Melting of antiferromagnetic ordering in spinel oxide CoAl2O4

The magnetic and thermal properties of the spinel oxides CoB2O4 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, ΘCW/TN, depends strongly on B3+ cations. CoRh2O4 was a typical antiferromagnet with ΘCW/TN = 31 K/25 K = 1.2. The ratio was enhanced up to ΘCW/TN = 110 K/30 K = 3.7 for Co3O4 (B = Co) and ΘCW/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 T2.5 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