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The hysteresis of multilayer nanowires composed by a soft magnetic cylindrical wire, a non-magnetic spacer layer and an external hard magnetic shell is investigated. The external magnetic shell originates a non-homogeneous magnetic field on the inner wire, which is responsible for a displacement and a change of the width of the hysteresis curve of the wire. Moreover, different reversal modes occur at each branch of the hysteresis loop, which can be understood by analyzing the interaction magnetostatic field along the wire. Our results open the possibility of controlling two parameters of the hysteresis loop, the coercivity and the bias, providing an interesting system to be investigated.

The angular dependence of the remanence and coercivity of Ni nanowire arrays produced inside the pores of anodic alumina membranes has been studied. By comparing our analytical calculations with our measurements, we conclude that the magnetization reversal in this array is driven by means of the nucleation and propagation of a transverse wall. A simple model based on an adapted Stoner–Wohlfarth model is used to explain the angular dependence of the coercivity. [ABSTRACT FROM AUTHOR]

We have investigated numerically the angular dependence of the coercivity and remanence of cylindrical diameter modulated Ni80Fe20 nanowires. We observed that the system always starts reversing its magnetization through the thickest segment by means of a quite complex reversal process, considering the propagation of two vortex domain walls. Furthermore, we observed a transition from vortex domain walls to coherent-mode rotation for the thinnest segment as a function of the angle in which the external magnetic field is applied. In addition, we obtained a non-monotonic behavior for the coercivity and saturation field as a function of the angle at which the external magnetic field is applied. Finally, diameter modulation is an attractive way to handle over the motion of magnetic domain walls, a phenomenon proposed as a future data storage platform. [ABSTRACT FROM AUTHOR]

We have investigated the current-driven domain wall motion in nanostructures comprised of a pair of nanotube and nanowire segments. Under certain values of external magnetic fields, it is possible to pin a vortex domain wall in the transition zone between the wire and tube segments. We explored the behavior of this domain wall under the action of an electron flow applied in the opposite direction to the magnetic field. Thus, for a fixed magnetic field, it is possible to release a domain wall pinned simply by increasing the intensity of the current density, or conversely, for a fixed current density, it is possible to release the domain wall simply decreasing the magnetic external field. When the domain wall remains pinned due to the competition between the current density and the magnetic external field, it exhibits a oscillation frequency close to 8 GHz. The amplitude of the oscillations increases with the current density and decreases over time. On the other hand, when the domain wall is released and propagated through the tube segment, this shows the standard separation between a steady and a precessional regime. The ability to pin and release a domain wall by varying the geometric parameters, the current density, or the magnetic field transforms these wire-tube nanostructures in an interesting alternative as an on/off switch nano-transistor. [ABSTRACT FROM AUTHOR]

Magnetization reversal in planar nanowires has been controlled using structures with a larger area pad connected to a nanowire or by means of patterned variations in the planar nanowire such as notches. In this letter, we have introduced a magnetic nanostructure defined as a planar nanostructure with wire-ring morphology. In particular, we have performed micromagnetic simulations to investigate how the magnetic properties (coercivity and remanence) change as a function of the geometric parameters of the nanostructure. Additionally, we observe that when the ring is very thin, the system reverses its magnetization by nucleation and propagation of domain walls along the nanowire. Conversely, when the ring has very thick walls, or directly turns into a solid cylinder, the system nucleates a vortex in the ring/cylinder, and then propagates the domain walls toward the nanowire sections. This reversal process is characterized by a step or plateau in the hysteresis curve, that is, a region in which differential magnetic susceptibility presents a local minimum or, ideally, vanishes. Finally, this nanostructure can be used in many potential applications related to the control of domain walls in planar nanowires. [ABSTRACT FROM AUTHOR]

The magnetization reversal in ordered arrays of Co nanowires with tailored hcp-phase texture, controlled by pH synthesis and nanowires length, has been investigated. The angular dependence of coercivity has been experimentally determined for different crystal textures, and the corresponding magnetization reversal mode is interpreted by analytical modelling. The results show that reversal takes place by propagation of a transverse-like domain wall mode. The fitting of experimental and calculated data allows us the quantitative evaluation of the magnetocrystalline anisotropy constant strength whose magnetization easy direction evolves from parallel to the wires toward in-plane orientation with the change of hcp-phase texture. [ABSTRACT FROM AUTHOR]

