Soft Magnetic Nanowires Research Articles

Modeling Propulsion of Soft Magnetic Nanowires.

The emergent interest in artificial nanostructures that can be remotely navigated a specific location in a fluidic environment is motivated by the enormous potential this technology offers to biomedical applications. Originally, bio-inspired micro-/nanohelices driven by a rotating magnetic field were proposed. However, fabrication of 3D helical nanostructures is complicated. One idea to circumvent complex microfabrication is to use 1D soft magnetic nanowires that acquire chiral shape when actuated by a rotating field. The paper describes the comprehensive numerical approach for modeling propulsion of externally actuated soft magnetic nanowires. The proposed bead-spring model allows for arbitrary filament geometry and flexibility and takes rigorous account of intra-filament hydrodynamic interactions. The comparison of the numerical predictions with the previous experimental results on propulsion of composite two-segment (Ni-Ag) nanowires shows an excellent agreement. Using our model we could substantiate and rationalize important and previously unexplained details, such as bidirectional propulsion of three-segment (Ni-Ag-Au) nanowires.

Controlled magnetic behaviour of Ni80Fe20 nanowire by a selection of nucleation pads

Magnetic nanowires are of technological importance as their magnetic properties can be controlled by their dimensions and shape. In soft magnetic nanowires (e.g. Ni 80 Fe 20 alloy or Permalloy), shape anisotropy has a very strong effect on their magnetic properties and thus the wire width and thickness control the domain patterns, nature of magnetization reversal, and the switching field. The switching field of a permalloy (Py) nanowire (NW) can be reduced substantially by attaching a microsized nucleation pad (NPD) to it. In this work, the OOMMF software package was used to perform micromagnetic simulations for selected NPD-NW structures. In this present study, we proved that the shape of the attached NPD is an important factor to consider. For instance, the switching field of structure comprises a triangular NPD (area = 0.25 μm2) with its head connected to a Py NW (20 nm wide) is about one fourth of the switching field of a square-like NPD-NW structure of the same size. Also the NPD size has an effect on the quality of the obtained hysteresis loop and thus it has to be comparable to the size of the NW for the sake of having a step-free hysteresis loop. These findings suggest that the rectangular NPD with its head connected to the NW may serve better than other shapes as it provides a directed motion for the domain walls to reach the NPD/NW interface easily. Thus it gives the highest reduction in the switching field of the Py nanowire in addition to a step-free hysteresis loop when the NPD size is 0.01 μm2.

Probing domain walls in cylindrical magnetic nanowires with electron holography

We probe magnetic domain walls in cylindrical soft magnetic nanowires using electron holography. We detail the modelling of expected contrast for both transverse and Bloch point domain walls and provide comparison with experimental observations performed on NiCo nanowires, involving also both magnetic and electrostatic contribution to the electron holography map. This allows the fast determination of the domain wall type without the need for uneasy and time-consuming experimental removal of the electrostatic contribution. Finally, we describe and implement a new efficient algorithm for calculating the magnetic contrast.

Domain Walls in Bent Nanowires

Controlling the magnetic fine structure between two domains in a wire forms the basis of many promising applications, such as ``racetrack'' memory, sensors, and logic concepts for magnetic computing. This paper presents a study of V-shaped soft-magnetic nanowires, with regard to the influence of wire width, thickness, and bending angle on the prevalent type of domain wall. Domain patterns are imaged at high resolution via scanning electron microscopy with polarization analysis (SEMPA), and compared to micromagnetic simulations. Based on the results, a phenomenological model enables the predictable tuning of remanent wall type in such a nanowire.

One dimensional FexCo1-x nanowires; ferromagnetic resonance and magnetization dynamics

Soft magnetic nanowires (NWs) are widely used for microwave and mm-wave components. The investigation of magnetization damping behavior of NWs have attracted great interest due to large influence of loss to the device, like integrated microwave device, magnetic sensors, and magnetic random access memory. With increasing operational frequency and degree of integration, the requirements to characterize 1-dimensional NWs become increasingly high. The purpose of this work is to study the magnetization dynamics in FexCo1-x NWs. A series of FexCo1-x (x=0, 0.25, 0.5, 0.75, 1) NWs were grown by controlled electro-deposition. By adjusting FexCo1-x concentration (x=0 to 1), the saturation magnetization, increased more than 20%. Ferromagnetic resonance (FMR) both in field and frequency sweep mode are employed to characterize the NWs in flip-chip geometry. It is observed that FMR field (Hr) increases with increase in applied frequency. At a fixed frequency, Fe NWs resonate at a lower field than the Co substituted NWs. FMR field linewidth (ΔH) as well as frequency width (Δf) are largest for Co NWs and decreased for Fe NWs. Whereas ΔH and Δf decreased further for FexCo1-x nanowires with increasing x.

