Magnetic Structure of Er2O3 and Yb2O3

Abstract

Neutron‐diffraction measurements on single crystals and powders of Er₂O₃ and Yb₂O₃ have been used to establish the magnetic structures of these materials. Crystallographic structure parameters were determined by single‐crystal measurements at 4.2°K and the magnetic intensities were measured at 1.3°K. The Néel temperatures are 3.36°K for Er₂O₃ and 2.25°K for Yb₂O₃. A noncollinear antiferromagnetic model is proposed for Er₂O₃ which gives good agreement with the observed magnetic intensities. In this model the eight rare earth sites with point symmetry C_3i have moments along the local 〈111〉 symmetry axis. The remaining 24 Er ions, with point symmetry C₂, have moments along the local 〈100〉 symmetry axis. Sites related by the body‐center translation have moments in the same direction. This model is identical to that proposed by Bertaut and Chevalier,¹ except they conclude that the two sites have equal moments, while we find 6.1±0.1 _μB on the C_3i sites and 5.4±0.1 _μB on the C₂ sites. This configuration is consistent with g‐factor measurements which indicate highly anisotropic g tensors for both sites with the major axis for each site along the local symmetry axis. The dipole field produced by this array is self‐consistent in that the resultant field at each site, due to dipoles at all the other sites, is in the direction of the dipole moment at that site. The dipole‐dipole energy for the C_3i sites is 4.0°K but is only 0.4°K for the C₂ sites, so that superexchange coupling must be invoked to explain the observed transition temperature. A calculation of the expected moment for the C₂ sites, based on the ground‐state wavefunction given by Gruber, Krupke, and Poindexter,² gives a value of 2.1 _μB, in definite disagreement with the neutron diffraction result. For Yb₂O₃ a structure is proposed in which moments of 1.06±0.06 _μB at the C_3i sites are oriented along 〈111〉 directions but not along the particular 〈111〉 local symmetry axis. The C₂ sites have moments of 1.88±0.06 _μB, oriented along 〈110〉 directions perpendicular to the local symmetry axis. Sites related by the body‐center translation have their moments directed oppositely. This configuration is not consistent with a coupling scheme based on dipole‐dipole interactions.