Arrays Of Cylindrical Nanowires Research Articles

Cylindrical nanowire arrays: From advanced fabrication to static and microwave magnetic properties

This article reviews information on arrays of cylindrical magnetic nanowires grown electrochemically inside ordered nanoporous alumina membranes, summarizing some of the most relevant aspects about the advanced synthesis of nanowires and their overall static and microwave magnetic properties. The electrochemical details for the preparation of templates with well-defined hexagonal ordering are addressed focusing on the different anodization methods including mild, hard, pulsed or anodization of aluminum rods. A section follows on the growth of metallic nanowires, either as single element (i.e., Fe, Ni, Co), alloys (i.e., CoFe, CoNi, FeNi) or modulated in composition (i.e., Ferromagnetic/Metal). The static magnetic properties as anisotropy, coercivity and magnetostatic interactions are overviewed with particular emphasis on the influence of magnetocrystalline anisotropy and on the temperature dependence of those properties. The last section is devoted to the ferromagnetic resonance of nanowire arrays of different composition. The dependence of microwave absorption on the orientation of applied field is also a useful tool to determine the magnetic anisotropy parameters and the significance of magnetostatic interactions. The magnetization easy axis can be switched from the nanowire direction to the plane of the membrane as the nanowire filling factor increases or at a compensation temperature, in both cases it derives from the energetic balance of anisotropy and magnetostatic interactions among nanowires. A final subsection is devoted to the spin waves resonance spectra, and the perspectives of applications making use of microwave properties. We expect the article can be useful for researchers interested on the topic and provide a wide overview on the matter to those starting a new research line.

A Novel Design of a 3D Racetrack Memory Based on Functional Segments in Cylindrical Nanowire Arrays.

A racetrack memory is a device where the information is stored as magnetic domains (bits) along a nanowire (track). To read and record the information, the bits are moved along the track by current pulses until they reach the reading/writing heads. In particular, 3D racetrack memory devices use arrays of vertically aligned wires (tracks), thus enhancing storage density. In this work, we propose a novel 3D racetrack memory configuration based on functional segments inside cylindrical nanowire arrays. The innovative idea is the integration of the writing element inside the racetrack itself, avoiding the need to implement external writing heads next to the track. The use of selective magnetic segments inside one nanowire allows the creation of writing and storage sections inside the same track, separated by chemical constraints identical to those separating the bits. Using micromagnetic simulations, our study reveals that if the writing section is composed of two segments with different coercivities, one can reverse its magnetization independently from the rest of the memory device by applying an external magnetic field. Spin-polarized current pulses then move the information bits along selected tracks, completing the writing process by pushing the new bit into the storage section of the wire. Finally, we have proven the efficacy of this system inside an array of 7 nanowires, opening the possibility to use this configuration in a 3D racetrack memory device composed of an array of thousands of nanowires produced by low-cost and high-yield template-electrodeposition methods.

Correlation between structure and magnetic properties in CoxFe100−x nanowires: the roles of composition and wire diameter

The structural and magnetic characteristics of CoxFe100−x (0 ≤ x ≤ 100) cylindrical nanowire arrays are investigated for two series of nanowires with diameters of 20 and 40 nm, respectively. The crystalline structure evolves with Co content from bcc Fe through a mixed bcc–fcc phase to a final polycrystalline fcc or hcp phase for 40 and 20 nm diameter Co nanowires, respectively. A monocrystalline structure is found only in a few nanowires with a 40 nm diameter.The magnetic characterization under axial magnetic field reveals an increase in coercivity and remanence for increasing Co content as the crystalline structure evolves from bcc Fe to fcc Co. These parameters decrease when hcp Co with a stronger magnetocrystalline anisotropy and nearly perpendicular ‘c’ axis is formed. Overall higher values are observed in nanowires when the nanowire diameter decreases from 40 to 20 nm.An increase of the total magnetic anisotropy energy density is found with decreasing temperature, especially for Co wires where the strong magnetocrystalline anisotropy plays the most significant role.

A model for conductive percolation in ordered nanowire arrays

The combined processes of anodization and electrodeposition lead to highly ordered arrays of cylindrical nanowires. This template-based self-assembly fabrication method yields nanowires embedded in alumina. Commonly, chemical etching is used to remove the alumina and free the nanowires. However, it has been experimentally observed during the etching process that the nanowires tend to form clumps. In this work, the nanowires are modeled as elastic rods subject to surface interaction forces. The dynamics of the model give rise to the aforementioned clumping behavior, which is studied via percolation theory. This work finds that percolation takes place with probability P∼(t−tc)x, where the exponent x=2.8 and tc is the time at which percolation takes place. The critical exponents that entirely determine the system are found to be for (dimension) d=2, β=2.1, γ=0.57, Δ=2.7, α=−2.8, ν=2.4, and δ=1.3.