Serious discrepancies between nuclear magnetic resonance and neutron scattering measurements in the critical region of classic two-dimensional magnets and anomalous behaviors at low temperatures

A characterization of 2D critical behavior in the classic 2D antiferromagnets K2NiF4, Rb2MnF4, and K2MnF4 made originally by Birgeneau et al. [Phys. Rev. B 1, 2211 (1970)], using neutron scattering (NS), is used to review NMR data in these compounds and in some classic 2D ferromagnets. On analyzing the NMR data, very serious discrepancies were found between the NMR and NS results for the critical exponent β, with the values of the NMR βs being some 50% higher. Some novel results were also found, including 2D cases where β=0.33 (but characterized by a non-3D parameter, D∼1). The discrepancies surely need to be resolved, and if the present analysis of the NMR data is further validated, these results could present a clear and nontrivial hurdle for current critical-region theories. Additionally, from the NMR data it was found that the low-temperature behaviors of these classical 2D systems are at odds with the predictions of conventional spin-wave theory analyses.