Intrinsic Insulating Ferromagnetism in Manganese Oxide Thin Films

Recently, LaMnO$_{\mathrm{3}}$ thin films attract considerable attentions not only because LaMnO$_{\mathrm{3}}$ is the most common magnetic component in all fabricated oxide superlattices/interfaces, but also because experiment observed exotic insulating ferromagnetism in LaMnO$_{\mathrm{3}}$ thin film grown on SrTiO$_{\mathrm{3}}$. However, there is no any model or theory/calculation to explain such striking insulating ferromagnetism. In this work, by means of genetic algorithm optimization, first-principles calculations and the orbital-degenerate double-exchange model studies, we successfully find the insulating ferromagnetic phase of the epitaxially strained LaMnO$_{\mathrm{3}}$ film grown on the cubic SrTiO$_{\mathrm{3}}$ substrate. The unexpected insulating ferromagnetism, which was observed experimentally but not fully understood, originates from the G-type orbital order $d_{{3z}^{2}-r^{2}} \mathord{\left/ {\vphantom {d_{{3z}^{2}-r^{2}} d_{x^{2}-y}}} \right. \kern-\nulldelimiterspace} d_{x^{2}-y}$ and the insulating gap opens as a result of both the orbital ordering and the strong electron-phonon coupling. Our work provides new insight into how a prototypical antiferromagnetic Mott insulator transforms into the ferromagnetic insulator.