2D-to-1D modulated-to-lock in successive magnetic phase transitions associated with ferroelectricity in ErMn2O5

We report results of dielectric and neutron diffraction measurements in zero field on magnetic ordering of weakly ferroelectric ErMn2O5. Around the ferroelectric transition temperature of T-C1 similar to 38 K below T-N1 = 44K, we observed successive magnetic phase transitions from a two-dimensionally-modulated (2D) incommensurate magnetic (ICM) phase with the propagation vector q = (q(x), 0, q(z)) to another 1D-ICM one with q = (q(x), 0, 1/4) at T = 39.1 K, and then to a commensurate magnetic (CM) phase with q = 4 (1/2, 0, 1/4) at T = 37.7 K. These two magnetic phase transitions (39.1 K and 37.7 K) were found to be 2 4 associated with an anomaly in the dielectric constant along the b axis. In addition, we found that in the case of a rapid-cooling procedure a low-temperature ICM state characterized by q = (1 0, 0.265) shows 2 up and coexists with the CM phase below T-N2 = 11.5 K where evolution of magnetic order of the Er ions is inferred from measurements of the magnetoelectric effect. These observations indicate the strong coupling between ferroelectricity and magnetic ordering in this system.