Magnetic, thermal and hyperfine behaviours of Tm³⁺ in TmPO₄, YPO₄ and LuPO₄: a comparative study

Abstract

Single crystals of thulium phosphates (TmPO₄) are grown and the principal magnetic susceptibility perpendicular to the symmetry axis ‘c’ of the crystal $\left( {{\text{ $ \chi $ }}_ \bot } \right)$ and the magnetic anisotropy $\Delta {\text{ $ \chi $ }}\left( {{\text{ = $ \chi $ }}_\parallel - {\text{ $ \chi $ }}_ \bot } \right)$ are measured in the temperature range 300–13.5 K and 300–80 K, respectively. Though ${\text{ $ \chi $ }}_ \bot$ increases rapidly with the decrease of temperature, ${\text{ $ \chi $ }}_\parallel$ increases very slowly with the lowering of temperature. The tetragonal crystal structure of TmPO₄ is similar to that of Tm³⁺ in YPO₄ and LuPO₄ and in all the cases the non-Kramers Tm³⁺ ion occupies a site of D_2d symmetry. Our observed magnetic data on TmPO₄ are analyzed using crystal field analysis where the Hamiltonian includes the atomic free ion and crystal field (one-electron) interaction term. The computed and measured paramagnetic susceptibilities of TmPO₄ agree very well in the temperature range of our study. The magnetic behaviours of Tm³⁺ in YPO₄ and LuPO₄ are also studied and the results are compared with that of TmPO₄. In all the cases the natures of thermal variations of average susceptibilities and magnetic anisotropies are mostly governed by the perpendicular susceptibilities as their values are higher and rapidly increase with the lowering of temperature compared to parallel susceptibilities. The computed nuclear quadruple splittings, electronic heat capacities of Tm³⁺ in three different hosts give some interesting results.