Spin-orbit-entangled $J_{\mathrm{e}\mathrm{f}\mathrm{f}}=\frac{1}{2}$ magnetism and unconventional spin freezing in the bond-disordered pyrochlore antiferromagnet NaCdCo${}_2$F${}_7$

Bond disorder in frustrated pyrochlore antiferromagnets can give rise to fundamentally different quantum ground states depending on the nature of the local magnetic moments. Here, we show that the bond-disordered pyrochlore NaCdCo${}_2$F${}_7$ realizes an unconventional spin-glass-like state with continued dynamics, in stark contrast to its isostructural $S=\frac{1}{2}$ NaCdCu${}_2$F${}_7$ counterpart. High-field magnetization and Co $L_{2,3}$-edge XAS/XMCD establish spin-orbit-entangled $J_{\mathrm{e}\mathrm{f}\mathrm{f}}=\frac{1}{2}$ Co${}^{2+}$ moments with a substantial unquenched orbital contribution, consistent with local $XY$ anisotropy seen in the isostructural Na$A^{\mathrm{\prime }\mathrm{\prime }}$Co${}_2$F${}_7$ ($A^{\mathrm{\prime }\mathrm{\prime }}$ = Ca, Sr) analogues. $\mu$SR and ${}^{23}$Na NMR measurements reveal progressive slowing of spin fluctuations below $\sim 10$ K, culminating in a partially frozen state with persistent low-temperature dynamics that deviates from a canonical spin glass. Comparison with the isostructural bond-disordered pyrochlore NaCdCu${}_2$F${}_7$, which realizes a random-singlet state, reveals a fundamentally different response of spin-orbit-entangled Co${}^{2+}$ moments to bond disorder. These results identify spin-orbit coupling as a key ingredient governing the fate of bond-disordered frustrated pyrochlore magnets.