Tweaks to a widely used scientific tool allow it to detect spin, a fundamental quality of atoms and subatomic particles.

A simple technique allows a high-tech microscope to sense an individual atom’s spin, the microscopic equivalent of a bar magnet’s magnetic axis and a crucial component of a material’s magnetic and chemical properties.

The widely used instrument called the scanning tunnelling microscope (STM) runs an electric current between its sharp probe, or tip, and a sample’s surface. By sensing slight fluctuations in the current, an STM can map surfaces with single-atom resolution.

Markus Ternes at RWTH Aachen University in Germany and his collaborators modified their STM tip by attaching a molecule of a compound called nickelocene. Each nickelocene molecule contains one nickel atom.

When spins were present on a sample, they prompted changes to the nickel atom’s energy levels, which are attributes of the ‘shells’ containing an atom’s electrons. The STM could detect this change as fluctuations in the current.

The strategy enabled the team to map spins without applying external magnetic fields, a step that can mask a sample’s magnetic properties. Any laboratory that owns an atomic-resolution STM can easily employ the technique, Ternes says, and it could help to image exotic spin arrangements such as those called skyrmions.