  We demonstrate the use of the magnetic-field-dependence of highly spatially
confined, GHz-frequency ferromagnetic resonances in a ferromagnetic
nanostructure for the detection of adsorbed magnetic nanoparticles. This is
achieved in a large area magnonic crystal consisting of a thin ferromagnetic
film containing a periodic array of closely spaced, nano-scale anti-dots. Stray
fields from nanoparticles within the anti-dots modify resonant dynamic
magnetisation modes in the surrounding magnonic crystal, generating easily
measurable resonance peak shifts. The shifts are comparable to the resonance
linewidths for high anti-dot filling fractions with their signs and magnitudes
dependent upon the modes' localisations (in agreement with micromagnetic
simulation results). This is a highly encouraging result for the development of
frequency-based nanoparticle detectors for high speed nano-scale biosensing.
