Measurements of the magnetic susceptibility and anisotropy of Tb 2Ge 2O 7 single crystal

Abstract

Magnetic susceptibilities χ a and the anisotropy χ a − χ c =Δ χ in the ac-plane of the tetragonal crystal of Tb 2Ge 2O 7 or TbPG were measured between 300 and 23 K. The crystal field (CF) at the site of the Tb 3+ ion has a unique D 5h symmetry and the D 5h axis coincides with the c-axis of the tetragonal (D 4 4) crystal, which permitted easy determination of the thermal characteristics of molecular susceptibilities K ‖, K ⊥, K ̄ and the anisotropy Δ K= K ⊥− K ‖. It was found that at 300 K, Δ K was 31.3% of K ̄ and increased by 34.5 times at 23 K, becoming 107% of K ̄ , suggesting strong CF effect in TbPG. For analysing these results and the optical spectra reported earlier, the total Hamiltonian, consisting of the atomic and CF interactions, was diagonalized considering 66 intermediately coupled (IC) basis states of the 4f 8 ground configuration of Tb 3+ in the PG host, allowing J-mixing between the IC states. The best-fitted values of the atomic and CF parameters were E 1=6030, E 2=35, E 3=605, ξ so=1749, α=18, β=−673, γ=1918, B 20=470, B 40=571, B 60=2500, B 65=915 (all in cm −1). The lowest CF level was found to be a singlet followed by a doublet at 19.5 cm −1 above and the total CF splitting of the 7 F 6 ground term was 280 cm −1. The calculated value of the nuclear quadrupole splitting 〈Δ E Q〉 of the ground nuclear level of 159 Tb in TbPG changed from 3.186×10 −4 cm −1 (=0.204 mm/s) to 89.65×10 −4 cm −1 between 300 and 2 K. The Schottky specific heat C sh showed a maximum at 26 K and a hump is expected around 14 K in the C p versus T curve, calculated by substituting the value of the lattice component C L/ R=3.2±0.2 T 3×10 −5 of GdPG.