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

Abstract

The electric and magnetic properties of a series of highly correlated two-dimensional organic \ensuremath{\pi} conductors incorporating magnetic ions (${\mathrm{Fe}}^{3+}$ with $S=\frac{5}{2}$), $\ensuremath{\lambda}\ensuremath{-}{\mathrm{BETS}}_{2}{\mathrm{FeBr}}_{x}{\mathrm{Cl}}_{4\ensuremath{-}x}$ [$\mathrm{B}\mathrm{E}\mathrm{T}\mathrm{S}=\mathrm{b}\mathrm{i}\mathrm{s}(\mathrm{ethylenedithio})\mathrm{tetraselenafulvalene},$ $x=0--0.8$] were examined by controlling the Br content (x). A broad resistivity maximum at 100 $(x\ensuremath{\approx}0)--50\mathrm{K}$ $(x\ensuremath{\approx}0.7)$ indicating the strong correlation of \ensuremath{\pi} conduction electrons becomes prominent with increasing x. At the same time the metal-insulator (MI) transition temperature ${(T}_{\mathrm{MI}})$ 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 ${(T}_{N}\ensuremath{\approx}{T}_{\mathrm{MI}}).$ A large magnetization drop was observed at ${T}_{\mathrm{MI}}$ for the magnetic field parallel to the c axis $(H\ensuremath{\parallel}c),$ which indicates the appearance of localized \ensuremath{\pi} spins $(S=\frac{1}{2})$ and a strong AF coupling of \ensuremath{\pi} and d spin systems. For $0.3<x<0.5,$ two anomalies were observed in the magnetization-temperature $(M\ensuremath{-}T)$ curve. The high-temperature anomaly corresponds to a MI transition ${(T}_{\mathrm{MI}})$ and the low-temperature one corresponds to an AF ordering of the ${\mathrm{Fe}}^{3+}$ spins ${(T}_{N}).$ In this case, the relatively small magnetization drop observed at ${T}_{\mathrm{MI}}$ suggests a small coupling of \ensuremath{\pi} and d spin systems. For $x>0.6,$ the magnetization drop at ${T}_{\mathrm{MI}}$ disappeared, which shows that the \ensuremath{\pi} and d electron systems are decoupled and the \ensuremath{\pi} electron system undergoes MI transition independently of the d spin systems. The disappearance of the susceptibility anomaly at ${T}_{\mathrm{MI}}$ indicates that the \ensuremath{\pi} electron system transforms to a nonmagnetic insulating state below ${T}_{\mathrm{MI}}.$ 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 $x=0.3\ensuremath{-}0.5,$ 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 $\ensuremath{\pi}\ensuremath{-}d$ coupling. Magnetoresistance measurements showed that ${T}_{\mathrm{MI}}$ of $\ensuremath{\lambda}\ensuremath{-}{\mathrm{BETS}}_{2}{\mathrm{FeBr}}_{0.7}{\mathrm{Cl}}_{3.3}$ is almost independent of the magnetic field below 1 kbar. But a field-restored highly conducting state similar to that of $\ensuremath{\lambda}\ensuremath{-}{\mathrm{BETS}}_{2}{\mathrm{FeCl}}_{4}$ was observed at high pressure. The Weiss temperature (\ensuremath{\theta}) estimated from the $M\ensuremath{-}T$ curve at ${T}_{\mathrm{MI}}<T<30\mathrm{K}$ decreases with increasing x.