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In the present work, we report on the in situ magnetic force microscopy (MFM) study of the magnetization reversal in two-dimensional arrays of ferromagnetic Ni80Fe20 and Co55Fe45 nanowires (NW) with different diameters (40, 50, 70, and 100 nm) deposited inside low porosity (P < 1%) nanoporous polycarbonate membranes. In such arrays, the nanowires are sufficiently isolated from each other so that long range dipolar interactions can be neglected. The MFM experiments performed for different magnetization states at the same spot of the samples are analysed to determine the switching field distribution (SFD). The magnetization curves obtained from the MFM images are relatively square shaped. The SFD widths are narrower compared to those obtained for high density arrays. The weak broadening of the curves may be ascribed to the NW intrinsic SFD. The influence of diameter and composition of the ferromagnetic NW is also investigated. [ABSTRACT FROM AUTHOR]

Using low density arrays of bistable magnetic nanowires as a model dipolar system, it is shown that the dipolar interaction field coefficient can be measured from the remanence curves as well as from other functions of the isothermal remanent magnetization and the DC demagnetization remanence obtained as an affine transformation of the Wohlfarth relation. Based on mean field arguments, these measurements are used to subtract and remove the contribution of the configuration dependent dipolar interaction field from the major loop and remanence curves. The corrected remanence curves are first used to obtain the intrinsic switching field distribution of the nanowire array and then to validate this approach showing that they yield results consistent with the Wohlfarth relation for an assembly of noninteracting particles, thus providing a self-consistent procedure to verify the measured values of the interaction field and its removal from the measurements. [ABSTRACT FROM AUTHOR]

The microwave power density absorbed in the normal state by continuously irradiated superconducting Nb and NbN nanostrips is extracted from their electrical transport properties. The procedure is based on the reduction of the retrapping current (i.e., the minimum applied current required to sustain a dissipative region inside a superconducting microbridge) that results from the microwave dissipation. The power effectively absorbed by the nanostrips varies linearly with the input power level and falls in the μWμm-3 range. At a given microwave frequency, the relation between the input power and the absorbed power does not depend on the coolant temperature. In addition, the upper limit of the detection range is given by the heat removal capabilities through the substrate. When the absorbed microwave power exceeds this temperature-dependent heat removal threshold, normal hotspot domains are stabilized down to zero bias current. The determination of the microwave power absorbed by superconducting samples in the normal state may be of practical interest in the field of incident power detection. [ABSTRACT FROM AUTHOR]

The static magnetization experimental behavior of cobalt (Co), Permalloy (Py), and nickel (Ni) nanocylinders is obtained from vibrating sample magnetometry while the dynamic behavior for the Co and Py ones is analyzed by means of Brillouin light scattering spectroscopy. Assuming the presence at remanence of two populations of cylinders with up and down magnetizations and including the dipolar coupling between the cylinders, a single analytical model based on a mean-field approach allowed us to satisfactorily analyze both series of experimental results. The model requires three physical parameters, allowing us to derive the in-plane saturation field, the eigenfrequency in the absence of applied field, and the eigenfrequency at the in-plane saturation field; these parameters enable us to adjust the whole variation of the eigenfrequency versus the applied field. Moreover, the effect of the magnetocrystalline anisotropy on the softening of the frequency in the nonsaturated state is clearly evidenced: it is more pronounced when the magnetocrystalline anisotropy is not vanishing and adds to the shape anisotropy (Co c-axis parallel to the cylinder axis); the softening being weak in the other cases (Co c-axis perpendicular to the cylinder axis or Permalloy). [ABSTRACT FROM AUTHOR]

We report on exchange bias effects and time evolution in Co nanowires electrodeposited into the pores of track etched polycarbonate membranes. The slow variation of the exchange bias anisotropy, as extracted from field cooled hysteresis loops, is due to the delayed in time oxidation of the nanowires embedded in the polymer template. A model has been proposed to account for the drop of magnetization due to the presence of an oxide layer at the periphery of the nanowires, within the nanowires/polymer interface. The measured exchange bias energy is about four times smaller than the energy in planar bilayers because of the cylindrical geometry of the wires. [ABSTRACT FROM AUTHOR]

