Magnetoelectric coupling tuned by competing anisotropies in Mn1-xNixTiO3

A flop of electric polarization from P parallel to c (P-c) to P parallel to a (P-a) is observed in MnTiO3 as a spin-flop transition is triggered by a c-axis magnetic field, H-parallel to c = 7 T. The critical magnetic field H-parallel to c for P-a is significantly reduced in Mn1-xNixTiO3 (x = 0.33). P-a and P-c have been observed with both H-parallel to c and H-parallel to a. 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 T-R. A magnetic field, H-parallel to c, aligns the spins along c for T-R<T<T-N. The rotation of the collinear spins away from the c axis for T<T-R 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.