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 = 11.2 emu K/mol   ErTiO3.5 and θ CW = − 13 K . A crystal field (CF) analysis of our experimental χ ¯ data, g -values ( g ∥ = 0.27 and g ⊥ = 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 θ CW CF = − 12.5 K . Single-ion magnetic anisotropy ( 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 θ CW 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 C HF vs.  T  (K) for T < T N matches the observed results. Mössbauer lines expected for 166ErT are also predicted.