A Novel Antiferromagnetic Organic Superconductor κ-(BETS)2FeBr4 [Where BETS = Bis(ethylenedithio)tetraselenafulvalene]

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Abstract
The electrical and magnetic properties of κ-(BETS)2FeBr4 salt [where BETS = bis(ethylenedithio)tetraselenafulvalene] showed that this system is the first antiferromagnetic organic metal at ambient pressure (TN = 2.5 K). The characteristic field dependence of the magnetization at 2.0 K indicates a clear metamagnetic behavior. The small resistivity drop observed at TN clearly shows the existence of the interaction between π metal electrons and localized magnetic moments of Fe3+ ions. In addition, this system underwent a superconducting transition at 1.1 K. That is, κ-(BETS)2FeBr4 is the first antiferromagnetic organic metal exhibiting a superconducting transition below Néel temperature. The magnetic field dependence of the superconducting critical temperature indicated that the superconductivity in this system is strongly anisotropic also in the conduction plane because of the existence of the metamagnetically induced internal field based on the antiferromagnetic ordering of the Fe3+ 3d spins in contrast to the cases of the other conventional organic superconductors. Furthermore, the specific heat measurement exhibited a λ-type large peak of zero-field specific heat corresponding to the three-dimensional antiferromagnetic ordering of high-spin Fe3+ ions. The lack of distinct anomaly in the Cp vs T curve at Tc suggests the coexistence of the superconductivity and the antiferromagnetic order below Tc.