Frustration!

Neutron powder diffraction data for TbMn2O5

Neutron powder diffraction data for TbMn2O5 collected on the Gem instrument at 1 K intervals. Changes in periodicity of the spin structure are clearly visible from the sudden shift of positions of magnetic Bragg peaks
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Minerals prefer to form a perfectly ordered crystal structure when they are cooled, but for some this is a hard task. In these cases any given way of organising produces some atoms that are not happy with the way they are arranged with respect to their neighbours. These minerals are known as ‘frustrated systems’ – frustrated because no matter how much wiggling they do they can’t lower their energy any further.

Studying frustrated systems using neutrons at ISIS has enabled scientists to discover new classes of magnets and exotic forms of matter, with wide ranging applications.
Ice, magnetite and silica are all good examples of naturally occurring frustrated systems. Their molecular geometry means that they find it very hard to reach a perfectly ordered ground state. “Molecules can still make moves, but they are unable to lower the energy further, because the gain they make in one part of the crystal is exactly compensated by a loss somewhere else,” explains ISIS scientist Paolo Radaelli.
For Radaelli and his colleagues their interest lies in understanding how these unhappy minerals respond to being cooled. “How does the crystal "break the bondage" imposed by its geometry and find true happiness?” he asks.
In particular Radaelli is interested in a class of materials known as multiferroics – minerals that are both ferroelectric (their atoms can order electrically, just like a ferromagnet’s atoms order magnetically) and magnetic. Such materials are exceedingly rare and until recently it was unclear what caused this novel magnetic-ferroelectric coupling.
Using neutron diffraction at ISIS, Radaelli and his colleagues have been able to probe the atomic and magnetic structure of these unusual minerals. “Neutrons are an ideal tool for this job because they are sensitive to light atoms and to magnetic spins,” explains Radaelli.
They found that the properties of multiferroic materials were intimately related to their frustration geometry. “The molecules distort to escape their geometry, which generates the ferroelectric properties,” says Radaelli. Potentially multiferroic minerals could be used as an alternative form of data storage for computer memories. Understanding how the frustration geometry controls the mineral properties brings applications like this one step closer.
In addition scientists working at ISIS have used neutron diffraction to discover new forms of frustrated matter which exist at extreme temperatures and pressure. These systems have very strange behaviours and some of them, know as ‘spin-ices’, have quantum properties at low temperatures, raising the possibility that they could be exploited to build a quantum computer in the future.

Prof. Paolo Radaelli (ISIS)

Research date: December 2005

Further Information

Spin structure and magnetic frustration in multiferroic RMn2O5 (R=Tb,Ho,Dy), GR Blake et al., Phys Rev B 71 (2005) 214402 

Structural anomalies and multiferroic behavior in magnetically frustrated TbMn2O5, LC Chapon et al., Phys Rev Lett 93 (2004) 177402

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