Solving a 20 Year Mystery: The Frustrating Case of β-Manganese

Beta manganese

The lattice structure of β-Manganese, showing groups of atoms arranging themselves into triangular “windmill” geometries.
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Manganese is a common element, the 12th most abundant element in the Earth's crust. It’s essential to all forms of life, and used in industrial processes such as steel production. But manganese has some unusual properties, a collaboration between scientists at the ISIS, Oxford University and the Institut Laue Langevin (ILL) have just presented the results of a 20 year study, explaining how the apparently complex crystal structure of a particular form of manganese called β-manganese leads to surprisingly simple properties. Their research has been published in the journal Physical Review Letters.

Look across the periodic table, especially the transition metals, and manganese stands out from the others. While most elements have 2 or 3 atoms in a unit cell, the most common form of manganese, α-manganese, has 58. β-manganese has 20. α and β-manganese have quite different properties – for example α-manganese is magnetic, whereas β-manganese is not. This is one of the problems the study set out to solve.

Pure β-manganese is very difficult to study due to the lack of single crystals. The group used the world leading crystal growth facilities at the ILL to grow two large single crystals of manganese doped with cobalt. They then used polarised neutrons on the D7 instrument at the ILL to measure the magnetic structure. They then used new software, developed at ISIS and Oxford, to extract the magnetic interactions in the material.

They found strong evidence that underlying the complex crystal structure was a simple system of frustrated magnets. Atoms behave like tiny magnets. Where two magnets are present, they try and line up so the north and south cancel out. However, when a third magnet is added a triangular lattice can form where two align but the third is frustrated – it cannot align with both other magnets.

They concluded that β-manganese has a simple emergent spin structure in which rod-shaped clusters of manganese atoms forming large “collective” magnetic objects interact with each other on such a triangular lattice.  Ross Stewart from ISIS took part in the study. He says, “What’s exciting about this is that we have an element, manganese, that should be simple to understand but in fact displays very strange behaviour. We’ve shown that despite the complex crystal structure there is a simple collective magnetic state, and this may help us in realising new states of correlated quantum matter.”

Laurent Chapon from the ILL adds,” Over the past half a century neutrons have proved themselves the perfect tool for probing magnetism at a microscopic level, and facilities like the ILL and ISIS are to this day involved in world-leading studies investigating the magnetic structure and spin dynamics of materials. This study from Dr Stewart follows on from ground-breaking work on magnetic monopole quasiparticles found in spin-ices which were first investigated on the same instrument at the ILL only a few years ago. In the present study, it is fascinating to observe that the complex magnetic properties of a certain form of Mn metal - poorly understood to date despite progress in ab-initio theory - is now remarkably well modelled using the results of magnetic diffuse scattering obtained with polarized neutrons.”

Sara Fletcher

Research date: July 2013

Further Information

Emergent Frustration in Co-doped β-Mn, Joseph A. M. Paddison, J. Ross Stewart, Pascal Manuel, Pierre Courtois, Garry J. McIntyre, Brian D. Rainford, and Andrew L. Goodwin,  Phys. Rev. Lett. 110, 267207 (2013)

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