Hyperfine field and ground-state spin alignment in antiferromagnetic KMnF3

The specific heat of antiferromagnetic KMnF3 at low temperatures has been measured and the contribution of the Mn55 polarization extracted. The hyperfine field, when expressed as a frequency, is found to be A55Sh=686±10 Mc/sec. A55 has been separately measured for KMgF3: Mn2+, and the result is corrected to give A55 for KMnF3 at low temperature. By comparison of the two experiments, the ground-state spin alignment of the antiferromagnet is found to be SS=(99.8±1.5)%, in disagreement with spin-wave predictions. © 1962 The American Physical Society.