Interfaces between ferromagnetic (FM) and antiferromagnetic (AFM) materials constitute one of the active elements of new magneto-electronic devices, which exploit the electron’s 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 [1]. 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 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 polarisation dependence of the XAFS cross section in order to separately probe the in-plane and out-of-plane structure [2].

Fig. 109: a) Background subtracted Fe K-edge XAFS spectra for the 2 ML Fe film in the two geometries. b) Corresponding k3 weighted magnitude of the Fourier transforms (solid line) and fit (dashed line).

The background subtracted XAFS spectra in the two geometries are shown in Figure 109a and the corresponding magnitude of the Fourier transforms and the results of structural fits are shown in Figure 109b. The data demonstrate the formation of a planar FeO-like layer at the Fe/NiO interface and fitting allowed to determine the interface geometry (see Figure 110). 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.

Fig. 110: Model of the Fe/NiO(001) interface.

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 linearised augmented plane wave method + local orbital in the generalised 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 us to evaluate the spin magnetic moment of the Fe atoms at the interface, providing an 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 oxidised 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 polarisation is achieved. Uncompensated moments coming from the interfacial FeO layer, which may couple ferromagnetically with the Fe layer, are expected to dramatically influence the exchange interaction at the Fe/NiO interface, with significant consequences also for 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 the spin magnetic moment of Fe atoms is increased compared to the abrupt interface. This work provides the atomic level characterisation necessary in order to provide a structural basis for a physical understanding of exchange bias in FM/AFM interfaces.



[1] J. Nogués and I.K. Schuller, J. Magn. Magn. Mater. 192 203 (1999).
[2] E. Groppo et al., J. Phys. Chem. B 107 4597 (2003).

Principal Publication and Authors

P. Luches (a), V. Bellini (a), S. Colonna (b), L. Di Giustino (a), F. Manghi (a,c), S. Valeri (a,c), F. Boscherini (d), Physical Review Letters 96, 106106 (2006).
(a) S3, CNR-INFM, Modena (Italy)
(b) ISM-CNR, Roma (Italy)
(c) Department of Physics, University of Modena and Reggio Emilia, Modena (Italy)
(d) Department of Physics and CNISM, University of Bologna (Italy)