Jean-Philippe Ansermet

Anisotropic magnetoresistance as a probe of magnetization reversal in individual nono-sized nickel wires

Magnetization reversal of individual Ni nanowires has been studied by anisotropic magnetoresistance (AMR) measurements. The wires are homogeneous cylinders 6/spl mu/ long and averaging 80 nm in diameter. Nanowires were produced by electrodeposition in track-etched membrane templates. Electrical contacts to as-deposited single nanowires were obtained by a novel in-situ method where contact to a single wire is accomplished automatically and reliably during electrodeposition. Steps of the magnetoresistance are detected, corresponding to irreversible switching of the magnetization in a single wire. The dependence of the switching field on the orientation of the applied field as seen by AMR is identical to that recently observed by using micro-SQUID techniques.

Bridging the gap between template synthesis and microelectronics: spin-valves and multilayers in self-organized anodized aluminium nanopores

Current perpendicular to the plane, giant magnetoresistance (GMR) and the spin-valve effect were observed in Co/Cu multilayered nanowires electrodeposited into self-organized nanoporous anodized aluminium templates grown at the surface of bulk aluminium. Pores as short as 2000 nm could be used. The pore bottom oxide layer was removed or thinned by chemical etching. Alternating Co and Cu layers of 10 nm in thickness were synthesized by pulse plating. 20% of GMR was observed in 100 Co/Cu bilayer nanowires at room temperature. Co/Cu/Co tri-layered nanowires clearly showed the typical resistance switching of spin-valves.

Electrochemical synthesis and magnetoresistance properties of Ni, Co and Co/Cu nanowires in a nanoporous anodic oxide layer on metallic aluminium

The layer thickness of anodized aluminium oxide was controlled by anodization time and voltage to obtain templates with the desired pore-length . The barrier layer at the pore bottom of anodized aluminium templates was chemically etched to make good electrical contacts for nanowires electrodeposited in the pores thus formed on metallic aluminium substrates. Ni and Co homogeneous nanowires and Co/Cu multilayered nanowires were fabricated in the anodized aluminium templates using the electrodeposition technique and also in ion-track etched polycarbonate templates for comparison. The thicknesses of the Cu- and Co-rich layers were controlled in the range of 5 to 15 nm by varying each deposition time. Ni and Co homogeneous nanowires in aluminium templates showed typical anisotropic magnetoresistance (AMR) of around 1.0%. Co/Cu multilayered nanowires in aluminium templates reached a giant magnetoresistance (GMR) of 20% in current perpendicular to the plane geometry.

The microstructure of electrodeposited cobalt-based nanowires and its effect on their magnetic and transport properties

The microstructure of Co in electrodeposited magnetic nanowires was characterised by X-ray diffraction and nuclear magnetic resonance (NMR). A mixture of FCC and HCP Co was seen when electrodeposition was performed under standard conditions. Predominantly, HCP Co was obtained only when using electrolyte of low Co ions concentration at low deposition rate. When Cu ions were added to the Co electrolyte, the FCC phase of Co dominated. NMR investigations revealed that Cu was co-deposited in the form of clusters of about 20 atoms in the Co layers. The consequences of these observations on the transport and magnetic properties are discussed.

Recent progress in surfaceNMR-electrochemistry

NMR spectroscopy is one of the newer spectroscopic techniques for investigating the static, dynamic and electronic structures of molecules adsorbed onto metal catalyst surfaces. We review recent progress in the application of solid-state NMR methods to the investigation of molecules adsorbed onto metal surfaces in an electrochemical environment: in the presence of electrolyte, and at an electrified interface under external potentiostatic control. While at a very early stage of development, the NMR-electrochemistry approach has considerable potential for investigating otherwise inaccessible aspects of electrode and adsorbate structure, and should enable a comparison of results obtained from different spectroscopies, in particular from IR spectroscopy. We present a brief review of the development of the subject, followed by details of the instrumentation necessary for NMR-electrochemistry studies. We show how spin–spin relaxation can give information on surface structure and surface diffusion, how spin–lattice relaxation can give information on the presence of conduction electron spillover onto the adsorbate, and how the NMR of surface species responds to an externally applied electric field. The 13 C-NMR of CO on Pt in an electrochemical environment is compared with the 13 C-NMR of CO on Pt catalysts in vacuum, which are well characterized. In the case of CN on Pt, we show large spectral shifts of the resonance as the electrode potential is varied, providing an independent measurement of the effects of the electrified interface on the chemisorption bond. Spectral sensitivity is also now adequate to observe nuclei which produce even weaker signals than 13 C, such as 15 N. The NMR-electrochemistry method thus opens up a broad new array of possibilities for probing static structures (from T 2 ), surface diffusion (from the temperature dependence of T 2 ) as well as electronic properties of the chemisorption bond (from T l , and from electrode potential effects) at electrochemical interfaces, and for studying reactive intermediates and poisons on high-surface-area catalysts, such as those utilized in hydrogen and organic fuel cells.

