MAGNETIC EXCITATIONS IN RARE-EARTH ANTI-FERROMAGNETS WITH BILINEAR AND BIQUADRATIC PAIR COUPLINGS - APPLICATION TO DYSB

A Green's-function formalism is constructed for the purpose of computing the elementary excitation energies in a type-II antiferromagnet with Heisenberg exchange and quadrupolar couplings in a cubic crystal field. Three types of excitation modes are found: a longitudinal mode (L mode) associated with O01 and O02 operators (Δm=0), a transverse mode (T1 mode) associated with O±11 and O±12 operators (Δm=±1), and a second transverse mode (T2 mode) associated with O±22 operators (Δm=±2). In the ordered phase the L-mode and T1-mode excitations are mixed magnetic dipolar and quadrupolar excitations. In the disordered phase, as a consequence of cubic symmetry, the magnetic dipolar modes decouple from the quadrupolar modes, giving rise to the possibility of observing a pure quadrupolar excitation. Cubic symmetry also demands that in the disordered phase certain of the excitation energies in the L, T1, and T2 modes have identical dispersion curves. In general, the dispersion in both the ordered and disordered phase is complicated owing to the inclusion of next-nearest-neighbor coupling. The theory is applied to DySb, a type-II antiferromagnet with strong evidences of quadrupolar coupling. © 1979 The American Physical Society.