New insights into the use of magnetic force microscopy to discriminate between magneticand nonmagnetic nanoparticles

Abstract

Magnetic force microscopy (MFM) is a very powerful technique, which can potentially beused to detect and localize the magnetic fields arising from nanoscopic magnetic domains,such as magnetic nanoparticles. However, in order to achieve this, we must be able to useMFM to discriminate between magnetic forces arising from the magnetic nanoparticles andnonmagnetic forces from other particles and sample features. Unfortunately, MFM canshow a significant response even for nonmagnetic nanoparticles, giving rise topotentially misleading results. The literature to date lacks evidence for MFMdetection of magnetic nanoparticles with nonmagnetic nanoparticles as a control.In this work, we studied magnetite particles of two sizes and with a silica shell,and compared them to nonmagnetic metallic and silica nanoparticles. We foundthat even on conducting, grounded substrates, significant electrostatic interactionbetween atomic force microscopy probes and nanoparticles can be detected, causingnonmagnetic signals that might be mistaken for a true MFM response. Nevertheless, weshow that MFM can be used to discriminate between magnetic and nonmagneticnanoparticles by using an electromagnetic shielding technique or by analysis ofthe phase shift data. On the basis of our experimental evidence we propose amethodology that enables MFM to be reliably used to study unknown samplescontaining magnetic nanoparticles, and correctly interpret the data obtained.