Deviation from the dipole-ice model in the spinel spin-ice candidate MgEr2Se4

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, MgEr2Se4, 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 → 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 CdEr2Se4, and point to the importance of quantum fluctuations in these materials.arXiv:1703.04267v3 [cond-mat.dis-nn]

Neutron scattering measurement of crystalline-electric fields in magnesium rare-earth selenide spinels

“…The advantage of such a spectroscopic analysis is not only increased precision, but also symmetry-driven modeling which is largely insensitive to the presence of impurity phases, defects and other mechanisms which adversely affect bulk thermodynamic data. In a recent publication, we showed how a similar analysis of INS data can be used to confirm the Ising-like effective spins in the material MgEr 2 Se 4 , and additionally identified several low-lying CEF excitations capable of inducing quantum fluctuations [31]. Below, we extend this analysis to three other closely related spinel selenide systems.…”

“…The CEF parameters of the compounds MgRE2Se4 for RE = Ho, Tm and Yb from the best fits of the INS data shown in this paper, and results for RE = Er taken from our previous paper[31].…”

“…The initial fit parameters were found by rescaling the published CEF parameters found from our previous work on the material or MgEr 2 Se 4 [31] using…”

“…This substantial difference in local environment allows specific rare earth ions to adopt a drastically different symmetry in the two material families. For example, Er 3+ has XYlike moments in the '227' pyrochlores [32] and Ising-like moments in the spinels [31].…”

“…In each of the above cases, the exact nature of the magnetic ground state is closely entwined with the local CEF environment of the constituent RE moments, as has been acknowledged on several occasions [31,47,49,50,52,55]. A necessary condition for realizing spin ice physics in MgEr 2 Se 4 [31] and CdEr 2 Se 4 [47,52] is the presence of a ground state Kramers doublet with Ising symmetry. The ordered states in MgYb 2 X 4 and CdYb 2 X 4 have been discussed in the context of frustrated anisotropic exchange models, in which the particular choice of CEF parameters can select from a variety of distinct ordered or spin liquid phases [55].…”