  Thermomagnetic measurements are used to obtain the size distribution and
anisotropy of magnetic nanoparticles. An analytical transformation method is
described which utilizes temperature-dependent zero-field cooling (ZFC)
magnetization data to provide a quantitative measurement of the average
diameter and relative abundance of superparamagnetic nanoparticles. Applying
this method to self-assembled MnAs nanoparticles in MnAs-GaAs composite films
reveals a log-normal size distribution and reduced anisotropy for nanoparticles
compared to bulk materials. This analytical technique holds promise for rapid
assessment of the size distribution of an ensemble of superparamagnetic
nanoparticles.
