  The complex permittivity and permeability of superparamagnetic iron-oxide
nanoparticles has been quantified using a circular waveguide assembly with a
static magnetic field to align the nanoparticle's magnetization. The high
sensitivity of the measurement provides the precise resonant feature of
nanoparticles. The complex permeability in the vicinity of ferromagnetic
resonance (FMR) is in agreement with the nanoparticle's measured magnetization
via conventional magnetometry. A rigorous and self-consistent measure of
complex permittivities and permeabilities of nanoparticles is crucial to
ascertain accurately the dielectric behaviour as well as the frequency response
of nanoparticle magnetization, necessary ingredients when designing and
optimizing magnetic nanoparticles for biomedical applications.
