Spin Wave Localization in Random Anisotropy Systems: Amorphous (DyxGd1-x)Ni

Abstract

Magnetization measurements performed in the random anisotropy (amorphous) system (DyxGd1-x)Ni are analysed in terms of spin wave excitations in the temperature range 1.5 to 20 K and magnetic fields up to 60 kOe. Besides the Zeeman term, the field-dependent spin wave gap contains a contribution coming from the random magnetic anisotropy. This contribution decreases with H, from a zero-field value proportional to the random anisotropy field, D2/J. This observation, in quantitative agreement with the theory, constitutes the first evidence for the existence of a random anisotropy gap in amorphous alloys. The field decreasing dependence of this gap induces a progressive delocalization of spin waves with the application of a magnetic field, opposing to the effect of the disorder of local anisotropy directions. For zero field the excitations are localized in space, in regions of the order of the ferromagnetic correlation length (Imry and Ma domains). Field-induced reorientations of the magnetization of these domains favour the propagation of spin waves to much more extended regions (spin wave delocalization).