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

Abstract

Magnetic force microscopy (MFM) is a very powerful technique, which can potentially be used 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 use MFM to discriminate between magnetic forces arising from the magnetic nanoparticles and nonmagnetic forces from other particles and sample features. Unfortunately, MFM can show a significant response even for nonmagnetic nanoparticles, giving rise to potentially misleading results. The literature to date lacks evidence for MFM detection 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 found that even on conducting, grounded substrates, significant electrostatic interaction between atomic force microscopy probes and nanoparticles can be detected, causing nonmagnetic signals that might be mistaken for a true MFM response. Nevertheless, we show that MFM can be used to discriminate between magnetic and nonmagnetic nanoparticles by using an electromagnetic shielding technique or by analysis of the phase shift data. On the basis of our experimental evidence we propose a methodology that enables MFM to be reliably used to study unknown samples containing magnetic nanoparticles, and correctly interpret the data obtained.

Magnetic force microscopy

“…All the measurements were conducted in air and at room conditions without applying an external magnetic field. We would explicitly note that while some authors pointed out the importance of the presence of an external field 29 , 88 when performing MFM images of MNPs, other authors successfully performed MFM experiments without the external field, the magnetization of the MNPs being obtained as the result of the magnetic field produced by the MFM tip 30 , 70 , 89 , 90 . In this work, in order to verify if the localized magnetic field produced by our tips were capable of magnetizing the MNPs preliminary tests have been performed using a permanent magnet under the sample.…”

Magnetic and conductive magnetite nanowires by DNA-templating

“…Similar findings relating to other non-magnetic materials have previously been commented upon in MFM studies reported by other groups. 38,64,65 Upon increasing the lift height to >10 nm however, this positive contrast associated with the uncoated DNA regions was rapidly lost, whilst the negative contrast correlating with the Fe 3 O 4 -coated DNA regions remained clearly evident. The persistence of this negative contrast at increased lift heights confirms these features arise as a consequence of long-range magnetic interactions between the probe and sample, substantiating the room-temperature magnetic behaviour of the Fe 3 O 4 when prepared in such a nanostructured form.…”

Distinguishing magnetic and electrostatic interactions by a Kelvin probe force microscopy–magnetic force microscopy combination

“…However, MFM is nowadays proposed as a valuable technique to characterize more complex systems such as organic nanomagnets [6], magnetic oxide films [7], superparamagnetic particles [8–9] and carbon based materials [10–11]. In general, these materials present low magnetic moment at room temperature.…”