Phase Transitions and Magnetic Correlations in Two-Dimensional Antiferromagnets

Abstract

Recent neutron scattering work on the planar antiferromagnet K₂NiF₄ has indicated that this system undergoes a true two‐dimensional antiferromagnetic phase transition. In this paper we review these studies together with measurements of the magnons at low temperatures and in the vicinity of T_N. New high precision measurements of the sublattice magnetization around T_N are presented. It is shown that the power law $${\text{M(T)/M(4.2)=0.973(1−T/92.23)}}^{\text{0.138±0.004}}$$ holds for 2×10⁻¹ > 1‐T/T_N > 3×10⁻⁴. Preliminary measurements of the wavevector‐dependent staggered susceptibility χ^αβ (q) in K₂NiF₄ show that only χ^zz, the component parallel to the anisotropy axis, has pronounced diffusive character, diverging at T_N. This provides definitive proof that the phase transition is anisotropy‐induced.