Coexistence of magnetic order and spin-glass-like phase in the pyrochlore antiferromagnet Na₃Co(CO₃)₂Cl

We present comprehensive investigations on a pyrochlore antiferromagnet Na₃Co(CO₃)₂Cl. Performed dc magnetization indicates a broad maximum at around 4 K. The field dependence of the peak temperature of this maximum follows the de Almeida-Thouless line, suggesting a spin-glass-like phase transition. The ac susceptibility measurements determine the glassy transition temperature to be 4.5 K and reveal a frequency-independent peak at 17 K. The temperature dependence of the specific heat shows a sharp peak at 1.5 K and a broad hump at around 5 K, which are attributed to a long-range magnetic phase transition and a spin-glass-like freezing process, respectively. The average crystallographic structure of Na₃Co(CO ₃)₂Cl has been determined using neutron-powder diffraction. No obvious site disorder has been detected within the experimental resolution. The diffuse neutron scattering with polarization analysis indicates short-range spin correlations characterized by dominating antiferromagnetic coupling between nearest neighbors and weak ferromagnetic coupling between next-nearest neighbors. The long-range magnetic order below 1.5 K is evidenced by the magnetic reflections observed at 50 mK and can be well explained with an all-in-all-out spin configuration. Inelastic neutron scattering of Na ₃Co(CO₃)₂Cl exhibits collective magnetic excitations at 3.5 K, suggesting that the spin-glass-like transition temperature T_g = 4.5 K does not correspond to a complete spin-glass freezing as expected in canonical spin glasses. The peak observed in magnetic susceptibility at 17 K is attributed to the onset of an intermediate partially ordered phase transition, qualitatively consistent with the theoretical predictions for pyrochlore antiferromagnets with weak ferromagnetic next-nearest-neighbor interactions.