Ultrathin oxide films as templates for magnetic nanostructures

Abstract

The study of ultrathin oxide film as templates for magnetic nanostructures is presented. The ultrathin oxide thin films are investigated for the surface morphology, electronic properties, surface terminations, and ferroelectric properties by the combination of various local and non-local surface science techniques. Ultrathin oxide films, only a few unit cells in thickness, have significant advantages over bulk oxides, such as atomically defined, unreconstructed and stoichiometric surfaces. The high structural quality makes them suitable templates for the fabrication of metal-oxide heterostructures with unprecedented interface quality. This is expected to enable the studies of fundamental interactions between the oxide and metallic adsorbates, such as the theoretically predicted magnetoelectric coupling based on interfacial Fe-Ti bonds. Two kinds of oxide films, BaTiO₃ and Cr₂O₃, are discussed. Scanning tunneling microscopy is utilized to investigate their structure and electronic properties. Firstly, studies of magnetism of Fe nanoclusters deposited on BaTiO₃ by X-ray absorption and photoelectron spectroscopy demonstrate that the oxidization of Fe is largely suppressed by the molecular beam epitaxy growth at low temperature so that even isolated Fe atoms on the BaTiO ₃ have significant magnetic moments. As a second example, the growth and characterization of ultrathin chromia film are discussed. The thin chromia films exhibit highly ordered and atomically smooth surfaces. The electronic structure is locally explored with scanning tunneling spectroscopy, to resolve surface termination and polarization. The presented studies on the ultrathin oxide films are expected to advance the fundamental understanding of interface effects in multiferroics and may help improving magneto-electric effects. In a third example, the formation of magnetic Co nanoclusters on boron nitride nanotemplates is analyzed. The analytic models and Monte-Carlo simulations for the analysis of the coverage of Co clusters show that interactions between particles are the determining factor in template-assisted cluster deposition experiments. Growth models are discussed that include inter-cluster interaction to describe the layer formation and predict layer filling.