Dipolar interactions in Fe: A study with the neutron Larmor precession technique MIEZE in a longitudinal field configuration

While the influence of dipolar interactions on the spin-wave dispersion in ferromagnets with localized magnetic moments has been studied in detail, similar studies in itinerant electron systems are rather scarce due to experimental difficulties. Using the newly developed neutron Larmor precession technique MIEZE in a longitudinal field configuration, we succeeded to map out the spin-wave dispersion in Fe at small momentum q and energy transfers E. The results demonstrate an excellent agreement of the magnon dispersion with the Holstein-Primakoff theory, which takes the dipolar interactions into account. At larger q, the data is in agreement with previous investigations by Collins, Phys. Rev. 179, 417 (1969)PHRVAO0031-899X10.1103/PhysRev.179.417. The q dependence of the linewidth of the magnons is proportional to q2.5 in agreement with dynamical scaling theory. The critical exponent for the stiffness, μ=0.35±0.01, agrees with field theory. The spin dynamics in Fe is now explored by neutron scattering over an energy range 15μeV<Esw<120meV, i.e., over about four orders of magnitude in energy.