The applicability of the anisotropy field distribution method and the conditions required for an accurate determination of the effective anisotropy field in arrays of magnetic nanowires have been evaluated. In arrays of magnetic nanowires that behave as ideal uniaxial systems having only magnetostatic contributions to the effective anisotropy field, i.e., shape anisotropy and magnetostatic coupling, the method yields accurate values of the average anisotropy field at low-moderate dipolar coupling and accuracy decreases as wire packing increases. When an additional non-negligible magnetocrystalline anisotropy is present, the method is less accurate, as shown for the case of hcp Co nanowires. [ABSTRACT FROM AUTHOR]

Spin-transfer torque effects are reported in nanowires consisting in Co/Cu/Co trilayers electrodeposited on an anodic alumina template. Using a nanolithography process based on electrically controlled nanoindentation of the alumina template, we are able to investigate the spin transport properties of single nanowires at room temperature. For small applied magnetic fields, we have measured resistance changes above a critical direct-current (dc) injected current that corresponds to the change in resistance observed in the magnetoresistance curves at low current. We conclude that magnetic reversals are driven by a spin-polarized current. The critical current densities needed for the magnetization reversals are in the 107 A/cm2 range and the dependence of the critical currents with the applied field is consistent with the spin-transfer mechanism. For large applied magnetic fields, the differential resistance exhibits some peaks that we attribute to the onset of high-frequency excitations of the free-layer magnetization. According to the high density of electrodeposited nanowires in alumina templates, our results are promising for synchronized spin-transfer oscillators. [ABSTRACT FROM AUTHOR]

Presents a study which investigated the physical properties of carbon-boron nitride prepared from mixtures of boron trichloride and nitrogen hydrogen[sub3] and selected hydrocarbons by codeposition methods. Methods; Results; Discussion.

Interfaces are a major issue when designing ferroelectric nanostructured materials with tailored properties. In a context of integration and multifunctionality in the field of electronics, several strategies have been developed to control the microstructure and defect chemistry of interfaces that strongly impact the macroscopic properties. The suitability of the core-shell approaches that allow a subtle tuning of interface phenomena at different scales has been widely demonstrated. We focus here on the flexibility of the core-shell approach devoted to the processing of nanostructured ferroelectric composites. Our strategy relies on the use of advanced synthesis processes to design ferroelectric grains coated with shells of different nature, morphology and crystallinity. Typical examples will be reviewed with a specific attention on their impact on both microstructure and dielectric properties. Our approach, based also on the use of fast sintering technique, provides a guidance to design 3D bulk nanostructured ferroelectrics while controlling and/or exploiting size, interface and defects chemistry. The contribution of specific spectroscopies to probe interfacial chemistry and defects is underlined. The high density of interfaces in core-shell materials is obviously an advantage to target additional functionality such as magneto-electric coupling. This is illustrated in 3D composites and one dimensional nanostructures that coaxially combine electric and magnetic materials. The core-shell approach described here could be transferred to a much broader range of materials covering many functionalities provided a deeper understanding of the interfaces at the atomic scale is achieved and a further development of low temperature processing is reached. [ABSTRACT FROM AUTHOR]

We present conductance and magnetoresistance measurements in magnetic Ni-Ni and Co-Ni nanocontacts prepared by electrodeposition within the pores of a track-etched polymer membrane. At room temperature, Ni-Ni constrictions show broad quantization plateaus of conductance during their dissolution in units of e[SUP2]/h, as expected for ferromagnetic ballistic nanocontacts. Additionally, positive and negative magnetoresistance has been measured in Co-Ni nanocontacts. [ABSTRACT FROM AUTHOR]

