Effect of chemical pressure on the crystal electric field states of erbium pyrochlore magnets

We have carried out a systematic study of the crystal electric field excitations in the family of cubic pyrochlores Er2B2O7 with B=Ge, Ti, Pt, and Sn, using neutron spectroscopy. All members of this family are magnetic insulators based on 4f11Er3+ and nonmagnetic B4+. At sufficiently low temperatures, long-range antiferromagnetic order is observed in each of these Er2B2O7 pyrochlores. The different ionic sizes associated with different nonmagnetic B4+ cations correspond to positive or negative chemical pressure, depending on the relative contraction or expansion of the crystal lattice, which gives rise to different local environments at the Er3+ site. Our results show that the g-tensor components are XY-like for all four members of the Er2B2O7 series. However, the XY anisotropy is much stronger for Er2Pt2O7 and Er2Sn2O7(g⊥/gz>25) than for Er2Ge2O7 and Er2Ti2O7(g⊥/gz<4). The variation in the nature of the XY anisotropy in these systems correlates strongly with their ground states as Er2Ge2O7 and Er2Ti2O7 order into Γ5 magnetic structures, whereas Er2Pt2O7 and Er2Sn2O7 order in the Γ7 Palmer-Chalker structure.