MnWO 4 is a bulk multiferroic showing three different magnetic phases at low temperature. The intermediate AF2 phase (7.8 K < T < 12.6 K) is also ferroelectric (multiferroic) due to the inversion symmetry breaking helical Mn-spin order and the strong coupling of the spins to the lattice. The complex phase sequence and the multiferroic properties are a consequence of a delicate balance of competing magnetic exchange interactions, anisotropy, and the resulting frustration. It is therefore of interest to tune and control these interactions by substituting the magnetic Mn ions with other magnetic or nonmagnetic ions. We have investigated the effects of chemical substitutions (Co, Zn, and Fe for Mn) on the multiferroic phases. In Mn1- xFe xWO 4 the helical (multiferroic) phase is quickly suppressed by less than 5% Fe. In contrast, Co substitution in MnWO 4 of only 2% is sufficient to stabilize the helical (multiferroic) phase as the ground state. The substitution of nonmagnetic Zn for Mn also stabilizes the multiferroic phase. The multiferroic phase is surprisingly stable with regard to the dilution of the magnetic system of more than 50% which indicates the long-range nature of the magnetic exchange interactions.
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