Magnetic order and spin-flop transitions in the cobalt-doped multiferroic Mn _1-xCo _xWO ₄

We present a comprehensive single-crystal neutron diffraction investigation of the Mn _1-xCo _xWO ₄ with 0.02≤x≤0.30. At lower concentration x≤0.05, the system is quickly driven into the multiferroic phase with spin structure forming an elliptical spiral order similar to the parent compound. The reduction of electric polarization is ascribed to the tilting of the spiral plane. For x∼0.075, the magnetic structure undergoes a spin-flop transition that is characterized by a sudden rotation of the spin helix envelope into the ac plane. This spin structure persists for concentration up to x=0.15, where additional competing magnetic orders appear at low temperature. For 0.17≤x≤0.30, the system experiences another spin-flop transition and recovers the low-x spiral spin configuration. A simple commensurate spin structure with q =(0.5,0,0) is found to coexist with the incommensurate spiral order. The complex evolution of magnetic structure in Co doped MnWO ₄ contrasts sharply with other transition metal ion doped Mn _1-xA _xWO ₄ (A=Zn, Mg, Fe) where the chemical substitutions stabilize only one type of magnetic structure. The rich phase diagram of Mn _1-xCo _xWO ₄ results from the interplay between magnetic frustration and spin anisotropy of the Co ions.