Carbon-Substituted Hematite and Magnetite Nanoparticles
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Carbon-Substituted Hematite and Magnetite Nanoparticles Monica Sorescu and Richard Trotta Duquesne University, Department of Physics, Fisher Hall, Pittsburgh, PA 15282 ABSTRACT
Graphite-doped hematite and magnetite nanoparticles systems (~50 nm) were prepared by mechanochemical activation for milling times ranging from 2 to 12 hours. Their structural and magnetic properties were studied by 57Fe Mössbauer spectroscopy. The spectra corresponding to the hematite milled samples were analyzed by considering two sextets, corresponding to the incorporation of carbon atoms into the iron oxide structure. For ball milling time of 12 hours a quadrupole split doublet has been added, representing the contribution of ultrafine particles. The Mössbauer spectra of graphite-doped magnetite were resolved considering a sextet and a magnetic hyperfine field distribution, corresponding to the tetrahedral and octahedral sublattices of magnetite, respectively. A quadrupole split doublet was incorporated in the fitting of the 12-hour milled sample. The recoilless fraction for all samples was determined using our previously developed dual absorber method. It was found that the recoilless fraction of the graphite-doped hematite nanoparticles decreases as function of ball milling time. The f factor of graphite-containing magnetite nanoparticles for the tetrahedral sites stays constant, while that of the octahedral sublattice decreases as function of ball milling time. These findings reinforce the idea that carbon atoms exhibit preference for the octahedral sites of magnetite. INTRODUCTION The iron-carbon system plays a significant role in both basic and applied research. Thus, iron inclusions in a carbon matrix have been studied by X-ray diffraction (XRD), electron microscopy, Mӧssbauer spectroscopy and SQUID magnetometry [1]. Preparation of iron/graphite core-shell structured nanoparticles by laser pyrolysis has been reported [2] and synthesis and characterization of iron, iron oxide and iron carbide nanostructures with different morphologies has been achieved [3]. This was followed by the laser synthesis of magnetic ironcarbon nanocomposites with size dependent properties; onion-like graphenic layers were found to cover the buried iron cores [4]. Moreover, iron/iron carbide core and carbon shell nanoparticles with improved magnetic properties were successfully synthesized by laser pyrolysis and analyzed by temperature dependent Mӧssbauer spectroscopy [5]. The Mӧssbauer parameters of graphite-encapsulated iron and iron compound nanocrystals prepared by carbonarc method were determined and it was found that the superparamagnetic relaxation of the iron nanocrystals encapsulated in graphite is much weaker than that of the iron oxide nanocrystals encapsulated in graphite [6]. Recently, pure and impure graphite powders have been studied by
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different techniques in order to identify the distribution of iron impurities and establish their role in the weak ferromagnetism of these materials [7]. Reduced graphene oxide/iron carbide nanocomposit
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