Structure analysis of lattice modulation of multiferroic compound YMn2O5 by SR x-ray

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University* NDESRC, Faculty of Engineering, Iwate University** Photon Factory, Institute Materials Structure Sciemce, KEK,*** Photon Factory, Institute Materials Structure Sciemce, KEK**** Department of Physics, Okayama University***** Waseda University******
â—‹Yukio Noda* Hiroyuki Kimura* Youichi Kamada* Yoshihisa Ishikawa* Satoru Kobayashi** Yusuke Wakabayashi*** Hiroshi Sawa**** Naoshi Ikeda***** Kay Kohn******

RMn2O5 (R=Y and rare earth) are well known multi-ferroic compounds, and they show successive magnetic and ferroelectric phase transitions at about 45K, 40K, 39K, 20K and 10K (TN1, TC, TS, TC2, TN2) associated with the large magneto-electric coupling[1]. The precise structure giving the spontaneous polarization is still unclear at the moment.
We surveyed the existence of superlattice reflections in YMn2O5 and TbMn2O5 by SR x-ray at Photon Factory BL4C in KEK, and performed the structure analysis at SPring-8 BL02B1. We found very weak superlattice and satellite reflections. The ratio between the fundamental and the superlattice reflection is about 10-6. 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 structure analysis of modulation in ferroelectric and magnetically commensurate phase of YMn2O5 was carried out. Since we failed to observe the order parameter reflections at q=0 position, our analysis is limited only to the modulated structure and will not give the final result corresponding to the ferroelectric atomic displacements. The R-factor for fundamental Bragg reflection (304 data points and 22 parameters) is 2.51% and that for weak superlattice reflection (180 data points and 14 parameters) is 7.9%. Obtained atomic displacement shows some characteristic pattern relating to the magnetic spin configuration of Mn4+ and Mn3+ atoms.
[1] Y. Noda, Y. Fukuda, H. Kimura, I. Kagomiya, S. Matsumoto, K. Kohn, T. Shobu and N. Ikeda,
J. Korean Phys. Soc. 42 (2003) S1192.