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Effects of ferromagnet–molecule chemical bonding on spin injection in an Fe3O4–molecule granular system

Spin injection at the interface of Fe3O4/stearic acid molecule is investigated in a comparative study of Fe3O4 nanoparticles chemically bonded with molecules (ChemNPs) and Fe3O4 nanoparticles with physically absorbed molecules (PhyNPs). The resistance of PhyNPs is two orders of magnitude larger than that of ChemNPs, indicating that the resistance of PhyNPs is dominated by the energy barrier at the Fe3O4–molecule interface. A magnetoresistance of −12% under a field of 5.8 kOe at room temperature is observed in ChemNPs, in sharp contrast to the zero magnetoresistance in PhyNPs, reflecting that the chemical bonding is crucial for spin injection. We attribute this result to the induced magnetic moment in molecules by proximity effect, which is likely the origin of the spin-dependent tunnelling through molecules. In addition, the estimated relatively large spin polarization of ChemNPs suggests electronic hybridization at the interface and improved oxygen stoichiometry of the Fe3O4 surface.