Ultracold atoms close-in on Stoner ferromagnetism

Using ultracold atoms to simulate a model of ferromagnetism that was first proposed 85 years ago has come one step closer because of work done by Matteo Zaccanti of the University of Florence and colleagues in Italy and the US. The team has managed to separate experimental signals from atoms that pair-up to create molecules from signals from free atoms that have aligned their magnetic moments with their neighbours. As well as providing insights into the fundamental nature of magnetic interactions, the research could be used to simulate other interesting systems such as “quantum emulsions”.

While iron is the most familiar magnetic material, the origins of its magnetism are rather murky. It cannot be described simply as a collection of magnetic moments that are fixed in a crystalline lattice because its magnetism arises from the spins of its conduction electrons, which are free to move throughout the material.

In 1933 the British physicist Edmund Stoner came up with a theory of “itinerant ferromagnetism” to explain how these electrons become magnetic. Wolfgang Pauli had already pointed out that electrons with spins pointing in opposite directions can get much closer to each other than electrons with spins pointing in the same direction. However, electrons are charged particles and being up close has a huge cost in terms of electrostatic energy. If the spins point in the same direction, the electrons cannot get close together and the electrostatic energy is much lower — but this comes with the cost of increased kinetic energy. Stoner worked out that at a certain electron density, alignment wins-out and the material becomes a ferromagnet.

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