Crystal field effect and geometric frustration in Er2Ti2O7 —an XY antiferromagnetic pyrochlore

Magnetic susceptibility χ¯ of powder Er2Ti2O7 (ErT) is measured between 300 K and 80 K. χ¯ shows a Curie–Weiss (CW) type behaviour with C=11.2emu K/mol  ErTiO3.5 and θCW=−13K. A crystal field (CF) analysis of our experimental χ¯ data, gg-values (g∥=0.27g∥=0.27 and g⊥=7.8g⊥=7.8) and the positions of two CF levels (reported earlier from an inelastic neutron scattering study) provide CF parameters and CF levels of the ground 4I15/2 and excited multiplets of ErT. The theoretical χ¯ follows a CW-type behaviour, with θCWCF=−12.5K. Single-ion magnetic anisotropy (χ⊥−χ∥)(χ⊥−χ∥) is 9500×10−6 emu/mol ErTiO3.5 at 300 K, which increases by ∼54 times at 10 K and ErT resembles an XY planar system. It can be inferred from CF analysis that the earlier observed change of θCW from −13 K to −22 K below 50 K is not due to the CF effect. Nuclear hyperfine (HF) levels of 167ErT and 166ErT are calculated and the theoretical curve of CHF vs. TT (K) for T