Ferromagnetic Resonance in Polycrystalline Magnetostrictive Films with Large Isotropic Planar Strain

Generally, in a magnetic material the magnetostriction constants ${\lambda }_{100}$ and ${\lambda }_{111}$ are quite different. In the present paper we take this fact into account in a new calculation of the ferromagnetic resonance in polycrystalline magnetostrictive films under large isotropic strain. The results are quite different from those predicted by the MacDonald theory, where it was supposed that ${\lambda }_{100}={\lambda }_{111}$. The calculation presented here is based on the independent-grain approach with randomly oriented grains and is expected to be valid when the strain-induced anisotropy field is of the same order of magnitude or is larger than the saturation magnetization. The calculation is developed for the case when the applied field is perpendicular to the plane of the film, and it is assumed that the dc field is large enough to keep the transverse components of the magnetization one order of magnitude lower than the longitudinal one. An analytical study shows that the absorption line presents singularities arising from those values of the resonance field that are stationary with respect to grain orientation. The singularities are located near $H_{100}$, $H_{110}$, and $H_{111}$, which are the resonance fields corresponding, respectively, to crystallites with [100], [110], and [111] direction along the applied field. Near $H_{110}$ the absorption line shows a logarithmic singularity; the condition that a secondary peak should be resolved near $H_{100}$ is determined. The results are illustrated by a numerical computation of the resonance line where the strain-induced antisotropy field, the elastic constants, and the crystalline anisotropy are varied independently. Finally, it is shown that the contribution of the crystalline anisotropy to the total linewidth can be completely canceled out for a given value of the strain-induced anisotropy. The theory explains several results obtained in various magnetostrictive thin films, which are listed in the paper.