A general perspective on the magnetization reversal in cylindrical soft magnetic nanowires with dominant shape anisotropy

Peculiarities of the magnetization reversal process in cylindrical Ni-Cu soft magnetic nanowires with dominant shape anisotropy are analyzed via both static and time dependent micromagnetic simulations. A reversible process involving a coherent-like spin rotation is always observed for magnetic fields applied perpendicularly to the easy axis whereas nucleation of domain walls is introduced for fields applied along the easy axis. Simple criteria for making distinction between a Stoner-Wohlfarth type rotation and a nucleation mechanism in systems with uniaxial magnetic anisotropy are discussed. Superposed reversal mechanisms can be in action for magnetic fields applied at arbitrary angles with respect to the easy axis within the condition of an enough strong axial component required by the nucleation. The dynamics of the domain wall, involving two different stages (nucleation and propagation), is discussed with respect to initial computing conditions and orientations of the magnetic field. A nucleation time of about 3ns and corkscrew domain walls propagating with a constant velocity of about 150m/s are obtained in case of Ni-Cu alloy (Ni rich side) NWs with diameters of 40nm and high aspect ratio.

Diameter-modulated ferromagnetic CoFe nanowires

Highly ordered nanoporous anodic aluminium oxide templates with uniform or periodically modulated pore diameters have been fabricated by the hard-pulse anodization technique. Straight and diameter-modulated CoFe nanowires with diameters of 60 nm and 60/120 nm, respectively, were AC-pulse electrodeposited into the templates. A comparison of magnetisation curves of the two types of nanowires show a typical coercivity (μ0Hc) of 74 mT and squareness (SQ) of 92% for the modulated wires, which are significantly squarer but have 12% lower coercivity compared to the 60 nm straight CoFe wires (μ0Hc = 84 mT, SQ = 60%). The difference is attributed to magnetostatic interactions between the segments due to the stray field of the modulated wires. Diameter modulation is an attractive way of producing soft magnetic nanowires with a controllable hysteresis loop shape. The time required for the nanoporous template fabrication by hard-pulse anodization is typically one tenth of that needed for conventional template fabrication methods, hence making it suitable for large-scale nanowire production.

Surface magnetization processes in soft magnetic nanowires

The surface magnetization processes taking place in simple permalloy (Py) and FeGa nanowires, Py/Cu, CoFeB/Cu, CoNiP/Cu, FeGa/Py, and FeGa/CoFeB multilayered nanowires have been studied by magneto-optical Kerr effect (MOKE) magnetometry. The results indicate a strong correlation between the direction of the anisotropy axis relative to the direction of the applied field and the plane of incidence of the laser spot, as well as the effect of dipolar interactions between the nanowires or between the ferromagnetic layers on the magnetization reversal. The larger laser spots are inducing more noise in the MOKE hysteresis loops because of the dimensional imperfections along the nanowires.

Probing vortex-core dynamics using current-induced resonant excitation of a trapped domain wall

A magnetic vortex trapped by a notch in a nanowire is no larger than 10 nm, but both the direction and polarity of the vortex can now be measured without applying a magnetic field. In this set-up, the vortex is therefore both stable and switchable for use in possible device applications. Magnetic domain walls in soft magnetic nanowires often exhibit a structure in which the magnetization curls within the plane of the nanowire around a singular point with out-of-plane magnetization, the vortex core1,2. Although the core is a small object, with a diameter of only ∼10 nm in permalloy2, its motion controls the dynamics of the entire wall, which can be several hundred nanometres in size. In particular, when a domain wall trapped at a pinning site is driven out of equilibrium by either a magnetic field or a spin-polarized current, the vortex core gyrates around its equilibrium position. The sense of gyration is determined by the polarity of the core3,4,5,6,7. Here, we show that spin-polarized a.c. currents can resonantly excite a vortex domain wall trapped at a notched site in a nanowire. The shape and magnitude of the resonance, measured from the nanowire’s resistance, reveal both the elliptical trajectory of the vortex core as well as its polarity.

Current-induced domain wall motion

The present understanding of domain wall motion induced by spin-polarized electric current is assessed by considering a subset of experiments, analytical models, and numerical simulations based on an important model system: soft magnetic nanowires. Examination of this work demonstrates notable progress in characterizing the experimental manifestations of the “spin-torque” interaction, and in describing that interaction at a phenomenological level. At the same time, an experimentally verified microscopic understanding of the basic mechanisms will require substantial future efforts, both experimental and theoretical.

Domain walls and magnetization reversal process in soft magnetic nanowires and nanotubes

The micromagnetic structure of static and moving head to head domain walls (DW) in soft magnetic nanotube is calculated by means of 2D numerical simulation. Approximate formula is obtained for stationary DW velocity in external magnetic field. The numerical results for the DW width and stationary velocity are in agreement with the analytical estimations. Also the structure of vortex and transverse types of head to head DWs in soft magnetic nanowires is calculated by means of 3D numerical simulations. The vortex propagation mode is found to be similar to that of head to head DW in nanotube. The propagation of the transverse DW is accompanied by a procession of wall magnetization. For a thin nanowire this propagation mode can be successfully described based on the Walker's 1D solution.

Remanent Magnetization States in Soft Magnetic Nanowires

Remanent Magnetization States in Soft Magnetic Nanowires