Magnetomechanical damping in amorphous ribbons with uniaxial anisotropy

We have investigated the magnetomechanical damping and the resonant amplitude of the longitudinal vibrations of amorphous ribbons with a uniaxial anisotropy induced transverse to the ribbon axis by magnetic field annealing. The damping and herewith the resonant amplitude are governed by eddy current losses. Yet, classical eddy current theory fails to describe the experimental results, in particular, when the sample is biased by a constant magnetic field. This failure ultimately originates in the commonly used assumption of an isotropic permeability tensor. This is not true in uniaxial ferromagnets where magnetization changes by rotation. Thus, within a domain, a change of magnetization along the ribbon axis is inevitably accompanied by a change of magnetization transverse to the ribbon axis. The latter produces excess eddy current losses which become increasingly important the more the equilibrium position of the magnetization vector is declined towards the ribbon axis by the bias field. Taking this into account, a straight forward calculation ends up in analytic expressions capable to describe correctly the experimental findings starting from the basic material parameters.