Hidden spin-order-induced room-temperature ferroelectricity in a peculiar conical magnetic structure

A novel mechanism of spin-induced ferroelectricity is unraveled in the alternating longitudinal conical (ALC) magnetic structure. Because the noncollinear ALC structure possesses a c-axis component with collinear ↑–↑–↓–↓ spin order, spin-driven ferroelectricity along the c axis due to the exchange striction mechanism is predicted. Our experiments verify this prediction in the Y-type hexaferrite Ba0.3Sr1.7Co2Fe11AlO22, where ferroelectricity along the c axis is observed up to room temperature. Neutron diffraction data clearly reveal the ALC phase and its evolution with magnetic fields. The c -axis electric polarization can be well modulated by applying either ab-plane or c-axis magnetic fields, even at 305 K. This kind of spin-induced ferroelectricity associated with the ALC magnetic structure provides a new resource of type II multiferroics.