Hyperfine Field and Ground-State Spin Alignment in Antiferromagnetic KMnF3

The specific heat of antiferromagnetic KMn${\mathrm{F}}_3$ at low temperatures has been measured and the contribution of the ${\mathrm{Mn}}^{55}$ polarization extracted. The hyperfine field, when expressed as a frequency, is found to be $\frac{A^{55}〈S〉}{h}=686\mathrm{}\pm 10$ Mc/sec. $A^{55}$ has been separately measured for KMg${\mathrm{F}}_3$: ${\mathrm{Mn}}^{2+}$, and the result is corrected to give $A^{55}$ for KMn${\mathrm{F}}_3$ at low temperature. By comparison of the two experiments, the ground-state spin alignment of the antiferromagnet is found to be $\frac{〈S〉}{S}=(99.8\mathrm{}\pm 1.5)\%$, in disagreement with spin-wave predictions.