Fabrication and Funcntionality of Complex Oxide Superlattices

In this talk, we focus on three-constituent magnetic oxide superlattice (`tricolor' superlattice), where the asymmetric stacking like $\cdot \cdot$ ABCABC $\cdot \cdot$ artificially breaks the space-inversion symmetry. The purpose of the study is to realize artificial polar ferromagnets. The polar/noncentrosymmetric magnets, represented by multiferroic compounds, recently attract a considerable interest, because we can expect novel phenomena, such as magneto-electric (ME) effect, or magnetization-induced second harmonic generation (MSHG). In the artificial superlattice, the gigantic MSHG was first realized with a `tricolor' consisting of a ferromagnet La$_{0.6}$Sr$_{0.4}$MnO$_{3}$ and band insulators LaAlO$_{3}$ (LAO) and SrTiO$_{3}$ [H. Yamada,\textit{ et al.}, APL \textbf{81}, 4793 (2002), Y. Ogawa, \textit{et al.}, PRL \textbf{90}, 217403 (2003)]. From the temperature dependence of MSHG in this superlattice, we found that the MSHG originate from the interface magnetism. By utilizing the MSHG as a probe for interface magnetism, we optimized the various oxide interfaces, leading to the discoveries of huge tunneling magnetoresistance in a junction with engineered interfaces [H. Yamada \textit{et al.}, Science \textbf{305}, 646 (2004)], or enhanced optical ME effect in the `tricolor' superlattice composed of LaMnO$_{3}$, SrMnO$_{3}$ and LAO. In those functionalities, crucial roles are played by the interface effects characteristic of correlated electron oxides, such as charge transfer or orbital-state-mediated magnetism [H. Yamada, et al., APL\textbf{89}, 052506 (2006)].