Magnetic and magnetoelastic properties in tetragonal TbPO4.

Abstract

The magnetic and magnetoelastic properties (third-order magnetic susceptibility and parastriction) of ${\mathrm{TbPO}}_{4}$, which has the zircon-type tetragonal structure, are analyzed in the paramagnetic phase. The susceptibility formalism is then used to describe all the symmetry-lowering modes in a rare-earth insulator. The dominant magnetoelastic coupling is associated with the \ensuremath{\delta} symmetry and is, at least partly, responsible for the transition occuring at 2.15 K in the antiferromagnetic phase and clearly dominates the \ensuremath{\gamma}-symmetry mode. However, none of the different symmetry-lowering modes can be completely neglected and we determine parameters for the \ensuremath{\alpha}1 and \ensuremath{\alpha}2 tetragonal modes and the \ensuremath{\varepsilon} monoclinic mode. Owing to the close vicinity of the first excited singlet to the ground-state doublet, the \ensuremath{\alpha} quadrupolar interactions play an important role in determining the temperature dependence of the level spacing at low temperature, in particular in the ordered phases. The determination of all the magnetoelastic modes of ${\mathrm{TbPO}}_{4}$ is an essential step towards the understanding of the complex magnetic properties in the ordered phases, where numerous interactions coexist.