Lisbon, Portugal
Lisbon, Portugal

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Dias C.,University of Porto | Lv H.,INESC MN and IN Institute of Nanoscience and Nanotechnology | Picos R.,University of the Balearic Islands | Aguiar P.,University of Porto | And 3 more authors.
Applied Surface Science | Year: 2017

Resistive switching devices are being intensively studied aiming a large number of promising applications such as nonvolatile memories, artificial neural networks and sensors. Here, we show nanoscale bipolar resistive switching in Pt/Si/Ag/TiW structures, with a dielectric barrier thickness of 20nm. The observed phenomenon is based on the formation/rupture of metallic Ag filaments in the otherwise insulating Si host material. No electroforming process was required to achieve resistive switching. We obtained average values of 0.23V and -0.24V for the Set and Reset voltages, respectively. The stability of the switching was observed for over 100 cycles, together with a clear separation of the ON (103 Ω) and OFF (102 Ω) states. Furthermore, the influence of the Set current compliance on the ON resistance, resistances ratio and Set/Reset voltages percentage variation was also studied. © 2017 Elsevier B.V.

Timopheev A.A.,University of Aveiro | Sobolev N.A.,University of Aveiro | Pogorelov Y.G.,University of Porto | Talalaevskij A.V.,University of Porto | And 6 more authors.
Journal of Applied Physics | Year: 2013

We use the resonant and non-resonant microwave absorption to probe the dynamic and static magnetic parameters of weakly coupled spin valves. The sample series include spin valve structures with varying thickness of the non-magnetic metallic spacer and reference samples comprised only a free or fixed magnetic layer. Beside the common resonance absorption peaks, the observed microwave spectra present step-like features with hysteretic behavior. The latter effect is a direct manifestation of the interlayer coupling between the ferromagnetic layers and provides two static magnetic parameters, the switching field and coercivity of the fixed layer. The analysis of the microwave absorption spectra under in-plane rotation of the applied magnetic field at different spacer thicknesses permits a deeper insight in the magnetic interactions in this system as compared to the conventional magnetometry. We combine the standard Smit-Beljers formalism for the angular dependence of the resonance fields with a Landau-Lifshitz-Gilbert dynamics extended to describe in detail the intensity of microwave absorption in the spin valves. In this way, we extract a set of parameters for each layer including the effective magnetization and anisotropy, exchange bias and interlayer coupling, as well as Gilbert damping. The model reproduces well the experimental findings, both qualitatively and quantitatively, and the estimated parameters are in a reasonable agreement with the values known from the literature. The proposed theoretical treatment can be adopted for other multilayered dynamic systems as, e.g., spin-torque oscillators. © 2013 AIP Publishing LLC.

Teixeira J.M.,University of Porto | Ventura J.,University of Porto | Fernandez-Garcia M.P.,University of Porto | Araujo J.P.,University of Porto | And 4 more authors.
Applied Physics Letters | Year: 2012

In this study, we demonstrate that, beyond the standard magnon excitations, the electronic band structure of the electrodes plays a significant role on the low bias voltage window (0 V 0.4 V) of the tunnel magnetoresistance (TMR) in thin MgO-CoFeB junctions. The tunneling conductance in the parallel state presents a minimum at about 0.35 and 0.3 V for the negative and positive bias, respectively. The presence of this minimum indicates a related decrease in the TMR(V). These observations are explained by the electronic band structures of bcc-Fe and Co. © 2012 American Institute of Physics.

Leitao D.C.,University of Porto | Leitao D.C.,INESC MN and IN Institute of Nanoscience and Nanotechnology | Ventura J.,University of Porto | Sousa C.T.,University of Porto | And 4 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

Understanding the physical properties of magnetic nanowires (NWs) is of crucial importance due to their potential technological applications. In this paper we report a detailed study on the temperature dependence of the magnetic [M(T)] and magnetotransport [MR(T)] properties of Ni and NiFe NWs grown on anodic aluminum oxide templates. While the behavior of the NiFe NWs reflected the presence of a strong shape anisotropy, Ni NWs showed anomalous M(T) and MR(T). The deviations from the expected M(T) and MR(T) behaviors suggest a reorientation of the magnetization easy axis with decreasing temperature. We then extracted the temperature variation of the angle between the magnetization and the NW longitudinal axis, and found an increase from 0? at 370 K to ∼43? at 5 K. Using a fourth-order magnetic anisotropy energy model we were able to successfully explain our results and show that the presence of a magnetoelastic anisotropy contribution due to the compressive stress acting in the NWs is the main origin of the observed magnetization reorientation. © 2011 American Physical Society.

Costa J.D.,University of Porto | Huisman T.J.,Radboud University Nijmegen | Mikhaylovskiy R.V.,Radboud University Nijmegen | Razdolski I.,Radboud University Nijmegen | And 9 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2015

The ultrafast laser-induced response of spins and charges in CoFe/Al2O3 multilayers are studied using THz and optical pump-probe spectroscopies. We demonstrate the possibility of ultrafast manipulation of the transport and magnetic properties of the multilayers with femtosecond laser excitation. In particular, using time-resolved THz transmission experiments we found that such an excitation leads to a rapid increase of the THz transmission (i.e., electric resistivity). Our experiments also reveal that femtosecond laser excitation results in the emission of broadband THz radiation. To reveal the origin of the emitted THz radiation, we performed magnetic-dependent measurements of the THz emission. We also compared the observed electric field of the THz radiation to calculations performed using subpicosecond laser-induced demagnetization measurements. The good agreement between the experimentally obtained spectra and the calculations corroborates that the measured THz emission originates from the demagnetization process. © 2015 American Physical Society.

