Vila Franca do Campo, Portugal


Vila Franca do Campo, Portugal
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Martins L.,IFIMUP IN | Ventura J.,IFIMUP IN | Ferreira R.,INL | Freitas P.P.,INL
Applied Surface Science | Year: 2017

Due to their high tunnel magnetoresistance (TMR) ratios at room temperature, magnetic tunnel junctions (MTJs) with a crystalline MgO insulating barrier and CoFeB ferromagnetic (FM) layers are the best candidates for novel magnetic memory applications. To overcome impedance matching problems in electronic circuits, the MgO barrier must have an ultra-low thickness (∼1. nm). Therefore, it is mandatory to optimize the MTJ fabrication process, in order to prevent relevant defects in the MgO barrier that could affect the magnetic and electrical MTJ properties. Here, a smoothing process aiming to decrease the roughness of the buffer surface before the deposition of the full MTJ stack is proposed. An ion beam milling process was used to etch the surface of an MTJ buffer structure with a Ru top layer. The morphologic results prove an effective decrease of the Ru surface roughness with the etching time. The electrical and magnetic results obtained for MTJs with smoothed buffer structures show a direct influence of the buffer roughness and coupling field on the improvement of the TMR ratio. © 2017 Elsevier B.V.

Carvalho P.S.,University of Porto | Rodrigues M.J.,IFIMUP IN
European Journal of Physics | Year: 2017

The interpretation of complex trajectories of rigid bodies by the identification of their centre of mass (CM), has a large potential for improving the understanding of the concept of CM at college and university level. Therefore, it is not surprising that there are several techniques described in the literature concerning how to identify the CM of rigid bodies. However, these techniques fail when the CM's position in the body's frame of reference changes when the body is at motion. In this work we present a computational model that allows the identification of the CM with very good accuracy, either when the CM's position changes or is fixed in the body's frame of reference. This model can be used for a system of bodies moving in a plane, for which the CM of each body coincides with its geometric centre. The effectiveness of this model is tested with experiments using video acquisition and numerical analysis, and can be done in experimental classes under controlled conditions. Students are then able to compare the computed CM with the experimental CM, and investigate why the bodies sometimes present weird trajectories. This property applies in particular to sports, so the model can be also very useful as an educational resource for the explanation of the motion of athletes, namely as a tool for optimizing their performance. © 2016 IOP Publishing Ltd.

Carvalho P.S.,University of Porto | Rodrigues M.J.,IFIMUP IN
Physics Education | Year: 2017

Bodies can have very weird trajectories when tossed into the air. The amazing motion of a water bottle flip is qualitatively described as related to the position of the centre of mass (CM) with respect to the body's frame of reference. Although this is rather easy to guess from simple observations, it is not so obvious to verify quantitatively with the naked eye. In this work, we use video analysis and a computational model to show that the position of the CM changes in the frame of reference of the bottle, while it describes a parabolic trajectory in the frame of reference of the laboratory. We also show that the behaviour of the CM also explains the weird motion of the bottle. It can be observed graphically and discussed in the classroom with students, as it only needs some critical thinking and practically no math. © 2017 IOP Publishing Ltd.

Dias C.,IFIMUP IN | Guerra L.M.,IFIMUP IN | Bordalo B.D.,IFIMUP IN | Lv H.,IN Institute of Nanoscience and Nanotechnology | And 5 more authors.
Physical Chemistry Chemical Physics | Year: 2017

Resistive switching in metal-insulator-metal nanosctructures is being intensively studied for nonvolatile memory applications. Here, we report unipolar resistive switching in Pt/MgO/Ta/Ru structures, with a 30 nm oxide barrier. A forming process was needed to initiate the resistive switching, which was then observed for all Set and Reset voltage polarity combinations. We studied the influence of the voltage polarity on the variability of the Set/Reset voltages and ON/OFF resistances and revealed the importance of a thin TaOx layer working as an oxygen revervoir for resistive switching. The mechanism behind this phenomenon can be understood in terms of conductive filaments formation/rupture with a contribution from Joule heating. Resistance change is thus caused by a voltage-driven oxygen vacancy motion in the MgO layer and a filament model was proposed for each polarity mode. A OFF/ON resistance ratio of at least 2 orders of magnitude was obtained with resistive states stable up to 104 s. Our results open the prospect to improve switching performance in other resistive switching systems, by proving a better understanding of the differences between operation modes. © 2017 the Owner Societies.

De Sousa N.,IFIMUP IN | Apolinario A.,IFIMUP IN | Vernay F.,University of Perpignan | Monteiro P.M.S.,IFIMUP IN | And 4 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We investigate equilibrium properties of an exchange-spring magnetic system constituted of a soft layer (e.g., Fe) of a given thickness on top of a hard magnetic layer (e.g., FePt). The magnetization profile M (z) as a function of the atomic position ranging from the bottom of the hard layer to the top of the soft layer is obtained in two cases with regard to the hard layer: (i) in the case of a rigid interface (the FePt layer is a single layer), the profile is obtained analytically as the exact solution of a sine-Gordon equation with Cauchy's boundary conditions. Additional numerical simulations also confirm this result. Asymptotic expressions of M (z) show a linear behavior near the bottom and the top of the soft layer. In addition, a critical value of the number of atomic planes in the soft layer, that is necessary for the onset of spin deviations, is obtained in terms of the anisotropy and exchange coupling between the adjacent plane in the soft layer. (ii) In the case of a relaxed interface (the FePt layer is a multilayer), the magnetization profile is obtained numerically for various Fe and FePt films thicknesses and applied field. © 2010 The American Physical Society.

