Monitoring the evolution of Fe3O4 mesocrystal morphology using first-order reversal curves

Abstract

We report the results of systematic measurements of first-order reversal curves (FORCs) for magnetite (Fe3O4) mesocrystals during the reaction process, where small nanoparticles, all with the same crystallographic orientation, form a cluster, and as the reaction time increases, grow in size. The FORC distribution exhibits a single peak in the FORC diagram, whose position gradually shifts toward a higher coercivity as the reaction time increases, which is associated with the broadening of the peak along both the coercivity and local interaction field axes. In addition, the FORC distribution peak is slightly displaced toward a negative local interaction field. These observations suggest the magnetic hardening and the presence of a positive (magnetizing) mean field acting on an individual nanoparticle from the surrounding nanoparticles within a mesocrystal. As the reaction time is further increased, nanoparticles constituting a mesocrystal split into porous nanoparticles, which is associated with a further shift of the FORC peak toward a higher coercivity. The present study demonstrates that FORCs can be a powerful tool for monitoring the morphology evolution of magnetic mesocrystals composed of small nanoparticles.