Strategy for achieving multiferroic E-type magnetic order in orthorhombic manganites RMnO3 (R = La–Lu)
Based on first-principles calculations, multiferroic properties of orthorhombic manganites (RMnO3, R = La–Lu) with E-type ground state have been achieved by lanthanide contraction (chemical pressure) and/or external strain. Our research demonstrates that a smaller R radius within the octahedral voids in RMnO3 results in the increase in the tilts of the octahedra but only a gentle change in the Jahn–Teller (JT) distortion. The reduction of the intraplane octahedral rotation angle and the narrowed eg states and lifting t2g band edge are mainly responsible for the intraplane magnetic transition from ferromagnetic (La–Gd) to zigzag-like spin arrangement (Ho–Lu). In turn, the center-broken E-type RMnO3 bulk characterizes the dominated electronic polarization behavior, benefiting from their distortion response to small R substitution, which gives rise to the strong magnetoelectricity. Subsequently, we have figured out the strain strategy for obtaining an E-type transition in light rare-earth manganites (La–Gd) by imposing a series of hypothetical strains, where the small intraplane rotation angle (Θ) and large JT distortion favor the small aspect ratios of a/b and c/b, respectively. The strained LaMnO3 and GdMnO3 achieve E-type transitions successfully by imposing a modest compressive strain along the a- and c-axes and remaining free along the b-direction. Simultaneously, their polarization behaviors were comparatively studied. It was found that the size of the A-site rare-earth ions has a great influence on the external strain response, in addition to its effect on the magnetic phase transition.