Abstract
Multiferroic BiFeO3 (BFO) that exhibits a gigantic off-centering polarization (OCP) is the most extensively studied material among all multiferroics. In addition to this gigantic OCP, the BFO having R3c structural symmetry is expected to exhibit a couple of parasitic improper polarizations owing to coexisting spin-polarization coupling mechanisms. However, these improper polarizations are not yet theoretically quantified. Herein, we show that there exist two distinct spin-coupling-induced improper polarizations in the R3c BFO on the basis of the Landau-Lifshitz-Ginzburg theory: ΔPLF arising from the Lifshitz gradient coupling in a cycloidal spin-density wave, and ΔPms originating from the biquadratic magnetostrictive interaction. With the help of ab initio calculations, we have numerically evaluated magnitudes of these improper polarizations, in addition to the estimate of all three relevant coupling constants. We further predict that the magnetic susceptibility increases substantially upon the transition from the bulk R3c BFO to the homogeneous canted spin state in a constrained epitaxial film, which satisfactorily accounts for the experimental observation. The present study will help us understand the magnetoelectric coupling and shed light on design of BFO-based materials with improved multiferroic properties.
Highlights
BFO plays some important roles as ferroelectric substrate, AFM pinning layer for exchange bias, and interfacial quantum modulation donor2
We have further predicted that the magnetic susceptibility increases substantially upon the transition from the bulk R3c BFO to the homogeneous canted spin state in a constrained epitaxial film
Having theoretically identified the two spin-coupling-induced improper polarizations in the R3c BFO (i.e., ΔPms, ΔPLF), we focus on the numerical estimate of these values with the help of ab initio density-functional theory (DFT) calculations and experimental measurements
Summary
BFO plays some important roles as ferroelectric substrate, AFM pinning layer for exchange bias, and interfacial quantum modulation donor. Room-temperature exchange coupling between the BFO AFM order and the Co overlayer with ~90° in-plane Co-moment rotation upon single-step ferroelectric switching of the monodomain BFO. This has important consequences for practical, low power non-volatile ME devices utilizing BFO20. We show unequivocally that there exist two distinct spin-coupling-induced improper polarizations in the R3c BFO on the basis of the Landau-Lifshitz-Ginzburg phenomenological theory. We have further predicted that the magnetic susceptibility increases substantially upon the transition from the bulk R3c BFO to the homogeneous canted spin state in a constrained epitaxial film These studies will help us comprehensively understand the ME coupling mechanisms in the R3c BFO and shed light on design of BFO-based materials with improved multiferroic properties. We have to evaluate the position-dependent (Mi × Mj) to assess ΔPDM due to the cycloidal variation of M with the propagation vector Q along [110]h
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