High temperature ferrielectricity and ferrimagnetism in LnACrOsO$_{6}$ by design

Despite intense efforts over the last decade, there are surprisingly few multiferroics in which a net magnetization coexists with a switchable polarization at room temperature. Since magnetism tends to be the harder problem, one approach to solve this challenge is to start with a material that is magnetically ordered at room temperature and drive it ferroelectric. In this regard, the double perovskite Sr$_{2}$CrOsO$_{6}$ is a promising candidate; it is ferromagnetic and insulating with a $T_{\mathrm{c}} =$ 725 K, the highest known $T_{\mathrm{c}}$ of any magnetic insulating oxide with appreciable uncompensated magnetic moment. Here we discuss our first-principles study of the ferroic properties of as not yet synthesized 3$d$-5$d$ double perovskites, LnACrOsO$_{6}$ (Ln $=$ La, Y, Ce-Lu; A $=$ Na, K). We identify polar compounds that have moderate polarization switching barriers and display ferrimagnetism that is expected to persist above room temperature.