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abstracts
RMn2O5 (R = Y, Bi, rare-earth) is one of the prototypical multiferroic materials that exhibits a rich variety of magnetoelectric effects. Since the successive magnetic and ferroelectric phase transitions simultaneously take place, magnetic order has been thought to be a primary order parameter for the ferroelectricity in this system. We recently have found that in neutron diffraction study of 153EuMn2O5, magnetic phase transition is induced by applying hydrostatic pressure. As temperature decreases upon p = 1.4 GPa, the magnetic propagation wave vector changes from qM = (1/2, 0, 1/3) to (1/2, 0, 1/2), indicating that the period of magnetic unit cell as well as the magnetic structure change at the phase transition. We have also carried out the dielectric and polarization measurements under pressure and established magnetic and dielectric phase diagram as functions of temperature and pressure as shown in the figure. This study has revealed that the ferroelectric (FE1) - ferroelectric (FE2) phase transition concomitantly occurs at the magnetic phase transition, where the electric polarization is enhanced. To clarify the relevance between the ferroelectricity and the magnetic structure, we carried out single crystal magnetic structure analysis of 153EuMn2O5 upon ambient- and high-pressure. In the magnetic phase with qM = (1/2, 0, 1/3), cycloidal magnetic structure of manganese spins propagating along c-axis is realized. On the contrary in the magnetic phase with qM = (1/2, 0, 1/2), the spins arrange almost collinearly along c-axis. The result indicates that the presence of the cycloidal spin structure plays an important role for inducing (or reducing) the electric polarization in this compound. This study was supported by "KAKENHI"-programs of Scientific Research (B) (24340064), Scientific Research (A) (21244051), Challenging Exploratory Research (23654098) and of Scientific Research on Priority Areas "Novel States of Matter Induced by Frustration" (19052001).
abstracts
It might not be well recognized, most reflections are contaminated by multiple diffractions (MD). Therefore high redundancy data could not coincide with high accuracy data when MDs are not avoided. We collected both data set of MD-avoided and no MD-avoided ones and investigated its effectiveness in electron density measurement. For data collection, four-circle diffractometer at KEK-PF BL14A (Tsukuba, Japan) was used. In MD-avoided measurement, each reflection is collected at angle setting of least of MD contamination which calculated by psi-scan simulation software MDC [1]. In no MD-avoided measurement, usual bisect setting were used. In no MD-avoided measurement, intensities of forbidden reflections of YMn2O5 are more than 10 times largely observed than for MD-avoided one, and resulting residual density map is also highly contaminated reflecting the tendency of Fo>>Fc which is typical for reflections of weak intensity. Figure 1 shows this situation. Figure 2 is the deformation density of YTiO3 for MD-avoided data. Where model density of without Ti-3d1 valence electrons is subtracted from experimentally observed electron density. In the figure, quenching of angular momentum of Ti-3d1 electron is clearly observed. Although Rint could not be an ideal indicator of data accuracy since it cannot perceive Fo>>Fc, Rint(F) of MD-avoided measurement for YTiO3 is significantly reduced to ~0.5%. For no MD-avoided one, Rint(F) is ~1.2%. Since accuracy of MD-avoidance technique is confirmed, the next step is to exploit informations of only a few numbers of valence electrons among F(000) electrons. To accomplish this, wave function based refinement such as XAO [2] should be applied and studied.
