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

Abstract

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 $J_{\rm eff}=\frac{1}{2}$ Co$^{2+}$ moments with a substantial unquenched orbital contribution, consistent with local $XY$ anisotropy seen in the isostructural Na$A''$Co$_2$F$_7$ ($A''$ = Ca, Sr) analogues. $μ$SR and $^{23}$Na NMR measurements reveal progressive slowing of spin fluctuations below $\sim10$ 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.