Quenched Disorder in the Triangular Lattice Antiferromagnet YbZn$_2$GaO$_5$

TL;DR

This study reveals that site disorder in YbZn$_2$GaO$_5$ causes broadening of crystal electric field excitations and influences its magnetic properties, highlighting the role of heterogeneity in its unusual magnetism.

Contribution

It demonstrates how Ga/Zn site mixing induces heterogeneity in YbZn$_2$GaO$_5$, affecting CEF excitations and magnetic behavior, a novel insight into disorder effects in this material.

Findings

Significant broadening of CEF excitations due to heterogeneity.

Approximately 35% of Ga and 60% of Zn site mixing.

Disorder influences magnetic excitations and prevents magnetic ordering.

Abstract

We investigate the crystal electric field (CEF) excitations of Yb$^{3+}$ ions in powder samples of the triangular-lattice rare-earth-based antiferromagnet YbZn$_2$GaO$_5$ using inelastic neutron scattering (INS). Three CEF excitations from the ground-state Kramers doublet were observed, each exhibiting significant broadening beyond instrumental resolution. Combining temperature-dependent INS and neutron powder diffraction, we identify a significant static contribution to this broadening and attribute it to heterogeneous coordination of Yb$^{3+}$ ions due to Ga$^{3+}$/Zn$^{2+}$ site mixing. Rietveld refinement of neutron powder diffraction indicates that 35% of Ga occupies the Zn site and 60% of Zn occupies the Ga site. We show with a point charge model for the CEF Hamiltonian that heterogeneous coordination of Yb$^{3+}$ ions leads to broadened CEF peaks. First-principles calculations demonstrate that the random Ga$^{3+}$/Zn$^{2+}$ distribution can produce the distortions of the YbO$_6$ octahedra observed from neutron diffraction. Because the documented heterogeneity will extend to exchange interactions, our results suggest that disorder is a significant factor in the unusual magnetism previously reported in YbZn$_2$GaO$_5$, including broad low-energy magnetic excitations and the absence of magnetic ordering down to 0.3K.