Origin of ferromagnetic exchange interactions in a fullerene–organic compound

Abstract

Organic ferromagnets, which exhibit exchange interactions between unpaired electrons in pi-orbitals, are rare, and the origin of ferromagnetism in these compounds has so far remained unexplained. Tetrakis(dimethylamino)ethylene-fullerene[60] (TDAE-C60) shows a transition to a ferromagnetic state with fully saturated s = 1/2 molecular spins at the relatively high Curie temperature (for organic materials) of 16 K (ref. 4). It has been suggested that the orientations of the C60 molecules may be important for ferromagnetism in this material, but in the absence of structural data at low temperatures there has been little progress towards understanding these microscopic interactions. Here we report the results of a comparative structural study of two different magnetic forms of TDAE-C60 crystals at low temperatures, correlating the structural properties--in particular, the intermolecular orientations--with the magnetic properties. We find that both ferromagnetism and spin-glass-like ordering are possible in this material, and depend on the orientational state of C60 molecules. This resolves the apparent contradictions posed by different macroscopic measurements, and opens the way to a microscopic understanding of pi-electron ferromagnetic exchange interactions in organic materials.

Defect-induced magnetism: Codoping and a prescription for enhanced magnetism

“…In some cases, the lattice relaxations of Jahn-Teller form are found to underlie the establishment of the bipartite structure [30][31][32] In the McConnell-type systems, the intrinsic bipartite structure is modified by the charge transfer processes. The effective exchange interactions (J ab ) between free radicals can be considered as consisting of two major contributions, i.e.,…”

“…Roth 29 : This model is known as the orbital ordering model and was subsequently used by Kawamoto to explain the ferromagnetism in the TDAE-C 60 and layered perovskite-type materials [30][31][32] . This is found in the case of a system exhibiting narrow d-or impurity band and spatial orbital ordering which, by its nature, has the potential to favor a FM ground state.…”

Magnetic Ordering in an Organic Polymer

“…Recent investigations into the properties of organic magnets have generally concentrated on materials, such as crystalline solids of small molecule radicals or charge transfer salts, in which the exchange interaction involves s-and p-orbitals (1)(2)(3)(4)(5). An alternative approach to organic magnets may be based on -conjugated polymers, as envisioned by Mataga in 1968 (6 ).…”

Development of Conductive Organic Molecular Assemblies: Organic Metals, Superconductors, and Exotic Functional Materials

“…1062 Recently, it has been reported that C 60 molecules in TDAE Á C 60 are polymerized by applying pressure (''-phase''), 1027,1028 although the structural information including the polymerized pattern is currently absent. Curie temperature gradually decreases as the applied pressure increases, and the ferromagnetic ordering is completely suppressed above 1 GPa 1027 or 0.7 GPa.…”