Theory of Room Temperature Ferromagnet V(TCNE)x ($1.5 < x < 2$): Role of Hidden Flat Bands

概要

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Theoretical studies on the possible origin of room temperature ferromagnetism (ferromagnetic once crystallized) in the molecular transition metal complex, V(TCNE)x ($1.5 < x < 2$) have been carried out. For this family, there have been no definite understanding of crystal structure so far because of sample quality, though the effective valence of V is known to be close to +2. Proposing a new crystal structure for the stoichiometric case of $x = 2$, where the valence of each TCNE molecule is $-1$ and resistivity shows insulating behavior, exchange interaction among d-electrons on adjacent V atoms has been estimated based on the cluster with 3 vanadium atoms and one TCNE molecule. It turns out that Hund's coupling among d orbitals within the same V atoms and antiferromagnetic coupling between d oribitals and LUMO of TCNE (bridging V atoms) due to hybridization result in overall ferromagnetism (to be precise, ferrimagnetism). This view based on localized electrons is supplemented by the band picture, which indicates the existence of a flat band expected to lead to ferromagnetism as well consistent with the localized view. The off-stoichiometric cases ($x < 2$), which still show ferromagnetism but semiconducting transport properties, have been analyzed as due to Anderson localization.
Physical Society of Japanの論文
2010-03-15