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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.

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.

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.

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.

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