Staggered spin order of localized π-electrons in the insulating state of the organic conductor κ-(BETS)2Mn[N(CN)2]3

The magnetic properties of the conduction π-electron system of κ-(BETS)2Mn[N(CN)2]3 have been probed using 13C NMR. At ambient pressure, the metal-insulator transition observed in the resistivity measurements below T ≃ 23 K is shown to be accompanied by ordering of the π-spins in a long-range staggered structure. As the metal-insulator transition is suppressed by applying a small pressure of ∼0.5 kbar, the π-spin system maintains the properties of the metallic state down to 5 K.