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Description
The family of $A^\prime A^{\prime\prime}B_2$F$_7$ pyrochlore fluoride antiferromagnets represents a unique but understudied class of materials containing the three-dimensional frustrated network of corner-sharing tetrahedra. While the rare-earth-based $A_2B_2$O$_7$ pyrochlore oxide counterparts have long been the main focus of study for their exotic magnetic ground states (spin glass, spin ice, spin liquid, order-by-disorder etc.), studies of these systems require extremely low temperatures due to the weak dipolar interactions between the magnetic $4f$ ions (|$θ_{CW}$| $\sim 10^0$ – $10^1$ K).
Conversely, the $A^{\prime}A^{\prime\prime}B_2$F$_7$ pyrochlore fluorides [1] overcome this limitation by replacing oxygen (O$^{2-}$) with fluorine (F$^{1-}$), enabling the stabilization of divalent magnetic $3d$-transition-metal ions (from Mn$^{2+}$ to Cu$^{2+}$) with stronger super-exchange interactions via F$^-$ ligands (|$θ_{CW}$| $\sim 10^1$ – $10^2$ K) on the pyrochlore $B$-site. Charge balancing and structure stability constraints, however, require a mixed occupancy of the pyrochlore $A$-site by monovalent A$^{\prime +}$ (Na$^+$) and divalent $A^{\prime\prime2+}$ (Ca$^{2+}$/Sr$^{2+}$/Cd$^{2+}$) cations, leading to chemical disorder. Consequently, this chemical mixing introduces magnetic bond disorder – local variations in the magnetic exchange energy $J$ - due to the $A^{\prime +}$/$A^{\prime\prime 2+}$ ionic size mismatch.
In our contribution, we report the successful synthesis and magnetic characterisation of novel frustrated NaCd$M_2$F$_7$ pyrochlore ($M$ = Co$^{2+}$, Ni$^{2+}$, Cu$^{2+}$) and defect-fluorite ($M$ = Mn$^{2+}$) antiferromagnets. [2,3] While $M$ = Co$^{2+}$ ($J_{eff} = 1/2$), Ni$^{2+}$ ($S = 1$) and Mn$^{2+}$ ($S = 5/2$) indicate a frozen spin-glass ground-state below $T_f\sim 2$ – $4$ K by means of AC susceptibility measurements, $M$ = Cu$^{2+}$ ($S = 1/2$) shows no magnetic transition in magnetisation (DC & AC) or specific heat, with continued spin dynamics down to $50$ mK confirmed by ${\rm \mu}$SR and NMR measurements. Furthermore, the power-law scaling of $\chi$($T$) and data collapse of $C_{mag}/T$ and $M$($H$) in NaCdCu$_2$F$_7$ hint at the realisation of a $S = 1/2$ random-singlet quantum spin liquid ground-state on the pyrochlore lattice [4].
References
[1] D. Reig-i-Plessis and A. M. Hallas, “Frustrated magnetism in fluoride and chalcogenide pyrochlore lattice materials,” Physical Review Materials, vol. 5, no. 3. American Physical Society (APS), Mar. 29, 2021. https://doi.org/10.1103/physrevmaterials.5.030301
[2] A. Kancko et al., “Structural and spin-glass properties of single crystal J eff = ½ pyrochlore antiferromagnet NaCdCo2F7: correlating T f with magnetic-bond-disorder,” Physica Scripta, vol. 98, no. 7. IOP Publishing, p. 075947, Jun. 29, 2023. https://doi.org/10.1088/1402-4896/acdeb7
[3] A. Kancko et al., “Spin-glass ground states in the frustrated pyrochlore and fluorite antiferromagnets NaCd$M_2$F$_7$ ($M$ = Ni$^{2+}$, Mn$^{2+}$),” 2024, arXiv. https://doi.org/10.48550/ARXIV.2411.11579
[4] I. Kimchi et al., “Scaling and data collapse from local moments in frustrated disordered quantum spin systems,” Nature Communications, vol. 9, no. 1. Springer Science and Business Media LLC, Oct. 22, 2018. https://doi.org/10.1038/s41467-018-06800-2
