Magnetic Structure of Neodymium

Neutron diffraction measurements have been made on a single crystal of neodymium at temperatures between 1.6° and 20°K. The onset of magnetic ordering occurs at 19°K and the magnetic structure changes at 7.5°K. The crystal structure is hexagonal with a four-layer stacking sequence of type ABAC. Thus, alternate layers have different nearest-neighbor environments, either hexagonal or face-centered cubic. The main features of the diffraction pattern can be interpreted on the basis of an approximate model in which the hexagonal sites order at 19°K with an antiferromagnetic arrangement between alternate hexagonal layers and with a sinusoidal modulation within each layer. The propagation vector for this modulation and the direction of the magnetic moments are both in the basal plane along a b1 reciprocal lattice vector. The magnitude of the propagation vector changes from 0.13 b1 at 18°K to 0.11 b1 at 7.5°K. The cubic sites order at 7.5°K with a similar structure, except that the propagation vector is 0.15 b1 and the moments are in the basal plane at an angle of 30° from b1. The amplitude of the modulated moments is 2.3±0.2 Bohr magnetons for the hexagonal sites and 1.8±0.2 Bohr magnetons for the cubic sites. This model fails to account for all the observed intensities but may be improved by adding a small modulated moment on the cubic sites in the upper temperature region which is shifted in phase by ½π from the modulation in the hexagonal layers, and by allowing the hexagonal moments to deviate slightly from the b1 direction.