News and notices

Magnets in a spin


mq5020thumbnailWhile we might be familiar with magnets in everyday life, the phenomena that underpin magnetism are not yet fully understood. Magnetism is all about how the "spins" of the electrons align in a material. If the axes of those spinning electrons are all aligned, then the material has magnetic order, but often they're randomly arranged. The alignment of electron spins in any given material depends sensitively on the structure of the material, the arrangement of atoms within it, a property that can be extracted by crystallographic techniques.

According to Simon Billinge of Columbia University, New York, and colleagues, the behaviour of exotic materials often depends not on the crystallographic structure, but on short-range deviations from the overall symmetry. Examples, are materials such as high-temperature superconductors, which have zero electrical resistance, materials with colossal magnetoresistance that might be used for high-density computer data storage and those that have several types of magnetism in the same material, multiferroics, which could also lead to new types of fast computational and memory devices. But what about the magnetism? Is the magnetic order sensitive to these deviations?

The deviations from the symmetrical norm help give rise to the phenomena in which scientists are interested, but unfortunately this puts them off-limits to conventional crystallographic techniques. Ultimately, crystallography relies on a repeating pattern in the material, translational symmetry, in order to generate a strong enough diffraction pattern that reflects the internal structure of the material. Billinge has been working on this "nanostructure problem" by developing new tools for studying structure at the nanoscale. But studying magnetism on the nanoscale is a step beyond.

Now, Billinge and graduate student Ben Frandsen have moved closer to overcoming this inherent problem of exotic materials [Frandsen et al. (2014), Acta Cryst. A70, 3-11; DOI: 10.1107/S2053273313033081]. "This theoretical development, the magnetic pair distribution function (mPDF) analysis, allows us to 'see' the spin arrangements even when they are only ordered over short-range," said Frandsen, opening the door to a better understanding of the relationship between local magnetism and material properties.

"The next step is to collect neutron diffraction data and show that the theory works in practice," he adds. By using a mathematical process known as a Fourier transform, the researchers can convert the diffraction pattern into the mPDF, a picture of the arrangement of pairs of spins. The magnetic pairs tell the researchers how those electrons and their spins are aligned within the structure and the range of the alignments.

The team has successfully developed their approach to a range of materials with various magnetic structures such as ferromagnets, anti-ferromagnets, so-called spin-ice materials, nanomagnets and molecular magnets.

David Bradley