  Biosensing with ferromagnet-based magnetoresistive devices has been dominated
by electrical detection of particle-induced changes to the devices' static
magnetic configuration. There are however potential advantages to be gained
from using field dependent, high frequency magnetization dynamics for magnetic
particle detection. Here we demonstrate the use of nano-confined ferromagnetic
resonances in periodically patterned magnetic films for the detection of
adsorbed magnetic particles with diameters ranging from 6 nm to 4 $\mu$m. The
nanopatterned films contain arrays of holes which can act as preferential
adsorption sites for small particles. Hole-localized particles act in unison to
shift the resonant frequencies of the various modes of the patterned layer with
shift polarities determined by the localization of each mode within the
nanopattern's repeating unit cell. The same polarity shifts are observed for a
large range of coverages, even when hole-localized particles are covered by
quasi-continuous particle sheets. For large particles however, preferential
adsorption no longer occurs, leading to resonance shifts with polarities which
are independent of the mode localization. Analogous shifts are seen in
continuous layers where, for small particles, the shift of the layer's
fundamental mode is typically about 10 times less than in patterned systems and
induced by relatively weak fields emanating beyond the particle in the
direction of the static applied field. This highlights the importance of having
confined modes consistently positioned with respect to nearby particles.
