Magnetostriction in mixed valent magnetic oxides

摘要

Since the early discovery of the magnetostriction effects on iron by Joule and the Invar alloys by Ch. E. Guillaume, a vast discipline emerged in solid state physics. From the basic point of view based on thermodynamic and symmetry considerations, the first phenomenological explanation of the magnetostriction was established by Becker R., Doring W. (1939), Lee E. W. (1955). An important contribution to the explanation of the magnetostriction incorporating a Quantum-Mechanics formalism is due to Callen E. R. & Callen H. B. (1963) who really established the grounds of the modern magnetostriction theory for localised magnetic moment systems. In a broad sense, it is considered that magnetostriction has as origin the spin-orbit coupling, which is manifested in two different kinds of magnetostriction behaviours. One is isotropic, giving rise to volume effects, and the other of anisotropic nature, having its origin in a local distortion of the lattice, due to a preferential orientation of the angular moment of the magnetic electronic charge cloud. Those mechanisms are well explained in systems with localised magnetic moments. Good examples of this effect are observed in rare earth metals, and their alloys, mainly with 3d metals (Fe, Co, Ni), in the Laves phase structure (RM{sub}2) and also in cubic RZn. The largest room temperature effect was found in Terfenol (Th-Dy) Fe{sub}2 alloy, which is the base of the commercial materials for magnetostrictive applications. In these systems, the isotropic effect, i.e. volume magnetostriction, is small and originated either by field induced change of the intrinsic magnetisation (paraprocess), giving rise to the forced magnetostriction, or by the change with the distance of the exchange interaction between localised magnetic moments.