Measuring Demagnetization Factors in Magnetic Nanostructure Assemblies

Some magnetic nanostructure assemblies exhibit no magnetic moment in zero field, but readily magnetize in applied fields due to a property known as superparamagnetism, which allows these assemblies to be directed to a site or heated by application of an alternating field. The study of such assemblies represents an expanding field with applications including the targeted release of drugs, cancer treatments, and medical imaging.

McCann et al. presented two new transverse susceptibility techniques for experimentally measuring the demagnetization factor. Enabling comparison with experiment, these techniques are also applicable to superparamagnetic assemblies without the magnetization features normally required using conventional methods.

“The demagnetization factor determines the size of an internal field within the sample that always opposes the applied field, thereby reducing it, which can give false features in magnetic characterization curves,” said author Tim Mercer. “Until now, it has only been possible to take this into account using theoretical calculations assuming an average value.”

Knowing a sample’s demagnetization factor is key to calculating the effective field to which it is subjected. Using a numerical polar-based model, the scientists found the mean demagnetization factor in near perfect agreement with current theory. The model provided other valuable information as well, including full distributions of particles’ demagnetization factors with locations in the sample.

“It raised questions about the overall demagnetization factor to be used,” said McCann. “Should it be the mean, median, or mode?”

On further examination, the scientists found that modal values of the model are closest to those of the measured results. They also uncovered additional results that provide important insight into superparamagnetism and its various effects on magnetic nanostructure assemblies.

Source: “Methods of investigating the demagnetization factors within assemblies of superparamagnetic nanoparticles,” by Steven M. McCann, James Leach, Subrayal M. Reddy, and Tim Mercer, AIP Advances (2022). The article can be accessed at http://doi.org/10.1063/5.0095899 .