Magnetic nanowires

We review recent developments in the research on magnetic nanowires electrodeposited into pores of membranes. Typical nanowires fabricated by this method have a diameter in the range 30–500 nm for a length of the order of 10 μm, and can be composed of a stack of layers of different metals with thicknesses in the nanometer range (multilayered nanowires). We describe the preparation methods and present typical examples of structural characterization. We review the magnetic properties with examples of results on both arrays of nanowires and isolated nanowires. We then describe the magnetoresistance properties of multilayered nanowires, and their interest for their understanding of the CPP–GMR and the determination of spin diffusion lengths. The last section is an overview on the perspectives of future research.     

Magnetic properties of one-dimensional embedded nickel nanostructures in gold nanowires

Magnetic nanostructures of nickel embedded in gold were successfully fabricated by electrochemical deposition in porous alumina templates. Structural characterization of the samples confirmed the formation of pure phase, crystalline multi-segmented Au-Ni-Au nanowires. Magnetic characterization of the wires reveals that ferromagnetism arises as a result of Ni embedded in Au segments. An interesting behavior of coercivity was observed that showed a rapid decrease of coercivity for smaller Ni segments while a monotonic decrease was found for the larger segments. Finally, the saturation magnetization of the wires exhibited a slower increase for smaller Ni segments while a sharp increase was observed for larger Ni segments.     

Nickel nanofibers and nanowires: Elaboration by reduction in polyol medium assisted by external magnetic field

Nickel nanowires, with diameter 250 nm and a length of several microns, were prepared by the polyol process (chemical reduction) while an external magnetic field of 1.4 T has been applied during preparation. This combination has allowed the elaboration of Ni nanowires with a yield of over 90%. X-ray diffraction (XRD) showed that these nanowires crystallize with the face-centered-cubic structure. Magnetic static measurements showed the effect on the nanoparticles’ morphology of the external magnetic field applied during the synthesis. They also allowed studying the effect of the external magnetic field on the magnetic properties of nanowires as a function of their orientation. When nanowires are aligned parallel with magnetic field, the hysteresis loop obtained is very open with a coercivity field (Hc) value of 385 Oe and a high remanence to saturation ratio Mr/Ms of 0.85.     

Electronic Transport in Magnetic Domain-wall Structure

The resistivity due to the domain wall in the presence of impurities in a ferromagnetic metallic wire is calculated based on the linear response theory. Assuming the local spin distribution to be spin-spiral the modification of the band structure has been obtained exactly. With this modified band structure we calculate the conductivity of the system as a function of electronic filling and show that the domain-wall contribution to the resistance can be positive or negative depending on the dimensionality, the position of the Fermi energy on the band structure and the strength of the electron local spin interaction of the system.     

Experimental determination technique for magnetic anisotropy of individual nanowires

We present an experimental technique to determine the magnetic anisotropy of ferromagnetic nanowires. In the technique, the magnetization state is monitored by measuring the anisotropic magnetoresistance with rotating the external magnetic field. The measured magnetoresistance curves exhibit basically the same curves typically appeared in the torque magnetometric measurements, which are then readily analyzed based on the Stoner–Wohlfarth theory with a single fitting parameter – the magnetic anisotropy. By applying the present technique to Permalloy nanowires, it is shown that the shape anisotropy in real nanowires is significantly influenced by the edge roughness.     

Magnetic properties of cobalt nanowires: Study by polarized SANS

Structural and magnetic properties of two-dimensional spatially ordered system of ferromagnetic cobalt nanowires embedded into Al₂O₃ matrix have been studied using polarized small-angle neutron scattering. A comprehensive analysis of contributions to the scattering intensity was carried out, including nonmagnetic (nuclear) contribution, magnetic contribution depending on the magnetic field, and nuclear-magnetic interference indicating the correlation between the magnetic and nuclear structures. Experiments have revealed an anomalously low value of the magnetic contribution as compared to the nuclear one. This behavior is interpreted in terms of low coherence of the magnetic structure caused by the anisotropy of Co crystallites as compared with the large coherency of nuclear structure of nanowires.     

