Enhancement of low-field magneto-dielectric response in Co–Al₂O₃ nanogranular films via controlling their nanostructure

Magnetic metal–insulator nanogranular films that can host multiple outstanding functionalities have been widely studied, among which the tunneling magneto-dielectric (TMD) effect is a novel magneto-electric phenomenon discovered by our group. Enhancing their TMD response in a low magnetic field is necessary to practically apply magnetic metal–insulator nanogranular films. Herein, we focused on the effect of film annealing on TMD effect. The nanostructures, magnetic and dielectric properties, and the relationship between annealing temperature were investigated. The film nanostructure (i.e., Co granule size and intergranular distance), which is the key factor in realizing the TMD effect, was discussed. After optimizing the annealing temperature and Co content, a TMD ratio of 3.0 %, which is 75 times higher than that of the reported value, was obtained under a magnetic field of 200 kA/m in annealed Co0.37–(Al2O3)0.63 nanogranular films. We have revealed that post-annealing is an effective route toward enhancing the low-field sensitivity of the TMD response of nanogranular films.