Thickness dependent structural ordering and magnetic properties of Co₂FeSi films with or without a Cr buffer layer

We have performed a detailed study on the structure and magnetic properties for Co₂FeSi (CFS) alloy films deposited on the MgO(0 0 1) substrates with or without a Cr buffer layer. With the increase of CFS thickness, the structure ordering degree increases, leading to an improved biaxial magnetic anisotropy. Nevertheless, in spite of the reduced lattice misfit between MgO and CFS, XRD results reveal that the insertion of a 2.5 nm thick Cr layer could give rise to the loss of CFS ordering, which has been ascribed to the interfacial diffusion of Cr atoms caused by thermal annealing. As the Cr buffer thickness is increased more than 10 nm, it is found that the chemically ordered CFS structure reappears, showing a weak four-fold symmetry. Nevertheless, as compared to the CFS films without a Cr buffer, the biaxial magnetic anisotropy rotates by 45, with two magnetic easy axes transforming from the directions of MgO1 1 0/CFS1 0 0 to MgO1 0 0/CFS1 1 0. We consider that the observed orientation transition of biaxial magnetic anisotropy results from the competition between the Cr/CFS interfacial contribution and the intrinsic bulk CFS contribution. Our results provide a better understanding of the evolution of crystalline structure and in-plane magnetic anisotropy with increasing the CFS and Cr layer thicknesses, which will be helpful for designing advanced spintronic devices.