Microscopic Observation of Spin Polarization by d Localized Spin in Type BETS Based Organic Superconductors

In this study, Se-77 NMR measurements were carried out to detect the pi spin polarization of the organic BETS (BETS = Bis(ethylenedithio)tetraselenafulvalene) molecule of the field induced superconductor, lambda-(BETS)(2)Fe1-xGaxCl4, which shows a superconducting transition at relatively low magnetic field compared to the non-doped lambda-(BETS)(2)FeCl4. From the analysis of the NMR spectrum at low temperature, it was clarified that the exchange interaction between pi and 3d spins in the Ga doping system is smaller than that in the Fe salt. It is also clarified that the conduction pi spins feel the "averaged" exchange field from the localized 3d spins at the dilute Fe sites.