Improvement of high-frequency properties of Co₂FeSi Heusler films by ultrathin Ru underlayer

Heusler Co₂FeSi films with a uniaxial magnetic anisotropy and high ferromagnetic resonance frequency f_r were deposited by an oblique sputtering technique on Ru underlayers with various thicknesses t_Ru from 0 nm to 5 nm. It is revealed that the Ru underlayers reduce the grain size of Co₂FeSi, dramatically enhance the magnetic anisotropy field H_K induced by the internal stress from 242 Oe (1 Oe=79.5775 A·m^-1) to 582 Oe with an increment ratio of 2.4, while a low damping coefficient remains. The result of damping implies that the continuous interface between Ru and Co₂FeSi induces a large in-plane anisotropic field without introducing additional external damping. As a result, excellent high-frequency soft magnetic properties with f_r up to 6.69 GHz are achieved.