Complex oxide materials are intriguing because their properties span an enormous range of distinct physical states, including conductors, insulators, superconductors, ferromagnets, anti-ferromagnets, ferro-electrics, piezo electrics, and multiferroics. Part of the attraction of these materials is that their properties can be modified by introducing a mixed valence state — that is, a composition that includes metal ions with more than one oxidation state.
Scientists from DOE's Brookhaven and Lawrence Berkeley National Laboratories, SLAC National Accelerator Laboratory, and collaborators at the University of California, Berkeley, and the Science and Technology Facilities Council of the UK are taking a closer look at one such material with a particular composition of lanthanum strontium and magnesium oxide.
La0.7Sr0.3MnO3 (LSMO) is a mixed-valence, complex oxide (containing a mix of Mn3+ and Mn4+ ions) whose properties have been examined extensively. LSMO is thought to be a simple metallic ferromagnet and it has been used in prototype, thin-film electrical devices (such as magneto-resistance junctions) that seek to exploit the many intriguing properties of complex oxides for new applications.
To further explore LSMO, the scientists grew single-layer films with a variable number of unit cells epitaxially on strontium titanate (STO). Using a combination of x-ray magnetic circular dichroism, x-ray absorption spectroscopy, and x-ray reflectivity measurements at Brookhaven’s National Synchrotron Light Source (NSLS), the scientists discovered that, due to doping instabilities and/or charge transfer at the interface, an intermediate enriched Mn3+ layer of a few unit cells develops at the LSMO/STO interface. The presence of this intermediate layer may provide a mechanism for antiferromagnetic coupling across the interface — which, in turn, may lead to the reversed magnetic configuration observed in thicker LSMO films.
The findings demonstrate the rich variety of interfacial spin couplings that can occur in complex oxide thin films that may be utilized in engineering thin-film devices. This work was partially supported by the Semiconductor Research Corporation’s NRI-WIN program.