spin glasses

Spin Glass Half Full

Toughened Readers

I’m sure readers of New Scientist in the 1970s must been made of tougher stuff than their present day counter parts. This ambitious article which introduces spin glasses – a form of dilute magnetic phenemonon – is a case in point.

Not for the faint-hearted this article attempts to cover the history of magnetic science, magnetic susceptibility, central limit theorem, quantum mechanics and random walks.

All this of course makes this article a great introduction to the less well known magnetic phenomenon of the spin glass.

Spin Glasses

Spin glasses were discovered during attempts to better understand more conventional magnetic phenomena such as ferro-, anti-ferro- and para-magnetic behaviour. Whereas in structural glasses (e.g. window glass, or metglas) there is positional disorder of the atoms, in spin glasses the atoms are ordered but there is disorder in the spins on the atoms.

Spin glasses are typically metallic alloys with around 0.1 – 20% magnetic atomic content. These dilute magnetic moments interact with each other via the ‘RKKY‘ interaction which allows mediation of the magnetisation by conduction electrons. It is this same interaction which governs the magnetic behaviour in the dilute ferromagnetic semiconductor GaMnAs.

The weak interaction of this magnetic content means that they do not show any magnetic ordering except at very low temperatures (usually liquid helium: 4.2K), where the randomising effect of temperature is sufficiently reduced.

Dispite the dilute and random nature of the magnetic content in spin glasses, their properties still follow clear and definite laws. The magnetic susceptibility for example shows a clear cusp at the transition temperature between magnetic order and disorder.

A model system

The relatively simple random processes in spin glasses meant that it could be a test system for models of other magnetic processes. Spin glasses still pop up today as examples of simple magnetic systems.

A comprehensive review of spin glasses was published some time after this New Scientist article. You can find it here.