Theoretical studies on the isolated spin cluster complex, tetrameric cobalt (II) acetylacetonate Co4(C5H7O2)8: Effects of competing superexchange interactions

Abstract

According to x‐ray studies, Co₄(C₅H₇O₂)₈ is an assembly of linear, tetrameric Co²⁺ ion clusters, which are effectively magnetically isolated. An examination of superexchange paths suggests that this tetramer has two different kinds of nearest‐neighbor exchange constants plus a next‐nearest‐neighbor exchange constant. The only bulk experimental data available so far are the powder susceptibilities from room temperature down to about 0.35 K. This paper demonstrates that this information is apparently sufficient to infer that all three exchange constants are antiferromagnetic in type and have values in the region of Jⁿⁿ₁ ∼ 5 K, Jⁿⁿ₂ ∼ 10 K, and Jⁿⁿⁿ₃ ∼ 4 K. The interactions therefore compete; and they compete so as to give a paramagnetic character to the low temperature behavior. This is in interesting contrast to the behavior of a sister compound, trimeric nickel (II) acetylacetonate, where the paramagnetic character results from a dominant nearest‐neighbor ferromagnetic exchange constant. The exchange constants for the cobalt tetramer are unusual in the light of current ideas concerning superexchange, and additional, confirmatory experiments would be desirable. Accordingly, specific, exact, theoretical predictions are made for the specific heat and single crystal magnetization for comparison with future experiments.