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

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 $(2J+1)=16$ 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${}_7$ ($\frac{g_{\perp }}{g_z}>25$), than for Er${}_2$Ge${}_2$O${}_7$ and Er${}_2$Ti${}_2$O${}_7$ ($\frac{g_{\perp }}{g_z}<4$). 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 ${\Gamma }_5$ magnetic structures, while Er${}_2$Pt${}_2$O${}_7$ and Er${}_2$Sn${}_2$O${}_7$ order in the ${\Gamma }_7$ Palmer-Chalker structure.

12 pages, 8 figures and 4 tables

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Cited by in corpus (4)

• Quantum Spin Ice Dynamics in the Dipole-Octupole Pyrochlore Magnet Ce${}_2$Zr${}_2$O${}_7$
• Absence of long-range order in an $XY$ pyrochlore antiferromagnet Er${}_2$AlSbO${}_7$
• Intertwined Magnetic Dipolar and Electric Quadrupolar Correlations in the Pyrochlore Tb${}_2$Ge${}_2$O${}_7$
• The Spectral and Magnetic Properties of Er${}^{3+}$ and Yb${}^{3+}$ Ions in Y${}_2$Ti${}_2$O${}_7$ Crystals with a Pyrochlore Structure