π-d interaction in the field induced superconductor λ-(BETS)2FeCl4: Studied by77Se NMR

77Se NMR measurements have been performed in the normal state of the organic, magnetic field induced superconductor λ(BETS)2FeCl4, to examine the interaction between Fe local spins and π conduction electrons. Large, negative Knight shift values were observed at fields lower than the one corresponding to onset of field induced superconductivity. This evidences that the conduction electrons feel a negative internal field coming from the Fe spins through the exchange interaction {\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $$J\vec s$$ \end{document}} · {\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $$\vec S$$ \end{document}}. This suggests that the superconductivity induced by high magnetic fields has its origin in the Jaccarino-Peter compensation mechanism. The estimated value of the exchange interaction J, is 30% smaller than that predicted by the resistivity measurements.