MAGNETIC-STRUCTURES AND INTERACTIONS IN ERBIUM

The magnetic structures in a single crystal of isotopically enriched erbium have been studied using a high-resolution neutron diffraction technique at the HFIR reactor at Oak Ridge National Laboratory. In addition, these structures have been examined by mean-field calculations, in which the primary interactions were derived from magnetization and spin-wave measurements. The agreement between theory and experiment is generally very good. At intermediate temperatures, the structure is based on an elliptical cycloid in the a-c plane, and blocks of moments with alternating positive and negative components in the c-direction give rise to a series of commensurable structures in the manner proposed by Gibbs and co-workers. These structures are, however, distorted by two-ion couplings of trigonal symmetry, which reflect the different orientation of the two HCP sublattices and have a magnitude which is a substantial fraction of the isotropic exchange interaction. The result is a wobbling cycloid, in which there is an oscillating moment in the b-direction whose period differs from that of the basic cycloidal structure. In the low-temperature cone phase, the moments bunch around alternating a-directions in a pattern with trigonal rather than hexagonal symmetry. Some further consequences of the trigonal couplings on the low-symmetry magnetic structures in erbium and holmium are discussed.