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Lee S.,KAIST | Lee S.,Center for Electricity and Magnetism | In J.,KAIST | Chang J.-W.,Center for Electricity and Magnetism | And 5 more authors.
Chemistry - An Asian Journal | Year: 2012

We report unconventional magnetotransport properties of an individual Fe 1-xCo xSi nanowire. We have studied the dependence of the resistivity on the angle between the directions of the magnetization and electrical current below the Curie temperature (T C). The observed anisotropic magnetoresistance (MR) ratio is negative, thereby indicating that the conduction electrons in a minority spin band of the Fe 1-xCo xSi nanowire dominantly contribute to the transport. Unlike typical ferromagnets, positive MR is observed in the overall temperature range. MR curves are linear below T C and show a quadratic form above T C, which can be explained by the change of density of states that arises as the band structures of the Fe 1-xCo xSi nanowire shift under a magnetic field. The temperature dependence of the resistivity curve is sufficiently explained by the Kondo effect. The Kondo temperature of the Fe 1-xCo xSi nanowire is lower than that of the bulk state due to suppression of the Kondo effect. The high single crystallinity of Fe 1-xCo xSi nanowires allowed us to observe and interpret quite subtle variations in the prominent intrinsic transport properties. Down to the wire: Ferromagnetic single-crystalline Fe 1-xCo xSi nanowires (NWs) were synthesized by means of a vapor transport method with no catalyst. The magnetotransport properties of the Fe 1-xCo xSi NW reveal negative anisotropic magnetoresistance owing to dominant spin-down electrons. The various results (see figure) are of interest for spintronic devices. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Lee S.,KAIST | Lee S.,Center for Electricity and Magnetism | In J.,KAIST | Yoo Y.,KAIST | And 7 more authors.
Nano Letters | Year: 2012

A recent theoretical study suggested that Ag 2Te is a topological insulator with a highly anisotropic Dirac cone. Novel physics in the topological insulators with an anisotropic Dirac cone is anticipated due to the violation of rotational invariance. From magnetoresistance (MR) measurements of Ag 2Te nanowires (NWs), we have observed Aharanov-Bohm (AB) oscillation, which is attributed to the quantum interference of electron phase around the perimeter of the NW. Angle and temperature dependences of the AB oscillation indicate the existence of conducting surface states in the NWs, confirming that Ag 2Te is a topological insulator. For Ag 2Te nanoplates (NPLs), we have observed high carrier mobility exceeding 22 000 cm 2/(V s) and pronounced Shubnikov-de Haas (SdH) oscillation. From the SdH oscillation, we have obtained Fermi state parameters of the Ag 2Te NPLs, which can provide valuable information on Ag 2Te. Understanding the basic physics of the topological insulator with an anisotropic Dirac cone could lead to new applications in nanoelectronics and spintronics. © 2012 American Chemical Society.

Lee S.,Kyung Hee University | Lee S.,KAIST | In J.,KAIST | Kim S.-I.,KAIST | And 5 more authors.
Journal of Materials Chemistry C | Year: 2013

We have synthesized quaternary single crystalline (Nb0.94V 0.06)10(SixGe1-x)7 nanowires (NWs) (0.1 ≤ x ≤ 0.5) in high density by flowing a NbCl 5 precursor and placing vanadium (V) foil on a mixture of Si, Ge, and C powder. The composition of Si (x) in the NW could be modulated from 0.1 to 0.5 by changing the substrate temperature. We have investigated how the atoms comprising the quaternary NWs are arranged in a real-space using a spherical aberration corrected scanning transmission electron microscope. The filling of Si and Ge atoms in Ge atom columns is analyzed by comparing experiments and simulations. Electrical transport measurements show that (Nb 0.94V0.06)10(Si0.5Ge 0.5)7 NWs have an ultralow resistivity of ∼8.5 μΩ cm, lower than that of most conducting metal silicides, as well as a high failure current density of 1.1 × 108 A cm-2 at room temperature. The synthesis of quaternary single crystalline (Nb 0.94V0.06)10(SixGe 1-x)7 NWs (0.1 ≤ x ≤ 0.5) shows that Si and Ge composition can be easily modulated in metal germanosilicide nanostructures. The quaternary NWs may supply high quality nanoscale materials for the gate and interconnect in SiGe based nanoelectronics. This journal is © The Royal Society of Chemistry 2013.

Joo S.,Center for Electricity and Magnetism | Jung K.Y.,Korea University | Jung K.Y.,Spintronic Device Research Center | Jun K.I.,Spintronic Device Research Center | And 5 more authors.
Applied Physics Letters | Year: 2014

Tunneling magnetoresistance (TMR) dependence on the Al2O 3 barrier thickness was investigated for CoFe/Al2O 3/CoFe magnetic tunnel junctions (MTJs). MTJs with very thin Al 2O3 layers were grown by inserting an amorphous FeZr buffer layer whose role is only to reduce the roughness of bottom electrode. The TMR decreased as the thickness of the Al2O3 layer was reduced. The results are analyzed with the dependence of the spin-filtering effect on the Al2O3 thickness. It was found that a simple model of separating sp- and d-like electrons does not work, and it may suggest that the tunneling electrons are in rather hybridized state. © 2014 AIP Publishing LLC.

Kim K.-T.,Center for Electricity and Magnetism | Kwon S.-W.,Center for Electricity and Magnetism | Jung J.K.,Center for Electricity and Magnetism | Lee S.-H.,Center for Electricity and Magnetism | Lee H.G.,Center for Electricity and Magnetism
20th IMEKO World Congress 2012 | Year: 2012

The Josephson voltage standard and the quantum Hall resistance standard is a clear and solid basis to realize SI traceable calibrations for all electrical measurements of varieties of applications. However, the traceability link is necessarily required between the wellestablished SI ampere and the wide measurement range at application sites for routine calibration works. The step-up technology is a simple and useful method which can be applied in various electrical measurement fields. Copyright © (2012) by the International Measurement Federation (IMEKO).

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