Luc Piraux

Giant Magnetoresistance in Magnetic Multilayered Nanowires

Giant magnetoresistance (GMR) is observed in a new type of nanostructured material consisting of magnetic multilayered nanowires formed by electrodeposition into nanometer-sized pores of a template polymer membrane. The composition of these nanowires is modulated over nanometer length scales with distinct magnetic and nonmagnetic metallic layers. Magnetoresistance measurements with the current perpendicular to the layers were performed on the array of parallel nanowires. GMR of about 15% was observed at room temperature on Co/Cu multilayered nanowires

Spin Transition Charted in a Fluorophore-Tagged Thermochromic Dinuclear Iron(II) Complex

The first crystal structures of a dinuclear iron(II) complex with three N1,N2-1,2,4-triazole bridges in the high-spin and low-spin states are reported. Its sharp spin transition, which was probed using X-ray, calorimetric, magnetic, and (57)Fe Mossbauer analyses, is also delineated in the crystalline state by variable-temperature fluorimetry for the first time.

Perpendicular giant magnetoresistance of NiFe/Cu multilayered nanowires

We have prepared by electrodeposition Ni80Fe20/Cu multilayered nanowires into the pores of polymer membranes and performed giant magnetoresistance (GMR) measurements in the current perpendicular to the layer planes geometry. GMR ratios as high as 80% have been obtained at 4.2 K. Two types of structure have been studied: conventional Ni80Fe20/Cu multilayers and multilayers composed of Ni80Fe20/Cu/Ni80Fe20 trilayers magnetically isolated by long Cu rods

Controlled changes in the microstructure and magnetic anisotropy in arrays of electrodeposited Co nanowires induced by the solution pH

The effects of the electrolytic bath acidity, or pH, on the magnetic properties in arrays of electrodeposited Co nanowires and their correlation with the crystalline properties have been studied using ferromagnetic resonance. The results show that, depending on the value of the pH of the electrolyte, appreciable changes in the effective anisotropy can be induced. These changes are attributed to modifications in the microstructure of the Co nanowires. In particular, quantification of the effective anisotropy field shows that the microstructure of the deposited Co wires can be set to contain a dominant fraction of the Co-hcp phase with the c-axis oriented perpendicular to the wires, for pH values of 3.8-4.0, or parallel to the wires, for pH values >or=6.0. This results in a competitive or additive magnetocrystalline contribution to the total anisotropy field. Furthermore, at a pH value of 2.0, no contribution from the magnetocrystalline anisotropy is present, indicating a lack of texture in the Co microstructure. As a result, the effective anisotropy can be controlled over a field range of 5 kOe.

Magnetization reversal in cobalt and nickel electrodeposited nanowires

We have investigated the magnetization reversal processes in arrays of sub-micron Ni and Co wires by means of magnetization and torque experiments together with micromagnetic calculations. The wires were produced by electrodeposition in the cylindrical pores of track-etched polymer membranes. Diameters in the range 35–400 nm have been studied. The arrays of Co and Ni nanowires display different magnetic behaviors. Particular emphasis is given to the competing shape and crystal magnetic anisotropies that exist in the Co nanowire system. In both systems, explaining the experimental results requires domain formation, except for the smallest diameters where single domain behavior occurs.

Properties and Characterization of Codeposited Boron-nitride and Carbon Materials

The physical properties of carbon‐boron nitride (C‐BN) prepared from mixtures of BCl3, NH3, and selected hydrocarbons by codeposition methods have been investigated using x‐ray diffraction, electron diffraction, transmission electron spectroscopy, x‐ray photoelectron spectroscopy, Raman scattering, optical reflectivity, thermal conductivity, thermopower, and electrical resistivity. Taken collectively, the results of these experiments indicate that the compression‐annealed C‐BN materials studied here consist of separated domains of pyrolytic boronated graphite and pyrolytic boron nitride, while as‐deposited samples may possibly be a single‐phase mixture of C, B, and N. As‐deposited materials containing more than 20% carbon were found to be more highly oriented than unannealed pyrolytic graphite, and the crystallinity of these materials was greatly enhanced by uniaxial compression annealing. Results of the thermal conductivity, thermopower, and electrical resistivity measurements are consistent with a netwo...

Electrochemical control and selection of the structural and magnetic properties of cobalt nanowires

In this letter we present a convenient way of controlling the direction of the uniaxial magnetocrystalline anisotropy in arrays of electrodeposited hcp Co nanowires. Combining electron microscopy and ferromagnetic resonance measurements, it is shown that using an appropriate pH of the electrolytic solution, the hcp c axis can be oriented parallel or perpendicular to the wires axes simply by changing the deposition current density or deposition rate. This reorientation of the c axis leads to a drastic change in overall magnetic anisotropy as the crystal anisotropy either competes for perpendicular oriented c axis or adds to the shape anisotropy for parallel oriented c axis

Study of the magnetization reversal in individual nickel nanowires

The magnetization reversal of Ni nanowires was studied by anisotropic magnetoresistance measurements at temperatures between 15 and 300 K. The wires, synthesized by electrodeposition in a nanoporous polycarbonate membrane, are regular cylinders 22 mu m long with a diameter of 75 or 35 nm. The nucleation field was measured on individual nanowires as a function of the angle between the applied field and the wire axis. The results are quantitatively analyzed using classical magnetization reversal theories. Measurements of the nucleation field first obtained as a function of temperature evidence an extra uniaxial anisotropy induced by the contraction of the membrane at low temperature. Combining SQUID measurements and x-ray diffractometry at different temperatures, a clear picture of the large magnetoelastic effect was obtained

Template synthesis of nanoscale materials using the membrane porosity

The template strategy combined with electrodeposition techniques have been successfully used to produce nanoscale objects in the cylindrical pores of track-etched polycarbonate membranes. Using this method, nanometer-size metallic wires, conductive polymer nanotubules, superconducting nanowires and quasi-one-dimensional magnetic multilayers have been fabricated. These nanoscale materials exhibit physical properties different from those found in the bulk.

Microwave properties of metallic nanowires

We report on the microwave properties of arrays of parallel magnetic nanowires constituted of nickel, cobalt, or Ni/Fe alloy embedded in nanoporous track-etched polymer membranes. The experiments consist of transmission measurements carried out on microwave stripline structures using a magnetically loaded membrane as the substrate. Measurements were performed at frequencies ranging from 100 MHz to 40 GHz and under static magnetic fields up to 5.6 kOe applied along the wires axis. Resonance phenomena have been observed in the magnitude of the complex transmission coefficient at frequencies which depend on the nature of the material and applied static magnetic field. Results are consistent with those expected for a ferromagnetic resonance (FMR) experiment and the observed behaviors are analyzed in the framework of the classical FMR theory