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.

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• Short-range order and hidden energy scale in geometrically frustrated magnets
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