Neutron Diffraction Studies on Iron at High Temperatures

Neutron diffraction measurements have been made on iron at a series of elevated temperatures up to about 1000°C to establish the magnetic scattering effects through the Curie temperature and in the $\gamma$-phase region. Diffraction patterns obtained for the face-centered-cubic $\gamma$ phase offer no evidence for an antiferromagnetic structure in $\gamma$ iron at high temperatures, and the diffuse scattering above $T_c$ suggests that both phases of the metallic lattice are paramagnetic in that temperature region. At temperatures up to the $\alpha -\gamma$ phase transformation, the paramagnetic scattering suggests the existence of considerable ferromagnetic short range order. Pronounced small-angle scattering was found to develop in the temperature region around the Curie transition, and this type of scattering is explained on the basis of the critical magnetic scattering suggested by Van Hove. Detailed investigations, which included experiments on nickel and magnetite near their Curie transitions, were performed to determine the general characteristics of the critical magnetic near their Curie transitions, were performed to determine the general characteristics of the critical magnetic scattering.