Critical Magnetic Scattering of Neutrons in Iron

Measurements of the angular and energy distributions of 4.28 Å neutrons scattered at small angles from iron at temperatures above the Curie temperature are described. The results are interpreted in terms of Van Hove's theory of critical magnetic scattering and yield information on the range of spin correlations and the dynamics of the spin-ordering process. For the dimensionless parameter $\frac{2m\Lambda }{\hslash }$ which describes the time dependence of the spin fluctuations, we obtain the value 11.0±0.6 at $T-T_c=2\mathrm{}$ and 18°C. The zero-field magnetic susceptibility, as determined by the parameters ${\kappa }_1$ and $r_1$ (which represent the range and strength, respectively, of the spin correlations), is found to vary as ${(T-T_c)}^{-1.30\mathrm{}\pm 0.04}$. Near the Curie temperature there was sufficient intensity to measure the ratio of the coefficients of the $K^4$ and $K^2$ terms of the angular distribution. The value of this ratio, 29 ${\mathrm{\AA }}^2$, is related to the existence of long-range couplings within the spin system. Details of certain recent modifications of the theory of critical systems are discussed and compared with the experimental results.