Modulated and enhanced magnetoelectric coupling in multiferroic [inline-graphic not available: see fulltext]

We investigate the effects of doping on the magnetism, ferroelectricity, and magnetoelectric coupling in the . The lower Co doping (at ) suppresses the commensurate collinear antiferromagnetic phase (AF1 phase) and stabilizes the ferroelectric phase down to . For higher Co substitution levels and 0.07, the ferroelectric polarization P reaches a peak at and , respectively, and decreases to nonzero (measured at ) with further cooling, which can be attributed into a continuous rotation of the spiral plane driven by the small change in temperature. The polarization P at increases monotonically with cooling, which suggests the orientation of the spin helix unchanged. Ferroelectric polarization P reverses synchronously with the applying the cyclic oscillation external magnetic field and suggests that the polarization P is strongly coupled with the magnetism in the sample.