Investigation on the Magnetic Interaction between Neighboring Grains in Insulating Granular Film of Co54Al19O27

The insulating granular film of Co±Al±O consists of nano-scaled magnetic granules of cobalt embedded in amorphous alumina. This kind of system shows a large tunnel-type giant magnetoresistance (TMR) effect at room temperature when its composition and microstructure are appropriate [1]. Similar to the other fine magnetic particle systems, the grains in this film may correlate each other. However, this correlation can not be detected by the usual magnetization measurement owing to the fact that the film is superparamagnetic at room temperature. Bearing in mind that the TMR ratio is directly related to the angle between magnetic moments of neighboring grains, we have investigated the magnetic interaction in this system by measuring the TMR ratio as a function of a low external magnetic field. The Co54Al19O27 film was prepared by radio frequency reactive sputtering. Its composition was analyzed via Rutherford Backscattering Spectrometry (RBS). The TMR ratio (R(0) ± R(H))/R(H) reaches 7% under a 1.5 T field. It shows no hysteresis in magnetization at room temperature, which is fitted reasonably well by the Langevin function. The TMR was measured under low and high fields at room temperature by the conventional fore-probe method. Taking into consideration the spin-dependent tunneling between neighboring grains, the conductance of insulating granular films was deduced [2]