Influence of annealing on microstructure and magnetic-transport of FeCo-Al_2O_3 nanogranular films

摘要

We have prepared a series of FeCo-Al_2O_3 nanogranular films using magnetron controlled sputtering method and have investigated the microstructure, tunneling giant magnetoresistance (TMR) and magnetic properties of FeCo-Al_2O_3 films by means of transmission electron microscopy (TEM), conventional four probes method and vibrated sample magnetometer. The experimental results showed that all FeCo-Al_2O_3 films were composed of FeCo granules with equiaxial shape uniformly dispersing in the Al_2O_3 matrix and the granule size increased monotonically with the annealing temperature. For FeCo (32.8vol.%)-Al_2O_3 film the size distribution satisfied the log-normal function at low annealing temperature but deviated gradually from the log-normal function as the annealing temperature increased. Meanwhile it was found that the TMR of films with lower volume fraction reached a peak value at a higher annealing temperature and the TMR of films with middle volume fraction reached a peak value at a lower annealing temperature. However for the films with higher volume fraction, TMR dropped dramatically. In addition, the TMR changed non-monotonically with the annealing time or the granule size and both the susceptibility and saturation magnetization increased with the elevation of annealing temperature for FeCo (32.8vol.%)-Al_2O_3 films.