Arrays Of Co Nanowires Research Articles

Micromagnetic simulation of electrochemically deposited Co nanowire arrays for wideband microwave applications

We study the magnetic properties of arrays of Co nanowires which exhibit zero bias-field ferromagnetic resonance absorptions in a 0–30 GHz range. Columnar arrays of Co nanowires with lengths of 8–15 µm were electrochemically grown using ∼20 µm thick anodic alumina membranes with 50 nm pore diameters. Microstructural, static magnetic, and microwave properties of five different nanowire arrays were characterized. The studied Co nanowires present different crystal structure textures and magnetic properties. The static magnetic loop shapes and the ferromagnetic resonance frequencies of the nanowire arrays were correctly reproduced using the Mumax3 micromagnetic software. For each sample input parameters dependent on the x-ray diffraction and microstructural data, were fine-tuned to allow the best fit of the experimental hysteresis loops and the related microwave spectra. Using this method, it was possible to analyze the rather complex interplay between geometry and magneto-structural features of the different arrays, defining which parameters play a key role in the development of nano-systems with specific microwave properties.

Effect of pH and Boric Acid on Magnetic Properties of Electrodeposited Co Nanowires

Arrays of Co nanowires of diameter ~ 200 nm have been prepared by electrodeposition into nanopores of anodic aluminum oxide templates. X-ray diffraction confirms that these nanowires consist of mostly hexagonal close-packed (hcp) or face-centered cubic (fcc) grains or a mixture of both. Scanning electron microscope confirms that these nanowires are continuous and parallel to each other. The effect of boric acid and pH on the structural and magnetic properties of electrodeposited cobalt nanowires has been studied. When boric acid is not introduced into the electrolyte, high concentration of hydrogen is liberated, which results into the formation of disordered fcc Co at low pH. The high value of coercivity for the nanowires deposited at low pH is attributed to the formation of higher fraction of disordered fcc Co nanowires. The induced anisotropy is weak at high pH compared to low pH. This is attributed to the competition between shape and magnetocrystalline anisotropy of hcp phase.

A Comparative Study of Magnetic Properties of Large Diameter Co Nanowires and Nanotubes.

A comparative study of the magnetic properties of the arrays of Co nanowires and nanotubes with large external diameters (180 nm) has been carried out. The nanowires/nanotubes were grown by electrodeposition into the self-assembled pores of anodic alumina membranes. The experimental study of their magnetic behavior was focused on the angular dependence of hysteresis loops and their parameters. In both nanowire and nanotube arrays, from the analysis of experimental data, effective longitudinal magnetic anisotropy is concluded, which is stronger in the case of the nanotube array. In addition, the extremely small remanence observed for all loops indicates the important role played by magnetostatic interactions. Micromagnetic simulations were first performed considering intrinsic shape and magnetocrystalline anisotropy terms, together with an effective easy-plane anisotropy to account for those magnetostatic interactions. A qualitative agreement between experiments and simulations is found despite the complexity introduced by the intrinsic and extrinsic array properties (i.e., large diameters, grain structure, and array configuration). In addition, simulations were also carried out for individual nanowire/nanotube with a particular emphasis to understand their differences at the remanence, due to pure geometry contribution.

Shape-dependent magnetic properties of Co nanostructure arrays synthesized by pulsed laser melting

One dimensional (1D) magnetic nanowires show unique magnetic behaviors, such as large coercivity and high remanence, in comparison to the bulk and thin film materials. Here, planar arrays of Co nanowires, nanorods and nanoparticles were fabricated from thin Co films by a nanosecond pulsed laser interference irradiation technique. Magnetic force microscopy (MFM) and surface magneto-optic Kerr effect (SMOKE) techniques were used to study the individual and average magnetic properties of the nanostructures. Magnetic domain orientation was found to depend on the in-plane aspect ratio of the nanostructure. The magnetic orientation was out-of-plane for in-plane aspect ratio ranging from 1 to 1.4 and transitioned to an in-plane orientation for aspect ratios greater than 1.4 (such as in nanorods and nanowires). Our results also showed that polycrystalline Co nanowires showed much higher coercivity and remanence as compared to bulk and thin film materials, as well as shapes with smaller aspect ratio. This result was attributed mainly to the shape anisotropy. This study demonstrated that nanosecond pulsed laser synthesis is capable of fabricating various nanostructures in a simple, robust and rapid manner and SMOKE is a reliable technique to rapidly characterize such magnetic nanostructures.

High-frequency ferromagnetic resonance of Co nanowire arrays

Highly ordered arrays of Co nanowires with different diameters have been fabricated by electrodeposition in nanoporous anodic aluminum oxide membranes. The prepared Co nanowires have hexagonal close-packed structure and are [002] preferred oriented crystalline with the c-axis parallel to the long axis of the nanowires. The high anisotropy of Co nanowires offers the possibility to control their magnetic properties and microwave properties, which are experimentally characterized by a vibrating sample magnetometer and frequency-swept ferromagnetic resonant flip-chip based techniques, respectively. The nanowire arrays exhibit strong magnetic anisotropies. As for the ferromagnetic resonance frequency, it increases with increasing value of the applied field, and zero-field ferromagnetic resonance frequencies up to 28–34 GHz. Also, the magnetic anisotropies and high ferromagnetic resonance frequency of Co nanowire arrays, are shown to be determined by the combined effect of shape anisotropy, crystalline anisotropy, and dipolar interactions between the nanowires.

