Key role in giant magnetoresistance of granular films: Single-domain ferromagnetic granules

The challenging problem on what kind of ferromagnetic granules plays a key role in giant magnetoresistance (GMR) was investigated. By analyzing the hysteresis curves and GMR of FeCo-Al2O3 films with different volume fraction of granules, we found that only the change of the fraction of single-domain granules over total ferromagnetic granules with the volume fraction of FeCo granules in FeCo-Al2O3 film is in agreement with that of GMR. This verifies the key role of single-domain granule in GMR. And it was also confirmed by our annealing experiments. However, the hysteresis curve of FeCo-Al2O3 film with low FeCo content exhibits superparamagnetic characteristic when it is measured at room temperature, which is attributed to the fact that the fraction of superparamagnetic granules is in majority.