Long-range dynamical magnetic order and spin tunneling in the cooperative paramagnetic states of the pyrochlore analogous spinel antiferromagnets CdYb2X4 ( X=S<

Abstract

Magnetic systems with spins sitting on a lattice of corner sharing regular tetrahedra have been particularly prolific for the discovery of new magnetic states for the last two decades. The pyrochlore compounds have offered the playground for these studies, while little attention has been comparatively devoted to other compounds where the rare earth R occupies the same sub-lattice, e.g. the spinel chalcogenides CdR2X4 (X = S, Se). Here we report measurements performed on powder samples of this series with R = Yb using specific heat, magnetic susceptibility, neutron diffraction and muon-spin-relaxation measurements. The two compounds are found to be magnetically similar. They long-range order into structures described by the Γ5 irreducible representation. The magnitude of the magnetic moment at low temperature is 0.77 (1) and 0.62 (1) µB for X = S and Se, respectively. Persistent spin dynamics is present in the ordered states. The spontaneous field at the muon site is anomalously small, suggesting magnetic moment fragmentation. A double spin-flip tunneling relaxation mechanism is suggested in the cooperative paramagnetic state up to 10 K. The magnetic space groups into which magnetic moments of systems of corner-sharing regular tetrahedra order are provided for a number of insulating compounds characterized by null propagation wavevectors.

Frustration and anisotropic exchange in ytterbium magnets with edge-shared octahedra

“…Of the possible ordered states expected for the nearest-neighbor anisotropic exchange model, this is only consistent with the (nearly) gapless spectrum expected in a Γ 5 state [90], as found in Er 2 Ti 2 O 7 [12,20,91,92]. Indeed the presence of Γ 5 order was recently directly observed in the CdYb 2 S 4 and CdYb 2 Se 4 spinels by neutron diffraction [86]. Given these considerations, the AYb 2 X 4 may represent a particular clean example of quantum order-bydisorder, free from some of the potential complications of the Er 2 M 2 O 7 family [25], similar to what has been proposed for Yb 2 Ge 2 O 7 [12,18,21].…”

“…Indeed, if we consider K = Γ = 0, thermal fluctuations select the ψ 2 state near T N , but ψ 3 near T = 0 (classically) [34,76,77]. Whether such an intermediate ψ 2 phase [86] 11.003 −13.0 1.92 CdYb 2 Se 4 [86] 11.455 −11.0 1. 75 TABLE I.…”

“…Note that this procedure ignores variations in the ligand structural parameter x. While this parameter is only reported in the literature for CdYb 2 S 4 (with x ∼ 0.2579 [26] and x ∼ 0.2594 [86]) and for CdYb 2 Se 4 (with x ∼ 0.2575 [86]), we note that this parameter does not appear to vary strongly upon variation of of A = Mg, Cd or the ligand X = S, Se [26,80,86]. With these rescaled parameters, one finds energy levels that are broadly consistent with that found from fitting the magnetic susceptibility [27] for CdYb 2 S 4 and MgYb 2 S 4 , as presented in Table I.…”

Frustrated Quantum Rare-Earth Pyrochlores

“…Early studies have shown much the same diversity seen in the R 2 M 2 O 7 pyrochlores, with examples of classical dipolar spin ice [145][146][147], as well as potential candidates for order-by-quantum-disorder as found in Er 2 Ti 2 O 7 [148,149]. More speculatively, it is possible that the Yb-spinels [148,149] may exhibit similar physics to Yb 2 Ge 2 O 7 and its cousins [29] -if so, they could shed new light on the underlying mystery of the Yb 2 M 2 O 7 family. Synthesis of single crystals as well as detailed studies of the excitations of these rare-earth pyrochlore spinels are thus needed.…”

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

“…Indeed, both MgEr 2 Se 4 [31] and CdEr 2 Se 4 [47,52] have independently been identified as strong candidates for a classical spin ice state. Ordered states have been observed in both MgYb 2 X 4 [50] and CdYb 2 X 4 [51] X ∈ {S, Se}, but are notable for their highly renormalized moments and the existence of persistent spin dynamics at low temperatures [50,51]. Similar anomalous fluctuations are reported in CdHo 2 S 4 [49] below a reported ordering transition, along with several features which draw parallels to the "partially ordered" pyrochlore system Tb 2 Sn 2 O 7 [53,54].…”