What is it about?

We report here two new methods for experimentally measuring the demagnetization factor of assemblies of superparamagnetic nanoparticles. To examine the underpinning mechanisms and to relate current theory to these new methods, a model was created that examined the free poles on the constituent particles’ surfaces.

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Why is it important?

Understanding how assemblies of magnetic nanoparticles respond when placed in a magnetic field is of great interest and importance in the area of bio-medical research and its application to treatment of cancers.


Previous techniques that we have used in the laboratory to measure demagnetization field of particulate samples were based upon materials that had classic ferromagnetic characteristics; strong hysteresis and anisotropy which superparamagnetic samples do not have. The two techniques we developed do not require these characteristics. The most edifying aspect of the modelling work was seeing that for a given simulated sample, the mean demagnetization factor was in near perfect agreement with current theory. But the model provided so much more information, producing full distributions of constituent particles’ demagnetization factors, with locations in the sample. The model raised questions about what is the overall demagnetization factor that we should use for a sample in the laboratory? Should it be the mean, median or mode?

Steven McCann

When a sample is magnetised, there is always an internal field within the sample that always opposes the applied field, thereby reducing it, which can give false features in magnetic characterization curves. This effect is typicaly measured using a single value demagnetisation factor. However, in reality there is a distribution of these factors throughout the volume of a sample and they can vary significantly with their position. Not only do we show the distribution is nearer to the modal value (the value that the majority of particles have) rather than the average value assumed by current theory, but we also develop two new experimental methods that allow this to be determined for samples of superparamagnetic particles. This class of particles are increasingly important in bio-medical applications such as treatment of cancers.

Tim Mercer
University of Central Lancashire

Read the Original

This page is a summary of: Methods of investigating the demagnetization factors within assemblies of superparamagnetic nanoparticles, AIP Advances, July 2022, American Institute of Physics,
DOI: 10.1063/5.0095899.
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