MULTIAXIAL MAGNETIC-ORDERING IN NDMG

NdMg is a cubic compound (CsCl-type) which orders antiferromagneticall y at T-N = 61 K. The magnetization measurements show a second transiti on at T-R = 35 K. Over the whole order range, the powder neutron diffr action pattern can be indexed on the hypothesis of a collinear structu re, the magnetic moments being parallel to the (1/2 00) wave vector. T he neutron diffraction experiments on a single crystal, under an appli ed magnetic field, establish that the transition at T-R corresponds to a change from a collinear structure to a multiaxial one. The magnetic moments successively point along a fourfold axis, in the collinear st ructure, and along twofold axes in the multiaxial one. This sequence o f magnetic structures is consistent with the coexistence of ferroquadr upolar gamma-couplings and antiferroquadrupolar epsilon-couplings.