Resonance-based Detection of Magnetic Nanoparticles and Microbeads Using Nanopatterned Ferromagnets

TL;DR

This paper introduces a novel method for detecting magnetic nanoparticles and microbeads using nano-confined ferromagnetic resonances in patterned magnetic films, offering advantages over traditional electrical detection methods.

Contribution

The study demonstrates the use of nano-confined ferromagnetic resonances in patterned films for magnetic particle detection, revealing mode shifts related to particle localization and size, with potential for improved biosensing.

Findings

Resonance frequency shifts depend on particle size and localization.

Patterned films show larger shifts compared to continuous layers.

Mode shifts are consistent across various particle coverages.

Abstract

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.