Spin

Going deeper

A magnet that is not spinning

In 1922 Otto Stern and Walther Gerlach sent a beam of silver atoms between the poles of a specially shaped magnet. A magnet pointing in a random direction would have smeared the beam into a line; instead it split cleanly in two. Three years later George Uhlenbeck and Samuel Goudsmit proposed that the electron itself carries a fixed amount of angular momentum, with a magnetic moment to match, which can only line up with or against any chosen direction.

The picture of a spinning ball does not survive a closer look: a particle as small as an electron would have to turn faster than light to carry that much angular momentum. Spin is better thought of as a property of the electron in its own right, like its charge – one that happens to behave, in a magnetic field, exactly like a tiny bar magnet.

Why most materials are not magnetic

Each electron state in an atom or a crystal can hold two electrons, and only if their spins point opposite ways – the Pauli exclusion principle. Paired spins cancel, so a material whose electrons all sit in pairs has no magnetism of its own. Magnetism needs unpaired electrons, like the three that each chromium ion carries in CrI₃.

Even then, the spins must agree on a direction. What makes neighbouring spins line up, or alternate, is not the weak magnetic pull between them but the exchange interaction, a consequence of the Pauli principle and the repulsion between electrons, and far stronger. In a single layer a preferred direction set by the crystal is needed as well; without one, heat scrambles the order at any temperature above absolute zero.

Spin as a carrier of information

The read head of a hard disk works by spin: its resistance depends on how the spins in two magnetic layers are aligned, an effect whose discovery won the 2007 Nobel Prize in Physics. Magnetic memory chips store bits the same way. 2D materials add new options. Graphene carries spins over long distances – tens of micrometres at room temperature in the best devices – because carbon is light, so spin–orbit coupling is weak, and its common isotope has no nuclear spin to disturb the electron’s. In TMDC monolayers spin–orbit coupling is strong instead and ties the spin to the valley, and single spin defects in hexagonal boron nitride can be set and read out with light at room temperature.