  Endohedral fullerenes are perfect nanolaboratories for the study of
magnetism. The substitution of a diamagnetic scandium atom in Dy2ScN@C80 with
gadolinium decreases the stability of a given magnetization and demonstrates Gd
to act as a single atom catalyst that accelerates the reaching of thermal
equilibrium. X-ray magnetic circular dichroism at the M4,5 edges of Gd and Dy
shows that the Gd magnetic moment follows the sum of the external and the
dipolar magnetic field of the two Dy ions and compared to Dy2ScN@C80 a lower
exchange barrier is found between the ferromagnetic and the antiferromagnetic
Dy configuration. The Arrhenius equilibration barrier as obtained from
superconducting quantum interference device magnetometry is more than one order
of magnitude larger, though a much smaller prefactor imposes faster
equilibration in Dy2GdN@C80. This sheds light on the importance of the angular
momentum balance in magnetic relaxation.
