Browsing by Author "Conde, O."
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- Argon assisted chemical vapor deposition of CrO2: An efficient process leading to high quality epitaxial filmsPublication . Duarte, A. C.; Franco, N.; Viana, A. S.; Polushkin, N. I.; Silvestre, António Jorge; Conde, O.A comparative study of the structural, microstructural and magnetic properties of CrO2 thin films grown onto (110) and (100) TiO2 rutile single crystal substrates by chemical vapor deposition (CVD), using CrO3 as chromium precursor and either oxygen or argon as carrier gas is presented. Our results show that growth under argon carrier gas leads to high quality CrO2 epilayers with structural and magnetic properties similar to those obtained using the more standard oxygen carrier gas. Furthermore, we interpret the larger magnetic coercivity observed for the (110) oriented films in terms of their microstructure, in particular of the highest strain and edge roughness of the building structures of the CrO2 epilayers, which are settled by the substrate crystallographic orientation.
- Coexistence of Universal and Topological Anomalous Hall Effects in Metal CrO2 Thin Films in the Dirty LimitPublication . Branford, W. R.; Yates, Karen A.; Barkhoudarov, E.; Moore, J. D.; Morrison, K.; Magnus, F.; Miyoshi, Y.; Sousa, P. M.; Conde, O.; Silvestre, António Jorge; Cohen, L. F.The scaling exponent of 1.6 between anomalous Hall and longitudinal conductivity, characteristic of the universal Hall mechanism in dirty-metal ferromagnets, emerges from a series of CrO2 films as we systematically increase structural disorder. Magnetic disorder in CrO2 increases with temperature and this drives a separate topological Hall mechanism. We find that these terms are controlled discretely by structural and magnetic defect populations, and their coexistence leads to apparent divergence from exponent 1.6, suggesting that the universal term is more prevalent than previously realized.
- Cr2O3 thin films grown at room temperature by low pressure laser chemical vapour depositionPublication . Sousa, P. M.; Silvestre, António Jorge; Conde, O.Chromia (Cr2O3) has been extensively explored for the purpose of developing widespread industrial applications, owing to the convergence of a variety of mechanical, physical and chemical properties in one single oxide material. Various methods have been used for large area synthesis of Cr2O3 films. However, for selective area growth and growth on thermally sensitive materials, laser-assisted chemical vapour deposition (LCVD) can be applied advantageously. Here we report on the growth of single layers of pure Cr2O3 onto sapphire substrates at room temperature by low pressure photolytic LCVD, using UV laser radiation and Cr(CO)(6) as chromium precursor. The feasibility of the LCVD technique to access selective area deposition of chromia thin films is demonstrated. Best results were obtained for a laser fluence of 120 mJ cm(-2) and a partial pressure ratio of O-2 to Cr(CO)(6) of 1.0. Samples grown with these experimental parameters are polycrystalline and their microstructure is characterised by a high density of particles whose size follows a lognormal distribution. Deposition rates of 0.1 nm s(-1) and mean particle sizes of 1.85 mu m were measured for these films. (C) 2011 Elsevier B.V. All rights reserved.
- CVD of CrO2 thin films: Influence of the deposition parameters on their structural and magnetic propertiesPublication . Mota, Ana Filipa; Silvestre, António Jorge; Sousa, P. M.; Conde, O.; Rosa, M. A.; Godinho, M.This work reports on the synthesis of CrO2 thin films by atmospheric pressure CVD using chromium trioxide (CrO3) and oxygen. Highly oriented (100) CrO2 films containing highly oriented (0001) Cr2O3 were grown onto Al2O3(0001) substrates. Films display a sharp magnetic transition at 375 K and a saturation magnetization of 1.92 mu(B)/f.u., close to the bulk value of 2 mu(B)/f.u. for the CrO2.
- Evidences for direct magnetic patterning via diffusive transformations using femtosecond laser interferometryPublication . Polushkin, N. I.; Oliveira, Vitor; Conde, O.; Vilar, R.; Drozdov, Y. N.; Apolinario, A.; Garcia-Garcia, A.; Teixeira, J. M.; Kakazei, G. N.The application of femtosecond laser interferometry to direct patterning of thin-film magnetic alloys is demonstrated. The formation of stripe gratings with submicron periodicities is achieved in Fe1-xVx (x=18-34wt. %) layers, with a difference in magnetic moments up to Delta mu/mu similar to 20 between adjacent stripes but without any significant development of the topographical relief (<1% of the film thickness). The produced gratings exhibit a robust effect of their anisotropy shape on magnetization curves in the film plane. The obtained data witness ultrafast diffusive transformations associated with the process of spinodal decomposition and demonstrate an opportunity for producing magnetic nanostructures with engineered properties upon this basis.
