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Distinct magnetic phase transition at the surface of an antiferromagnet

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journal contribution
posted on 24.07.2015 by S. Langridge, G.M. Watson, D. Gibbs, Joseph 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.

Publisher

© American Physical Society

Version

AM (Accepted Manuscript)

Publisher statement

This work is made available according to the conditions of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) licence. Full details of this licence are available at: https://creativecommons.org/licenses/by-nc-nd/4.0/

Publication date

2014

Notes

This article was published in the journal, Physical Review Letters [© American Physical Society] and the definitive version is also available at: http://dx.doi.org/10.1103/PhysRevLett.112.167201.

ISSN

0031-9007

Language

en

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