Deviation from the dipole-ice model in the new spinel spin-ice candidate, MgEr${}_2$Se${}_4$

Abstract

In spin ice research, small variations in structure or interactions drive a multitude of different behaviors, yet the collection of known materials relies heavily on the `227' pyrochlore structure. Here, we present thermodynamic, structural and inelastic neutron scattering data on a new spin-ice material, MgEr${}_2$Se${}_4$, which contributes to the relatively under-explored family of rare-earth spinel chalcogenides. X-ray and neutron diffraction confirm a normal spinel structure, and places Er${}^{3+}$ moments on an ideal pyrochlore sublattice. Measurement of crystal electric field excitations with inelastic neutron scattering confirms that the moments have perfect Ising character, and further identifies the ground state Kramers doublet as having dipolar-octupolar form with a significant multipolar character. Heat capacity and magnetic neutron diffuse scattering have ice-like features, but are inconsistent with Monte Carlo simulations of the nearest-neighbor and next-nearest-neighbor dipolar spin-ice (DSI) models. A significant remnant entropy is observed as $T\to 0$~K, but again falls short of the full Pauling expectation for DSI, unless significant disorder is added. We show that these observations are fully in-line with what is recently reported for CdEr${}_2$Se${}_4$, and point to the importance of quantum fluctuations in these materials.

arXiv comment12 pages with 12 figures, including appendix. Changes since last revision include, new monte carlo simulations fitting to the neutron powder diffraction data, update of crystal field parameter fitting method to find error bars, update of magnetization calculation for higher accuracy at high field, and numerous updates to wording for better clarity and fixing spelling and formatting issues

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• Distinguishing thermodynamics and spectroscopy for octupolar U(1) spin liquid of Ce-pyrochlores
• Orientation-dependent stabilization of MgCr${}_2$O${}_4$ spinel thin films
• Single Crystal Diffuse Neutron Scattering Study of the Dipole-Octupole Quantum Spin Ice Candidate Ce${}_2$Zr${}_2$O${}_7$: No Apparent Octupolar Correlations Above $T=0.05$ K