Critical Scattering of Neutrons from Iron

Abstract

<p>Dynamical critical phenomena is iron were studied by the technique of inelastic neutron scattering.</p> <p>Over the range of wave vector and temperature measured the spin-wave energy varied as the square of the wave vector and as the reduced temperature raised to the power 0.36±0.03. The spin-wave damping diverged as the critical temperature is approached from below according to a simple power law with exponent -0.96±0.10. The wave vector dependence of the damping was proportional to the fourth power of the wave vector. These results are in good agreement with the predictions of dynamic scaling and hydrodynamic theory. Anomalous damping was observed at the largest wave vector measured, 0.26 A‾¹, this is tentatively ascribed to the effect of a Fermi surface minimum at wave vector 0.25 A‾¹.</p> <p>The spin waves became over-critically damped a few degrees below the critical temperature. At the critical temperature the scattering was diffusive in character with no evidence of remnant spin waves; the energy width varied as a simple power of the wave vector with the exponent predicted by dynamic scaling. Above the critical temperature the width agreed with the general prediction of dynamic scaling and with the detailed numerical calculations of Résibois and Piette.</p> <p>Below the critical temperature there was no evidence in the scattering of longitudinal spin fluctuations. This is in accord with other observations on isotopic ferromagnets; such a result is not physically understood at the present time.</p>