Sublattice Magnetization of Quadratic Layer Antiferromagnets

Abstract

The temperature variation of sublattice magnetization in the quadratic layer antiferromagnets ${\mathrm{K}}_2$Ni${\mathrm{F}}_4$ and ${\mathrm{K}}_2$Mn${\mathrm{F}}_4$ has been determined by measuring the NMR frequency of out-of-layer ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ nuclei in these compounds. The results are found to agree with a simple two-dimensional spin-wave theory for $T\lesssim \frac{T_{\mathrm{N}}}{2}$, with $T_{\mathrm{N}}=\mathrm{N}\acute{\mathrm{c}}\mathrm{eel} \mathrm{temperature}$. Some evidence is found for residual exchange coupling between second-neighbor layers in ${\mathrm{K}}_2$Ni${\mathrm{F}}_4$; upper limits for such coupling are set. Evidence is also presented for a large zero-point spin reduction in ${\mathrm{K}}_2$Ni${\mathrm{F}}_4$.