Magnetization Measurements of Two-Dimensional Planar Ferromagnet K₂CuF₄

Linear and nonlinear responses of magnetization in K 2 CuF 4 ( T c =6.25 K) have been measured as functions of magnetic field and temperature for both directions parallel and normal to the easy plane. The measured isothermal magnetizations for the direction parallel to the plane strongly suggest that the transition point in K 2 CuF 4 is fundamentally of the Kosterlitz-Thouless type but modified slightly by the 3D interactions. When the field is applied normal to the plane, a critical line indicating the lowering of the T c is observed. This again suggests the transition is of characteristic for the XY-system.