Melting of antiferromagnetic ordering in spinel oxide CoAl2O4

The magnetic and thermal properties of the spinel oxides CoB 2 O 4 have been studied for B = Rh, Co and Al. The compounds consist of magnetic Co 2+ ( S = 3/2) in the A site and non-magnetic B 3+ 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 B 3+ cations. CoRh 2 O 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 3 O 4 (B = Co) and |Θ CW |/ T * = 89 K/9 K = 10 for CoAl 2 O 4 , indicating the evolution of magnetic frustration. The specific heat for CoAl 2 O 4 exhibited a broad peak at T * = 9 K and T 2.5 behaviour at low temperatures, suggesting the most frustrated magnet CoAl 2 O 4 is in the critical vicinity of a quantum melting point of the antiferromagnetically ordered state.