Magnetic hysteresis and strong ferromagnetic coupling of sulfur-bridged Dy ions in clusterfullerene DyS@C.

Two isomers of metallofullerene DyS@C with sulfur-bridged Dy ions exhibit broad magnetic hysteresis with sharp steps at sub-Kelvin temperature. Analysis of the level crossing events for different orientations of a magnetic field showed that even in powder samples, the hysteresis steps caused by quantum tunneling of magnetization can provide precise information on the strength of intramolecular Dy⋯Dy inter-actions. A comparison of different methods to determine the energy difference between ferromagnetic and antiferromagnetic states showed that sub-Kelvin hysteresis gives the most robust and reliable values. The ground state in DyS@C has ferromagnetic coupling of Dy magnetic moments, whereas the state with antiferromagnetic coupling in and cage isomers is 10.7 and 5.1 cm higher, respectively. The value for the isomer is among the highest found in metallofullerenes and is considerably larger than that reported in non-fullerene dinuclear molecular magnets. Magnetization relaxation times measured in zero magnetic field at sub-Kelvin temperatures tend to level off near 900 and 3200 s in and isomers. These times correspond to the quantum tunneling relaxation mechanism, in which the whole magnetic moment of the DyS@C molecule flips at once as a single entity.