Field-Induced Dispersion in the Amorphous Antiferromagnet

A magnetic-field-induced exchange stiffness and dispersion have been observed in the spin-flip Raman Spectra of localized donor electrons in $n$-type CdS, a model amorphous antiferromagnet. At high applied fields, there is a $q$-dependent shift in the peak position of the Stokes line. With increasing field, the peak and spectral weight of the excitations move to the low-energy side of the Zeeman energy as $q$ increases. This negative dispersion is consistent with the antiferromagnetic coupling.