High-sensitivity planar Hall effect micro-magnetometry for probing weak magnetic responses of gold nanoparticles at room temperature.

The magnetic responses of gold nanoparticles (AuNPs) continue to attract considerable interest because they contrast with the diamagnetic nature of bulk gold and remain challenging to investigate experimentally. The detection and comparative analysis of these weak magnetic responses are often limited by the sensitivity of conventional magnetometry techniques and by the relatively large sample quantities they require. Here, we employ a high-sensitivity Planar Hall Effect (PHE) micro-magnetometry platform to investigate weak magnetic responses from gold nanoparticle assemblies at room temperature. The system achieves a magnetic moment sensitivity down to 10-14 emu, under the present experimental conditions, enabling measurements on samples containing approximately 2 µg of AuNPs. Two nanoparticle sizes (3.4 nm and 5.0 nm) and two surface states (bare and thiol-terminated polyethylene glycol (PEG-SH)-functionalized) were investigated. Real-time measurements performed during droplet evaporation reveal reproducible differences in the detected magnetic response as the solvent contribution progressively disappears. Smaller nanoparticles exhibit stronger responses than larger particles, while PEG-SH functionalization results in a larger response than the corresponding bare AuNPs. These trends are consistent with previously reported size- and surface-dependent magnetic effects in gold nanoparticle systems and suggest a significant contribution of nanoscale surface effects and interfacial interactions. Overall, the results demonstrate that PHE-based micro-magnetometry provides a highly sensitive, compact, and room-temperature-compatible approach for detecting and comparatively investigating weak magnetic responses from nanoscale materials using microgram-scale sample quantities. Beyond its instrumental capability, the observed size- and surface-dependent variations provide additional insights into the factors influencing weak magnetic responses in gold nanoparticles, highlighting the importance of surface-related effects and nanoparticle organization. This platform complements conventional magnetometry techniques and offers new opportunities for the experimental investigation of weak magnetic phenomena in nanoscale magnetic systems.