Sign reversal of the dielectric polarization of MnWo4 in very high magnetic fields

Dielectric polarization P of a geometrically frustrated spin system MnWO4 has been examined for B || x and E || b* in pulsed magnetic fields up to 52 T. We have observed two new phased IV (38 ≤ B ≤ 48 T) and V (B ≥ 48 T). Moreover, we found that the IV-phase also exhibits ferroelectricity, in addition to the AF2-phase already known. Interestingly, the sign of P of the IV-phase is found to be always opposite to that of AF2 irrespective of the direction aof a poling electric field. The results implied that the ferroelectric domain is preserved between the AF2 and IV phases.