Organic Radical Based Magnetic Materials with Strong Intermolecular Magnetic Interactions

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Recently acquirement of rare earth metals in industrial circles grows into a serious problem. At some future date, it will be important to develop high performance magnets without use of heavy rare elements. In fact we should consider that we make the high performance materials from abundant elements such as carbon, nitrogen, oxygen, hydrogen, sulfur, iron, etc. Then, there are organic ferromagnets as candidate for materials satisfying the above demand. Since the first organic ferromagnet, p-NPNN, was found in 1991, more than 30 organic ferromagnets with intermolecular ferromagnetic interactions have been synthesized. However, their Curie temperatures of molecule-based ferromagnets with the ferromagnetic ordered state remained low because intermolecular ferromagnetic interactions were generally weak in organic magnetic materials. In this study, a new approach to construction of molecule-based ferrimagnets which have strong antiferromagnetic interactions and which have high transition temperatures is reported. In an ionic crystal, cations and anions are usually aligned alternatively due to the Coulomb's interaction. If the spin multiplicity of the cation will be different from that of the anion, desirable molecular alignment from ferrimagnetism in ionic crystals will be obtained. We demonstrated ferrimagnetic ordering below 12. 2 K in an ionic crystal, BBDTA_2ReBr_6, consisted of an organic radical cation BBDTA^+(S=1/2) and a transition metal anion ReBr_6^<2->(S=3/2). Although the magnetic transition temperature is relatively low, compared to those of traditional molecule-based ferrimagnets, we believe that this new approach using the Coulombic interactions will be useful for the construction of ferrimagnetic materials. Combination of various magnetic cations and anions will give rise to ferrimagnets with high transition temperature. The above aspects will attract the interest of readers working in the various fields on materials science.