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

Abstract

The symmetry of local moments plays a defining role in the nature of exotic grounds states stabilized in frustrated magnetic materials. We present inelastic neutron scattering (INS) measurements of the crystal electric field (CEF) excitations in the family of compounds MgRE2Se4 (RE ∈ {Ho, Tm, Er and Yb}). These compounds form in the spinel structure, with the rare earth ions comprising a highly frustrated pyrochlore sublattice. Within the symmetry constraints of this lattice, we fit both the energies and intensities of observed modes in the INS spectra to determine the most likely CEF Hamiltonian for each material and comment on the ground state wavefunctions in the local electron picture. In this way, we experimentally confirm MgTm2Se4 has a non-magnetic ground state, and MgYb2Se4 has effective S = 1 2 spins with g = 5.188(79) and g ⊥ = 0.923(85) µB. The spectrum of MgHo2Se4 indicates a ground state doublet containing Ising spins with g = 2.72(46), though low-lying CEF levels are also seen at thermally accessible energies δE = 0.591(36), 0.945(30) and 2.88(7) meV, which can complicate interpretation. These results are used to comment on measured magnetization data of all compounds, and are compared to published results on the material MgEr2Se4. :1906.10767v1 [cond-mat.dis-nn] arXiv

Frustrated Magnetism in Fluoride and Chalcogenide Pyrochlore Lattice Materials

“…The accuracy of these studies was limited by their assumption of a perfect octahedral environment and also because magnetization data alone was used for the refinements of the CEF Hamiltonian. More recent studies have used inelastic neutron scattering to directly measure the CEF levels and find that although the ligands are a near perfect octahedra, the field from the next nearest neighbor creates a significant anisotropy along the local [111] directions, which is the axis that connects the centers of two adjacent tetrahedra [48][49][50]. The other major omission of the works prior to the new millennia is that they did not take into account the importance of geometric frustration, which explains their findings that none of these materials magnetically order above 2 K [43][44][45]51].…”

“…There are conflicting reports on the spin anisotropy of the Yb 3+ moments, which can only partially be explained by material dependent parameters (the specific A and X ions). While one study found that the in-and out-of-plane components are nearly equal [62] and thus, Heisenberg-like, others have reported that it is weakly [63], or moderately Ising-like [50]. An exact identification of the ground state anisotropy is difficult because direct measurements of the CEF energy levels via neutron scattering leads to an underconstrained problem.…”

“…An exact identification of the ground state anisotropy is difficult because direct measurements of the CEF energy levels via neutron scattering leads to an underconstrained problem. An additional constraint can be added by considering the magnetization as a function of applied field [50], which led to the most Ising-like result. However the accuracy of this method is debatable, as the strength of short range correlations at low temperature may confound the fits [63].…”

“…The CEF ground state was initially suggested to be a non-magnetic singlet [45], due to low temperature susceptibility measurements showing temperature independent paramagnetism in CdHo 2 S 4 . More recently, direct measurement of the CEF levels in MgHo 2 Se 4 via inelastic neutron scattering reveals a non-Kramers doublet ground state, in close proximity to an excited doublet and singlet below 1 meV, and yet another singlet below 3 meV [50]. The complicated CEF scheme with many low lying levels bares some resemblance to the terbium oxide pyrochlores, whose ground states are formed by two closely spaced doublets and the competing energy scales of single ion effects and the ion-ion interactions leads to rich and complex physics that has eluded description by a microscopic theory [72][73][74][75].…”

Crystal electric field excitations in the quantum spin liquid candidate NaErS2

“…In order to facilitate the QSL search in the delafossites, it is crucial to have an overview of their CEF environment. As exemplified by the spin ice compounds [40][41][42][43], the CEF parameters in systems with similar structures normally obey the scaling rule. Therefore, compared to the Yb 3+ ions with J = 7/2 [35,39], rare earth ions with a larger J allow more CEF transitions, which will enable a more accurate fit of the CEF parameters and thus provide a reference in the study of the similar delafossite compounds.…”