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DDC: 540 × Subject: magnetoresistance ×

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    Control of Positive and Negative Magnetoresistance in Iron Oxide : Iron Nanocomposite Thin Films for Tunable Magnetoelectric Nanodevices
    (2020) Nichterwitz, Martin; Honnali, Shashank; Zehner, Jonas; Schneider, Sebastian; Pohl, Darius; Schiemenz, Sandra; Goennenwein, Sebastian T.B.; Nielsch, Kornelius; Leistner, Karin
    The perspective of energy-efficient and tunable functional magnetic nanostructures has triggered research efforts in the fields of voltage control of magnetism and spintronics. We investigate the magnetotransport properties of nanocomposite iron oxide/iron thin films with a nominal iron thickness of 5-50 nm and find a positive magnetoresistance at small thicknesses. The highest magnetoresistance was found for 30 nm Fe with +1.1% at 3 T. This anomalous behavior is attributed to the presence of Fe3O4-Fe nanocomposite regions due to grain boundary oxidation. At the Fe3O4/Fe interfaces, spin-polarized electrons in the magnetite can be scattered and reoriented. A crossover to negative magnetoresistance (-0.11%) is achieved at a larger thickness (>40 nm) when interface scattering effects become negligible as more current flows through the iron layer. Electrolytic gating of this system induces voltage-triggered redox reactions in the Fe3O4 regions and thereby enables voltage-tuning of the magnetoresistance with the locally oxidized regions as the active tuning elements. In the low-magnetic-field region (<1 T), a crossover from positive to negative magnetoresistance is achieved by a voltage change of only 1.72 V. At 3 T, a relative change of magnetoresistance about -45% during reduction was achieved for the 30 nm Fe sample. The present low-voltage approach signifies a step forward to practical and tunable room-temperature magnetoresistance-based nanodevices, which can boost the development of nanoscale and energy-efficient magnetic field sensors with high sensitivity, magnetic memories, and magnetoelectric devices in general. Copyright © 2020 American Chemical Society.
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    Theoretical Prediction of a Giant Anisotropic Magnetoresistance in Carbon Nanoscrolls
    (Washington, DC : ACS Publ., 2017-4-12) Chang, Ching-Hao; Ortix, Carmine
    Snake orbits are trajectories of charge carriers curving back and forth that form at an interface where either the magnetic field direction or the charge carrier type are inverted. In ballistic samples, their presence is manifested in the appearance of magnetoconductance oscillations at small magnetic fields. Here we show that signatures of snake orbits can also be found in the opposite diffusive transport regime. We illustrate this by studying the classical magnetotransport properties of carbon tubular structures subject to relatively weak transversal magnetic fields where snake trajectories appear in close proximity to the zero radial field projections. In carbon nanoscrolls, the formation of snake orbits leads to a strongly directional dependent positive magnetoresistance with an anisotropy up to 80%.
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    Voltage-Controlled ON-OFF-Switching of Magnetoresistance in FeOx/Fe/Au Aerogel Networks
    (Washington, DC : ACS Publications, 2023) Nichterwitz, Martin; Hiekel, Karl; Wolf, Daniel; Eychmüller, Alexander; Leistner, Karin
    Voltage control of magnetoresistance (MR) in nanoscale three-dimensional (3D) geometries is interesting from a fundamental point of view and a promising route toward novel sensors and energy-efficient computing schemes. Magneto-ionic mechanisms are favorable for low-voltage control of magnetism and room-temperature operation, but magneto-ionic control of MR has been studied only for planar geometries so far. We synthesize a 3D nanomaterial with magneto-ionic functionality by electrodepositing an iron hydroxide/iron coating on a porous nanoscale gold network (aerogel). To enable maximum magneto-ionic ON-OFF-switching, the thickness of the coating is adjusted to a few nanometers by a self-terminating electrodeposition process. In situ magnetotransport measurements during electrolytic gating of these nanostructures reveal large reversible changes in MR, including ON-OFF-switching of MR, with a small applied voltage difference (1.72 V). This effect is related to the electrochemical switching between a ferromagnetic iron shell/gold core nanostructure (negative MR at the reduction voltage) and an iron oxide shell/gold core nanostructure (negligible MR at the oxidation voltage).