Reinvestigation of simultaneous magnetic and ferroelectric phase transitions in YMn₂O₅

Magnetic ordering around the ferroelectric transition temperature T _C1 of weakly ferroelectric YMn₂O₅ has been reinvestigated by single-crystal neutron diffraction and dielectric measurements. Besides a two-dimensionally modulated incommensurate magnetic (2D-ICM) phase with the propagation vector q = (q_x, 0, q_z) and a lock-in commensurate magnetic (CM) one with q = (1/2, 0, 1/4) reported previously, we observed a 1D-ICM phase characterized by q = (q_x, 0, 1/4) with q_x∼ 0.492 that appears in a narrow temperature interval of ∼1 K and separates the 2D-ICM and CM phases. Corresponding to the 2D-ICM-to-1D-ICM and 1D-ICM-to-CM phase transitions at T = 40.8 K and 40.0 K, a sharp peak and a shoulder in the dielectric constant along the b-axis were detected. These results suggest that the ferroelectricity appears simultaneously with the magnetic phase transition from the 2D-ICM to 1D-ICM phase in YMn ₂O₅.