title: Distinct magnetic phase transition at the surface of an antiferromagnet.
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.
authors.affiliation: ISIS Facility, STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Oxon OX11 0QX, United Kingdom.
authors.forename: S
authors.initials: S
authors.lastname: Langridge
authors.affiliation: Brookhaven National Laboratory, Upton, New York 11973-5000, USA.
authors.forename: G M
authors.initials: GM
authors.lastname: Watson
authors.affiliation: Brookhaven National Laboratory, Upton, New York 11973-5000, USA.
authors.forename: D
authors.initials: D
authors.lastname: Gibbs
authors.affiliation: Department of Physics, University of Loughborough, Loughborough LE11 3TU, United Kingdom.
authors.forename: J J
authors.initials: JJ
authors.lastname: Betouras
authors.affiliation: Department of Physics, Durham University, South Road, Durham DH1 3LE, United Kingdom.
authors.forename: N I
authors.initials: NI
authors.lastname: Gidopoulos
authors.affiliation: Max Planck Institute for Physics of Complex Systems, Noethnitzer Strasse 38, 01187 Dresden, Germany.
authors.forename: F
authors.initials: F
authors.lastname: Pollmann
authors.affiliation: School of Physics, Birmingham University, Edgbaston, Birmingham B15 2TT, United Kingdom.
authors.forename: M W
authors.initials: MW
authors.lastname: Long
authors.affiliation: European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France.
authors.forename: C
authors.initials: C
authors.lastname: Vettier
authors.affiliation: European Commission, Joint Research Center, Institute for Transuranium Elements, Postfach 2340, D-76125 Karlsruhe, Germany.
authors.forename: G H
authors.initials: GH
authors.lastname: Lander