Multiaxial magnetic ordering in NdMg

NdMg is a cubic compound (CsCl-type) which orders antiferromagnetically at TN=61 K. The magnetization measurements show a second transition at TR=35 K. Over the whole order range, the powder neutron diffraction pattern can be indexed on the hypothesis of a collinear structure, the magnetic moments being parallel to the <1/2 0 0> wave vector. The neutron diffraction experiments on a single crystal, under an applied magnetic field, establish that the transition at TR corresponds to a change from a collinear structure to a multiaxial one. The magnetic moments successively point along a fourfold axis, in the collinear structure, and along twofold axes in the multiaxial one. This sequence of magnetic structures is consistent with the coexistence of ferroquadrupolar γ-couplings and antiferroquadrupolar ε-couplings. (author)