Investigations of Two-Dimensional Ferromagnet K₂CuF₄ by Neutron Scattering

Abstract

The magnetic properties of the insulating Heisenberg ferromagnet K₂CuF₄ have been studied by neutron diffraction. A strong paramagnetic diffuse scattering of the form of ridge was observed near T_c=6.25 K supporting the two-dimensional nature of this compound. From the asphericity of this scattering, the ratio of the exchange constants both in the weakest and the strongest directions was determined as J′⁄J=0.00066. Below T_c, ferromagnetism appears with the spin aligned parallel to the c-plane. As K₂CuF₄ has no Ising-like symmetry, though the crystal structure is the same to that of K₂NiF₄, marked contrast appears. For instance, it behaves as a typical simple three-dimensional ferromagnet like M_s(T)∝(1−CT^3⁄2) up to T⁄T_c∼0.86, β=0.33 and 2ν=1.35.