interactions could be attributed to the presence of a supertransferred hyperfine field in which spin density is transferred via Fe-O${\mathrm{\ensuremath{-}}}^{111}$Cd bonds. The predictions of a quantum-chemistry theory agree within a factor of 2 with the magnitude of this field. The direction of this field makes an angle of approximately 40\ifmmode^\circ\else\textdegree\fi{} with the principal z axis of the EFG. For ${\mathrm{NdFeO}}_{3}$, similar results were obtained. The results of these experiments indicate that combined interactions can be measured in highly distorted crystals and that they can be analyzed. Moreover, these results indicate that PAC spectroscopy can provide new information about the magnitudes and directions of supertransferred hyperfine fields, which can be used as benchmarks for theoretical calculations.