Nature of the magnetic order and origin of induced ferroelectricity in TbMnO$_3$

S. B. Wilkins, T. R. Forrest, T. A. W. Beale, S. R. Bland, H. C. Walker, D. Mannix, F. Yakhou, D. Prabhakaran, A. T. Boothroyd, J. P. Hill, P. D. Hatton, D. F. McMorrow

Published 2009-10-16, updated 2009-10-20Version 2

The magnetic structures which endow TbMnO$_3$ with its multiferroic properties have been reassessed on the basis of a comprehensive soft x-ray resonant scattering (XRS) study. The selectivity of XRS facilitated separation of the various contributions (Mn $L_2$ edge, Mn 3d moments; Tb M$_4$ edge, Tb 4f moments), while its variation with azimuth provided information on the moment direction of distinct Fourier components. When the data are combined with a detailed group theory analysis, a new picture emerges of the ferroelectric transition at 28 K. Instead of being driven by the transition from a collinear to a non-collinear magnetic structure, as has previously been supposed, it is shown to occur between two non-collinear structures.