Arrays of Ni nanowires have been studied by means of first order reversal curves (FORC) diagrams. The dependence of the coercivity of the arrays as a function of the length of the nanowires has been experimentally investigated. We have shown that the FORC diagrams provide detailed information about the distribution of interactions and coercivities and allowed us to observe the reversible component of magnetization. Our results are in good agreement with analytical calculations obtained by means of a simple model. [ABSTRACT FROM AUTHOR]

In this work, we have theoretically explored the spin-waves modes that can be excited in cylindrical nanostructures with wire-tube morphology. Using micromagnetic simulations, we have found up to 5 resonance modes whose existence and position depend on the geometric parameters of the nanostructure. The lower and higher frequency modes remain invariant in their position as a function of the size of the tube segment hole, although their intensity increases as the length of the wire segment increases. Other modes vary in their frequency, even producing a crossover between them. Finally, we found that the number and amplitude of the reported spin-wave modes have a complex dependence on the geometry of the system, converting the wire-tube nanostructure into a promising piece for potential magnonic devices. [ABSTRACT FROM AUTHOR]

Ordered hexagonal arrays of Co nanowires (NWs) and nanotubes (NTs), with diameters between 40 and 65 nm, were prepared by potentiostatic electrodeposition into suitably modified nanoporous alumina templates. The geometrical parameters of the NW/NT arrays were tuned by the pore etching process and deposition conditions. The magnetic interactions between NWs/NTs with different diameters were studied using first-order reversal curves (FORCs). From a quantitative analysis of the FORC measurements, we are able to obtain the profiles of the magnetic interactions and the coercive field distributions. In both NW and NT arrays, the magnetic interactions were found to increase with the diameter of the NWs/NTs, exhibiting higher values for NW arrays. A comparative study of the magnetization reversal processes was also performed by analyzing the angular dependence of the coercivity and correlating the experimental data with theoretical calculations based on a simple analytical model. The magnetization in the NW arrays is found to reverse by the nucleation and propagation of a transverse-like domain wall; on the other hand, for the NT arrays a non-monotonic behavior occurs above a diameter of ∼50 nm, revealing a transition between the vortex and transverse reversal modes. [ABSTRACT FROM AUTHOR]

In this paper we have calculated analytically the magnetostatic interaction between two wire-tube nanostructures as a function of their magnetic and geometric parameters. As expected, the interaction energy increases as the nanostructures approach, but interestingly when the nanostructures are close enough, a non-monotonic behavior with the wire-tube portions is reported. Besides, we investigate the hysteresis loop for two interacting Ni 81 Fe 19 wire-tube nanostructures by micromagnetic simulations in order to study how the interaction affects the magnetic properties of these nanostructures. This work allows for the study of magnetostatic interactions between wire-tube nanostructures that have been proposed as an interesting alternative to store information or even perform logic functions, because to their ability to pin a domain wall. [ABSTRACT FROM AUTHOR]

Abstract: We have investigated the evolution of the magnetization reversal mechanism in asymmetric Ni nanowires as a function of their geometry. Circular nanowires are found to reverse their magnetization by the propagation of a vortex domain wall, while in very asymmetric nanowires the reversal is driven by the propagation of a transverse domain wall. The effect of shape asymmetry of the wire on coercivity and remanence is also studied. Angular dependence of the remanence and coercivity is also addressed. Tailoring the magnetization reversal mechanism in asymmetric nanowires can be useful for magnetic logic and race-track memory, both of which are based on the displacement of magnetic domain walls. Finally, an alternative method to detect the presence of magnetic drops is proposed. [Copyright &y& Elsevier]

Ordered arrays of CoxNi1-x nanowires (0 < x < 1) were fabricated by a template-assisted method using electrodeposition into anodic aluminum oxide membranes. Tuning of the Co-alloy composition by changing the Ni content enables control of the effective anisotropy axis, which is determined by the balance between the hep and fee magnetocrystalline and shape anisotropies. We report on the nanowires' structural and magnetic properties (e.g., hysteresis curves and their parameters as well as first-order reversal curve analysis), paying particular attention to their angular dependence. It is confirmed that the crystal phase of nanowires with length 2.5 &mgr;m and diameter 35 nm shifts from hep to fee as the Ni content increases. That results in a significant modification of the magnetization process and, accordingly, of the magnetic properties of the array. Analytical calculations of the angular dependence of the coercivity allow us to confirm that the magnetization reversal is mostly ascribed to the propagation of a transverse domain wall. Fitting of the experiment to these calculations indicates the presence of a transverse crystalline anisotropy (ascribed to the hep phase) in Co wires, while this changes to an axial anisotropy (fee phase) as the Ni content increases. [ABSTRACT FROM AUTHOR]