Thermopower and GMR of a single Co/Cu multilayer nanowire

We introduce here a novel technique specifically designed to measure the thermoelectrical properties of nanoscale ferromagnetic wires, prepared by electrodeposition in nanoporous membranes. The conditions of measurements have been determined through experiments performed on uniform Co and Cu wires made in different kinds of membranes at different temperatures, from 300 K down to 70 K. We then investigated the magnetic field dependence of Co-Cu multilayer nanowires in the current-perpendicular-to-plane geometry: the relation between the giant magnetoresistance and the magnetothermopower demonstrated the efficiency of this experimental technique.

Electrochemical Synthesis and Magnetic Properties of CoFe2O4 Nanowire Arrays

We report the preparation and magnetic properties of Co2FeO4nanowires grown by electrodeposition inside the pores of anodic aluminum oxide (AAO). The synthesis requires two successive steps: the electrodeposition of fairly amorphous Fe-Co oxides or hydroxides species followed by a heat-treatment. According to X-ray diffraction and magnetic properties, a 24-h heat-treatment is necessary to obtain well-crystallized Co2FeO4 nanowires with interesting magnetic properties. The diameter of the CoFe2O4 nanowires is equivalent to the diameter size of the pores (200 nm) and their length depends on the electrodeposition time used in the preparation of the precursor (here we prepared up to 30-μm-long wires).

Space- and time-resolved Seebeck and Nernst voltages in laser-heated permalloy/gold microstructures

Thermoelectric effects in microstructured permalloy (Py)/Au wires are investigated using space- and time-resolved measurements based on scanning focused laser heating. Supported by numerical simulations of the temperature distribution, we identify two major contributions to the laser-induced signals: (i) the Seebeck effect due to thermocouples of Py/Au and (ii) the anomalous Nernst effect (ANE) in Py with a coefficient of NANE≈1.6 μV/K. ANE-based magnetic imaging of magnetic domains and magnetization reversal is demonstrated with a lateral resolution on the μm scale.

Buffer influence on magnetic dead layer, critical current, and thermal stability in magnetic tunnel junctions with perpendicular magnetic anisotropy

We present a detailed study of Ta/Ru-based buffers and their influence on features crucial from the point of view of applications of Magnetic Tunnel Junctions (MTJs) such as critical switching current and thermal stability. We study buffer/FeCoB/MgO/Ta/Ru and buffer/MgO/FeCoB/Ta/Ru layers, investigating the crystallographic texture, the roughness of the buffers, the magnetic domain pattern, the magnetic dead layer thickness, and the perpendicular magnetic anisotropy fields for each sample. Additionally, we examine the effect of the current induced magnetization switching for complete nanopillar MTJs with lateral dimensions of 270 × 180 nm. Buffer Ta 5/Ru 10/Ta 3 (thicknesses in nm), which has the thickest dead layer, exhibits a much larger thermal stability factor (63 compared to 32.5) while featuring a slightly lower critical current density value (1.25 MA/cm2 compared to 1.5 MA/cm2) than the buffer with the thinnest dead layer Ta 5/Ru 20/Ta 5. We can account for these results by considering the differen...

Thermopower measurement of single isolated metallic nanostructures

A method was developed to determine the absolute thermopower coefficient of metallic wires without ensuring a preset temperature gradient. The temperature gradient is determined by a lock-in detection of the voltage caused by the simultaneous flows of an AC heat current and a DC electric current through the wire. This technique gives access to the thermopower of nanostructures. It is illustrated with measurements from room temperature down to 15 K of Ni nanowires made by template synthesis. Data are discussed with regards to temperature calibration issues, the frequency limit and accuracy.