Browsing by Author "Aragon, Fermin H."

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    Stoichiometry and Orientation- And Shape-Mediated Switching Field Enhancement of the Heating Properties of Fe3 O4 Circular Nanodiscs
    (American Physical Society, 2021-01-28T00:00:00) Niraula, Gopal; Coaquira, Jose A. H.; Aragon, Fermin H.; Bakuzis, Andris F.; Villar, Bianca M. G.; Garcia, Flavio; Muraca, Diego; Zoppellaro, Giorgio; Ayesh, Ahmad I.; Sharma, Surender K.
    The generation of topological magnetic vortex-domain structures in iron-oxide nanomaterials has promising applications in biomedical scenarios, such as heat generators for hyperthermia treatments. In this report we describe alternative kinds of magnetic-vortex nanoparticles, circular Fe3O4 nanodiscs (NDs), and dissect their heating properties by in-depth investigation of their shape and size, stoichiometry, orientations, and switching field "HS"behaviors, through experiments and theoretical simulation. We find that the stoichiometric NDs show better heating performance than nonstoichiometric materials because of the significant electron hopping between Fe3+ and Fe2+ ion. The higher heating efficiency (in terms of specific absorption rate, SAR) is observed only for the higher switching field regime, an effect that is associated with the parallel and perpendicular alignment of nanodiscs with respect to low and high ac magnetic field, respectively. A higher SAR of approximately 270 W/g is observed at a higher switching field (approximately 700 Oe) for NDs of diameter 770 nm, which increases by a factor of 4 at a switching field of approximately 360 Oe for NDs of diameter 200 nm. The reported results suggest that the heating efficiency in these systems can be enhanced by controlling the switching field, which is, in turn, tuned by size, shape, and orientation of circular magnetic vortex nanodiscs. � 2021 American Physical Society.
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    Tuning the shape, size, phase composition and stoichiometry of iron oxide nanoparticles: The role of phosphate anions
    (Elsevier Ltd, 2020-09-17T00:00:00) Niraula, Gopal; Coaquira, Jose A.H.; Aragon, Fermin H.; Galeano Villar, Bianca M.; Mello, Alexandre; Garcia, Flavio; Muraca, Diego; Zoppellaro, Giorgio; Vargas, Jose M.; Sharma, Surender K.
    This work describes a microwave synthetic approach for the controlled assembly of ?-Fe2O3 nanosystems with defined morphologies, such as hollow nanotubes (NTs), solid nanorods (NRs) and nanodisks (NDs). The morphological control is aided during the crystallization processes by using phosphate anions as key surfactants in solution. Furthermore, the thermal reduction under H2 atmosphere of these NTs, NRs and NDs ?-Fe2O3 systems to the correspondent Fe3O4 nanomaterials preserved their initial morphologies. It was observed that the concentration of phosphate anions and volume of solvent had significant impact not only on controlling the shapes and sizes, but also phase composition and stoichiometry of the NTs, NRs and NDs nanoparticles. X-ray Rietveld refinement analysis of the NTs, NRs and NDs systems, after reduction in H2, revealed the presence of zero-valent iron (Fe0) in the final materials, with Fe0 fractions that decreased gradually in % from NTs (?16%), NRs (?11%) to NDs (?0%) upon increasing amount of phosphate anions. Bulk magnetic susceptibility measurements showed clear alterations of the Verwey transition temperatures (TV) and the development of unusual magnetic phenomena, such as magnetic vortex states in NDs, which was subsequently verified by micro-magnetic simulations. From the combination of XRD analysis, bulk magnetic susceptibility and M�ssbauer results, we provide herein a detailed mechanistic description of the chemical processes that gated the development of shape-controlled synthesis of NTs, NRs and NDs and give a detailed correlation between specific morphology and magneto-electronic behaviors. � 2020 Elsevier B.V.