Wisniowski P.,AGH University of Science and Technology | Dabek M.,AGH University of Science and Technology | Cardoso S.,INESC MN and in Institute of Nanoscience and Nanotechnology | Freitas P.P.,INESC MN and in Institute of Nanoscience and Nanotechnology
Sensors and Actuators, A: Physical | Year: 2013

We developed spin valve tunneling magnetoresistance devices based on MgO barrier and two compositions of CoFeB electrodes capable of sensing magnetic field in tunable ranges with high sensitivity and low nonlinearity. The tunable field ranges are due to varying strength of perpendicular anisotropy in a sensing electrode induced by changing its thickness. The sensing field ranges span from ±0.1 mT to ±100 mT. In the narrowest field range devices showed sensitivity up to 91%/mT and nonlinearity below 1.5% of full scale and in the widest field range sensitivity up to 0.076%/mT and nonlinearity below 2% of full scale. The sensing characteristics and their dependence on the electrode thickness suggest that these device structures are useful for design low to medium magnetic field sensors. © 2013 Elsevier B.V.

Wisniowski P.,AGH University of Science and Technology | Wrona J.,AGH University of Science and Technology | Stobiecki T.,AGH University of Science and Technology | Cardoso S.,INESC MN and IN Institute of Nanoscience and Nanotechnology | Freitas P.P.,INESC MN and IN Institute of Nanoscience and Nanotechnology
IEEE Transactions on Magnetics | Year: 2012

We have studied hysteresis, nonlinearity and sensitivity of CoFeB-MgO based magnetic tunnel junctions with out-of-plane and in-plane magnetizations configuration in the CoFeB free and reference layers. The configuration was achieved by using perpendicular interface anisotropy in the free and exchange bias structure in the pinned layers. For the CoFeB thickness range from 1.05 to 1.4 nm the junctions show linear transfer curves. In this CoFeB thickness range devices show linear range from ± 3 Oe to ± 1 kOe, hysteresis better than 0.6 Oe, nonlinearity 1% of the full scale and sensitivity in the range from 0.0045%/Oe to 3.8%/Oe. © 1965-2012 IEEE.

Ventura J.,IFIMUP and IN Institute of Nanoscience and Nanotechnology | Teixeira J.M.,IFIMUP and IN Institute of Nanoscience and Nanotechnology | Paz E.,International Iberian Nanotechnology Laboratory | Amaral J.S.,IFIMUP and IN Institute of Nanoscience and Nanotechnology | And 8 more authors.
Physica Status Solidi - Rapid Research Letters | Year: 2013

The exchange bias effect at the interface between antiferromagnetic (AFM) and ferromagnetic (FM) layers is of paramount importance in state-of-the-art spintronic devices. However, a complete account of the physics behind exchange bias remains elusive and new effects are constantly unraveled. In particular, a bimodal distribution of blocking temperatures $(T_{\rm B})$ was recently discovered, associated with the bulk of the AFM layer and interfacial AFM/FM regions with spin-glass-like properties. Here we study exchange bias in MnIr (25, 60 Å)/CoFe (50 Å) bilayers annealed at high temperatures (623 K and 673 K). We observe, for all samples and annealing temperatures, the existence of a large exchange bias variation at low temperatures associated with interfacial disorder. Such variation is more significative in the thinnest samples, where it is found to be independent on annealing temperature. On the other hand, in the thickest samples the contribution of the low temperature distribution largely increases with annealing temperature, due to enhanced disorder arising from Mn diffusion. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Teixeira J.M.,University of Porto | Costa J.D.,University of Porto | Ventura J.,University of Porto | Sousa J.B.,University of Porto | And 2 more authors.
Applied Physics Letters | Year: 2014

We report the observation of spin-dependent quantum well (QW) resonant tunneling in textured CoFeB free layers of single MgO magnetic tunnel junctions (MTJs). The inelastic electron tunneling spectroscopy spectra clearly show the presence of resonant oscillations in the parallel configuration, which are related with the appearance of majority-spin Δ1 QW states in the CoFeB free layer. To gain a quantitative understanding, we calculated QW state positions in the voltage-thickness plane using the so-called phase accumulation model (PAM) and compared the PAM solutions with the experimental resonant voltages observed for a set of MTJs with different CoFeB free layer thicknesses (tfl = 1.55, 1.65, 1.95, and 3.0 nm). An overall good agreement between experiment and theory was obtained. An enhancement of the tunnel magnetoresistance with bias is observed in a bias voltage region corresponding to the resonant oscillations. © 2014 AIP Publishing LLC.

Sousa C.T.,University of Porto | Leitao D.C.,University of Porto | Leitao D.C.,INESC MN and IN Institute of Nanoscience and Nanotechnology | Ventura J.,University of Porto | And 2 more authors.
Nanoscale Research Letters | Year: 2012

We report an innovative strategy to obtain cylindrical nanowires combining well established and low-cost bottom-up methods such as template-assisted nanowires synthesis and electrodeposition process. This approach allows the growth of single-layer or multi-segmented nanowires with precise control over their length (from few nanometers to several micrometers). The employed techniques give rise to branched pores at the bottom of the templates and consequently dendrites at the end of the nanowires. With our method, these undesired features are easily removed from the nanowires by a selective chemical etching. This is crucial for magnetic characterizations where such non-homogeneous branches may introduce undesired features into the final magnetic response. The obtained structures show extremely narrow distributions in diameter and length, improved robustness and high-yield, making this versatile approach strongly compatible with large scale production at an industrial level. Finally, we show the possibility to tune accurately the size of the nanostructures and consequently provide an easy control over the magnetic properties of these nanostructures. © 2012 Sousa et al.