Structure and tunneling magneto-dielectric properties of Co-SrF₂nano-granular thin films

Nano-granular thin films with a new composition designed to enhance the tunneling magneto-dielectric (TMD) effect of Co-SrF2 were successfully deposited using the co-sputtering method. This study explored the influence of the crystallinity of the matrix material on nano-granular structures. Cobalt contents were controlled from 0 to 19 at. % by changing the power of the cobalt cathode from 0 to 205 W. The structures in which the Co granules (φ 3-4 nm) were homogeneously dispersed in the crystallized SrF2 matrix were clearly observed by transmission electron microscopy and its diffraction patterns. The dielectric constant of the Co19-(SrF2)81 thin film peaked at the giant value of 648 and the saturation magnetization peaked at 3.1 kG, but this material still exhibited a TMD response Δϵ′/ϵ′0 of 1.0%. For the Co16-(SrF2)84 thin film, the TMD response peaked at 3.5%, which is higher than previously reported. This study confirmed that increased matrix crystallinity can enhance the TMD response. It also provides motivation for other microstructure improvements.