Role of magnetic chirality in polarization flip upon a commensurate- incommensurate magnetic phase transition in YMn₂O₅

We have performed simultaneous measurements of magnetic chirality by using polarized neutrons and electric polarization along the b axis of single crystals of YMn4⁺(Mn_1-xGa_x)3⁺O₅ with x=0.047 and 0.12, in which nonmagnetic Ga ions dilute Mn3⁺ spins. The x=0.047 sample exhibits high-temperature incommensurate (HT-ICM), commensurate (CM), and low-temperature incommensurate (LT-ICM) magnetic phases in order of decreasing temperature, whereas the x=0.12 sample exhibits only HT-ICM and LT-ICM phases. Here, the CM and LT-ICM phases are ferroelectric and weak ferroelectric, respectively. Measurements conducted under zero-field heating after various field-cooling conditions evidence that the microscopic mechanisms of the spin-driven ferroelectricity in the CM and LT-ICM phases are different: The magnetic chirality of Mn4⁺ cycloidal spins plays a dominant role in the LT-ICM phase, whereas the magnetic exchange striction by the Mn4⁺-Mn3⁺ chain plays a dominant role in the CM phase. The polarization of YMn₂O₅ flips upon a CM to LT-ICM phase transition because the ferroelectricity driven by the magnetic chirality and the exchange striction provides opposite directions of polarization.