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 frequencybased nanoparticle detectors for high speed nano-scale biosensing.Magnetic biosensors, in which biological analytes are tagged with magnetic nanoparticles (MNPs), have excellent potential for solid-state point-of-care medical diagnostics 1-3 . The technique is intrinsically matrix-insesntive 1 , can compete with industry-standard immunoassays 4 and can be combined with magnetic separation methods 5 . The central element of a magnetic biosensor is a detector for the stray or 'fringing' magnetic fields generated by magnetised MNPs which are used to label, typically in-vitro, analytes of interest within a biological sample. Previously used sensors include SQuIDs 6 , Hall sensors 7 , ferromagnetic rings 8,9 and magneto-impedance devices 10 . However one of the most widely used methods is that employing magnetoresistive (MR) magnetic field sensors [1][2][3][4][5][11][12][13][14] which are typically fabricated with at least one
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 µm. 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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.