Control of improper ferroelectricity by chemical substitution and magnetic fields in multiferroic Mn_1-xCu_xWO₄

Multiferroic materials, like MnWO₄, exhibit improper ferroelectricity driven by an inversion symmetry breaking magnetic order and strong spin lattice interactions. We show that the three magnetic phases in MnWO₄, including one ferroelectric phase, can be controlled by substitution of copper for manganese and by application of external magnetic fields. Upon increasing Cu substitution, the paraelectric low-temperature phase is replaced by the ferroelectric phase which becomes the ground state at higher doping levels. We conclude that a mixed (para-/ferroelectric) ground state exists below 10% of Cu substitution.