Magneto-elasticity in amorphous ferromagnets: Basic principles and applications

Ferromagnetic metallic glasses (FMG) show a very special combination of magnetic, elastic, and magnetostrictive properties that have rocketed their most extended technological applications, mainly in the field of sensors and surveillance labels. The amorphous structure results in the cancellation of the magnetic anisotropy as a result of the averaging out of the atomic scale local anisotropy in a region of the order of the exchange correlation length. Magnetostriction, however, does not cancel out, and values similar to the corresponding crystalline alloys are displayed by the amorphous ones. The absolute values of magnetostriction are not very high, but the extremely magnetic softness of the alloys does balance the competition between stress and magnetic field. In particular, the magnetoelastic coupling coefficient can reach values as high as 0.98, the highest in any known material. Magnetoelasticity, defined as the magnetization change caused by stress, reaches outstanding values in FMGs and a number of applications do use such properties for sensing applications. We can briefly review some of them, either based on static or magnetoelastic wave resonance operation. The most widely extended application is their use as magnetoelastic resonant elements in electronic surveillance labels. New applications of FMGs are under development. Hybrid magnetoelectric (magnetostrictive/piezoelectric) materials have been devised, using highly magnetostrictive amorphous alloys working at resonance. Possible applications of such new materials are briefly discussed. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim