Physics

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15 May 2014

International team of researchers discovers unexpected magnetic phase transition

In a recent international collaboration with experimentalists from Rutherford-Appleton Lab in UK, Brookhaven National Lab in USA, and theorists colleagues a distinct and unexpected surface magnetic phase transition, uniquely at a lower temperature than the Neel temperature of the compound UO2 was reported, see the full details → 

This material is a non-collinear, triple-q, antiferromagnet with the U ions on an face-centered-cubic lattice. The experimental method used was grazing incidence X-ray resonant magnetic scattering. Theoretical investigations establish that at the surface the energy increase, due to the lost bonds, reduces when the spins near the surface rotate, gradually losing their normal to the surface component. 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.

Dr Betouras, who participated in the discovery, proposed and showed that these experimental results support a distinct surface magnetic transition in the Kosterlitz-Thouless universality class.