Magnetization reversal dynamics in Co nanowires with competing magnetic anisotropies

We present the magnetization reversal dynamics of Co nanowires with competing magnetic anisotropies. The aspect ratio (R) of the nanowires is varied between 2.5 and 60, and we observe a cross-over of the directions of the magnetic easy and hard axes at R=6.8. Micromagnetic simulations qualitatively reproduce the observed cross-over and give detailed insight into the reversal mechanisms associated with the cross-over. The reversal mechanism for a field applied along the long axis of the nanowire exhibits a quasi-coherent rotation mode and a corkscrew-like mode, respectively, above and below the cross-over, with the formation of a Bloch domain near the cross-over region. For a field applied along the short axis, the reversal occurs by nucleation and propagation of reversed domains from the two ends of the nanowires for very high values of the aspect ratio down to the cross-over region, but it transforms into quasi-coherent rotation mode for smaller aspect ratios (below the cross-over region).     

Heating of magnetic fluid systems driven by circularly polarized magnetic field

A theory is presented to calculate the heat dissipation of a magnetic suspension, a ferrofluid, driven by circularly polarized magnetic field. Theory is tested by in vitro experiments and it is shown that, regardless of the character of the relaxation process, linearly and circularly polarized magnetic field excitations, having the same root-mean-square magnitude, are equivalent in terms of heating efficiency.     

Electronic and magnetic properties of single-wall GeC nanotubes filled with iron nanowires

Under GGA, the structural, electronic and magnetic properties of single-wall (8, 8) GeC nanotubes filled with iron Fe_n nanowires (n = 5, 9, 13 and 21) have been investigated systematically using the first-principles PAW potential within DFT. We find that the initial shapes of the Fe₅@(8, 8), Fe₉@(8, 8) and Fe₁₃@(8, 8) systems are preserved without any visible changes after optimization. But for the Fe₂₁@(8, 8) system, the initial shapes are distorted largely for both nanowire and nanotube. The binding processes of Fe_n@(8, 8) systems are exothermic, and Fe₅@(8, 8) system is the most stable structure. The pristine (8, 8) GeCNT is nonmagnetic and direct semiconductor with a wide band gap of about 2.65 eV. Projected densities of states onto different shell Fe atoms show that the separation between the bonding and antibonding d states is reduced as going from the core Fe atom to the outermost shell Fe atom. The spin polarization of the Fe_n@(8, 8) systems and free-standing nanowires are higher than that in bulk Fe. And the spin polarization generally decreases with the number n of the Fe atoms increasing for both the Fe_n@(8, 8) systems and free-standing nanowires. Both the largest spin polarization value itself and not more decrease with respect to value of free-standing Fe₅ nanowire suggest the Fe₅@(8, 8) system could be of interest for the use in electron spin injection. The magnetism is mainly confined within the inner Fe nanowire for these combined systems. More importantly, the Fe₅ nanowire encapsulated inside (8, 8) GeCNT is under the protection of the GeCNT to prevent from oxidation, thus may stably exist in atmosphere for long time and can be expected to have potential applications in building nanodevices.     

Preparation of few-layer graphene-capped boron nanowires and their field emission properties

Large-area boron nanowire(BNW) films were fabricated on the Si(111) substrate by chemical vapor deposition(CVD). The average diameter of the BNWs is about 20 nm, with lengths of 5–10 μm. Then, graphene-capped boron nanowires(GC-BNWs) were obtained by microwave plasma chemical vapor deposition(MPCVD). Characterization by scanning electron microscopy indicates that few-layer graphene covers the surface of the boron nanowires. Field emission measurements of the BNWs and GC-BNW films show that the GC-BNW films have a lower turn-on electric field than the BNW films.     

Orientation-controlled synthesis and magnetism of single crystalline Co nanowires

Orientation control and the magnetic properties of single crystalline Co nanowires fabricated by electrodeposition have been systematically investigated. It is found that the orientation of Co nanowires can be effectively controlled by varying either the current density or the pore diameter of AAO templates. Lower current density or small diameter is favorable for forming the (1 0 0) texture, while higher current values or larger diameter leads to the emergence and enhancement of (1 1 0) texture of Co nanowires. The mechanism for the manipulated growth characterization is discussed in detail. The orientation of Co nanowires has a significant influence on the magnetic properties, resulting from the competition between the magneto-crystalline and shape anisotropy of Co nanowires. This work offers a simple method to manipulate the orientation and magnetic properties of nanowires for future applications.     

