Resistive properties and phase diagram of the organic antiferromagnetic metal $\kappa$-(BETS)$_2$FeCl$_4$

Abstract

The low-temperature electronic state of the layered organic charge-transfer salt $\kappa$-(BETS)$_2$FeCl$_4$ was probed by interlayer electrical resistance measurements under magnetic field. Both above and below $T_{\mathrm{N}}=0.47\,$K, the temperature of antiferromagnetic ordering of $3d$-electron spins of Fe$^{3+}$ localized in the insulating anion layers, a non-saturating linear $R(T)$ dependence has been observed. A weak superconducting signal has been detected in the antiferromagnetic state, at temperatures $\leq 0.2\,$K. Despite the very high crystal quality, only a tiny fraction of the sample appears to be superconducting. Besides a small kink feature in the resistivity, the impact of the antiferromagnetic ordering of localized Fe$^{3+}$ spins on the conduction $\pi$-electron system is clearly manifested in the Fermi surface reconstruction, as evidenced by Shubnikov-de Haas oscillations. The "magnetic field -- temperature" phase diagrams for the field directions parallel to each of the three principal crystal axes have been determined. For magnetic field along the easy axis a spin-flop transition has been found. Similarities and differences between the present material and the sister compound $\kappa$-(BETS)$_2$FeBr$_4$ are discussed.