Spiral spin structures and origin of the magnetoelectric coupling in YMnO

Kim, J.-H.*; Lee, S.-H.*; Park, S. I.*; Kenzelmann, M.*; Harris, A. B.*; Schefer, J.*; Chung, J.-H.*; Majkrzak, C. F.*; Takeda, Masayasu  ; Wakimoto, Shuichi  ; Park, S. Y.*; Cheong, S.-W.*; Matsuda, Masaaki; Kimura, Hiroyuki*; Noda, Yukio*; Kakurai, Kazuhisa

By combining neutron four-circle diffraction and polarized neutron-diffraction techniques we have determined the complex spin structures of a multiferroic YMnO that exhibits two ferroelectric phases at low temperatures. The obtained magnetic structure has spiral components in both the low-temperature ferroelectric phases that are magnetically commensurate and incommensurate, respectively. Among proposed microscopic theories for the magnetoelectric coupling, our results are consistent with both the spin-current mechanism and the magnetostriction mechanism. Our results also explain why the electric polarization changes at the low-temperature commensurate-to-incommensurate phase transition.