How the surface affects the electronic and magnetic properties of magnetite nanoparticles

Abstract

Magnetite (Fe3O4) nanoparticles are objects of intense research activities due to their broad range of applications covering technological, medical, and environmental applications. They are used e.g. for rotary shaft sealing, oscillation damping, position sensing, magnetic inks for jet printing, as contrast agents in magnetic resonance imaging, and to remove heavy metals from wastewater. In addition, magnetite is a half-metal with a predicted negative spin polarisation making magnetite interesting for spintronics. For all applications, a high quality of magnetite is crucial to obtain the desired properties. In this work, we studied the influence of the surface on the electronic and magnetic properties of magnetite nanoparticles by means of x-ray absorption near-edge spectroscopy (XANES) and its associated magnetic circular dichroism (XMCD). In particular, we investigated ensembles of magnetite nanoparticles with a mean diameter of 3nm, 6nm or 9nm and the influence of capping the particles with a 3nm thick silica shell or organic ligands. XANES and XMCD gives the unique possibility to distinguish between the three different Fe species in magnetite, i.e. tetrahedrally coordinated Fe3+ ions, octahedrally coordinated Fe3+, and octahedrally coordinated Fe2+, by using different photon energies for hysteresis measurements. Besides changes in the electronic structure, i.e. in the density of unoccupied 3d states monitored by XANES, and the effective spin magnetic moments, we obtained a different spin canting behaviour of Fe ions in magnetite at different lattice sites from the magnetic field dependent XMCD as shown in the figure below. The results are discussed regarding different exchange mechanisms and possible advantages and drawbacks for applications. Measurements were performed at beamline UE46-PGM1, HZB – BESSY II synchrotron radiation facility. We thank the BESSY II staff for kind support during beamtimes. Financially supported by BMBF (05 ES3XBA/5) and DFG (WE2623/3-1)