⁷⁷Se NMR measurements of the π-d exchange field in the organic conductor λ-(BETS)₂FeCl₄

⁷⁷Se-NMR spectrum and frequency shift measurements in the paramagnetic metal and antiferromagnetic insulating phases are reported for a small single crystal of the organic conductor λ-(BETS)₂FeCl₄ as a function of temperature (T) and field alignment for an applied magnetic field B₀=9 T. The results show that in the low T limit, where the localized Fe³⁺ spins (S_d=5∕2) are almost fully polarized, the conduction electrons (Se π electrons, spin s_π=1∕2) in the BETS molecules experience an exchange field (B_πd) from the Fe³⁺ spins with a value of −32.7±1.5 T at 5 K and 9 T aligned opposite to B₀. This large negative value of B_πd is consistent with that predicted by the resistivity measurements and supports the Jaccarino-Peter internal field-compensation mechanism being responsible for the origin of field-induced superconductivity.