Temperature Dependence of the Hyperfine Field and Hyperfine Coupling Constant of Iron

The hyperfine field of ⁵⁷Fe in natural iron has been measured by continuous-wave NMR in the temperature range 4.2-400 ^∘{}K. The measurements below 32 ^∘{}K follow a T3/2 law and are in good agreement with the magnetization data. A single extra term in T5/2 is sufficient to fit the hyperfine-field measurements, reduced to zero spin-wave gap, between 32 and 400 ^∘{}K. The hyperfine coupling constant is temperature dependent, despite the agreement in the T3/2 term found by magnetization and hyperfine field measurements, with the form AT=1-6.58×10^-9T5/2+8.9×10^-12T7/2.