Our ability to quickly access the vast amounts of information available on the internet is at least in part owed to the miniaturization of magnetic data storage. From the invention of the hard disk drive at IBM in 1956 to our present day life, magnetic bits have shrunk from millimeter size to a few tens of nanometers. If magnets shrink even further, down to a point where only a few atoms comprise the magnetic structure, they undergo a fundamental change in their behavior. While classical magnets allow for a continuous rotation of their magnetization direction, small spin systems behave like quantum objects. Studying excitations between the discreet energy eigenstates of such nanomagnets allows insight into the quantum mechanics of spin systems.

Here we show how the magnetic properties of individual atoms and artificially created nanostructures can be probed with a low-temperature, high-field scanning tunneling microscope (STM) when the atoms are placed on a thin insulator. Using inelastic tunneling spectroscopy, we find clear evidence of large magnetic anisotropy – the energy of the spin system depends strongly on the spatial orientation of the spin. The STM allows further to place magnetic atoms in close proximity and study their interactions. In one-dimensional chains we find a surprisingly good match to the simple theory of the isotropic Heisenberg interaction. Finally, when spin-polarized currents are employed, one can pump a spin system far out of thermal equilibrium and probe the spin lifetimes of excited states. Taken together, the STM allows one to see, build, and probe magnetic nanostructures - all in the same tool and with atomic-scale precision.