Magnetic hysteresis is a direct manifestation of nonequilibrium physics that has to be understood if a system is to be used for information storage and processing. The dilanthanide endofullerene is shown to be a single-molecule magnet with a remanence time on the order of 100 s at 400 mK. Three different temperature-dependent relaxation barriers are discerned. The lowest 1 K barrier is assigned to intermolecular dipole-dipole interaction, the 10 K barrier to intramolecular exchange and dipolar coupling, and the 50 K barrier to molecular vibrations as was observed for . The 4 orders of magnitude difference in the prefactor between the Tb and the Dy compound in the decay process across the 10 K barrier is assigned to the lack of Kramers protection in . The sub-Kelvin hysteresis follows changes in the magnetization at level crossings of the four possible ground-state configurations. Comparison to a hysteresis model, with magnetic relaxation at level crossings only, reveals cooperative action between nearby molecules.