Forward Magnetic Scattering Amplitude of Iron for Thermal Neutrons

The forward magnetic scattering amplitude of iron for thermal neutrons was determined on an absolute scale in two different experiments: the refractive bending of a polarized neutron beam by a small-angle prism (apex angle 32°) and the refractive bending of an unpolarized neutron beam by a large-angle biprism (apex angle 150°) of polycrystalline iron magnetized to saturation. This magnetic scattering amplitude, determined at 301 °K, was established as being 0.589(6) × ${10}^{-12\mathrm{}}$ cm/atom and is in excellent agreement with the value of 0.587 (${10}^{-12\mathrm{}}$ cm/atom that is calculated from magnetization measurements. The measurement confirms the discrepancy between this value and that extrapolated by a $3d$-electron form factor from earlier Bragg reflection measurements, namely 0.645 × ${10}^{-12\mathrm{}}$ cm/atom, and this supports the presence of an anomalous magnetic form factor in iron at scattering angles smaller than permitted in Bragg reflection. Simultaneous measurement of the nuclear scattering amplitude yielded 0.954(6) × ${10}^{-12\mathrm{}}$ cm/atom in good agreement with previous powder measurements. The small refractive bending angles over a range of 20 sec of arc were measured by placing the prisms between the crystals of a double perfect-crystal spectrometer with torsion goniometer control.