THE ROLE OF ULTRA-SMALL MAGNETIC GRAINS IN DISORDERED GRANULAR FERROMAGNETS

We present magnetoresistance (MR) measurements on two unique granular ferromagnetic geometries: nanosized granular systems and magnetic tunnel junctions employing a granular electrode. In both systems the saturation fields of the magnetoresistance as well as the MR magnitudes are much larger than that of usual granular films. We interpret these results as being due to the impact of ultrasmall grains. The presence of such nanograins in a random granular array is usually undetectable since their influence on transport, magnetization and magnetoresistance is negligible. Our samples emphasize the role played by these grains leading to a number of unique effects and potential applications.