Obtaining the demagnetization factors within assemblies of superparamagnetic nanoparticles

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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?

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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.

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