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The Neutron as Magnetic Probe
Neutrons are particularly sensitive to internal magnetic fields in condensed matter since they possess a magnetic
moment. They allow us to determine not only the structural arrangement of static magnetic moments in
solids or molecules, but also the magnetic excitations and fluctuations that provide information about quantum
mechanical magnetic interactions. Neutron scattering is the method of choice - quite frequently even the
only method - by which to obtain microscopic information on the atomic scale about magnetic phenomena
which can be compared quantitatively with predictions of ab-initio calculations. Some examples of novel materials
and material systems currently being intensively investigated are:
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Highly correlated electron systems that exhibit unexpected phenomena and
functionalities (such as high-temperature superconductivity, colossal magnetoresistance
effects or multiferroic ordering) which cannot be explained using the "standard-model" of solid state physics.
Among the variety of different experimental methods, neutron scattering
is uniquely characterised by the fact that it allows the determination of pair correlations in solids at all relevant
lengths and time-scales.
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Nanomagnetism with properties that are essentially determined by atoms at surfaces or interfaces leads
to novel phenomena and functionalities (such as enhanced anisotropy of nanoparticles or the giant
magnetoresistance effect in layered systems). Neutrons provide unique insight into these systems by depth-resolved
vector magnetometry and polarised diffraction for the determination of the spin density distribution
in molecular magnets or by inelastic scattering for the measurement of relevant excitations - information that
cannot be obtained by any other method. Magnetism can also be spatially mapped: tomography using polarised
neutrons allows the 3D imaging of free magnetic fields as well as of internal fields in magnetic materials.
It has recently even become possible to visualize magnetic domains in the interior of magnets in three
dimensions using phase contrast tomography.
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Nano-structurered systems of materials with strong electronic correlations - a
field where the two major challenges of modern solid state physics meet:
interfacial phenomena and electronic correlations. Examples of unusual interfacial
phenomena that have been investigated using neutron scattering methods are ferromagnetic interfaces
between two anti-ferromagnets or superconducting interfaces that form between two insulators.
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Source:
Perspektiven der Neutronenforschung in Deutschland, 2011
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