No abstract

Magnetic ordering around the ferroelectric transition temperature T C1 of weakly ferroelectric YMn 2 O 5 has been reinvestigated by single-crystal neutron diffraction and dielectric measurements. Besides a twodimensionally modulated incommensurate magnetic (2D-ICM) phase with the propagation vector q ¼ ðq x ; 0; q z Þ and a lock-in commensurate magnetic (CM) one with q ¼ ð 1 2 ; 0; 1 4 Þ reported previously, we observed a 1D-ICM phase characterized by q ¼ ðq x ; 0; 1 4 Þ 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-ICMto-1D-ICM and 1D-ICM-to-CM phase transitions at T ¼ 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 2 O 5 .

We report results of dielectric and neutron diffraction measurements in zero field on magnetic ordering of weakly ferroelectric ErMn 2 O 5 . Around the ferroelectric transition temperature of T C1 $ 38 K below T N1 ¼ 44 K, we observed successive magnetic phase transitions from a two-dimensionally-modulated (2D) incommensurate magnetic (ICM) phase with the propagation vector q ¼ ðq x ; 0; q z Þ to another 1D-ICM one with q ¼ ðq x ; 0; 1 4 Þ at T ¼ 39:1 K, and then to a commensurate magnetic (CM) phase with q ¼ ð 1 2 ; 0; 1 4 Þ at T ¼ 37:7 K. These two magnetic phase transitions (39.1 K and 37.7 K) were found to be 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 ¼ ð 1 2 ; 0; 0:265Þ shows up and coexists with the CM phase below T N2 ¼ 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.

No abstract

No abstract