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Linear magnetoelectricity at room temperature in perovskite superlattices by design

Discovering materials that display a linear magnetoelectric effect at room temperature is challenge. Such materials could facilitate novel devices based on the electric-field control of magnetism. Here we present simple, chemically intuitive design rules to identify a new class of bulk magnetoelectric materials based on the 'bicolor' layering of $Pnma$ ferrite perovskites, e.g., LaFeO$_3$/ LnFeO$_3$ superlattices for which Ln = lanthanide cation. We use first-principles density-functional theory calculations to confirm these ideas. Additionally, we elucidate the origin of this effect and show it is a general consequence of the layering of any bicolor, $Pnma$ perovskite superlattice in which the number of constituent layers are odd (leading to a form of hybrid improper ferroelectricity) and Goodenough- Kanamori rules. Here, the polar distortions induce both weak ferromagnetism and a linear magnetoelectric effect. Our calculations suggest that the effect is 2-3 times greater in magnitude than that observed for the prototypical magnetoelectric material, Cr$_2$O$_3$. We use a simple mean field model to show that the considered materials order magnetically above room temperature.