Title: Distinct Magnetic Phase Transition at the Surface of an Antiferromagnet
Authors: LANGRIDGE S.WATSON G. M.GIBBS DoonBETOURAS J. J.GIDOPOULOS N.POLLMANN F.LONG M. W.VETTIER C.LANDER G.h.
Citation: PHYSICAL REVIEW LETTERS vol. 112 no. 16 p. 167201
Publisher: AMER PHYSICAL SOC
Publication Year: 2014
JRC N°: JRC84374
ISSN: 0031-9007
URI: http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.167201
http://publications.jrc.ec.europa.eu/repository/handle/JRC84374
DOI: 10.1103/PhysRevLett.112.167201
Type: Articles in periodicals and books
Abstract: In the majority of magnetic systems the surface is required to order at the same temperature as the bulk. In the present Letter, we report a distinct and unexpected surface magnetic phase transition at a lower temperature than the Néel temperature. Employing grazing incidence x-ray resonant magnetic scattering, we have observed the near-surface behavior of uranium dioxide. UO2 is a noncollinear, triple-q, antiferromagnet with the U ions on a face-centered cubic lattice. Theoretical investigations establish that at the surface the energy increase—due to the lost bonds—is reduced when the spins near the surface rotate, gradually losing their component normal to the surface. At the surface the lowest-energy spin configuration has a double-q (planar) structure. With increasing temperature, thermal fluctuations saturate the in-plane crystal field anisotropy at the surface, leading to soft excitations that have ferromagnetic XY character and are decoupled from the bulk. The structure factor of a finite two-dimensional XY model fits the experimental data well for several orders of magnitude of the scattered intensity. Our results support a distinct magnetic transition at the surface in the Kosterlitz-Thouless universality class.
JRC Directorate:Nuclear Safety and Security

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