Neutron-Diffraction Study of Antiferromagnetic FeCl3

Neutron diffraction measurements were made on polycrystalline and single-crystal samples of anhydrous Fe${\mathrm{Cl}}_3$ at sample temperatures from 298 to 1.3°K. Antiferromagnetic reflections were observed which indicate a complicated magnetic structure with a Néel temperature of 15±2°K. In this structure the magnetic moments of near-neighbor atoms in the $c$-axis direction are oppositely oriented, but superimposed on this order there is a modulation of the moment distribution in the [$14\overline{5}0$] direction. From the absence of satellite reflections other than first order and from the relative intensities of the first order satellites, it is concluded that the modulation is of the helical type in which the moments lie within ($14\overline{5}0$) planes and rotate by $\frac{2\pi }{15}$ in successive planes along the [$14\overline{5}0$] direction. Absolute intensity measurements yield a moment of $(4.3\mathrm{}\pm 0.4){\mu }_B$ per ${\mathrm{Fe}}^{+3\mathrm{}}$ ion, and indicate a slight distortion of the helix.