Melting of antiferromagnetic ordering in spinel oxide CoAl ₂O₄

The magnetic and thermal properties of the spinel oxides CoB ₂O₄ have been studied for B ≤ Rh, Co and Al. The compounds consist of magnetic Co²⁺ (S ≤ 3/2) in the A site and non-magnetic B³⁺ in the B site, which form diamond and pyrochlore sublattices, respectively. We found that the ratio of the Curie-Weiss temperature to the magnetic ordering temperature, |Θ_CW|/T _N, depends strongly on B³⁺ cations. CoRh₂O ₄ was a typical antiferromagnet with |Θ_CW|/T _N ≤ 31 K/25 K ≤ 1.2. The ratio was enhanced up to |Θ_CW|/T_N ≤ 110 K/30 K ≤ 3.7 for Co ₃O₄ (B ≤ Co) and |Θ_CW|/T ^* ≤ 89 K/9 K ≤ 10 for CoAl₂O₄, indicating the evolution of magnetic frustration. The specific heat for CoAl₂O₄ exhibited a broad peak at T^* ≤ 9 K and T^2.5 behaviour at low temperatures, suggesting the most frustrated magnet CoAl₂O₄ is in the critical vicinity of a quantum melting point of the antiferromagnetically ordered state.