Negative magnetoresistance of γ-Fe2O3 observed in γ-Fe2O3/Ag granular nanocomposites

The magnetoresistance of (γ-Fe2O3)xAg100-x (x=50–90), 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 (x=71), samples are insulators. Our data suggest that direct tunneling between silver grains across γ-Fe2O3 barriers dominates at low temperature and variable range hopping becomes the main transport mechanism at high temperature. The variable range hopping within γ-Fe2O3 is believed to be associated with the presence of Fe21 impurities which is determined by Mössbauer spectroscopy. Negative magnetoresistance up to -2% is found in (γ-Fe2O3)xAg100-x (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 Fe2+ to Fe3+ which is enhanced due to the alignment of their moments by an applied magnetic field.