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In the majority of magnetic systems the surface is required to order at thesame temperature as the bulk. In the present study, we report a distinct andunexpected surface magnetic phase transition, uniquely at a lower temperaturethan the N\'eel temperature. Employing grazing incidence X-ray resonantmagnetic scattering we have observed the near surface behavior of uraniumdioxide. UO$_2$ is a non-collinear, triple-q, antiferromagnet with the U ionson an face-centered-cubic lattice. Theoretical investigations establish that atthe surface the energy increase, due to the lost bonds, reduces when the spinsnear the surface rotate, gradually losing their normal to the surfacecomponent. At the surface the lowest-energy spin configuration has a double-q(planar) structure. With increasing temperature, thermal fluctuations saturatethe in-plane crystal field anisotropy at the surface, leading to softexcitations that have ferromagnetic $XY$ character and are decoupled from thebulk. The structure factor of a finite two-dimensional $XY$ model, fits theexperimental data well for several orders of magnitude of the scatteredintensity. Our results support a distinct magnetic transition at the surface inthe Kosterlitz-Thouless universality class.

Distinct magnetic phase transition at the surface of an antiferromagnet