MAGNETIC AND MAGNETOELASTIC PROPERTIES IN TETRAGONAL TBPO4

The magnetic and magnetoelastic properties (third-order magnetic susce ptibility and parastriction) of TbPO4, which has the zircon-type tetra gonal structure, are analyzed in the paramagnetic phase. The susceptib ility formalism is then used to describe all the symmetry-lowering mod es in a rare-earth insulator. The dominant magnetoelastic coupling is associated with the delta symmetry and is, at least partly, responsibl e for the transition occuring at 2.15 K in the antiferromagnetic phase and clearly dominates the gamma-symmetry mode. However, none of the d ifferent symmetry-lowering modes can be completely neglected and we de termine parameters for the alpha 1 and alpha 2 tetragonal modes and th e epsilon monoclinic mode. Owing to the close vicinity of the first ex cited singlet to the ground-state doublet, the alpha quadrupolar inter actions play an important role in determining the temperature dependen ce of the level spacing at low temperature, in particular in the order ed phases. The determination of all the magnetoelastic modes of TbPO4 is an essential step towards the understanding of the complex magnetic properties in the ordered phases, where numerous interactions coexist .