Elastic and Quasi-Elastic Neutron Scattering from K 2 MnF 4 near the Critical Point

Elastic and quasi-elastic neutron scattering experiments on K 2 MnF 4 single crystal which is a typical two-dimensional antiferromagnet with Heisenberg exchange interaction, have been carried out. From the measurements of the temperature variation of magnetic Bragg intensities, existence of two types of the magnetic structure (K 2 NiF 4 -type and Ca 2 MnO 4 -type) with different Néel temperatures T N (42.37±0.02 K and 58.0±0.5 K, respectively) has been confirmed. The temperature variation of sublattice magnetization has been determined from the (1, 0, 0) magnetic Bragg inten-sity. The results can be expressed by a simple power law for 4.2 K< T < T N with β=0.188±0.01. After the quasi-elastic scattering measurements, two-dimensional spin correlation is found to be long-range over all the critical region. The diverging nature of the correlation length among intra-planar spins κ -1 at T N strongly suggests the two-dimensionality in this substance.