Abstract
The magnetic and thermal properties of the spinel oxides CoB{sub 2}O{sub 4} have been studied for B = Rh, Co and Al. The compounds consist of magnetic Co{sup 2+} (S = 3/2) in the A site and non-magnetic B{sup 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, {theta}{sub CW}/T{sub N}, depends strongly on B{sup 3+} cations. CoRh{sub 2}O{sub 4} was a typical antiferromagnet with {theta}{sub CW}/T{sub N} = 31 K/25 K = 1.2. The ratio was enhanced up to {theta}{sub CW}/T{sub N} = 110 K/30 K = 3.7 for Co{sub 3}O{sub 4} (B = Co) and {theta}{sub CW}/T* = 89 K/9 K = 10 for CoAl{sub 2}O{sub 4}, indicating the evolution of magnetic frustration. The specific heat for CoAl{sub 2}O{sub 4} exhibited a broad peak at T* = 9 K and T{sup 2.5} behaviour at low temperatures, suggesting the most frustrated magnet CoAl{sub 2}O{sub 4} is in the critical vicinity of a quantum melting point of the antiferromagnetically ordered state.
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