Influence of annealing on microstructure and magnetic-transport of FeCo–SiO2 nanogranular films

A series of FeCo–SiO2 nanogranular films were prepared using magnetron controlled sputtering method. The microstructure, tunneling giant magnetoresistance (TMR) and magnetic properties of FeCo–SiO2 films deposited at room temperature and then annealed at various temperatures were investigated using transmission electron microscopy (TEM), conventional four probes method and vibrated sample magnetometer (VSM) under room temperature, respectively. The results showed that all FeCo–SiO2 films consisted of FeCo granules with equiaxial shape uniformly dispersed in the SiO2 matrix and formed body-centered cubic (bcc) structure. With increasing the annealing temperature, FeCo granule size increased monotonically. For film with 30.5 vol% FeCo, the size distribution satisfied the log-normal function at lower annealing temperature. While with increasing annealing temperature, the size distribution deviated gradually from the log-normal function. Meanwhile, upon varying the annealing temperature, the TMR of films with lower volume fraction reached a peak value at higher annealing temperature and the TMR of films with higher volume fraction reached a peak value at lower annealing temperature. In addition, the results also indicated that the sensitivity of TMR changed non-monotonically with the increment of the annealing temperature and both the saturation magnetization and the susceptibility of FeCo (30.5 vol%)–SiO2 films increased with increasing the annealing temperature.