Resistance and magnetic torque measurements have been performed to investigate an internal field effect on vortex states for a layered organic superconductor $\lambda \text{−}({\mathrm{BETS})}_2{\mathrm{Fe}}_{1-x}{\mathrm{Ga}}_x{\mathrm{Cl}}_4$ $\left(x=0.37\right)$, 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 $\sim 14$ 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 $\sim 23$ 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.

• Received 2 February 2017

DOI:https://doi.org/10.1103/PhysRevB.95.165133