Tunneling-MR in Fe-Cr-O metal-nonmetal granular systems

Tunneling magnetoresistance (TMR) has been achieved for Fe-Cr-O granular fi lms prepared by RIF cosputtering methods. These films are composed of nanos caled Fe granules with an average diameter of 18-28 nm embedded in the crys talline insulating Cr2O3 matrix. and Fe volume fraction f(v), ranges from 1 5% to 55%. Ferromagnetic properties exist in all samples. With the decrease of f(v), TMR increases from - 0.24% for f(v) = 55% to -1.9% for f(v) = 39% , the maximum, then decreases to -0.23% at f(v), = 28%. It is interesting t hat TMR becomes positive 2% at f(v) = 15%. The relationship between ln rho (0) and 1/T-1/2 shows a clear linear relation in the temperature range from 200 to 290 K for the sample with the maximum negative TMR, indicating a tu nneling conductive mechanism. Beyond this temperature range (5-200K) the re lationship is obviously nonlinear, implying that some other effects may exi st. which needs to be further studied. Our results imply that superparamagn etic state of the metal granules is not the necessary condition for TMR, an d it may not be the main factor affecting the TMR, while the microstructure of films, such as the interfaces between the metal and oxide may be import ant to TMR as the electronic structure of metal-oxide interface can influen ce the spin polarization. (C) 2001 Published by Elsevier Science B.V.