Amorphous magnetic order

Abstract

A review is given of the collinear and random magnetic structures which may be found in magnetically-concentrated amorphous solids. General consequences of a non-crystalline lattice on the atomic moments, exchange interactions and single-ion anisotropy are presented. Magnetic structures with one and two magnetic subnetworks are then described, taking examples from the literature of each type. Some discussion is also given of the low energy excitations and the behavior at the spin freezing or ordering temperature. What happens to the familiar forms of magnetic order - ferromagnetism, ferrimagnetism, antiferromagnetism - in the absence of a crystalline atomic lattice? This article sets out to give an overview of magnetic order in non-crystalline solids in an attempt to answer the question. The magnetism of two classes of disordered metals have received particular attention during the past five years; amorphous ferromagnets, notably the Metglas-type alloys, and spin glasses, typically dilute solutions of transition-metal impurities in a non-magnetic crystalline matrix. The subject-matter here will be magnetically-concentrated amorphous materials, with the accent on non-collinear spin structures. A coherent picture of amorphous magnetism will be presented, but it must be admitted that not all the magnetic structures discussed have been equally well established. Some of them are directly analogous to those found in crystals. Others are peculiar to amorphous (or disordered) solids, whereas one crystalline magnetic structure seems impossible in a noncrystalline lattice.