Electronic and magnetic structure of epitaxial NiO/Fe${}_3$O${}_4$(001) heterostructures grown on MgO(001) and Nb-doped SrTiO${}_3$(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 NiO/Fe${}_3$O${}_4$(001) bilayers grown on MgO(001) and Nb-SrTiO${}_3$(001) substrates. We compare the results with those obtained on pure Fe${}_3$O${}_4$(001) thin films. It is found that the magnetite layers are oxidized and Fe${}^{3+}$ dominates at the surfaces due to maghemite (${\hat{I}}^3$-Fe${}_2$O${}_3$) formation, which decreases with increasing magnetite layer thickness. From a layer thickness of around 20 nm on the cationic distribution is close to that of stoichiometric Fe${}_3$O${}_4$. At the interface between NiO and Fe${}_3$O${}_4$ 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 NiFe${}_2$O${}_4$ interlayer can be excluded by means of XMCD. Magneto optical Kerr effect measurements reveal significant higher coercive fields compared to magnetite thin films grown on MgO(001), and a 45${}^{\circ }$ rotated magnetic easy axis. We discuss the spin magnetic moments of the magnetite layers and find that the moment increases with increasing thin film thickness. At low thickness the NiO/Fe${}_3$O${}_4$ films grown on Nb-SrTiO${}_3$ exhibits a significantly decreased spin magnetic moments. A thickness of 20 nm or above leads to spin magnetic moments close to that of bulk magnetite.