The magnetic and ferroelectric properties of multiferroic RMn 2 O 5 (R = Y, Tb, Ho, Er, Tm) are reviewed based on recent neutron diffraction and dielectric measurements. Successive phase transitions of magnetic and dielectric ordering were found to occur simultaneously in this system. The characteristic magnetic ordering of the system exhibits an incommensurate-commensurate phase transition, and again transitions to an incommensurate phase. Special attention is given to the magnetic structure in order to discuss the mechanism for the introduction of ferroelectric polarization. For all the compounds examined, the spin configuration for Mn 4+ and Mn 3+ ions in the commensurate magnetic phase, where spontaneous electric polarization occurs, was determined to be a transverse spiral spin structure propagating along the c-axis. By contrast, the alignment of the induced 4f moment of R 3+ ions showed variation, depending on the character of each of the elements. Corresponding responses to external fields such as a magnetic field, hydrostatic pressure etc at low temperature are strongly dependent on the rare earth element present in the RMn 2 O 5 system. The so-called colossal magnetoelectric effect in this system can be easily interpreted by the phase transition from the magnetic incommensurate and weak ferroelectric phase to the commensurate and ferroelectric phase.

No abstract

No abstract

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