A giant negative magnetoresistance effect in an iron tetrabenzoporphyrin complex

摘要

By measuring the electrical resistivity in TPPFe-III(tbp)(CN)(2)(2) (TPP = tetraphenylphosphonium and tbp = tetrabenzoporphyrin) under the application of a static magnetic field, a giant negative magnetoresistance (MR) effect with high anisotropy is observed. More specifically, the MR ratio at 13 K under a field of 9 T perpendicular to the c axis is -70, whereas the MR ratio under a field parallel to the c axis is -40. Furthermore, electron spin resonance (ESR) measurements indicate large anisotropy in the principal g-values of d spin (S = 1/2) in the Fe-III(tbp)(CN)(2) unit; the g(1) value almost perpendicular to the tbp plane and the g(2) and g(3) values almost parallel to the tbp plane are 3.60, 1.24, and 0.39, respectively. It is revealed that the anisotropy in the MR effect arises from the anisotropy in the d spin, suggesting that the d spins in TPP Fe-III(tbp)(CN)(2)(2) affect the pi-conduction electron via the intramolecular pi-d interaction. The anisotropy and magnitude in the giant negative MR effect for TPPFe-III(tbp)(CN)(2)(2) are smaller than the corresponding values for the isostructural phthalocyanine (Pc) analogue TPPFe-III(Pc)(CN)(2)(2). This is consistent with the fact that the intermolecular antiferromagnetic d-d interaction in TPPFe-III(tbp)(CN)(2)(2) (suggested by the Weiss temperature: Theta = -8.0 K) is weaker than that in TPPFe-III(Pc)(CN)(2)(2) (Theta = -12.3 K). This indicates that the minor modification in coordination complexes can significantly affect the MR effect via tuning the intermolecular d-d interaction as well as the intermolecular pi-pi overlap.