On the modulated structure of La₂Co_1.7: a neutron Laue diffraction study

A preliminary analysis is presented of neutron Laue diffraction data from a single crystal of La₂Co_1.7 at temperatures between 295 and 15 K, recorded on neutron-sensitive image plates using a thermal neutron beam. The data can be interpreted in terms of a charge (nuclear) density wave with a propagation vector qⁿ = (q_x, 0, q_z) with q_x = 0.113(1)a^* and q_z = 0.203(2)c^*. It has also been confirmed by using a band-pass filter with a cold neutron beam that antiferromagnetic Bragg reflections which have a magnetic propagation vector q^m = (1/3, 1/3, 0) exist below 144(1) K and correspond to a cobalt atom moment of 0.80(15) μ_B. Magnetic satellite reflections with wave vectors corresponding to qⁿ ± q^m have also been observed on the Laue diagrams below this temperature. The values of the propagation vector and magnetic moment have been confirmed from neutron powder diffraction diagrams taken at 150 and 1.5 K.