Internal Field Effect On Vortex States In The Layered Organic Superconductor Lambda-(Bets)(2)Fe1-Xgaxcl4 (X=0.37)

Resistance and magnetic torque measurements have been performed to investigate an internal field effect on vortex states for a layered organic superconductor lambda-(BETS)(2)Fe1-xGaxCl4 (x = 0.37), where BETS = bis(ethylenedithio) tetraselenafulvalene. Because of the internal field by the localized 3d spins of the Fe ions, the superconducting transition temperature has a maximum at 14 T. The strongest energy dissipation due to Josephson vortex dynamics and the largest pinning of pancake vortices are observed at similar to 14 T. The interlayer resistance in parallel fields shows a characteristic dip with decreasing temperature. The dip temperature decreases with increasing field, suggesting a Josephson vortex transition. At similar to 23 T, we observe another small dip in the field dependence of the interlayer resistance, steep deceases of the perpendicular critical field, and diamagnetism. These results show a phase transition of the superconductivity, which is likely ascribed to an inhomogeneous superconducting transition.