Magnetics Business & Technology - Winter 2017 - 12

FEATURE ARTICLE

Diving Into Magnets
By Laura Heydermann/Paul Scherrer Institute

First-time 3D imaging of internal
magnetic patterns
Magnets are found in motors, in energy production and in data
storage. A deeper understanding of the basic properties of magnetic materials could therefore impact our everyday technology. A
study by Scientists at the Paul Scherrer Institute PSI in Switzerland,
the ETH Zurich and the University of Glasgow has the potential to
further this understanding. The researchers have for the first time
made visible the directions of the magnetisation inside an object
thicker than ever before in 3D and down to details ten thousand
times smaller than a millimetre (100 nanometres). They were able
to map the three dimensional arrangement of the magnetic moments. These can be thought of as tiny magnetic compass needles
inside the material that collectively define its magnetic structure.
The scientists achieved their visualisation inside a gadolinium-cobalt magnet using an experimental imaging technique called hard
X-ray magnetic tomography which was developed at PSI. The result revealed intriguing intertwining patterns and, within them, socalled Bloch points. At a Bloch point, the magnetic needles abruptly
change their direction. Bloch points were predicted theoretically
in 1965 but have only now been observed directly with these new
measurements. The researchers published their study in the renowned scientific journal Nature.
A team of scientists from the Paul Scherrer Institute PSI, the ETH
Zurich and the University of Glasgow have for the first time been
able to image the magnetic structure within a small 3D object on
the nanometre scale. The magnetic structure is an arrangement
of magnetic moments, each of which can be thought of as a tiny
magnetic compass needle. The studied object was a micrometresized pillar (thousandth of a millimetre in diameter) made of the
material gadolinium-cobalt, which acts like a ferromagnet. Within
it, the scientists visualised the magnetic patterns that occur on a
scale ten thousand times smaller than a millimetre - in other words,
the smallest detail they could make visible in their 3D images was
around 100 nanometres. The sophisticated imaging was achieved by
a technique called hard X-ray magnetic tomography that was newly
developed at PSI in the course of this proof-of-principle study.
"Up to now, imaging magnetism and magnetic patterns at this
small scale could only be done in thin films or on the surfaces
of objects, explains Laura Heyderman, principal investigator of the
study, researcher at PSI and professor at ETH Zurich. We really feel
like we are diving inside the magnetic material, seeing and understanding the 3D arrangement of the tiny magnetic compass needles.
These tiny needles
'feel' each other
and hence are not
oriented randomly,
but instead form
well-defined patterns throughout
the magnetic object.

Swirling internal magnetic structure. A section of the investigated sample, which is a gadolinium-cobalt pillar of diameter
0.005 millimetres (5 micrometres), is shown. With magnetic tomography, scientists determined
its internal magnetic structure.
Here, the magnetisation is represented by arrows for a horizontal
slice within the pillar. In addition,
the colour of the arrows indicate whether they are pointing
upwards (orange) or downwards
(purple). (Graphics: Paul Scherrer
Institute/Claire Donnelly)
A vertical slice of the internal
magnetic structure of a sample
section. The sample is 0.005 millimetres (5 micrometres) in diameter and the section shown here is
0.0036 millimetres (3.6 micrometres) high. The internal magnetic
structure is represented by arrows for a vertical slice within
it. In addition, the colour of the
arrows indicate whether they are pointing towards (orange)
or away from the viewer (purple). (Graphics: Paul Scherrer Institute/Claire Donnelly)

Basic magnetic structures and first-time visualisation of Bloch-Points
The scientists quickly realised that the magnetic patterns consisted of tangled fundamental magnetic structures: They recognised
domains, in other words, 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. Seeing these basic and wellknown structures come together in a complex 3D network made
sense and was very beautiful and rewarding, says Claire Donnelly,
first author of the study.
One specific kind of pattern stood out and gave additional significance to the scientists' results: a pair of magnetic singularities,
so-called Bloch points. Bloch points contain an infinitesimally small
region within which the magnetic compass needles abruptly change
their direction. Singularities in general have fascinated scientists in
a variety of research fields. Well known examples are black holes in
space. In ferromagnets, the magnetisation can generally be considered continuous on the nanoscale. At these singularities, however,
this description breaks down, says Sebastian Gliga of the University of Glasgow and visiting scientist at PSI. Bloch points constitute
monopoles of the magnetisation and although they were first predicted over 60 years ago, they have never been directly observed.

The two PSI-scientists Claire Donnelly (left) and Laura Heyderman. (Photo: Paul Scherrer Institute/Markus Fischer)

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