Abstract
We present a comprehensive single crystal neutron diffraction investigation of
the $\rm Mn_{1-x}Co_{x}WO_4$ with $0.02\leq x \leq0.30$. At lower
concentration $x \leq 0.05$, 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 $x\sim 0.075$, 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 $0.17 \leq x \leq 0.30$, the system
experiences another spin flop transition and recovers the low-$x$ spiral spin
configuration. A simple commensurate spin structure with $\vec{q}=(0.5,0,0)$
is found to coexist with the incommensurate spiral order. The complex
evolution of magnetic structure in Co doped $\rm MnWO_4$ contrasts sharply
with other transition metal ion doped $\rm Mn_{1-x}A_xWO_4$ (A=Zn, Mg, Fe)
where the chemical substitutions stabilize only one type of magnetic structure. The rich phase diagram of $\rm Mn_{1-x}Co_{x}WO_4 $ results from
the interplay between magnetic frustration and spin anisotropy of the Co ions.
