Effects of Au nanoparticles on the magnetic and transport properties of La0.67 Sr0.33 MnO3 ultrathin layers

The effect of the proximity of Au nanoparticles on the transport and magnetic properties of ultrathin La2/3 Sr1/3 MnO3 (LSMO) films has been investigated. We find a huge increase in the resistivity of the manganite (by four orders of magnitude for a Au nominal thickness of 2 nm), which is accompanied by a strong decrease in the Curie temperature. A combined scanning transmission electron microscopy and electron energy-loss spectroscopy analysis shows that interfaces are coherent and atomically sharp, and that the structural quality is very high. On the other hand, a strong reduction in the Mn oxidation state is seen upon Au capping. NMR data show a strong attenuation of the double exchange signal upon formation of Au nanoparticles. Ab initio calculations indicate a negligible influence of Au on LSMO at an ideal interface, with the LSMO surface magnetic and electronic properties essentially unchanged upon creation of the Au/LSMO interface. In view of these calculations, the experimental results cannot be explained in terms of purely electrostatic effects induced by the proximity of a noble metal. Here we propose that the main driving force underlying the observed change in physical properties is the high reactivity of Au nanoparticles, which can locally pump oxygen from the manganite, thus favoring a phase separation ensuing from O inhomogeneity which deteriorates the transport and electrical properties. © 2010 The American Physical Society.