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Coupling between improper ferroelectricity and ferrimagnetism in hexagonal ferrites

Antisymmetric Dzyaloshinskii-Moriya (DM) interactions generating from the spin-orbit coupling induce various fascinating properties, like magnetoelectric (ME) effect, weak ferromagnetism and non-trivial topological spin textures like skyrmions, in real materials. Compared to their symmetric isotropic exchange counterpart, these interactions are generally of a weaker order of strength, creating modest twisting in the spin structure which results in weak ferromagntism or weak linear ME effect. Our proposed two-sublattice model, in contrast, predicts a hitherto unobserved, charge ordered non-collinear ferrimagnetic behavior with a considerably high magnetization $\textbf{M}$ coexisting with a ferroelectric (FE) order with an electric polarization $\textbf{P}$ and a strong cross coupling between them which is primarily driven by the inter-sublattice DM interactions. The key to realize these effects is the coupling between these microscopic interactions and the FE primary order parameter. We predict microscopic mechanisms to achieve electric field $\textbf{E}$ induced spin-reorientation transitions and 180$^{\circ}$ switching of the direction of $\textbf{M}$. This model was realized in the hexagonal phase of LuFeO$_3$ doped with electrons. This system shows $P \sim$ 15 $μ$C/cm$^2$, $M \sim$ 1.3 $μ_B$/Fe and magnetic transition near room temperature ($\sim$ 290 K). Our theoretical results are expected to stimulate further quest for energy-efficient routes to control magnetism for spintronics applications.