Abstract
The electric and magnetic properties of a series of highly correlated two-dimensional organic π conductors incorporating magnetic ions ( with ), [ ] were examined by controlling the Br content (x). A broad resistivity maximum at 100 indicating the strong correlation of π conduction electrons becomes prominent with increasing x. At the same time the metal-insulator (MI) transition temperature increases from 8.5 to 18 K The crystal with shows a semiconductor-insulator transition around 20 K. At the MI transition and the antiferromagnetic (AF) transitions take place cooperatively around 8.5 K A large magnetization drop was observed at for the magnetic field parallel to the c axis which indicates the appearance of localized π spins and a strong AF coupling of π and d spin systems. For two anomalies were observed in the magnetization-temperature curve. The high-temperature anomaly corresponds to a MI transition and the low-temperature one corresponds to an AF ordering of the spins In this case, the relatively small magnetization drop observed at suggests a small coupling of π and d spin systems. For the magnetization drop at disappeared, which shows that the π and d electron systems are decoupled and the π electron system undergoes MI transition independently of the d spin systems. The disappearance of the susceptibility anomaly at indicates that the π electron system transforms to a nonmagnetic insulating state below On the other hand, the anisotropy of M showing the development of a AF spin structure of the Fe spins was observed independently of Around the direction of the easy axis of the AF spin structure changes from parallel to the c axis to perpendicular to it In other words, the direction of easy axis is varied according to the magnitude of coupling. Magnetoresistance measurements showed that of is almost independent of the magnetic field below 1 kbar. But a field-restored highly conducting state similar to that of was observed at high pressure. The Weiss temperature (θ) estimated from the curve at decreases with increasing x.
- Received 26 May 1998
DOI:https://doi.org/10.1103/PhysRevB.58.9294
©1998 American Physical Society