Charge distribution consequences of the magnetic order in TbB₆

Abstract

The antiferromagnetic state of TbB6 has been investigated by way of magnetostriction and X-ray scattering measurements on a single crystal. The magnetostriction data reveal a well defined tetragonal symmetry lowering and a weak volume increase below TN . The X-ray scattering investigation shows charge satellites, related to the IntroductionIn rare-earth compounds, the crystal field reduces the orbital degeneracy of the magnetic ions. The 4f electronic distribution thus adapts to the rare-earth site symmetry [1, 2], yielding anisotropic magnetic properties. In high symmetry systems, the orbital degeneracy is only partly lifted and due to the spin-orbit coupling, the remaining orbital degrees of freedom are involved in the low temperature phenomena even in the ordered phase. In most rare-earth intermetallics concomitantly with the magnetic ordering, quadrupolar moments develop. Due to the magnetoelastic couplings, the quadrupolar components at the center of the Brillouin zone (ferroquadrupolar) lead to sizable magnetostriction phenomena. The quadrupolar components outside the zone center (antiferroquadrupolar) determine multiaxial magnetic structures. They also introduce a new charge periodicity that is observed by X-ray diffraction. It is worth noting that quadrupolar components may also influence the order of the magnetic transition [3][4][5][6].It turns out that, in special crystallographic and magnetic structures a slight displacement of the 4f ion can be very effective in minimizing the exchange energy with a low elastic energy cost [7]. The compromise stabilize static atomic displacement waves (DW) in antiferromagnetic systems, as recently shown in the ordered phase of GdB 6 [8]. In this compound displacements organize in rather complex multiaxial patterns, as revealed by X-ray diffraction on a single crystal. While L = 0 for Gd 3+ , displacement waves are responsible for the first-order type transition at T N .The model of exchange displacements, developed for GdB 6 , applies to other REB 6 compounds since their magnetic wave-vectors k belong to the particular