Abstract
Despite decades of advances in magnetic imaging, obtaining direct, quantitative information with nanometer scale spatial resolution remains an outstanding challenge. Current approaches, for example, Hall micromagnetometer and nitrogen-vacancy magnetometer, are limited by highly complex experimental apparatus and a dedicated sample preparation process. Here we present a new AC field-modulated magnetic force microscopy (MFM) and report the local and quantitative measurements of the magnetic information of individual magnetic nanoparticles (MNPs), which is one of the most iconic objects of nanomagnetism. This technique provides simultaneously a direct visualization of the magnetization process of the individual MNPs, with spatial resolution and magnetic sensitivity of about 4.8 nm and 1.85 × 10−20 A m2, respectively, enabling us to separately estimate the distributions of the dipolar fields and the local switching fields of individual MNPs. Moreover, we demonstrate that quantitative magnetization moment of individual MNPs can be routinely obtained using MFM signals. Therefore, it underscores the power of the AC field-modulated MFM for biological and biomedical applications of MNPs and opens up the possibility for directly and quantitatively probing the weak magnetic stray fields from nanoscale magnetic systems with superior spatial resolution.
Highlights
Despite decades of advances in magnetic imaging, obtaining direct, quantitative information with nanometer scale spatial resolution remains an outstanding challenge
Isolated particles below 35 nm were obtained with the help of anomalous Hall-effect (AHE)[11], superconducting quantum interference device (SQUID)[12,13], spin-polarized scanning tunneling microscopy[14], transmission X-ray microscopy[15] and nitrogen-vacancy magnetometer[16]
In this work, we introduce an AC field-modulated magnetic force microscopy (MFM) technique, which uses the frequency modulation of a cantilever oscillation by applying ac magnetic field to a mechanically oscillated tip, to locally characterize the magnetization behavior of individual magnetic nanoparticles (MNPs) with diameter of about 10 nm in atmosphere, as well as the nanoscale magnetic domains
Summary
Results revealed that an external magnetic field of several Oe is sufficient to induce a stable magnetic dipole in MNPs at room temperature and the presence of an external magnetic field is essential to detect and distinguish an MFM signal from the MNPs. It is indicated that the magnetization behavior of individual MPNs can be probed by observing the magnetic domains under an applied field and we can separately estimate the distributions of the dipolar fields and the local switching fields of individual MNPs using the described AC field-modulated MFM technique. Another advantage is that a larger stray field can be generated from the magnetic nanoparticles in the AC magnetic field modulation technique, which gives higher MFM signal amplitude. The AC field-modulated MFM technique can be applied to investigate the microscopic magnetic domain structures in a variety of magnetic materials, such as nanoparticles, high density recording media, , patterned elements, as well as other magnetic features and nanostructures, which opens up the possibility for directly and quantitatively probing the weak magnetic stray fields from nanoscale magnetic systems with superior spatial resolution
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