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

## TL;DR

This study introduces MgEr$_2$Se$_4$ as a new spin-ice candidate with a normal spinel structure, revealing deviations from classical dipolar-ice models and highlighting the role of quantum fluctuations in its magnetic behavior.

## Contribution

The paper reports the synthesis and characterization of MgEr$_2$Se$_4$, demonstrating its unique magnetic properties and deviations from traditional spin-ice models, emphasizing the importance of quantum effects.

## Key findings

- MgEr$_2$Se$_4$ has a normal spinel structure with Er$^{3+}$ moments on a pyrochlore sublattice.
- The ground state is dipolar-octupolar with significant multipolar character.
- Heat capacity and neutron scattering show ice-like features but deviate from classical dipolar spin-ice models.

## 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 $T\rightarrow0$~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.