Spin wave dispersion relation in the ferromagnetic and helical phases of the disordered alloy Tb0.5Ho0.5

Abstract

Neutron inelastic scattering measurements of the c axis dispersion relations of a disordered rare earth alloy of approximate composition Tb0.5Ho0.5 are reported. The experiments were carried out on a single-crystal sample at 10, 47 and 71K in the ferromagnetic phase, at 98 and 125K in the helical phase, and in a field-induced ferromagnetic phase at 100, 125 and 150K. The results have been analysed in terms of a model including Heisenberg exchange and crystal field interactions. No more than five interplanar exchange constants are needed to fit the measurements at any given temperature. These constants vary little within each phase but are substantially different in the ferromagnetic and helical phases. Exchange functions derived from the constants are compared with published functions for Tb, Tb0.9Ho0.1, Tb0.1Ho0.9 and Dy. The similarities and differences which are observed are discussed in terms of the various types of magnetic interactions which are present in these systems.