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Effect of substrate anisotropy on magnetic state and stress state in magnetostrictive multilayers

Abstract:

There is a strong interaction of stress and magnetization in magnetostrictive materials which can be traced back to the corresponding elastic energy and magnetoelastic energy terms. In particular, an applied stress or magnetic field can be used to provide a bias which can then ensure large magnetostrictive actuation at minimum coercive field. The use of stress is especially interesting in the case of thin films because it can be conveniently introduced by means of the substrate. In this work, the basic formulations for the elastic, magnetoelastic and potential magnetic energies of an isotropic material, and the assumption that the film is in state of plane stress are combined in a consistent model that is used to study the effect of a stress bias which is created by bending a substrate during deposition of a magnetostrictive multilayer. Using this framework, the biaxial stresses induced in the thin film after the substrate is released and the multilayer total energy are calculated. The effect of energy contributions from strains in the thickness direction is shown to be unimportant, since they provide a constant, orientation-independent contribution which does not affect in-plane magnetization switching. The multilayer total energy, which is shown to vary with magnetic domain orientation, is minimized to find the orientation of the easy axis as well as to estimate the magnetization along various hard axis directions when an external field is applied in those directions. The uniaxial anisotropy energy and the anisotropy field are also calculated and the results are in good agreement with reported data. These calculations are seen to offer significant improvement over the ones obtained with previous models that consider films subjected to uniaxial stresses and to be governed only by variations of the magnetoelastic energy. © 2003 Elsevier Science B.V. All rights reserved.