4 september 2009

Icy journey finally uncovers magnetic monopole


Data showing evidence of magnetic monopoles in spin ice - the bottom row shows predicted data and above is the real data gathered.
Evidence for magnetic monopoles in spin ice - the bottom row shows predicted data and above is the real data gathered by the team.

A team, including Oxford University scientists, has found the first evidence that magnetic monopoles – where the ‘North’ and ‘South’ poles of magnets are separated – do exist in nature.

The existence of the magnetic monopole was suggested by Paul Dirac back in 1931 but for the next seven decades physicists searched outer space and quantum collisions in vain for evidence of this separated magnetic charge. 

Then, in 2007, an Oxford University team publishing in the journal Nature theorised that monopoles could exist in a strange material dubbed ‘spin ice’ – so-called because the atomic electrons within it carry ‘spin’, a property which causes them to behave like tiny bar magnets, and arrange themselves in a similar way to water molecules in water ice.

It’s perhaps not too far fetched to imagine that we might find a way to build magnetic circuits using magnetic monopoles.

Professor Andrew Boothroyd

Now, scientists from the Institut Laue-Langevin in Grenoble, University College London and Oxford University, report in this week’s Science that their investigations have detected disturbances in the magnetism of a spin ice compound that strongly suggest the influence of magnetic monopoles.

The monopoles predicted to occur in spin ice are not elementary particles like protons or electrons but are disturbances of the arrangement of spins – similar to ripples on the surface of a pond.

‘We can’t see the monopoles directly but what we can detect is their magnetic influence on their surroundings,’ said Professor Andrew Boothroyd of Oxford University’s Department of Physics, an author of the Science paper. ‘It’s rather like the motion of a crowd as someone pushes their way through.’

The spin ice compound studied has very unusual magnetic properties – which helps explain why monopoles have not been found elsewhere – arising from a specific crystalline structure that allows a very large number of possible equivalent magnetic arrangements. The experiments used a technique called neutron scattering that makes it possible to look at the arrangements of spin at the atomic scale.

The Oxford scientists’ particular contribution to the work was to create a very high quality crystal of a spin ice compound – any imperfections would mean that the signature of the monopoles would be too faint to pick up.

Dr Prabhakaran, another member of the Oxford team, said: ‘It’s rather like how the flaws in a gemstone can spoil its optical qualities, flaws in this sort of substance can prevent us from detecting its subtle magnetic qualities – without the highest quality crystals that we can make here in Oxford we’d never have been able to detect evidence for monopoles.’

Apart from solving a 70-year magnetic mystery the discovery could have long-term consequences in technology – rather as the discovery of how to manipulate electron spin led to the field of spintronics and the sort of advanced memory technology found in Apple’s iPod.

Professor Boothroyd said: ‘It’s perhaps not too far fetched to imagine that, rather as we can build electrical circuits using positive and negative electrical charge, we might find a way to build magnetic circuits using magnetic monopoles. It might one day lead to new types of memory elements, electrical circuits or sensor devices.’