Nanoengineering room temperature ferroelectricity into orthorhombic SmMnO₃ films

Orthorhombic RMnO₃ (R = rare-earth cation) compounds are type-II multiferroics induced by inversion-symmetry-breaking of spin order. They hold promise for magneto-electric devices. However, no spontaneous room-temperature ferroic property has been observed to date in orthorhombic RMnO₃. Here, using 3D straining in nanocomposite films of (SmMnO₃)_0.5((Bi,Sm)₂O₃)_0.5, we demonstrate room temperature ferroelectricity and ferromagnetism with T_C,FM ~ 90 K, matching exactly with theoretical predictions for the induced strain levels. Large in-plane compressive and out-of-plane tensile strains (-3.6% and +4.9%, respectively) were induced by the stiff (Bi,Sm)₂O₃ nanopillars embedded. The room temperature electric polarization is comparable to other spin-driven ferroelectric RMnO₃ films. Also, while bulk SmMnO₃ is antiferromagnetic, ferromagnetism was induced in the composite films. The Mn-O bond angles and lengths determined from density functional theory explain the origin of the ferroelectricity, i.e. modification of the exchange coupling. Our structural tuning method gives a route to designing multiferroics.