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Browsing by Author "Cardoso, S."

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  • Magnetic and transport properties of diluted granular multilayers
    Publication . Silva, Hugo G.; Gomes, Henrique L.; Pogorelov, Y. G.; Pereira, L. M. C.; Kakazei, G. N.; Sousa, J. B.; Araújo, J. P.; Mariano, José F. M. L.; Cardoso, S.; Freitas, P. P.
    The magnetic and transport properties of Co80Fe20t /Al2O34 nm multilayers with low nominal thicknesses t=0.7 and 0.9 nm of Co80Fe20 granular layers are studied. Magnetic studies find a superparamagnetic state above the blocking temperature Tb of field-cooled/zero-field-cooled splitting that grows with t and decreases with H. The low-voltage Ohmic tunnel transport passes to non-Ohmic IV3/2 law for applied fields above 500 V/cm. At fixed V, the temperature dependence of conductance reveals an anomalous dip around 220 K, which can be attributed to the effect of surface contamination by supercooled water. Current-in-plane tunnel magnetoresistance MR ratio tends, at lower t, to higher maximum values 8% at room temperature but to lower field sensitivity. This may indicate growing discorrelation effect e.g., between shrinking areas of correlated moments in this regime and corroborates the deficit of granule magnetization estimated from the Inoue–Maekawa MR fit, compared to that from direct magnetization measurements. MR displays a mean-field-like critical behavior when t approaches the point of superparamagnetic/ superferromagnetic transition tc1.3 nm at room temperature from below, different from the formerly reported percolationlike behavior at approaching it from above.With growing temperature, MR reveals, beyond the common decrease, an anomalous plateau from Tb30–50 K up to some higher value T150–200 K, not seen at higher t.
    2009Journal article Open access Show more
  • Resistive switching in nanostructured thin films
    Publication . Silva, Hugo G.; Gomes, Henrique L.; Pogorelov, Y. G.; Stallinga, Peter; De Leeuw, Dago M.; Araújo, J. P.; Sousa, J. B.; Meskers, S. C. J.; Kakazei, G. N.; Cardoso, S.; Freitas, P. P.
    Planar capacitor structures based on granular films composed of nanometric ferromagnetic grains embedded in an insulating Al2O3 matrix can switch between a high-conductance and a low-conductance state. The switching properties are induced by a forming process. The ON/OFF resistance ratio is as high as 104 under an electrical field of only 15 kV/m. This resistive switching is accompanied by a capacitive switching between two well-defined voltage-independent states, a behavior that is not readily explained by the filamentary type of conduction.
    2009Journal article Open access Show more
  • Resistive switching of silicon-silver thin film devices in flexible substrates
    Publication . Dias, C.; Leitao, D. C.; Freire, C. S. R.; H, Gomes; Cardoso, S.; Ventura, J.
    Novel applications for memory devices demand nanoscale flexible structures. In particular, resistive switching (RS) devices are promising candidates for wearable and implantable technologies. Here, the Pt/Si/Ag/TiW metal-insulator-metal structure was fabricated and characterized on top of flexible substrates using a straightforward microfabrication process. We also showed that these substrates are compatible with sputtering deposition. RS was successfully achieved using both commercial cellulose cleanroom paper and bacterial cellulose, and polymer (PET) substrates. The bipolar switching behavior was observed for both flat and bent (under a radius of 3.5 mm) configurations. The observed phenomenon was explained by the formation/rupture of metallic Ag filaments in the otherwise insulating Si host layer.
    2020-03Journal article Restricted access Show more