The effect of substrate temperature on the microstructure and tunnelling magnetoresistance of FeCo–Al2O3 nanogranular films

A series of FeCo–Al₂O₃ granular films were prepared with a magnetron controlled sputtering system. The magnetic–transport properties and microstructure of films sputtered at various substrate temperatures were characterized by conventional four probes method, SQUID magnetometer, analytical electron microscope and transmission electron microscope, respectively. The results indicate that the tunnelling magnetoresistance reaches the peak value of about 6.9% for FeCo (41 vol.%)-Al₂O₃ granular films sputtered at 300 K, while for FeCo granular films sputtered at 473 K, the 6% peak value in tunnelling magnetoresistance vs. volume fraction curve displaces toward the lower FeCo content of about 35 vol.%. Meanwhile, FeCo (30vol.%)-Al₂O₃ granular films sputtered both at room temperature and at 473 K behave as superparamagnetic and the susceptibility of this film increases with increasing substrate temperature. Based on the feature of microstructure of FeCo (41vol.%)-Al₂O₃ granular films sputtered at both room temperature and 823 K, the evolution sequence of two-stage phase separation is suggested. In addition, the effect of the microstructure on tunnelling magnetoresistance has been discussed.