Large magnetoelectric response and its origin in bulk Co-doped BiFeO 3 synthesized by a stirred hydrothermal process

In this work, we study the effect of Co-doping on the magnetoelectric (ME) response of multiferroic BiFeO3 (BFO) ceramics made of powder synthesized using a stirred hydrothermal method known to facilitate chemical reactions and favor good chemical homogeneity and particles size distribution. An unprecedented ME coefficient up to 11.3 mV/(Oe.cm) i.e. 8 times higher than pure BFO ceramic, measured with a direct method is achieved for 3–5% Co-doping which makes bulk Co-doped-BFO among the largest single-phase ME materials. We show that Co-doping can be seen as a chemical pressure i.e. a proxy hydrostatic pressure resulting into an increase of Néel antiferromagnetic temperature and especially a destabilization of the cycloidal magnetic modulation because of magnetoelastic coupling. By suppressing the cycloidal arrangement that we evidenced unambiguously by using low energy Raman spectroscopy, the linear ME effect is no more inhibited and can then take place explaining the large ME response. We argue that the combination of mechanical and chemical means during the stirred hydrothermal approach we used might be at the origin of such response by favoring Co-distribution and avoiding parasitic phases and therefore such a chemical route might be further explored in the future.