Effect of annealing on the structural and magnetic properties of giant magnetostrictive multilayers

Exchange-coupled multilayered thin films which combine giant magnetostriction and soft magnetic properties are of growing interest for applications. TbFe2/Fe and TbFe2/FeCo nanometric multilayers were grown by RF-sputtering onto glass substrates. A bias field of 250 Oe was applied along the longitudinal axis, inducing an in-plane uniaxial magnetic anisotropy. For es-deposited multilayers, TbFe layers are amorphous while soft layers are nanocrystallized. The effect of annealing was investigated by 57Fe Mössbauer spectrometry, magnetic and magnetostriction measurements in the temperature (Tann) range from 150 to 350 °C. After annealing, the Mössbauer analysis evidences for an increase of the crystallized iron component, probably due to a thinning of the interfaces. A maximum of the magnetoelastic coefficient is found for Tann around 250 °C, with the highest value obtained in the case of TbFe/FeCo. Furthermore, stress relaxation within the film reduces the anisotropy field, leading to a large magnetoelastic susceptibility of about 250 MPa/kOe.