Effect of Chemical Pressure on the Crystal Electric Field States of Erbium Pyrochlore Magnets

J. Gaudet; A. M. Hallas; A. I. Kolesnikov; B. D. Gaulin

doi:10.1103/PhysRevB.97.024415

paper

Effect of Chemical Pressure on the Crystal Electric Field States of Erbium Pyrochlore Magnets

arXiv:1708.01176 · doi:10.1103/PhysRevB.97.024415

Abstract

We have carried out a systematic study of the crystal electric field excitations in the family of cubic pyrochlores Er $_{2} B_{2}$ O $_{7}$ , with $B =$ ~Ti, Ge, Pt, and Sn, using neutron spectroscopy. All members of this family are magnetic insulators based on 4 $f^{11}$ Er $^{3 +}$ and non-magnetic $B^{4 +}$ . At sufficiently low temperatures, long-range antiferromagnetic order is observed in each of these Er $_{2} B_{2}$ O $_{7}$ pyrochlores. Our inelastic neutron scattering measurements probe the transitions from the ground state doublet to excited crystal electric field states belonging to the $J = 15/2$ Hund's rules manifold. This allows us to quantitatively determine the energy eigenvalues and eigenfunctions of these $(2 J + 1) = 16$ states across the Er $_{2} B_{2}$ O $_{7}$ series. The different ionic sizes associated with different non-magnetic $B^{4 +}$ cations correspond to positive or negative chemical pressure, depending on the relative contraction or expansion of the crystal lattice, which gives rise to slightly different local environments at the Er $^{3 +}$ site. Our results show that the $g$ -tensor components, which characterize the anisotropy of the ground state doublet eigenfunctions, are XY-like for all four members of the Er $_{2} B_{2}$ O $_{7}$ series. However, the XY anisotropy is much stronger for Er $_{2}$ Pt $_{2}$ O $_{7}$ and Er $_{2}$ Sn $_{2}$ O $_{7}$ ( $\frac{g _{⊥}}{g _{z}} > 25$ ), than for Er $_{2}$ Ge $_{2}$ O $_{7}$ and Er $_{2}$ Ti $_{2}$ O $_{7}$ ( $\frac{g _{⊥}}{g _{z}} < 4$ ). The variation in the nature of the XY-anisotropy in these systems correlates strongly with their ground states, as Er $_{2}$ Ge $_{2}$ O $_{7}$ and Er $_{2}$ Ti $_{2}$ O $_{7}$ order into $Γ_{5}$ magnetic structures, while Er $_{2}$ Pt $_{2}$ O $_{7}$ and Er $_{2}$ Sn $_{2}$ O $_{7}$ order in the $Γ_{7}$ Palmer-Chalker structure.

arXiv comment12 pages, 8 figures and 4 tables

Effect of Chemical Pressure on the Crystal Electric Field States of Erbium Pyrochlore Magnets

Abstract

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