Angular first-order reversal curves: an advanced method to extract magnetization reversal mechanisms and quantify magnetostatic interactions

The magnetic properties of ordered hexagonal arrays of Co nanowires (NWs) and nanotubes (NTs) with diameters of 50 nm and interwire/tube distances of 105 nm were studied using first-order reversal curves (FORCs). We report an advanced analysis of angle dependent first-order reversal curves (AFORCs), measured by changing the angle of the applied magnetic field from θ = 0° (parallel to the wire/tube axis) to 90° (perpendicular). This method allowed us to determine the magnetization reversal mode and to retrieve quantitative information on the magnetostatic interactions between NWs and between NTs. In particular, we found a sharp increase in the coercivity distribution of the NT arrays for θ > 70°, which is attributed to a transition between vortex and transverse reversal modes. Local magnetic interactions are found to prevail in the Co NT arrays, steadily increasing from θ = 0° to 90°. However, in the Co NW arrays the mean magnetic interactions decrease as θ increases, going from ones similar to local interactions to ones smaller than them.

Magnetic interactions and reversal mechanisms in Co nanowire and nanotube arrays

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.

Magnetic anisotropy in ordered textured Co nanowires

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.

Angular dependence of the coercivity and remanence of ordered arrays of Co nanowires

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.

Structural and magnetic properties of planar nanowire arrays of Co grown on oxidized vicinal silicon (111) templates

We fabricated planar arrays of Co nanowires (NWs) on oxidized step-bunched Si (111) templates using shallow angle deposition. These planar NW arrays exhibit ferromagnetic behavior at room temperature for NW widths down to 25 nm. The NWs possess polycrystalline character with hcp-crystal structure, and present a lightly oxidized interface when capped with MgO. The magnetic anisotropy of the NW array is dominated by the shape anisotropy, which keeps the magnetization in-plane with easy axis parallel to the wires. By reducing the inter-wire separation, we obtain NW arrays with reduced coercivity demonstrating the importance of magneto-static interactions in determining the magnetic properties of the NWs.

Coercivity of ordered arrays of magnetic Co nanowires with controlled variable lengths

Ordered hexagonal arrays of Co nanowires were prepared using anodic aluminum oxide templates, varying the nanowire length in a controlled way. The measured coercivity values were shown to decrease with the increase in the nanowire aspect ratio, in contrast to the behavior expected from the shape anisotropy. The measurements were interpreted considering the change in the preferred crystalline structure, from fcc cubic to hcp hexagonal in small and high aspect ratio nanowires, respectively. The experimental evolution of coercivity with nanowires length is interpreted considering micromagnetic simulations taking into account the change in their crystalline structure.

Magnetic Properties of Cobalt Nanowire Arrays

Arrays of Co nanowires with different lengths and diameters have been prepared by electrodeposition into nanopores of alumina and polycarbonate membranes. The dependence of the coercivity and remanence of the arrays as a function of the length, the electrodeposition potential, and the electrolytic bath pH of the nanowires has been experimentally investigated through hysteresis curves performed in a VSM (vibrating sample magnetometer) and a SQUID (superconducting quantum interference device).

Superconducting Properties of Pb/Bi Films Quench-Condensed on a Porous Alumina Substrate Filled with Co Nanowires

Superconducting properties of a new ferro-magnet–superconductor hybrid structure have been investigated. Organized arrays of Co nanowires are first electroplated into the columnar pores of anodic aluminum oxide (AAO) membranes. Superconducting Pb/Bi (18 at.%) films are then quench-condensed onto the surface of the AAO membranes filled with Co nanowires. The Co nanowire array produces a magnetic field with a strong spatial variation in the superconducting film. Hysteretic superconducting properties and enhanced critical currents have been observed in applied external magnetic fields, which we explain based on the magnetic domain structure of the Co nanowire arrays.

Effects of applied magnetic field on microstructure and magnetic properties of template-based Co nanowire arrays

Arrays of Co nanowires were fabricated by ac electrochemical deposition of metal into high order anodic alumina templates prepared through two-step oxidation method. The effects of applied magnetic field and the pH value of the electrolyte on growth of nanowires were investigated. Arrays of Co nanowires of hcp structure were deposited in the electrolyte whose pH values are 6.0 and 6.5 without magnetic field, then 0.3 T magnetic field was applied along the long axis of nanowires. The experimental results show that external magnetic field and adjustment of pH value of the electrolyte impel the easy magnetization axis of the hcp Co grain to grow along length direction of nanowires. The Co nanowire arrays show higher perpendicular anisotropy, larger coercivity and high M/Ms because the superposition of the crystal anisotropy of hcp Co grains and shape anisotropy of nanowire along length direction.

Co nanowire arrays onN-terminated Cu(110) surfaces

We demonstrate, using scanning tunneling microscopy, that the growth of Co onto Cu(110) surfaces can be dramatically altered by first terminating the Cu(110) surface with an atomic nitrogen-induced $(2\ifmmode\times\else\texttimes\fi{}3)$ structure. Co growth onto such surfaces results in the formation of ordered arrays of Co nanowires. These results are contrasted with Co growth on clean Cu(110) surfaces and suggest that the N termination also prevents the diffusion of Cu atoms into the Co layers.

Structure and magnetization of arrays of electrodeposited Co wires in anodic alumina

We have produced arrays of Co nanowires in anodic porous alumina filters by means of electrodeposition. The structure and magnetization behavior of the wires was investigated with nuclear magnetic resonance (NMR) and magnetization measurements. NMR shows that the wires consist of a mixture of fcc and hcp texture with the (0001) texture of the hcp fraction oriented preferentially perpendicular to the wires. The magnetization direction is determined by a competition of demagnetizing fields and dipole–dipole fields and can be tuned parallel or perpendicular to the wires by changing the length of the wires.