Magnetic Scattering of Neutrons in MnF2 near the Critical Point

The angular distribution of slow neutrons (λ=4.34 Å) scattered from a single crystal of MnF₂ has been studied as a function of temperature in the vicinity of the Néel point, about the (100) and the (001) reciprocal lattice points. The (001) data have been analyzed to obtain the values of χ_s⊥, the transverse staggered susceptibility, and its variation with temperature above the transition point has been examined in the light of the theories of Moriya and Tomita. Moriya's theory shows that χ_s⊥ is proportional to (T−T_⊥)⁻¹ where (T_N−T_⊥) is calculated by him to be 1.36°K. To give a good fit to our values, an expression of the type (T−T_⊥)^−γ has been used for χ_s⊥ with Moriya's value for (T_N−T_⊥), and γ is found to be 0.85. A much better fit is obtained by using the expression for χ_s⊥ suggested by Tomita, namely, C{A + B(T − T_N)^γ}⁻¹, for A=0.016 and γ=1.22. The longitudinal staggered susceptibility, ${\chi }_{\mathrm{s\parallel }}$ , has been deduced from the measurements around the (100) point. Its temperature dependence above T_N is found to be of the form (T−T_N)⁻¹, and is not consistent with a four‐thirds power law.