posted on 2015-07-24, 14:34authored byS. Langridge, G.M. Watson, D. Gibbs, Joseph BetourasJoseph Betouras, N.I. Gidopoulos, Frank Pollmann, M.W. Long, C. Vettier, G.H. Lander
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.
Funding
J. J. B. gratefully acknowledges support from EPSRC under EP/H049797/1. Work performed at Brookhaven National Laboratory is supported by the U.S. DOE under Contract No. DE-AC02-CH7600016.
History
School
Science
Department
Physics
Published in
PHYSICAL REVIEW LETTERS
Volume
112
Issue
16
Pages
? - ? (5)
Citation
LANGRIDGE, S. ... et al., 2014. Distinct magnetic phase transition at the surface of an antiferromagnet. Physical Review Letters, 112, 167201.
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