Magnetic and crystal structure in connection with ferroelectric properties of multiferroic R Mn 2 O 5

Abstract

RMn2O5 (R=Y and rare earth) are well known multi-ferroic compounds. A colossal magnetoelectric effect is reported such that magnetic field induces an electric polarization. We found successive phase transitions of magnetic and dielectric ordering occurring simultaneously, using neutron and x-ray scattering as well as electrical measurements. The character of magnetic ordering is essentially incommensurate - commensurate and again to incommensurate phase transitions. From the magnetic and dielectric phase diagram in magnetic-field and temperature, we can easily say that the origin of colossal magnetoelectric effect is the phase transition from the magnetic incommensurate and weak ferroelectric phase to the commensurate and ferroelectric phase. We will show the magnetic structure to discuss the mechanism to introduce ferroelectric polarization. For all the compounds, the spin configuration for Mn4+ and Mn3+ 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 R3+ ions has a variety depending on the character of each element. The lattice modulation wavevector qL was carefully measured, and we found the lattice modulation vector qL is exactly twice of the magnetic propagation vector qM, qL=2qM, in all phases. The precise structure giving the spontaneous polarization is still unclear at the moment. The structure analysis of modulation (magnetostriction pattern) in ferroelectric and magnetically commensurate phase of YMn2O5 was successfully carried out.