Superparamagnetic versus blocked states in aggregates of Fe_3−xO₄ nanoparticles studied by MFM

Abstract

Magnetic domain configurations in two samples containing small aggregates of Fe(3-x)O4 nanoparticles of about 11 and 49 nm in size, respectively, were characterized by magnetic force microscopy (MFM). Two distinct magnetic behaviors were observed depending on the particle size. The aggregates constituted of nanoparticles of about 11 nm in size showed a uniform dark contrast on MFM images, reflecting the predominant superparamagnetic character of these particles and arising from the coherent rotation of the spins within the aggregate as the latter align along the tip stray-field. By applying a variable in-plane field, it is possible to induce magnetic polarization yielding an increasing dark/bright contrast as the strength of the applied field overcomes the stray-field of the tip, although this polarization completely disappears as the remanent state is recovered when the magnetic field is switched off. On the contrary, for aggregates of NPs of about 49 nm in size, dark/bright contrast associated with the existence of magnetic domains and magnetic polarization prevails in MFM images all along the magnetic cycle due to the blocking state of the magnetization of these larger particles, even in the absence of an applied field. All in all, we unambiguously demonstrate the capabilities of magnetic force microscopy to distinguish between blocked and superparamagnetic states in the aggregates of magnetic nanoparticles. Micromagnetic simulations strongly support the conclusions stated from the MFM experiments.

Understanding electrostatic and magnetic forces in magnetic force microscopy: towards single superparamagnetic nanoparticle resolution

“…However, characterization of different properties, such as spatial distribution, size and magnetic behavior of single superparamagnetic nanoparticles is difficult to perform at ambient conditions and mostly a combination of different measurement techniques is needed. Magnetic force microscopy (MFM) proved to be a promising technique to successfully image clusters of small superparamagnetic nanoparticles without labelling and provides all the information described above in a single pass [7][8][9][10][11]. Recent investigations demonstrate the capability of MFM for biological systems e. g. to evaluate the iron distribution in biological tissues [12] and to study the cellular uptake of magnetic nanoparticles [13].…”

Temperature-dependent dynamic correlations in suspensions of magnetic nanoparticles in a broad range of concentrations: a combined experimental and theoretical study

“…In the case of magnetic relaxation in magnetic soft materials [15][16][17][18] , the response to an AC magnetic field strongly depends on the type of the magnetic building blocks. Thus, for example, magnetic nanoparticles can be superparamagnetic or blocked 19,20 . In order to carefully control the properties of magnetic nanoparticles, many various approaches were recently proposed.…”

Magnetic Force Microscopy for Nanoparticle Characterization

“…Figure 2 shows the corresponding MFM phase and topographical images for bare as well as silica-coated SPIONs. A study in the Universitat de Barcelona in Spain demonstrated that MFM could be used to distinguish between superparamagnetic and blocked states in aggregates of iron oxide nanoparticles [21]. Another study used MFM to investigate the magnetic microstructure of nanoparticles with single-and multi-domain octahedral particles [22].…”