Sylvain Dubois

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

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.

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.

Fabrication and properties of arrays of superconducting nanowires

We report on the fabrication and structural characterization of arrays of superconducting nanowires by electroplating lead into the nanopores of track-etched polymer membranes. The diameters of the lead nanowires range from 400 down to 70 nm, whereas their length is about 20 mu m. Large enhancement of the critical field has been observed in good agreement with the Ginsburg-Landau-Silin theory. By comparing the predicted critical field enhancement for thin cylinders with experimental results, we have extracted the effective penetration depth. The dependence of the effective penetration depth and electron mean free path on the wire diameter is also discussed.

Superconducting properties of lead nanowires arrays

Mesoscopic superconducting lead nanowires with high aspect ratio and diameter ranging from 40 to 270 nm have been grown by electrodeposition inside nanoporous polycarbonate membranes. Nanowires with a diameter less than 50 nm were insulators due to a poor crystal structure. The others are shown to be type II superconductors because of their small electronic mean free path, instead of being type I which is usual for the bulk form of lead. An increase in the thermodynamic critical field H-c is observed and is attributed to the small transversal dimension leading to an incomplete Meissner effect. Finally, it is demonstrated that this enhancement agrees with numerical simulations based on the Ginzburg-Landau theory

Observing magnetic nanowires by means of magnetic force microscopy

Magnetic force microscopy (MFM) is being used to image [Co/Cu].N and pure Co nanowires with diameters in the 80-90 nm range. Symmetry considerations as well as a close inspection of defects in the magnetic sequence allow for detailed conclusions to be reached. Although necessarily resting on a highest likelihood basis, the latter are substantiated by a simple model excluding altogether perturbations of the: magnetization distribution within tip or sample. Crystallographic features are revealed indirectly as well as finely spaced antiferromagnetic type magnetic sequences. It is believed that the present observations reveal for the first time the coexistence of axially and transversally magnetized segments within a given nanowire. Lastly, it is shown from an instrumental point of view that topographic and magnetic imaging are intimately interwoven

Arrays of nanowires of magnetic metals and multilayers: Perpendicular GMR and magnetic properties

The template strategy combined with electrodeposition techniques have been used to fabricate arrays of nanowires of magnetic metals and multilayers in the cylindrical pores of track-etched polymer membranes, The giant magnetoresistance effects have been investigated in two different types of multilayered nanowires systems: Co/Cu and Ni80Fe20/Cu. In addition, a comparative study of the magnetic properties of sub-micron Ni, Co, Fe and Ni80Fe20 wires is made by means of anisotropic magnetoresistance and magnetization experiments.

Electrodeposition of patterned magnetic nanostructures

We report on fabrication and characterization of two types of devices, both with submicronic dimensions, and fabricated by combining lithography and electrodeposition. The first device, obtained by combining electron-beam lithography and electrodeposition, was devised to measure the current perpendicular to the plane giant magnetoresistance (CPP-GMR) of a single permalloy/ copper multilayered nanopillar (height similar to 0.3 mu m, diameter similar to 0.1 mu m). Besides the fundamental interest of the spin-dependent transport properties in such nanoscaled magnets, this system is a potential candidate as a CPP-GMR sensor used, for example, to read very high-density magnetic storage. The second device, relevant for high-density storage media, consists in large areas (4 x 4 mm(2)) of magnetic permalloy dots (diameter similar to 0.26 mu m, period similar to 0.4 mu m) electrodeposited in a x-ray patterned photoresist matrix. We study the magnetic behavior of such mesoscopic pillars as a function of their height. We emphasize that our processes are less damaging for the nanostructures, in comparison with samples prepared by high vacuum deposition followed by lithography. This is because our magnetic nanostructures are electrodeposited after the whole lithographic process

Slip line analysis around nanoindentation imprints in Ti3SnC2: a new insight into plasticity of MAX-phase materials

The plasticity of Ti3SnC2, a recently synthesized MAX phase, was investigated. Localized deformation was induced by nanoindentation in a polycrystalline sample, and the resulting surface topography was observed by atomic force microscopy (AFM). For several grains, buckling around the indent was observed, in agreement with the kink band deformation process often reported for MAX-phase materials. For other grains, slip lines have been revealed by AFM. The corresponding slip systems have been identified through the determination of the local crystalline orientation by electron backscatter diffraction. First- and second-order pyramidal slip systems are shown to be active for some grain orientations, as well as dislocation interactions and cross slip from one system to the other.