A New Method: Study on Individual Magnetic Nanoparticles by Frequency-Modulated Magnetic Force Microscopy

Individual magnetic Fe₃O₄ nanoparticles were imaged using the developed frequency-modulated magnetic force microscopy (FM-MFM) technique. High-resolution imaging of ac magnetic field from individual magnetic nanoparticles was demonstrated by FM-MFM with a high-coercivity FePt tip. It was found that the amplitude and phase images of ac magnetic field were obtained separately using the technique with a lock-in amplifier. The distribution of magnetic field gradient of the individual nanoparticles is clearly presented. It was observed that magnetic nanoparticles can be more precisely detected by FM-MFM. The results are significant for study of the real magnetic field distribution of individual magnetic nanoparticles (∼20 nm), which provide some valuable information for the analysis of nanoscale magnetic features.