MAGNETIC AND MAGNETOELASTIC PROPERTIES OF TMPO4

The rare-earth oxide compound TmPO4 (tetragonal zircon structure) is i nvestigated using the extended susceptibility formalism, which conside rs at any temperature all of the features of the crystalline electric field within the ground-state multiplet in the joint analysis of the m agnetic, magnetoelastic and elastic properties. The magnetoelastic coe fficients are determined from third-order magnetic susceptibility, par astriction and elastic constant measurements for the different symmetr y modes. The dominant magnetoelastic coupling is associated with the o rthorhombic delta-symmetry; however, tetragonal and symmetry-lowering modes are also observed to give rise to noticeable effects, in particu lar in the thermal expansion. The close coherency between the magnetoe lastic coefficients in TmPO4 and TbPO4 is then emphasized: the tetrago nal magnetoelastic coefficients are sizeable and all of the magnetoela stic couplings keep the same sign and order of magnitude in the two ph osphates. This coherency appears to be valid throughout the large fami ly of rare-earth zircons. Through these two examples, the occurrence o f a quadrupolar ordering is then discussed as being governed by the cr ystalline electric field, and the role of undercritical quadrupolar in teractions is emphasized in TmPO4.