Dias M.A.,Federal University of Rio de Janeiro | Dias M.A.,Instituto Oswaldo Cruz Programa Of Ensino Of Biociencias E Saude Ebs Fiocruz | Carvalho P.S.,University of Porto | Ventura D.R.,IFIMUP IN | Ventura D.R.,Federal University of Viçosa
Physics Education | Year: 2016

The Doppler effect is one of the recurring themes in college and high school classes. In order to contextualize the topic and engage the students in their own learning process, we propose a simple and easily accessible activity, i.e. the analysis of the videos available on the internet by the students. The sound of the engine of the vehicle passing by the camera is recorded on the video; it is then analyzed with the free software Audacity by measuring the frequency of the sound during approach and recede of the vehicle from the observer. The speed of the vehicle is determined due to the application of Doppler effect equations for acoustic waves. © 2016 IOP Publishing Ltd.

Rodrigues M.,IFIMUP IN | Marques M.B.,FCUP | Marques M.B.,INESC Porto | Simeao Carvalho P.,FCUP
Physics Education | Year: 2016

Optics is probably one on the most exciting topics in physics. However, it also contains some of the less understood phenomena by students - the light spectra obtained from the diffraction of light. The experimental study of light spectra for studying radiating bodies, usually requests sophisticated and expensive equipment that is not normaly affordable for schools, and only a few teachers know how to measure the wavelength of light in a spectrum. In this work we present a simple and inexpensive setup, with enough accuracy for measuring light spectra to be used both in physics and chemistry classes. We show how freeware software Tracker, commonly used for teaching mechanics, can serve to measure wavelengths with about 2 nm of resolution. Several approaches to the calibration of different setups are also provided, depending on the degree of accuracy demanded. © 2016 IOP Publishing Ltd.

Dias M.A.,Federal University of Rio de Janeiro | Dias M.A.,Instituto Oswaldo Cruz Programa Of Ensino Of Biociencias E Saude Ebs Fiocruz | Carvalho P.S.,University of Porto | Rodrigues M.,IFIMUP IN
Physics Education | Year: 2016

Image modelling is a recent technique in physics education that includes digital tools for image treatment and analysis, such as digital stroboscopic photography (DSP) and video analysis software. It is commonly used to analyse the motion of objects. In this work we show how to determine the position of the centre of mass (CM) of objects with either isotropic or anisotropic mass density, by video analyses as a video based experimental activity (VBEA). Strobe imaging is also presented in an educational view, helping students to visualize the complex motion of a rigid body with heterogeneous structure. As an example, we present a hammer tossed with translation and rotation. The technique shown here is valid for almost any kind of objects and it is very useful to work with the concept of CM. © 2016 IOP Publishing Ltd.

Mota D.A.,INESC Porto | Hema Chandra G.,INESC Porto | Hema Chandra G.,Vellore Institute of Technology | Ventura J.,IFIMUP IN | And 2 more authors.
Journal of Materials Science and Technology | Year: 2013

In this work, gallium phosphide thin films were deposited on glass substrates by radio frequency (RF) magnetron sputtering technique under different depositions conditions. The X-ray diffraction analysis showed a diversity of states: from amorphous in the films deposited at 175C to a nearly stoichiometric and polycrystalline films, exhibiting cubic phase with preferred orientation along (220), in the films deposited at temperatures higher than 250C. Scanning electron microscopy images revealed that all films were uniform with a smooth surface, while the energy-dispersive spectroscopy (EDS) analysis showed that there was a visible dependence on the Ga/P ratio in the deposition conditions and confirmed that a residual Ga metallic phase was presented in the surface of all the films. The Raman analysis showed the structural evolution of the GaP films was strongly dependent on the deposition conditions. The conductivity of the films was slightly dependent on the argon pressure and the rf power, but strongly dependent on the deposition temperature, mainly above 200C. The optical transmission and absorption analyses of the GaP films revealed an indirect band gap of ~1.70eV in the films deposited at temperatures less than 200C, which transited to a band gap of 2.26eV as the deposition temperature was close to 300C. © 2013.

Rodrigues M.,IFIMUP IN | Marques M.B.,Dep. de Fisica e Astronomia | Marques M.B.,INESC Porto | Simeao Carvalho P.,Dep. de Fisica e Astronomia
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2015

In this work we present a simple and low cost setup that allows obtaining the light spectra and measuring the wavelength of its features. It is based on a cheap transmission diffraction grating, an ordinary digital camera and using Tracker software to increase measuring accuracy. This equipment can easily be found in most schools. The experimental setup is easy to implement (the typical setup for a pocket spectroscope) replacing the eye with the camera. The calibration is done using a light source with a well-known spectrum. The acquired images are analyzed with Tracker (freeware software frequently used for motion studies). With this system, we have analyzed several light sources. As an example, the analysis of the spectra obtained with compact fluorescent lamp allowed to recognize the spectrum of mercury in the lamp, as expected. This spectral analysis is therefore useful in schools, among other topics, to enable the recognition of chemical elements through spectroscopy, and to alert students to the different spectra of illuminating light sources used in houses and public places. © COPYRIGHT SPIE.

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