Magnetoelectric coupling tuned by competing anisotropies in Mn1-x Nix TiO3

A flop of electric polarization from Pc (Pc) to Pa (Pa) is observed in MnTiO3 as a spin-flop transition is triggered by a c-axis magnetic field, Hc=7 T. The critical magnetic field Hc for Pa is significantly reduced in Mn1-xNixTiO3 (x=0.33). Pa and Pc have been observed with both Hc and Ha. Neutron diffraction measurements revealed similar magnetic arrangements for the two compositions where the ordered spins couple antiferromagnetically with their nearest intra- and interplanar neighbors. In the x=0.33 system, the uniaxial and planar anisotropies of Mn2+ and Ni2+ compete and give rise to a spin reorientation transition at TR. A magnetic field, Hc, aligns the spins along c for TR<T<TN. The rotation of the collinear spins away from the c axis for T<TR alters the magnetic point symmetry and gives rise to a new magnetoelectric (ME) susceptibility tensor form. Such linear ME response provides a satisfactory explanation for the behavior of the field-induced electric polarization in both compositions. As the Ni content increases to x=0.5 and 0.68, the ME effect disappears as a new magnetic phase emerges.