https://phys.org/news/2018-10-breakthrough-accessing-tiny-magnet-core.html

The two elements that were investigated in this work, iron and titanium, are atoms that can have a different number of neutrons in the atom's core, these are the so-called isotopes. Only certain isotopes of each element have a core with a nuclear spin. It is normally exceedingly hard to measure nuclear spins of individual atoms. Traditionally large numbers of nuclear spins are required, making this advancement so noteworthy.

In order to detect the presence of a nuclear spin within the core of a single atom, the team made use of the hyperfine interaction. This phenomenon describes the coupling between a single atom's nuclear spin and its electron counterpart, that is generally much easier to access. Dr. Philip Willke of the Center for Quantum Nanoscience (QNS), first author of the study, says: "We found that the hyperfine interaction of an atom changed when we moved it to a different position on the surface or if we moved other atoms next to it. In both cases, the electronic structure of the atom changes and the nuclear spin allows us to detect that."