Studies on high-moment soft magnetic FeCo/Co thin films

The dependences of soft magnetic properties and microstructures of the sputtered FeCo (=Fe_65Co_35) films on Co underlayer thickness t_\rm Co, FeCo thickness t_\rm FeCo, substrate temperature T_\rm s and target-substrate spacing d_\rm T-S are studied. FeCo single layer generally shows a high coercivity with no obvious magnetic anisotropy. Excellent soft magnetic properties with saturation magnetization \mu_0M_\rm s of 2.35 T and hard axis coercivity H_\rm ch of 0.25 kA/m in FeCo films can be achieved by introducing a Co underlayer. It is shown that sandwiching a Co underlayer causes a change in orientation and reduction in grain size from 70 nm to about 10 nm in the FeCo layer. The magnetic softness can be explained by the Hoffmann's ripple theory due to the effect of grain size. The magnetic anisotropy can be controlled by changing d_\rm T-S, and a maximum of 14.3 kA/m for anisotropic field H_\rm k is obtained with d_\rm T-S=18.0 cm.