- KrF pulsed laser deposition of chromium oxide thin films from Cr8O21 targetsPublication . Popovici, N.; Parames, M. L.; Da Silva, R. C.; Monnereau, O.; Sousa, P. M.; Silvestre, António Jorge; Conde, O.Chromium oxides, CrxOy, are of great interest due to the wide variety of their technological applications. Among them, CrO2 has been extensively investigated in recent years because it is an attractive compound for use in spintronic heterostructures. However, its synthesis at low temperatures has been a difficult task due to the metastable nature of this oxide. This is indeed essential to ensure interface quality and the ability to coat thermal-sensitive materials such as those envisaged in spintronic devices. Pulsed Laser Deposition (PLD) is a technique that has the potential to meet the requirements stated above. In this work, we describe our efforts to grow chromium oxide thin films by PLD from Cr8O21 targets, using a KrF excimer laser. The as-deposited films were investigated by X-ray diffraction and Rutherford backscattering spectrometry. Structural and chemical composition studies showed that the films consist of a mixture of amorphous chromium oxides exhibiting different stoichiometries depending on the processing parameters, where nanocrystals of mainly Cr2O3 are dispersed. The analyses do not exclude the possibility of co-deposition of Cr2O3 and a low fraction of CrO2.
- KrP laser CVD of chromium oxide by photodissociation of Cr(CO)(6)Publication . Sousa, P. M.; Silvestre, António Jorge; Popovici, N.; Parames, M. L.; Conde, O.This work reports on the synthesis of chromium oxide thin films prepared by photodissociation of Cr(CO)(6) in an oxidizing atmosphere, using a pulsed UV laser (KrF, lambda = 248 nm). The experimental conditions, which should enable the synthesis of CrO2, are discussed and results on the deposition of CrxOy films on Al2O3 (0001) substrates are presented.
- Laser surfasse patterning using a Michelson interferometer and a femtosecond laser radiationPublication . Oliveira, Vitor; Polushkin, N. I.; Conde, O.; Vilar, R.We report on a simple method to obtain surface gratings using a Michelson interferometer and femtosecond laser radiation. In the optical setup used, two parallel laser beams are generated using a beam splitter and then focused using the same focusing lens. An interference pattern is created in the focal plane of the focusing lens, which can be used to pattern the surface of materials. The main advantage of this method is that the optical paths difference of the interfering beams is independent of the distance between the beams. As a result, the fringes period can be varied without a need for major realignment of the optical system and the time coincidence between the interfering beams can be easily monitored. The potential of the method was demonstrated by patterning surface gratings with different periods on titanium surfaces in air.
- Laser-assisted deposition of thin films from photoexcited vapour phasesPublication . Conde, O.; Silvestre, António JorgeLaser-assisted chemical vapour deposition (LCVD) has been extensively studied in the last two decades. A vast range of applications encompass various areas such as microelectronics, micromechanics, microelectromechanics and integrated optics, and a variety of metals, semiconductors and insulators have been grown by LCVD. In this article, we review briefly the LCVD process and present two case studies of thin film deposition related to laser thermal excitation (e.g., boron carbide) and non-thermal excitation (e.g., CrO(2)) of the gas phase.
- Laser-induced diffusion decomposition in Fe-V thin-film alloysPublication . Polushkin, N. I.; Duarte, A. C.; Conde, O.; Alves, E.; Barradas, N. P.; García-García, A.; Kakazei, G. N.; Ventura, J. O.; Araújo, J. P.; Oliveira, Vitor; Vilar, R.We investigate the origin of ferromagnetism induced in thin-film (similar to 20 nm) Fe-V alloys by their irradiation with subpicosecond laser pulses. We find with Rutherford backscattering that the magnetic modifications follow a thermally stimulated process of diffusion decomposition, with formation of a-few-nm-thick Fe enriched layer inside the film. Surprisingly, similar transformations in the samples were also found after their long-time (similar to 10(3) s) thermal annealing. However, the laser action provides much higher diffusion coefficients (similar to 4 orders of magnitude) than those obtained under standard heat treatments. We get a hint that this ultrafast diffusion decomposition occurs in the metallic glassy state achievable in laser-quenched samples. This vitrification is thought to be a prerequisite for the laser-induced onset of ferromagnetism that we observe. 2014 Elsevier B.V. All rights reserved.