Spin frustration and magnetic ordering in the Mott insulating fcc-Cs$_3$C$_{60}$

The low-temperature magnetic state at ambient pressure has been investigated by specific heat and nuclear magnetic resonance (NMR) measurements in face-centered-cubic (fcc-) Cs$_3$C$_{60}$, which is characterized by a Mott insulating state with $S=1/2$ spins in C$_{60}^{3-}$ anions and a geometrical spin frustration inherent in the fcc lattice. Specific heat exhibited no sharp anomaly down to 0.4~K, but both magnetic specific heat and NMR relaxation rate revealed a broad peak around 2.5~K, indicating that the reported antiferromagnetic ordering is accompanied by a gradual freezing of electronic spins with distributed transition temperatures. These results are unexpected in the conventional fcc antiferromagnets. Interplay of geometrical frustration, orientational disorder of C$_{60}$ molecules, and weak Mottness gives rise to the unique magnetic ground state in fcc-Cs$_3$C$_{60}$.