Magnetism's standard model gets a facelift
Nov 16, 2000
A revamp of a classic - but flawed - theory of magnetism aims to give physicists a fuller picture of the behaviour of ferromagnetic materials. Ralph Chamberlin of Arizona State University, US, has incorporated the effects of local magnetic fluctuations to tackle a shortcoming in the 'standard model' of magnetism - its failure to accurately explain the characteristics of ferromagnets at certain temperatures (R V Chamberlin 2000 Nature 408 337).
The standard model - developed by Pierre Weiss in 1907 - treats paramagnets as systems of non-interacting magnetic dipoles. The theory successfully describes the behaviour of paramagnets - materials that become weakly magnetized inside an applied magnetic field - but cannot account for ferromagnets. Ferromagnets are strongly magnetic even when there is no external field, but the magnetism disappears above a critical temperature known as the Curie temperature.
According to the standard model, Curie temperatures should be much lower than they actually are. Weiss therefore proposed the existence of strong attractions among the dipoles - later found to be quantum 'exchange interactions' between atomic spins - to account for the discrepancy. The fix worked, but the model was still unable to predict accurately the behaviour of ferromagnets around the Curie temperature and at very low temperatures.
The simplicity of the standard model has made it popular and worthy of further refinement. Chamberlin addressed the blind spot by including the effects of nanothermodynamics - thermodynamic effects that cause magnetic fluctuations on a molecular scale. "I became intrigued by the similarities in the behaviour of glasses and magnets", Chamberlin told PhysicsWeb. "The breakthrough came when I noticed that essentially the same law describes the liquid-glass and the paramagnetic-ferromagnetic transitions".
The amended standard model now accurately describes ferromagnetism across the whole temperature range for the first time. Previous attempts to develop more accurate theories have had only limited appeal because they focused on narrow temperature ranges.