Magnetoresistance of mesoscopic granular ferromagnets

We have performed magnetoresistance (MR) measurements of granular ferromagnets having lateral dimensions smaller than 0.5μm and containing a small number of grains (down to about 100). Compared to macroscopic samples, these granular samples exhibit unusually large saturation fields and MR amplitudes. In addition, the evolution of the magnetoresistance curve as the intergrain distance decreases is qualitatively different from that of large samples. Computer simulations show that transport of such small samples is dominated by a quasi-one-dimensional channel, which is relatively insensitive to intergrain coupling. We suggest that our results reflect a transition from percolation to quasi-single-channel dominated transport as the sample size is reduced below the typical percolation radius.