Browsing by Author "Vojkovic, S."

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    Analysis on the stability of in-surface magnetic configurations in toroidal nanoshells
    (2019) Teixeira, A. W.; Castillo-Sepulveda, S.; Vojkovic, S.; Fonseca, J. M.; Alibir, D.; Nunez, A. S.; Carvalho-Santos, V. L.
    Curvature of nanomagnets can be used to induce chiral textures in the magnetization field. Here we perform analytical calculations and micromagnetic simulations aiming to analyze the stability of in-surface magnetization configurations in toroidal nanomagnets. We have obtained that despite toroidal vortex-like configurations are highly stable in magnetic nanotori, the interplay between geometry and magnetic properties promotes the competition between effective interactions yielding the development of a core in a vortex state when the aspect ratio between internal and external radii of nanoturus is >= 0.75.
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    Band gap determination in multi-band-gap CuFeO2 delafossite epitaxial thin film by photoconductivity
    (2019) Vojkovic, S.; Fernandez, J.; Elgueta, S.; Vega, F. E.; Rojas, S. D.; Wheatley, R. A.; Seifert, B.; Wallentowitz, S.; Cabrera, A. L.
    The photoconductivity within a wavelength range of 450-1100 nm was determined for a sample of epitaxial delafossite CuFeO2 film grown by pulsed laser deposition. The film thickness was estimated to be 75 nm. The resistance of the films was determined with four-contact van der Pauw's method and using monochromatic illumination of the film. The most significant change in resistance resulted in three rapid lineal conductivity increases at photon energies of similar to 1.5 eV (gap-1), similar to 2.1 eV (gap-2) and similar to 2.5 eV (gap-3). The conductivity properties are well correlated with prior optical absorption results obtained in the NIR-VIS region using transmittance spectroscopy.
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    Scattering modes of skyrmions in a bilayer system with ferromagnetic coupling
    (2021) Vojkovic, S.; Cacilhas, R.; Pereira, A. R.; Altbir, D.; Nunez, A. S.; Carvalho-Santos, V. L.
    Magnetic skyrmions are quasiparticle-like textures that are topologically different from a single domain magnetization state. Their topological protection, combined with the low current density needed to move them, make these objects relevant to be used as information storage structures. In such a context, the analysis of the interactions between skyrmions is interesting and relevant for future applications. In this work, through micromagnetic simulations and numerical calculations, we studied the interaction between two skyrmions living on different parallel ferromagnetic racetracks connected by an exchange-like interaction. The upper and lower racetracks are separated by a height offset and the interaction between the upper and the lower skyrmion is analyzed in terms of the magnetic and geometrical parameters. Three states are predicted, as a function of these parameters: scattered or free skyrmions, bound skymions, and annihilated skyrmions. Our results, presented in a phase diagram, demonstrate that even in the case here called free skyrmions, there is a small and brief interaction when both are close enough, but the skyrmion in the top layer does not drag the skyrmion in the bottom layer. For bound skyrmions, both keep linked during larger times. In the latter case, there are strong changes in the velocity of the skyrmions induced by the effect of a higher effective mass when both are coupled.
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    Structural, optoelectronic and photo-thermoelectric properties of crystalline alloy CuAlxFe1-xO2 delafossite oxide materials
    (2021) Wheatley, R. A.; Roble, M.; Gence, L.; Acuna, C.; Rojas-Aedo, R.; Hidalgo-Rojas, D.; Guzman-De La Cerda, D. E.; Vojkovic, S.; Seifert, B.; Wallentowitz, S.; Volkmann, U. G.; Diaz-Droguett, D. E.
    CuFeO2 and CuAlO2 are attractive candidate materials for solar energy harvesting applications such as photocatalysis and photovoltaics. This work describes the structural, optoelectronic, thermal and electric properties of alloyed CuAlxFe1-xO2 Delafossite material synthesized using solid-state sintering techniques. The alloyed samples of CuAlxFe1-xO2 Delafossite oxide consisted of substitution of Fe for Al ranging from x = 0.01 to x = 0.99. The inclusion of Al in low concentrations affects the crystallization rate during solid-state synthesis, dramatically changing the resultant sample morphologies. The addition of dilute amounts of Al also greatly improves the conductivity of the material to a maximum of 3.18 Scm(-1). The material absorption edge shows strong changes over the alloying range. The more highly conducting samples show a strong photoinduced thermoelectric response to Vis-NIR illumination. (C) 2020 Published by Elsevier B.V.