Symmetry Modulation and Enhanced Multiferroic Characteristics in Bi1-xNdxFeO3 Ceramics

BiFeO3 is recognized as the most important room temperature single phase multiferroic material. However, the weak magnetoelectric (ME) coupling remains as a key issue, which obstructs its applications. Since the magnetoelectric coupling in BiFeO3 is essentially hindered by the cycloidal spin structure, here efforts to improve the magnetoelectric coupling by destroying the cycloidal state and switching to the weak ferromagnetic state through symmetry modulation are reported. The structure is tuned from polar R3c to polar Pna2(1), and finally to nonpolar Pbnm by forming Bi1-xNdxFeO3 solid solutions, where two morphotropic phase boundaries (MPBs) are detected. Greatly enhanced ferroelectric polarization is obtained together with the desired weak ferromagnetic characteristics in Bi1-xNdxFeO3 ceramics at the compositions near MPBs. The change of magnetic state from antiferromagnetic (cycloidal state) to ferromagnetic (canted antiferromagnetic) is confirmed by the observation of magnetic domains using magnetic force microscopy. More interestingly, combining experiments and first-principles-based simulations, an electric field-induced structural and magnetic transition from Pna2(1) back to R3c is demonstrated, providing a great opportunity for electric field-controlled magnetism, and this transition is shown to be reversible with additional thermal treatment.