Staggered Spin Order of Localized $\pi$-electrons in the Insulating State of the Organic Conductor $\kappa$-(BETS)$_2$Mn[N(CN)$_2$]$_3$

Magnetic properties of the conduction $\pi$-electron system of $\kappa$-(BETS)$_2$Mn[N(CN)$_2$]$_3$ have been probed using $^{13}$C NMR. At ambient pressure, the metal-insulator transition observed in the resistivity measurements below $T\simeq23\,$K is shown to be accompanied by ordering of the $\pi$-spins in a long-range staggered structure. As the metal-insulator transition is suppressed by applying a small pressure of $\sim 0.5\,$kbar, the $\pi$-spin system maintains the properties of the metallic state down to $5$ K.