• We investigated the current-driven domain wall motion in a planar nanowire with a square hole. • We found four distinct regimes that give rise to different behaviors of the dynamics of the domain wall. • The regimes are distinguished according the hole size and the current density applied. • One of the regimes is analogous to the well-known Walker limit in planar nanowires. The current-driven domain wall motion in a planar nanowire with a square hole located at its center is studied by micromagnetic simulations. We show the existence of four regimes that depend both on the geometrical parameters of the hole and the nanowire, as well as on the applied current density. These regimes determine a pinning-depinning pattern characterized by: i) the domain wall remains pinned in its initial position; ii) the domain wall is released from its initial position and reaches the square hole, remaining pinned in that position; iii) the domain wall reaches the hole and passes it without major obstacles, reaching the other end of the wire; iv) the domain wall reaches the hole and suffers a series of complex transformations that annihilate it, giving rise to the nucleation of a new domain wall, which is what reaches the other end of the wire. Our finding could be of practical use in the engineering of logic devices based on the movement of domain walls. [ABSTRACT FROM AUTHOR]

Mosaic nanocomposites composed of nickel and cobalt nanowires arranged in different configurations were investigated using Monte Carlo simulations and a simple model that considers single-domain structures including length corrections due to the shape anisotropy. Our results showed that for an ordered array both the coercivity and the remanence decrease linearly as a function of the concentration of nickel nanowires. Besides, we obtained that the magnetic properties of an array of a certain hard magnetic material (cobalt) will not change, unless we have more than 50% of nanowires of other soft magnetic material (nickel) in the array. In principle the second material could be other soft magnetic material, but could also be a nonmagnetic material or could even be a situation in which some of the pore arrays were not filled by electrodeposition. Therefore, our results allow us to predict the behavior of magnetic mosaic nanocomposites that are promising candidates for functional electrodes, sensors, and model catalysts. [ABSTRACT FROM AUTHOR]

The magnetic behavior of arrays of Co and CoFe nanowire arrays has been measured in the temperature range between 100 and 300 K. We have paid particular attention to the angular dependence of magnetic properties on the applied magnetic field orientation. The experimental angular dependence of coercivity has been modeled according to micromagnetic analytical calculations, and we found that the propagation of a transversal domain wall mode gives the best fitting with experimental observations. That reversal mode holds in the whole measuring temperature range, for nanowires with different diameters and crystalline structure. Moreover, the quantitative strength of the magnetocrystalline anisotropy and its magnetization easy axis are determined to depend on the crystalline structure and nanowires diameter. The evolution of the magnetocrystalline anisotropy with temperature for nanowires with different composition gives rise to an opposite evolution of coercivity with increasing temperature: it decreases for CoFe while it increases for Co nanowire arrays. [ABSTRACT FROM AUTHOR]

Abstract: The magnetic properties in multisegmented cylindrical nanostructures comprised of nanowire and nanotube segments are investigated numerically as a function of their geometry. In this work we report systematic changes in the coercivity and remanence in these systems. Besides, we have found the ideal conditions for a magnetic configuration with two antiparallel domains that could be used to help to stabilize magnetic nanoparticles inside ferromagnetic multisegmented cylindrical nanoparticles. This magnetic behavior is due to the fact that the tube segment reverses its magnetization before the wire segment, allowing the control of the magnetic domain walls motion between two segments. In this way, these magnetic nanoobjects can be an alternative to store information or even perform logic functions. [Copyright &y& Elsevier]

Abstract: The angular dependence of the coercivity and remanence of ordered hexagonal arrays of Co nanowires prepared using anodic aluminum oxide templates was investigated. The experimental evolution of coercivity as a function of the angle, in which the external field is applied, is interpreted considering micromagnetic simulations. Depending on the angle between the axis of the wire and the applied magnetic field direction our results show that the magnetization reversal mode changes from vortex to a transverse domain wall. Besides, we observed that the dipolar interactions cause a reduction in coercive fields, mainly in the direction of easy magnetization of the nanowires. Good agreement between numerical and experimental data is obtained. [Copyright &y& Elsevier]