Magnetics Business & Technology - Winter 2017 - 15

RESEARCH & DEVELOPMENT
The scientists, from the University of
Glasgow and the Paul Scherrer Institute
and the ETH Zurich in Switzerland, observed complex internal magnetic patterns
and quickly realised that they consisted of
tangled fundamental magnetic structures.
They were able to see 'domains', or regions
of homogenous magnetisation, and 'domain
walls', the boundaries separating two different domains.
They also observed magnetic vortices,
which have a structure analogous to that of
tornadoes, and all of these structures intertwined to create a complex and unique pattern.
One specific kind of structure stood out
and gave additional significance to the scientists' results: a pair of magnetic singularities, or so-called 'Bloch points'. At a Bloch
point, the magnetisation abruptly changes
its direction and locally points in all possible
directions on the surface of a sphere, reminiscent of the spines on the back of a curled
up hedgehog. Bloch points were predicted
theoretically in 1965 but the structure directly surrounding them has only now been
observed with these new measurements.
The structure of Bloch points, like that
of other singularities, such as black holes in
space, can be measured through the effect
they have on their surroundings. The team
has measured particular configurations -
twists in the magnetisation - that had been
predicted to give away the presence of the
singularities.
Dr Sebastian Gliga, Marie Curie Research Fellow at the University of Glasgow,
played a key role in interpreting the measured magnetic structure based on micromagnetic theory and state-of-the-art simulations.
Dr Gliga said: "In ferromagnets, where
the magnetisation can be considered continuous on the mesoscopic scale, these singularities are points where this description
breaks down.
"Bloch points constitute monopoles
of the magnetic charge and although they
were first predicted over 50 years ago, they
have never been experimentally observed
before.
"'This is a breakthrough in magnetic imaging and was I was thrilled to collaborate
on understanding of the observed magnetic
structures."
Lead author of the study Claire Donnelly, who developed the reconstruction
code and performed the experiments along
with her colleagues at the ETH Zurich and
the Paul Scherrer Institute, is originally from
Glasgow. In fact she was a project student at

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the University of Glasgow before moving t
the ETH Zurich for her PhD.
Donnelly said: "Lower energy soft Xrays have already very successfully been
used to achieve a similar map of the magnetic moments. But soft X-rays hardly penetrate such magnetic materials, so you can
only use them to see the magnetisation patterns of thin films or at the surface of bulk
objects."
Principal investigator of the study Professor Laura Heyderman, of the Paul Scherrer Institute and the ETH Zurich, said: "Many
people did not believe that we would be

able to achieve this 3D magnetic imaging
with hard X-rays.
"We really feel like we are diving inside the
magnetic material, seeing and understanding the 3D arrangement of the tiny magnetic compass needles that gives rise to the
magnetic structure."
The team's paper, titled 'Three dimensional magnetisation structures revealed
with X-ray vector nanotomography', is published in Nature (doi:10.1038/nature23006).

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Winter 2017 * Magnetics Business & Technology

15
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