Spin Waves and Magnetic Ordering inK2MnF4

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Abstract
Elastic and inelastic neutron-scattering studies of the planar antiferromagnet K2MnF4 have been carried out. The magnetic ordering is confirmed to be of the K2NiF4 type with TN=42.14 K; no evidence is found for the additional TN=58 K phase reported by Ikeda and Hirakawa (IH). The sublattice magnetization is found to follow a single power law over the range 6×10−4<1−T42.14<3×10−1, with β=0.15±0.01. This contrasts with the results of IH, who report β=0.188 up to 1−TTN=5×10−3 with a crossover to three-dimensional behavior beyond 1−TTN=5×10−3. The effects of a distribution of Néel temperatures on neutron order-parameter determinations is discussed, and it is shown that the apparent crossover behavior observed by IH can be accounted for on the basis of a distribution of TN's with standard deviation 50 mK in their crystal. The spin-wave dispersion relations in the (qx,0,qz) plane have been measured at 4.5 K and at TN. The spin waves correspond precisely to those expected for a simple quadratic anisotropic Heisenberg antiferromagnet with J1=8.45±0.1 K, gμBHA=0.32 K, and all other exchange interactions below our resolution limit. These values are in excellent accord with those determined from the susceptibility by Breed and from the sublattice magnetization and AFMR by de Wijn et al., and they thence confirm that simple two-dimensional spin-wave theory gives a complete description of the low-temperature magnetic properties of K2MnF4. Well-defined spin waves are observed up to TN; at TN the (qx,0,0) dispersion relation is a simple sine wave with slope renormalized by ∼9% from the 4.5-K value; this renormalization is correctly predicted by the