J.-E. Wegrowe

Imaging the Fine Structure of a Magnetic Domain Wall in a Ni Nanocylinder

We present the first experimental imaging of the internal DW structure in 55 and 85 nm diameter Ni nanocylinders, using electron holography combined with micromagnetic calculations. We demonstrate the magnetic transition from a hybrid magnetic state with both vortex and transverse DW in 85 nm diameter Ni nanocylinders to a pure transverse wall in thinner nanowires. This is particularly important as DWs in nanocylinders are model systems to go beyond the classical Walker limit.

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.

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.

Electronic transport of silicon nanowires grown in porous Al2O3 membrane

Electronic transport properties of silicon nanowires grown by chemical vapor deposition, embedded in an insulating alumina nanoporous membrane are studied. Transport measurements were performed from 300to4.2K, which revealed a scaling law of the conductance as a function of the temperature and the dc bias voltage, which the authors interpreted as a Coulomb blockade manifestation. Magnetoconductive measurements at low temperature revealed a positive magnetoconductance which can be well fitted by quasi-one-dimensional (quasi-1D) weak localization theory. These results seem to indicate that electron-electron interactions and quasi-1D effect predominate on the electronic transport properties of these systems.

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.

Anisotropic magnetothermal transport and spin Seebeck effect

The angular dependence of the thermal transport in insulating or conducting ferromagnets is derived on the basis of the Onsager reciprocity relations applied to a magnetic system. It is shown that the angular dependence of the temperature gradient takes the same form as that of the anisotropic magnetoresistance, including anomalous and planar Hall contributions. The measured thermocouple generated between the extremities of the non-magnetic electrode in thermal contact to the ferromagnet follows this same angular dependence. The sign and amplitude of the magneto-voltaic signal is controlled by the difference of the Seebeck coefficients of the thermocouple.

Synthesis and magnetic reversal of bi-conical Ni nanostructures

Microstructure study of pinning sites of highly (0001) textured Sm(Co,Cu)5 thin films grown on Ru underlayer J. Appl. Phys. 111, 07B730 (2012) Spin-torque diode spectrum of ferromagnetically coupled (FeB/CoFe)/Ru/(CoFe/FeB) synthetic free layer J. Appl. Phys. 111, 07C917 (2012) Textured Nd2Fe14B flakes with enhanced coercivity J. Appl. Phys. 111, 07A735 (2012) Influence of Si Co-doping on electrical transport properties of magnesium-doped boron nanoswords Appl. Phys. Lett. 100, 103112 (2012) Additional information on J. Appl. Phys. Journal Homepage: http://jap.aip.org/ Journal Information: Template synthesis in polyethylene terephthalate (PET) membranes has been used to grow hour glass shaped nickel nanowires with a constriction in the range of tens of nanometers at the center. Anisotropic magnetoresistance measurements have been performed on a single nanowire to follow magnetization reversal of the structure. The results are explained via 3D micromagnetic simulations showing the appearance of a complex vortex state close to the constriction whose propagation depends on the angle between the cone axis and the applied field.

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.

The concept of entropic rectifier facing experiments

The transport of molecules in confined media is subject to entropic barriers. So theoretically, asymmetry of the confinement length may lead to molecular ratchets with entropy as the only driving force for the biased transport. We address experimentally this question by performing alternative ionic current measurements on electrolytes confined in neutral conical nanopores. In case anions and cations widely differ in size, we show that rectification of ionic current can be obtained that depends on ions size and cycle frequency, consistently with the entropic ratchet mechanism.