Structure at the Fe/NiO(001) interface probed by polarization dependent XAFS

Interfaces between ferromagnetic (FM) and antiferromagnetic (AFM) materials constitute one of the active elements of new magneto-electronic devices, which exploit the electron spin rather than its charge for information processing and transfer. The exchange interactions at the interface formed between the FM and the AFM layers result in an unidirectional magnetic anisotropy, the so called exchange bias, of which a quantitative explanation is presently lacking [ ]. Exchange bias leads to a shift of the magnetic hysterisis curve. In many cases, real FM/AFM interfaces are not ideal in their structure and chemistry and their atomic configuration is expected to have a very strong influence on the magnetic couplings between the two materials. We investigated the case of an epitaxial system, Fe/NiO(001), taken as a model for a FM metal / AFM oxide interface, and obtained a combined experimental and theoretical description for it, which provides new insight in the understanding of magnetic couplings in such systems. The sample used for this study was a UHV grown 2 ML Fe/10 ML NiO/Ag(001) multilayer, capped by a 10 nm thick Au layer to prevent the sample contamination by the atmosphere. Fe K-edge x-ray absorption fine structure (XAFS) measurements were performed at the BM08 “GILDA” beamline. We exploited the polarization dependence of the XAFS cross section in order to separately probe the in-plane and out-of-plane structure [ ]. The background subtracted XAFS spectra in the two geometries are shown in Figure1a. Figure 1b shows the corresponding magnitude of the Fourier transforms and the results of structural fits. The fitting of the extended range spectra was performed assuming the formation of a planar FeO-like layer at the Fe/NiO interface (see model in Figure 2). The FeO-like layer exhibits a buckling, with O and Fe atoms respectively shifted towards and away from the underlying NiO substrate. Moreover, the distance between the last NiO plane and the average position of the FeO plane is 7 % larger than the interplanar distance of bulk NiO. A body-centered-tetragonal (bct) Fe-Ni phase is present on top of the interfacial FeO layer. We have compared the structural parameters obtained by the XAFS analysis to the results of density functional theory calculations performed by means of the all-electron linearized augmented plane wave method + local orbital in the generalized gradient approximation. The atomic configurations of the structurally relaxed system compare very well with the experimental ones. In particular, the numerical agreement between the values for the buckling of the FeO layer (experiment: 0.29 ± 0.07 Å; theory: 0.34 Å) and for the expanded distance between the FeO layer and the underlying NiO (exp. 2.24 ± 0.08 Å; th. 2.25 Å) is notable. Our calculations also allowed to evaluate the spin magnetic moment of the Fe atoms at the interface, providing original insight into the relation between structure and magnetic properties. We compared the values obtained assuming the presence of a pure, pseudomorphic, Fe layer and the formation of an oxidized FeO layer. A significant increase of approximately 0.6 μB (from 2.6 μB to 3.2 μB) in the presence of the distorted FeO layer was found. The origin of this change lies in a depopulation of minority spin d orbitals involved in the Fe-O bonds. The Fe atoms of the interfacial FeO layer assumed in our model are in fact more coordinated with oxygen atoms than Fe atoms situated in the first layer of the ideal Fe/NiO interface, therefore a higher spin polarization is achieved. Uncompensated moments coming from the interfacial FeO layer, which may couple ferromagnetically with the Fe layer, are expected to influence dramatically the exchange interaction at the Fe/NiO interface, with significant consequences also on the exchange bias mechanism. In summary, we give evidence for the presence of a structurally distorted FeO layer at the Fe/NiO(001) interface, in which...