The S = ½ Heisenberg pyrochlore antiferromagnet is a notoriously complex quantum system, the true ground-state of which remains an unresolved problem. The theoretical ground-state predictions are strongly method-dependent and vary from dimer singlet phases, spin-liquids with short-range correlations to chiral spin liquids. Very few model materials exist to complement and confirm these predictions and ideal realisations of this system are still missing. 

The NaAM₂F₇ pyrochlore fluorides (A = Sr²⁺, Ca²⁺, Cd²⁺ and M = transition metal 2+ ion)  [1] are a family of near-ideal model materials but suffer from a small magnetic-bond-disorder due to random mixing between Na⁺ and A²⁺ ions on the pyrochlore A-site. The previous reports of M = Co²⁺ members of this family, NaCaCo₂F₇ and NaSrCo₂F₇ (both S_eff = ½) have shown strong antiferromagnetic interactions (θ_CW ≈ -140 K)  [2,3] with no magnetic order, instead freezing into spin-glass ground-states with XY-type anisotropy at low temperatures (T_f ≤ 3K). However, no ideal S = ½ realisations of this family have yet been studied except for the first successful synthesis report of the S = ½ Cu²⁺-based pyrochlore, NaCdCu₂F₇.  [4] 

We report a thorough magnetic-ground-state characterisation (thermodynamic measurements, ac-susceptibility, [high-field] dc-magnetisation, ²³Na-NMR, and μSR) of the two new A = Cd additions to this pyrochlore fluoride family, the random-spin-singlet quantum spin liquid candidate NaCdCu₂F₇ (S = ½) and the spin-glass NaCdCo₂F₇ (S_eff = ½). While these isostructural, magnetically-frustrated compounds share apparent similarities such as employing the low-spin-state (effective)-spin-½ magnetic ions (Cu²⁺ and Co²⁺) on the frustrated pyrochlore lattice, or showing similar exchange interaction strengths (θ_CW ≈ -100 K), they seem to show very different magnetic-ground-state properties. While NaCdCo₂F₇ undergoes a spin-freezing transition at T_f = 4 K  [5], NaCdCu₂F₇ shows continuous dynamics with no signs of any magnetic transition down to 50 mK. In addition, NaCdCu₂F₇ shows a power-law scaling of low-temperature susceptibility χ(T) ~ T ^-γ (with γ = 0.68), as well as data collapse of magnetic specific heat (μ₀H)^γ C_mag/T vs. T/μ₀H and isothermal magnetization MT^γ-1 vs. μ₀H/T. This suggests random-spin-singlet physics stemming from defect spins with a power-law distribution of exchange energies P[J] ~ T ^-γ. [6]

References
[1]       A. Kancko et al., ArXiV, arXiv:2411.11579, s. 1–14. (2024).
[2]       J. W. Krizan and R. J. Cava, NaCaCo₂F₇: A single-crystal high-temperature pyrochlore antiferromagnet, Phys. Rev. B 89, 214401 (2014).
[3]       J. W. Krizan and R. J. Cava, NaSrCo₂F₇, a Co²⁺ pyrochlore antiferromagnet, J. Phys. Condens. Matter 27, 296002 (2015).
[4]       R. Hansler and W. Rudorff, A₂B₂F₇-Verbindungen mit Pyrochlor- und Weberitstruktur, Z. Naturforsch. B 1306 (1967).
[5]       A. Kancko, G. Giester, and R. H. Colman, Structural and spin-glass properties of single crystal Jeff = ½ pyrochlore antiferromagnet NaCdCo₂F₇: correlating Tf with magnetic-bond-disorder, Phys. Scr. 98, 075947 (2023).
[6]       I. Kimchi, J. P. Sheckelton, T. M. McQueen, and P. A. Lee, Scaling and data collapse from local moments in frustrated disordered quantum spin systems, Nat. Commun. 9, 1 (2018).