Structural and spin-glass properties of single crystal $J_{eff}$ = 1/2 pyrochlore antiferromagnet NaCdCo$_2$F$_7$ : correlating $T_f$ with magnetic-bond-disorder

Abstract: Weak bond disorder disrupts the expected spin-liquid ground-state of the ideal $S$ = 1/2 Heisenberg pyrochlore antiferromagnet. Here we introduce a single crystal study of the structural and magnetic properties of the bond-disordered pyrochlore NaCdCo$2$F$_7$. The magnetic susceptibility appears isotropic, with a large negative Curie-Weiss temperature ($\theta{CW}$ = -108(1) K), however no magnetic order is observed on cooling until a spin-glass transition at $T_f$ = 4.0 K. AC-susceptibility measurements show a frequency-dependent shift of the associated cusp in $\chi$ at $T_f$, that can be fitted well by the empirical Vogel-Fulcher law. The magnetic moment of $\mu_{eff}$ = 5.4(1) $\mu_{B}$/Co$^{2+}$ indicates a significant orbital contribution and heat capacity measurements show that down to 1.8 K, well below $T_f$, only $S_{mag}$ ~2/3 Rln(2) of the magnetic entropy is recovered, suggestive of residual continued dynamics. Structural and magnetism comparisons are made with the other known members of the Na$A$Co$_2$F$_7$ family ($A$ = Ca$^{2+}$, Sr$^{2+}$), confirming the expected relationship between spin-glass freezing temperature, and extent of magnetic bond disorder brought about by the size mismatch between A-site ions.