Tunneling magneto-dielectric responses of nanogranular materials near the percolation threshold

The tunneling magneto-dielectric (TMD) effect describes a spin-dependent magneto-dielectric response arising from tunneling conduction in nanogranular materials consisting of superparamagnetic nanogranules embedded in an insulating matrix. Here, we report a co-separate-sputtering (CSS) method that sharpens the nanogranule–matrix interface and reduces diffusive scattering during tunneling. Electrical resistivity measurements show that the onset of percolative metallic transport is shifted to higher Co nanogranule contents in the CSS-prepared films. Consequently, the TMD effect is maintained over an expanded nanogranule-content range of 17–45 at. %, with a maximum measured TMD response of 5.6%. Even at the high nanogranule content of 45 at. %, the film retains large magnetization and high real permittivity together with a finite TMD response. These results demonstrate a simple interface-engineering strategy for improving TMD materials and reveal the critical role of the nanogranule–matrix interface in the TMD effect.