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) similar to 0.492 that appears in a narrow 4 temperature interval of similar to1 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 YMn2O5.