Coupling of metal-insulator and antiferromagnetic transitions in the highly correlated organic conductor incorporating magnetic anions, λ-BETS2FeBrxCl4-x[BETS = Bis(ethylenedithio)tetraselenafulvalene]

The electric and magnetic properties of a series of highly correlated two-dimensional organic rr conductors incorporating magnetic ions (Fe3+ with S=5/2), lambda-BETS2FeBrxCl4-x [BETS=bis(ethylene-dithio)tetraselenafulvalene, x=0-0.8] were examined by controlling the Br content (x). A broad resistivity maximum at 100 (x approximate to 0)-50 K (x approximate to 0.7) indicating the strong correlation of pi conduction electrons becomes prominent with increasing x. At the same time the metal-insulator (MI) transition temperature (T-MI) increases from 8.5 (x=0) to 18 K (x=0.7). The crystal with x=0.8 shows a semiconductor-insulator transition around 20 K. At 0<x<0.2, the MI transition and the antiferromagnetic (AF) transitions take place cooperatively around 8.5 K (T(N)approximate to T-MI) A large magnetization drop was observed at T-MI or the magnetic field parallel to the c axis (H parallel to c), which indicates the appearance of localized pi spins (S=1/2) and a strong AF coupling of pi and d spin systems. For 0.3<x<0.5, two anomalies were observed in the magnetization-temperature (M-T) curve. The high-temperature anomaly corresponds to a MI transition (TMI) and the low-temperature one corresponds to an AF ordering of the Fe3+ spins (T-N) In this case, the relatively small magnetization drop observed at TMI suggests a small coupling of pi and d spin systems. For x>0.6, the magnetization drop at TMI disappeared, which shows that the pi and d electron systems are decoupled and the pi electron system undergoes MI transition independently of the d spin systems. The disappearance of the susceptibility anomaly at TMI indicates that the rr electron system transforms to a nonmagnetic insulating state below T-MI. On the other hand, the anisotropy of M showing the development of a AF spin structure of the Fe spins was observed independently of x(0<x<0.8). Around x=0.3-0.5, the direction of the easy axis of the AF spin structure changes from parallel to the c axis (x<0.2) to perpendicular to it (x>0.6). In other words, the direction of easy axis is varied according to the magnitude of pi-d coupling. Magnetoresistance measurements showed that T-MI of lambda-BETS2FeBr0.7Cl3.3 is almost independent of the magnetic field below 1 kbar. But a field-restored highly conducting state similar to that of lambda-BET2FeCl4 was observed at high pressure. The Weiss temperature (theta) estimated from the M-T curve at T-MI<T<30 K decreases with increasing x. [S0163-1829(98)04438-5].