It took a while to discover the first endohedral single molecule magnet. In 2012 it was shown that DySc₂N@C₈₀ has, at a temperature of 2 K, magnetic relaxation times longer than 10⁴ s [1]. The Figure depicts an image of this molecule, where the J_z =15/2 groundsate of the Dy 4f⁹ shell is colored in red. This anisotropic electronic ground state is oriented and stabilized by the strong ligand field, mainly of the central nitrogen ion, which causes the single ion magnetism. After DySc₂N@C₈₀ a series of new endofullerene single molecule magnets with a variety of different magnetic ground states were reported. For example, one, two or three encaged Dy ions display tunneling of the magnetisation, long remanence, or a frustrated ground state, which indicates a strong magnetic interaction within the cage [2].
I will discuss x-ray magnetic circular dichroism (XMCD) as a particularly sensitive tool for the detection of the magnetism inside the carbon cages and how these experiments may be complemented with superconducting quantum interference device (SQUID) measurements down to 400 mK temperature.
It will furthermore be shown that endohedral units may get preferentially oriented on surfaces [3], and that this proves the carbon cages to be "leaky Faraday cages", with tunable interaction across the carbon shell.

References:
[1] An Endohedral Single-Molecule Magnet with Long Relaxation Times: DySc2N@C80, Westerström et al. J. Am. Chem. Soc. 134 (2012) 9840.
[2] Tunneling, remanence, and frustration in dysprosium-based endohedral single-molecule magnets, Westerström et al. Phys. Rev. B 9 (2014) 060406.
[3] Surface Aligned Magnetic Moments and Hysteresis of an Endohedral Single-Molecule Magnet on a Metal, Westerström et al. Phys. Rev. Lett. 114 (2015) 087201.

Figure: Model of the first endohedral single molecule magnet: DySc₂N@C₈₀ (see: www. spinshuttle.ch, courthesy Ari Seitsonen).