J.-Ph. Ansermet

Giant magnetoresistance of nanowires of multilayers

A new technique is required which enables tailoring of the morphology of a metallic nanostructured material down to the 10 nm length scale. Using nanoporous nuclear track etched membranes as templates for electrodeposition, an assembly of wires with diameters as low as 30 nm could be obtained. Alternating the electrodeposition of two metals resulted in multilayers grown perpendicular to the wire axis. Layer thicknesses as low as 2 nm could be reached. Application is demonstrated by making wires 6 μm long, 80 nm in diameter, having a succession of either Co and Cu layers or of (Ni,Fe) and Cu layers. Wires containing layers of 5–10 nm in thickness exhibited a giant magnetoresistance. The current was naturally perpendicular to the layers. At ambient temperature, a magnetoresistance of 14% for Co/Cu and of 10% for (Fe,Ni)/Cu was observed.

Magnetic properties of nanosized wires

Assemblies of ferromagnetic cylinders made of Ni with diameters ranging from 35 to 250 nm were produced by electrodeposition in nanoporous membranes. The large coercive fields of Ni nanowires at low temperature could be accounted for by the curling mode of magnetization reversal, taking into account the distributions of wire diameters and orientations. The coercive field of the nanowires of the smaller diameter range decreased from 1500 Oe at 20 K to 200 Oe at 300 K nearly linearly.

Spin-polarized current induced magnetization switch: Is the modulus of the magnetic layer conserved? (invited)

The direct effect of spin-polarized current on magnetization states is studied on various electrodeposited single contacted nanowires (diameter about 60 nm). Three kinds of samples have been studied: (1) Homogeneous Ni nanowires, (2) nanowires composed of both a homogeneous Ni part and a multilayered Co(10 nm)/Cu(10 nm) part, (3) pseudospin-valve pillars Co(30 nm)/Cu(10 nm)/Co(10) electrodeposited in Cu wires. The magnetization reversal due to the current injection is observed in the three cases. The effect is observed with using different experimental protocols, including current activated after-effect measurements. The results obtained suggest that two different mechanisms are able to account for the magnetization reversal: exchange torque and spin transfer. We propose a definition of the two mechanisms based on the conservation or nonconservation of the magnetic moment of the ferromagnetic nanostructure.

Exchange torque and spin transfer between spin polarized current and ferromagnetic layers

Magnetization reversal triggered by spin injection is measured in electrodeposited Co/Cu/Co pillars (diameter about 60 nm). Two protocols are used. (i) switching of magnetization after a current pulse is monitored as a function of applied field. The maximum offset from the switching field at which irreversible switching occurs is a measure of the strength of the effect; and (ii) irreversible and reversible magnetization changes are observed while the current is ramped at fixed applied field. (i) and (ii) show that irreversible transitions occur only from antiparallel to parallel magnetic configurations and for electrons flow from the polarizer to the analyzer.

Spin-dependent Peltier effect in Co∕Cu multilayer nanowires

Heat transport perpendicular to the plane of magnetic multilayers is monitored with ac temperature gradients in the presence of a direct charge current. A very strong dependence on the applied magnetic field of the voltage response to the ac gradient is observed using Co∕Cu multilayered nanowires. The effect is interpreted as a Peltier effect for a one-dimensional heat flux.

Tailoring anisotropic magnetoresistance and giant magnetoresistance hysteresis loops with spin-polarized current injection

Current pulses were injected into magnetic nanowires. Their effect on the magnetoresistance hysteresis loops was studied for three morphologies: homogeneous Ni wires, copper wires containing five cobalt/copper bilayers, and hybrid structures composed of a homogeneous Ni half wire and a multilayered Co/Cu half wire. The characteristic features of the action of the current on the magnetization are shown and discussed.

Real time probing of magnetization switching in magnetic nanostructures

Time-resolved anisotropic magnetoresistance (AMR) measurements of the irreversible switching of the magnetization were performed on isolated Ni nanowires. The magnetization reversal was triggered by injection of high current densities in a static magnetic field. The detection was achieved by means of a Wheatstone bridge with a 1 GHz bandwidth. Time-resolved switching was obtained in single shot measurements. Nanowires with diameter of about 100 nm that present a uniform rotation in the reversible regime detected in quasistatic AMR measurements are found to have switching in about 14 ns. This value can be accounted for in the framework of an uniform rotation model with value of the Gilbert damping coefficient of 0.005–0.01. Nanowires with larger diameters (typ. 200 nm) that manifest inhomogeneous magnetization in quasistatic AMR measurements have a switching time of about 37 ns.

Thermoelectrical study of ferromagnetic nanowire structures

The mixed effects of heat and charge transports have been studied at room temperature for Ni∕Cu and Co∕Cu multilayers with currents perpendicular to the interfaces as well as Ni and Co homogeneous nanowires. In order to carry out this analysis, magnetothermogalvanic voltage (MTGV) measurements have been performed. The method consists in monitoring an alternating voltage response that arises when an oscillation of the temperature of the nanostructure is applied and a steady current crosses the nanostructure. Different responses were observed for thicknesses of the ferromagnetic layer larger or shorter than the spin diffusion length. Qualitatively different MTGV profiles were also observed for Ni and Co homogeneous nanowires. These results demonstrate the importance of spin relaxation processes produced in ferromagnetic/nonferromagnetic (FM/NF) interfaces as well as in FM layers for the MTGV response.