Researchers using ISIS neutron and muon beams win prestigious Europhysics prize

North and south poles drifting apart

Artist impression of freely drifting north and south poles in magnetricity materials
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The 2012 Europhysics Prize for condensed matter physics has been awarded to ISIS neutron user Professor Steve Bramwell (London Centre for Nanotechnology and University College London), Claudio Castelnovo (Royal Holloway University of London and ISIS Theoretical Physics Group) and their colleagues, for the prediction and measurement of magnetic monopoles in spin ice.

Advanced materials research depends greatly on having access to central research labs the Science and Technology Facilities Council’s (STFC) world-leading ISIS neutron and muon source allowing the UK science community to flourish and make exciting discoveries with long-term impact.

Neutron scattering has been critical at all stages of this research giving unique information unmatched by any other experimental technique.

Steve Bramwell and his collaborator Mark Harris in experiments at ISIS first discovered and named the unusual magnetic material ‘spin ice’ in 1997, drawing attention to certain similarities found within water ice.

This laid the foundation for Claudio Castelnovo’s 2008 prediction of effective magnetic monopoles in spin ice, analogous to electrical charges, published in the journal Nature.  Then in October 2009, experiments at ISIS and elsewhere by Bramwell and others proved the existence of these monopoles. Bramwell coined the term ‘magnetricity’ to describe this similarity of currents of magnetic monopoles to electrical currents.

"It’s not often in physics you get the chance to ask 'How do you measure something?' and then go on to prove a theory unequivocally. It’s early stages but who knows what the applications of magnetic monopoles could be in 100 years time," said Professor Steve Bramwell

The Europhysics Prize – one of Europe’s most prestigious awards in the field of condensed matter physics – is awarded every two years, in recognition of excellent work by one or more individuals by the European Physics Society Condensed Matter Division.

 

Professor Steve Bramwell headshot

Professor Steve Bramwell (London Centre for Nanotechnology/ University College London)
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How it works

Magnetism arises from spin, a fundamental property of atoms and fundamental particles. Spin can be understood by thinking of each atom as a tiny bar magnet that can turn in any direction.

Most common magnets have two poles – north and south – caused by all of the atomic magnets pointing in the same direction, north-south, to create a permanent magnetic field.

Every magnet has these two poles, and bringing two magnets together, they will repel if the poles are alike and attract if they are different. Break a magnet in half and each half remakes the pole it lost. It seems that no magnet can break the two-pole rule and that magnetic monopoles, poles without their twins, don’t exist.

In 1997, a strange magnetic state known as ‘spin ice’ was discovered by Bramwell and Harris whilst carrying out neutron scattering experiments at ISIS. In the material holmium titanate they found that the crystal structure forced the arrangement of magnetic holmium atoms onto a network of tetrahedrons. This arrangement prevents the atomic magnets from lining up and pointing in the same direction. Instead, a compromise is made in which two holmium spins point into the centre of each tetrahedron and two spins point outwards.

The spin arrangement in holmium titanate mirrors the way that hydrogen ions are arranged in water ice, so Harris and Bramwell coined the term ‘spin ice’ to describe what they had found.

Claudio Castelnovo and colleagues discovered another remarkable property of spin ice. In an elegant paper, they predicted that individual tetrahedra could develop a slight imbalance in their spins and act as tiny, localised north or south poles. Even more remarkable, they predicted that the poles could break free from their usual north-south binding and independently drift around inside the crystal.

The prediction of free magnetic monopoles was rapidly verified in a series of beautifully conceived neutron scattering and muon spectroscopy experiments by Bramwell and others at ISIS and elsewhere.

Dr Martyn Bull

Research date: June 2012

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