Neutron Scattering Observation and Theory of Spin-Waves in Disordered Magnetic Insulators

Abstract

Neutron scattering provides a direct spectroscopic probe of the spin-wave response in substitutional disordered magnetic materials. The spin-wave frequencies, damping and detailed line shape may be determined from the measurements. For localized modes the measured structure factor may be Fourier transformed to give information on the extent to which the mode wave function is localized in space. Measurements have recently been made of two well-defined branches of the spin-wave dispersion relation for the split-band systems (Co,Mn)F₂ and K(Co,Mn)F₃, while for the single-band system (Mn, Zn) F₂ the spin-wave branch falls in frequency and broadens as the zinc concentration is increased. Near the critical zinc concentration for percolation, p_c, where the long range magnetic order disappears the spin-waves are found to be overdamped. The coherent potential approximation has recently been extended to apply to antiferromagnets, and it is now possible to calculate the spin-wave line shape for a quasi-binary alloy of any composition. The theory includes part of the effect of the off-diagonal randomness arising from the transverse exchange interactions as well as the differing interactions with the various neighboring clusters.

Research sponsored by the U. S. Atomic Energy Commission under contract with Union Carbide Corporation.