The microwave absorption properties of arrays of magnetic nanowires electrodeposited in nanoporous membranes have been studied using a strip transmission line configuration. From the measurements, the ferromagnetic resonance properties of the magnetic wires have been extracted at frequencies ranging from 100 MHz to 40 GHz for various angles between the wire axis and an external static magnetic field up to 8 kOe. We report on extensive data obtained on the ferromagnetic resonance properties in arrays of nanowires of Ni, NiFe, and NiFeCo alloys with various diameters and packing densities. In such arrays, apart from the shape anisotropy, a strong dipolar coupling between wires is present which can be modulated by the wire density. Both ferromagnetic resonance spectra and hysteresis loops show that this coupling acts as an additional uniaxial anisotropy favoring an easy axis perpendicular to the wires. In particular, it is shown that the easy axis of magnetization can be t! uned parallel or perpendicular to the wire axis by changing the wire packing density. Furthermore, ferromagnetic resonance occurs even at zero field and the resonance frequency is fixed by the saturation magnetization of the material independent of the packing density. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Measurements of the electrical resistivity, from 1.5 to 300 K, and of the low temperature magnetoresistance of highly conductive carbon nanotube (CNT) fibers, obtained by wet-spinning from liquid crystalline phase (LCP), are reported. At high temperature the results obtained on the raw CNT fibers show a typical metallic behavior and the resistivity levels without postdoping process were found to be only one order of magnitude higher than the best electrical conductors, with the specific conductivity (conductivity per unit weight) comparable to that of pure copper. At low temperature a logarithmic dependence of the resistivity and the temperature dependence of the negative magnetoresistance are consistent with a two-dimensional quantum charge transport--weak localization and Coulomb interaction--in the few-walled CNT fibers. The temperature dependence of the phase-breaking scattering rate has also been determined from magnetoresistance measurements. In the temperature range T < 100 K, electron-electron scattering is found to be the dominant source of dephasing in these highly conductive CNT fibers. While quantum effects demonstrate the two-dimensional aspect of conduction in the fibers, the fact that it was found that their resistance is mainly determined by the intrinsic resistivity of the CNTs--and not by intertube resistances--suggests that better practical conductors could be obtained by improving the quality of the CNTs and the fiber morphology. [ABSTRACT FROM AUTHOR]

A reliable and flexible synthesis route was used for processing high density Ni-BaTiO3 nanocable arrays based on wet chemical impregnation and subsequent electrodeposition within a highly ordered unidirectional porous alumina membrane. The core-shell structure was carefully investigated by bright field scanning transmission electronic microscopy coupled with energy dispersive X-ray spectroscopy. The strength of the dipolar interaction arising from the packing density of the magnetic nanowires was correlated with the BaTiO3 wall thickness through magnetometry and ferromagnetic resonance measurements. Our approach opens a pathway to obtain optimized nanostructured multiferroic composites exhibiting tunable magnetic properties. [ABSTRACT FROM AUTHOR]

Wefabricatedaperpendicularlymagnetizedbitpatternmediausingahexagonallyclose-packed auto-assembled anodic alumina template with 100 nm and 50 nm periods by depositing a Co/Pt multilayer toform an ordered array of ferromagnetic nanodots, so-called nanobumps. We used Hall resistance measurements and magnetic force microscopy to characterize the dot-by-dot magnetization reversal mechanism under applied field The role of interdot exchange coupling and dipolar coupling are investigated. Then we focus on separating the various origins of switching field distribution (SFD) in this system, namely dipolar interactions, intrinsic anisotropy distribution, and template packing faults. Finally we discuss the influence of triangular dipolar frustrations on the energy stability of demagnetized and half-switched states based on an Ising model, including local exchange coupling. The impact of SFD and lattice defects lines between misoriented ordered domains on the magnetic configurations is studied in detail. [ABSTRACT FROM AUTHOR]

A combined bottom-up assembly of electrodeposited nanowires and electron beam lithography technique has been developed to investigate the spin transfer torque and microwave emission on specially designed nanowires containing a single Co/Cu/Co pseudo spin valve. Microwave signals have been obtained even at zero magnetic field. Interestingly, high frequency vs. magnetic field tunability was demonstrated, in the range 0.4-2 MHz/Oe, depending on the orientation of the applied magnetic field relative to the magnetic layers of the pseudo spin valve. The frequency values and the emitted signal frequency as a function of the external magnetic field are in good quantitative agreement with the analytical vortex model as well as with micromagnetic simulations. [ABSTRACT FROM AUTHOR]

The requirements for imaging magnetic vortices properly by magnetic force microscopy are discussed. Special attention is devoted to the influence of the tip on the sample magnetic configuration. It is shown that one can obtain a quantitative understanding of the experiments using micromagnetic simulations within a general framework for reversible tip-induced perturbations. Finally, preliminary results obtained with specially prepared ‘nanowire tips’ are presented. [ABSTRACT FROM AUTHOR]

Beyond the well-known Abrikosov and giant vortex configurations, new solutions to the Ginzburg-Landau model corresponding to vortices of integer and half-integer winding number are described. Phase diagrams (B[sub ext], Energy) and magnetization curves have been determined, aiming towards an understanding of the magnetic properties of lead nanowires and the possible consequences of such solutions with respect to the switching mechanism between vortex states in mesoscopic superconductors. [ABSTRACT FROM AUTHOR]