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The magnetoresistance of (g -Fe2O3)xAg1002x ~x550–90!, a granular system where insulating magnetic nanoparticles and nonmagnetic metal are intimately mixed, has been studied at room temperature. For high silver concentration ~x<70!, samples are metallic and exhibit ordinary positive magnetoresistance. Below the percolation threshold, which is 11.4 vol% of silver ~x571!, samples are insulators. Our data suggest that direct tunneling between silver grains across g -Fe2O3 barriers dominates at low temperature and variable range hopping becomes the main transport mechanism at high temperature. The variable range hopping within g -Fe2O3 is believed to be associated with the presence of Fe21 impurities which is determined by Mo¨ssbauer spectroscopy. Negative magnetoresistance up to 22% is found in (g -Fe2O3)xAg1002x ~x>72! in an applied field of 10 kOe at room temperature. It is proposed that the observed negative magnetoresistance is due to the field-dependent hopping rate of electrons from Fe21 to Fe31 which is enhanced due to the alignment of their moments by an applied magnetic field. © 1999 American Institute of Physics.



Copyright 1999 AIP (American Institute of Physics) Publishing LLC.