Hall micromagnetometry of domain-wall depinning in Permalloy nanowires

Experimental results of field-induced domain-wall depinning in Permalloy nanowires of submicron width and thicknesses between 10 and 30 nm are presented. Single domain walls pinned at notches in nanowires are detected by Hall micromagnetometry. The technique allows to study domain-wall propagation and depinning non-invasively in the temperature range between 2 and 50 K. The influence of sample thickness on domain-wall propagation properties is investigated. In nanowires with two notches of different pinning strength single domain walls are pinned in a toggle mode. The temperature dependence of domain-wall depinning fields in two-notch wires is analyzed.     

Magnetic moment of phonons in graphene induced by a magnetic field

A magnetic field not only changes the electronic structure in graphene but also affects the phonon excitations via the electron-phonon interaction and even enables the phonons to generate magnetism. In this paper, we evaluate the magnetic moment of phonons in graphene using a generating-functional technique. The calculation results indicate that the phonon magnetic moment exists only in a weak magnetic field. The step-like change of the magnetic moment with the magnetic field reflects a macroscopic quantum effect.     

Period doubling toward chaos in a driven magnetic macrospin

The Landau-Lifshitz-Gilbert equation is analyzed in the case of a configuration involving easy plane isotropy under the influence of a sinusoidally oscillating magnetic field and a demagnetizing field. Through the use of numerical techniques, chaotic behavior is found and analyzed. By reducing the system to a discrete map (numerically), bifurcation diagrams for the system are computed. The system is found to exhibit a period doubling cascade route to chaos, and it obeys certain convergence rules for chaotic transitions outlined by Feigenbaum. A connection is drawn between the route to chaos and the geometry of the system, and comparisons are made with similar systems. Within the chaotic regime, windows of arbitrarily large period are suspected to exist, and explicitly illustrated and discussed for a period three window.     

Mode field diameter and nonlinear properties of air-core nanowires

Exact solutions of Maxwell's equations describing the lightwave through 3-layer-structured cylindrical waveguide are obtained and the mode field diameter and nonlinear coefficient of air-core nanowires (ACNWs) are numerically calculated. The simulation results show that ACNWs offer some unique optical properties, such as tight field confining ability and extremely high nonlinearity. At a certain wavelength and air core radius, we optimize the waveguide design to maximize the nonlinear coefficient and minimize the mode field diameter. Our results show that the ACNWs may be powerful potential tools for novel micro-photonic devices in the near future.     

Modification of domain-wall propagation in Co nanowires via Ga<Superscript>+</Superscript> irradiation

The propagation of domain walls in polycrystalline Co nanowires grown by focused-electron-beam-induced deposition is explored. We have found that Ga⁺ irradiation via focused ion beam is a suitable method to modify the propagation field of domain walls in magnetic conduits. Magneto-optical Kerr effect measurements show that global Ga⁺ irradiation of the nanowires increases the domain-wall propagation field. Additionally, we have observed by means of scanning transmission X-ray microscopy that it is possible to produce substantial domain-wall pinning via local Ga⁺ irradiation of a narrow region of the nanowire. In both cases, Ga⁺ doses of the order of 10¹⁶ ions/cm² are required to produce such effects. These results pave the way for the controlled manipulation of domain walls in Co nanowires via Ga⁺ irradiation.     

Surface dangling bonds dependent magnetic properties in Mn-doped GaAs nanowires

(Ga, Mn)As dilute magnetic semiconductor (DMS) is very promising for future spintronic devices, however, the lower Curie temperature ($T_c$) limits the applications. Here, using first-principles calculation based on density functional theory, we investigate the effect of the surface dangling bonds (SDBs) on the magnetic properties of Mn-doped GaAs nanowires (NWs). The results show that As (Ga)-SDBs are equivalent to holes (electrons) doping, giving rise to the magnetic moments on the surfaces of GaAs NWs. Further in the Mn-doped GaAs NWs, the SDBs can effectively regulate the total magnetic moments, due to charge transfers between the Mn-3d orbitals and the residual SDBs, which is analyzed by a carrier modulation model. Most importantly, the As-SDBs can stabilize the ferromagnetic (FM) states and enhance the $T_c$ in Mn-doped GaAs NWs because of their shallow acceptor level with lower energy compared with Mn-3d level.