Domain and Wall Hyperfine Fields in Ferromagnetic Iron

The Mössbauer effect and nuclear magnetic resonance are used to examine the temperature dependence of the domain and wall hyperfine fields in ferromagnetic iron. The difference between them is attributed to demagnetizing fields and exhibits a temperature dependence different than predicted from consideration of domain and wall spin-wave excitations. It is proposed that this temperature dependence arises from distortion of domain shapes with temperature. After correction to constant volume, the temperature dependence of the hyperfine field is compared with that of new magnetization measurements. It is shown that an intrinsic temperature dependence of the effective hyperfine coupling constant exists and may be qualitatively explained by three mechanisms: (i) phonon admixture of the $s$ and $d$ wave functions, (ii) Stoner-like excitations combined with a strongly energy-dependent hyperfine coupling constant, and (iii) changes in the intrinsic $s-d$ hybridization due to the changing magnetization. It is concluded that differentiation of the three mechanisms is not possible by the present type of experiment but requires reliable theoretical estimates of the magnitudes of the various effects.