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

Single crystals of thulium phosphates (TmPO4) are grown and the principal magnetic susceptibility perpendicular to the symmetry axis 'c' of the crystal and the magnetic anisotropy are measured in the temperature range 300-13.5 K and 300-80 K, respectively. Though increases rapidly with the decrease of temperature, 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 D2d 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.