Kosterlitz-Thouless transition in two-dimensional planar ferromagnet K2CuF4 (invited)

Abstract

We report that in 2D planar ferromagnet K₂CuF₄ the Kosterlitz‐Thouless transition occurs in a slightly modified form by the weak 3D interaction. K₂CuF₄ is a good 2D ferromagnet, but by no means a true XY magnet, the anisotropic part of interaction being only 1%. Nevertheless, it is approximated by a good 2D XY magnet at low temperatures. We have carried out static magnetization as well as neutron scattering measurements and found that all the critical parameters δ(T), χ(T), κ(T), ξ(T) and η(T) determined in the 2D XY‐like crossover regime are in good agreement with the Kosterlitz theory and the Monte Carlo simulation. The existence of the universal jump in J_eff is also indicated through the diffuse scattering measurements.