Laurent Gravier

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.

Template synthesis and magnetoresistance property of Ni and Co single nanowires electrodeposited into nanopores with a wide range of aspect ratios

Template synthesis and magnetoresistance property of Ni and Co single nanowires electrodeposited into nano-pores with wide range of aspect ratios

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.

Fe3O4 nanowires synthesized by electroprecipitation in templates

We present an electrochemical technique for growing Fe3O4 nanostructures. Hydrazine and iron compounds in an aqueous solution undergo chemical reactions with hydroxides and form magnetite at the surface of the working electrode of an electrochemical cell. Growth of magnetite in nanoscale templates is demonstrated. X-ray diffraction, transmission electron microscopy, electrical transport measurements, and magnetometry confirm the nature of the deposit. Giant magnetoresistance was observed when the nanowires were granular.

Current-induced two-level fluctuations in pseudo-spin-valve (Co/Cu/Co) nanostructures.

Two-level fluctuations of the magnetization state of pseudo-spin-valve pillars Co(10 nm)/Cu(10 nm)/Co(30 nm) embedded in electrodeposited nanowires ( approximately 40 nm in diameter, 6000 nm in length) are triggered by spin-polarized currents of 10(7) A/cm(2) at room temperature. The statistical properties of the residence times in the parallel and antiparallel magnetization states reveal two effects with qualitatively different dependences on current intensity. The current appears to have the effect of a field determined as the bias field required to equalize these times. The bias field changes sign when the current polarity is reversed. At this field, the effect of a current density of 10(7) A/cm(2) is to lower the mean time for switching down to the microsecond range. This effect is independent of the sign of the current and is interpreted in terms of an effective temperature for the magnetization.

Electrochemical Surface Modification of Aluminium Sheets for Application to Nano-electronic Devices: Anodization Aluminium and Electrodeposition of Cobalt-Copper

A nano-porous anodized aluminium oxide layer was synthesized on the surface of bulk aluminium at a wide range of anodization voltages. The barrier layer at the pore bottom of anodized aluminium oxide layer was chemically etched to make good electrical contact for nanowires electrodeposited in the pores thus formed on metallic aluminium substrates. Cathodic polarization was examined at a wide range of cathode potentials to investigate the electrodeposition behaviour of Cu and Co into the pores. Co81Cu19/Cu multilayered nanowires were fabricated using a pulse-plating technique into the templates. Co-alloy layer and Cu layer thicknesses were adjusted to 10 nm, by controlling the deposition times. The temperature dependence of the resistance of Co81Cu19/Cu multilayered nanowires grown on the template presented clean metallic characteristics and a giant magnetoresistance (GMR) of 23% was reached at 4 K.

Spin mixing processes in magnetic nanostructures detected by thermoelectric measurements

Spin-dependent transport properties of magnetic nanostructures have been investigated by means of magneto-thermogalvanic voltage measurements: the ac voltage response to an ac temperature oscillation is measured for various magnetic nanostructures under dc current. The samples studied include Co/Cu multilayered nanowires, homogeneous Ni nanowires and cobalt clusters embedded in copper films. The magnetic field dependence of this signal is always larger than the magneto-resistance (MR) and may be anisotropic even when the MR is not. A thermodynamic argument introduces spin mixing as the main process measured by this novel thermoelectric measurement technique. This effect is not observed in magnetite as can be justified by the absence of an accessible second spin channel.

Current-induced magnetization switching in pseudo spin-valves

The spin transfer torque model applied in the context of a Fokker-Planck analysis (Li and Zhang 2004 Phys. Rev. B 69 134416) is shown to account for a complete set of statistical data for switching times obtained with pseudo spin-valves (Fabian et al 2003 Phys. Rev. Lett. 91 257209). Current densities of the order of 10(7) A cm(-2) injected in Co/Cu/Co bilayers electrodeposited in nanoporous membranes gave rise to magnetization switching. Statistics could be accumulated on one single nanowire at a time: the field at which the average residence times in parallel and antiparallel configurations were equal, these times as a function of current, and the ratio of the times as a function of current and field.

YBCO/LSMO and LSMO/YBCO double-layer deposition by off-axis magnetron sputtering and strain effects

We report here on ferromagnetic/superconductor FM/SC and SC/FM double layers deposited without a buffer layer between FM and SC. Thin films of La0.7Sr0.3MnO3 (LSMO) are used for the FM, and YBa2Cu3O7-x (YBCO) for the SC. Both films can grow crystalline on SrTiO3 (STO) or LaAlO3 (LAO) substrates in high temperature and in oxygen atmosphere conditions. Magnetic and transport measurements and X-ray diffraction analysis are used to characterise film quality and properties. It is shown that off-axis magnetron sputtering can be used for high-quality double layer deposition. A prerequisite for this is the suppression of the interdiffusion process. This is achieved by lowering the deposition temperature and shortening the deposition time for the top layer. Lattice relaxation for LSMO films deposited on LAO substrates is seen. It is demonstrated that post-deposition annealing or additional top layer deposition enlarges the relaxed part of the lower LSMO film. LSMO films are smooth and free of imperfections and the stress is partly relieved by the formation of misfit dislocations. For YBCO films, SEM, XRD, EDX and magnetisation characterisations show that film lattice relaxation starts when single-element-oxide crystals start to grow into the film. Many experiments give evidence of a critical thickness of about 30nm of a strained layer, after which the top part of the YBCO film relaxes. Despite the small compositional deviation due to the interdiffusion, the YBCO/LSMO double films demonstrate high enough transport and magnetic properties to allow their application in the investigation of the injection of spin-polarised quasiparticles from FM to SC film. The critical current for YBCO film is Jc∼0.7 × 106A/cm2 if it is a bottom layer, and Jc∼1.7 × 106A/cm2 if YBCO is a top layer, whilst our optimal single-layer YBCO films have Jc∼(2.5-5) × 106A/cm2 at 77 K.

Linear response to a heat-driven spin torque

The existence of a heat-driven spin torque is demonstrated using Co/Cu/Co spin valves embedded in metallic nanowires. Heat currents flowing in one direction or its opposite were obtained by heating optically one end or the other of the nanowires. The spin torque associated with the heat-driven spin current pushes the magnetization out of equilibrium, resulting in a change of the magnetoresistance, which is detected using a charge current small enough not to cause heating or induced fields of any significance. The giant magnetoresistance response to this torque peaks with the magnetic susceptibility, whereas the spurious signal coming from the temperature dependence of the resistance produces merely a field independent baseline.