Observation of Zero-Point Spin Reduction in Quadratic Layer Antiferromagnets

Nuclear magnetic resonance of ${}_{}{}^{55}{}_{}{}^{}\mathrm{Mn}$ nuclei, detected through its effect on the antiferromagnetic resonance, has been used to measure the zero-point spin reduction in the quadratic layer antiferromagnets ${\mathrm{K}}_2$Mn${\mathrm{F}}_4$ and ${\mathrm{Rb}}_2$Mn${\mathrm{F}}_4$. The result, including a small correction for supertransferred hyperfine effects, is $S-{〈S_z〉}_{T=0\mathrm{}}=0.17\mathrm{}\pm 0.03$, in excellent agreement with spin-wave theory.