Inversed tunneling magnetoresistance in hybrid FePt/Fe3O4 core/shell nanoparticles systems

Abstract

Magnetotransport studies were performed on hybrid FePt/Fe3O4 core/shell nanoparticle systems. The linear relationship of LnR versus T−1/2 with and without a magnetic field confirms the physical picture of thermally assisted interparticle tunneling. A sign inversion of tunneling magnetoresistance (TMR) from negative to positive with decreasing temperature was observed. In the high temperature region, the core/shell nanoparticles show negative TMR similar to that of Fe3O4 nanoparticles. The magnitude of the TMR in the FePt/Fe3O4 core/shell nanoparticle systems is smaller than that in Fe3O4 nanoparticle systems, likely due to lower spin polarization of conducting electrons in FePt. However the MR becomes positive in the low temperature region. We propose that this behavior originates from the resonance tunneling between the FePt cores via the defect states near the Fermi level in the Fe3O4 shells which dominates the magnetotransport of the FePt/Fe3O4 nanoparticle system at low temperatures.