Estimation of single ion anisotropy in pyrochlore Dy2Ti2O7, a geometrically frustrated system, using crystal field theory

From accurate measurement of magnetic susceptibility (K) over bar of powder sample and single crystal of cubic Dy2Ti2O7 (DyT) between 300 and 70 K, a change of slope of 1/(K) over bar was detected at 130 K because of crystal field (CF) effect. For analysing (K) over bar, Mossbauer spectra and other available results on DyT consistently, the full form of the D-3d symmetry of CF was considered and J-mixing of the intermediately coupled states due to CF was done. The best fitted CF parameters were found as B-20 = -880, B-40 = -110, B-60 = 1040, B-43 = 120, B-63 = 175, B-66 = -1900 (all in cm(-1)). The corresponding multiplet of the ground H-6(15/2) has a lowest CF doublet level, well separated from the next doublet level (Delta similar to 100 cm(-1)) for which reason giant magnetostriction in DyT was not observed, unlike in TbT for which Delta similar to 12 cm(-1). Using the best fitted electronic and nuclear energy patterns, different quantities were calculated, viz., the Curie constant, saturated magnetisation and saturated magnetic moment per Dy spin at low temperatures. the values of which were respectively, 9 emu K/g ion, 3.74 x 10(5) A/m, 4.7 mu(B), being close to the measured values, g value of ground doublet, which was reduced by similar to8% from the free ion value, and the quadrupolar interaction parameter. which was nearly zero at 110 K and attained a constant value of 1240 +/- 30 MHz at 4 K as observed. The hyperfine magnetic field H-eff and the internal molecular field H-mol were found to be 540 and 0.2 T, respectively at 4 K. The calculated value of the single ion magnetic anisotropy DeltaK at low temperature was larger in DyT compared to HOT. Calculations also showed that compared to the classical Heisenberg exchange interaction (HEI), the ferromagnetic n.n. dipolar interaction (FDI) dominates in DyT being respectively, 1.53 and 0.3 K, whereas in HOT and in GdT the ratio between these interactions are much reduced. The magnetic energy gains associated with FDI and HEI were calculated and found to be 1.66 and 0.6 K, respectively. The large values of DeltaK and FDI, and weak HEI, are expected to make DyT a spin-ice system even at 0.2 K. (C) 2002 Elsevier Science B.V. All rights reserved.