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

The hyperfine field of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ 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 $T^{\frac{3}{2}}$ law and are in good agreement with the magnetization data. A single extra term in $T^{\frac{5}{2}}$ 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 $T^{\frac{3}{2}}$ term found by magnetization and hyperfine field measurements, with the form $A_T=1-6.58\mathrm{}\times {10}^{-9\mathrm{}}{T}^{\frac{5}{2}}+8.9\mathrm{}\times {10}^{-12\mathrm{}}{T}^{\frac{7}{2}}$.