Electronic and magnetic structure of epitaxial Fe3O4(001)/NiO heterostructures grown on MgO(001) and Nb-doped SrTiO3(001)

Abstract

We study the underlying chemical, electronic, and magnetic properties of a number of magnetite-based thin films. The main focus is placed onto Fe3O4(001)/NiO bilayers grown on MgO(001) and Nb-SrTiO3(001) substrates. We compare the results with those obtained on pure Fe3O4(001) thin films. It is found that the magnetite layers are oxidized and Fe3+ dominates at the surfaces due to maghemite (gamma-Fe2O3) formation, which decreases with increasingmagnetite layer thickness. For layer thicknesses of around 20 nm and above, the cationic distribution is close to that of stoichiometric Fe3O4. At the interface between NiO and Fe3O4 we find the Ni to be in a divalent valence state, with unambiguous spectral features in the Ni 2p core level x-ray photoelectron spectra typical for NiO. The formation of a significant NiFe2O4 interlayer can be excluded by means of x-ray magnetic circular dichroism. Magneto-optical Kerr effect measurements reveal significant higher coercive fields compared to magnetite thin films grown on MgO(001), and an altered in-plane easy axis pointing in the < 100 > direction. We discuss the spin magnetic moments of the magnetite layers and find that a thickness of 20 nm or above leads to spin magnetic moments close to that of bulk magnetite.