Inversed tunneling magnetoresistance in hybrid FePt/Fe₃ O ₄ core/shell nanoparticles systems

Magnetotransport studies were performed on hybrid FePt/ Fe₃ O₄ 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 Fe₃ O₄ nanoparticles. The magnitude of the TMR in the FePt/ Fe₃ O ₄ core/shell nanoparticle systems is smaller than that in Fe ₃ O₄ 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 Fe₃ O₄ shells which dominates the magnetotransport of the FePt/ Fe₃ O₄ nanoparticle system at low temperatures.