HCP-FCC phase transition in Co-Ni alloys studied with magneto-optical Kerr spectroscopy

Abstract

We have studied 100 nm thick electron beam evaporated Co1-xNix alloy films in the composition range 0≤x≤1 using magneto-optical spectroscopy and magnetic anisotropy measurements. For films with x≈0.2 we have also investigated the dependence of these quantities on the growth temperature, which was varied in the range 27≤TG≤408 °C. Both as function of the Ni content x and the growth temperature TG we observe the anticipated hcp→fcc phase transition, e.g. by monitoring the magneto-crystalline anisotropy constant Ku,1, which changes continuously from values of ≈0.3 MJ/m3 for Co rich hcp alloys (0≤x≤0.25) to ≈-0.05 MJ/m3 for Ni rich fcc alloy films, in good agreement with bulk literature data. The new and most striking result, however, is observed in the polar magneto-optical Kerr and ellipticity spectra, which were measured in the photon energy range 0.8≤hv≤5.5 eV. Changes by up to about 40% in Kerr rotation for films of constant composition and magnetization are observed when the structure changes from hcp→fcc. This demonstrates the sensitivity of magneto-optical effects to structural changes, making Kerr spectroscopy a useful electronic and physical structure probe.