  Elastic neutron scattering is used to study the spin correlations in the
multiferroic $\rm Mn_{1-x}Fe_{x}WO_4$ with $x=0.035, 0.05$ and 0.10. The
noncollinear, incommensurate (ICM) magnetic structure associated with the
ferroelectric (FE) phase in pure $\rm MnWO_4$ is suppressed at $x=0.035$ and
completely absent at $x=0.10$. The ICM spin order and FE phase can be restored
by applying a magnetic field along the spin easy-axis. The low-$T$ commensurate
magnetic structure extends in both H/T with increasing Fe concentration. The
systematic evolution of the magnetic and electric properties indicates that the
noncollinear ICM spin order results from competing magnetic interactions and
its stabilization can be tuned by the internal ($x$) or external (magnetic
field) perturbations.
