Microscopic origin of the simultaneous ferroelectric and magnetic phase transition of RMn_2O_5

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We performed the following investigations and published the results.(1)Magnetic propagation vectors q_M of RMn_2O_5(R=Er, Tb, Ho, Tm).(2)Developing the method to measure q_M, dielectric constant E and electric polarization Ps simultaneously.(3)Reinvestigation of the dielectric constant of RMn_2O_5,and found the generalized phase diagram.(4)Synchrotron x-ray experiments to find new Bragg reflections corresponding to the ferroelectric.(5)Polarized neutron scattering of HoMn_2O_5 to measure the spin-chirality.(6)Neutron scattering experiments of HoMn_2O_5 and ErMn_2O_5 at low temperature under external magnetic field.We confirmed that the successive magnetic and ferroelectric phase transitions in RMn_2O_5 occur at the same temperatures. Further, we show that the first ferroelectric phase transition has the same character regardless of the rare earth element R of RMn_2O_5. On the other hand, the giant electro-magnetic effect at the lowest temperature region strongly depends on R. We have s … More hown that the magnetic ordering under the external magnetic field induces such giant electro-magnetic effect. That is, magnetic ordering induces ferroelectric polarization. This result was awarded by Journal of the Physical Society of Japan. After the simultaneous measurements of q_M,ε and Ps, we have the feeling that the scenario we proposed must be changed slightly since very weak ferroelectric component is seen between the one-dimensionally moderated magnetic phase and magnetic commensurate phase. Measure part of the electric polarization shows up below the magnetic commensurate phase. Such phenomena might be related to the existence of the so-called discommensuration. We also performed synchrotron x-ray diffraction to know the true structure at the ferroelectric phase. The unit cell size is double along c-direction and the space group is Pb2_1m. Very weak new Brag reflections were measured at (001/2) and (000) positions, whose intensities are 8-order magnitude weaker than the fundamental Bragg reflections. Less