Magnetic nanowires

 

Nanowires are considered to be nearly ideal low dimensional systems, allowing one to address fundamental physical questions, such as magnetization reversal processes in quasi-ideal nanoparticles, quantized transport or electron localization effects. Besides, they have been the focus of research because of their promising applications in the perpendicular magnetic recording, as well as interconnects in future generations of nanoscale electronics or electrochemical devices. Also, there is an increasing interest in functionalizing one-dimensional (1D) nanostructures for biotechnological or sensor applications.

 

The electrochemical deposition in anodic alumina membranes (AAO) makes it possible to synthesize high aspect ratio nanowires. These membranes have attracted a huge scientific interest due to the outstanding features exhibited by these templates such as low cost, large self-ordering degree of the nanopores, high reproducibility and precise control over their morphological characteristics. The main advantage of AAO templates lies in their customized geometrical features, such as nanopore diameter, length and center-to-center distance, which are easily controlled by tuning the anodization conditions. In the case of nanowires, the diameter can be reduced below the limit of the single-domain state, while the length can be several orders of magnitude larger and results in strong shape anisotropy.

 

An additional degree of freedom, to tune the magnetic response of an array of nanowires, consists in using alloys instead of single materials. This would open the possibility to finely control the magnetic parameters of the alloy by varying its stoichiometry. For instance, nanowires of CoNi alloys have been found to exhibit outstanding properties since they can display either a soft or a hard magnetic behavior depending on the Co content in the alloy. At this point, we would like to emphasize that we have recently investigated CoNi nanowire arrays with 35 and 50 nm diameters [1,2]. Besides, It is well established that Ni nanowires present easy axes along the wire axis due to the predominant shape anisotropy contribution. However, the case of Co is especially interesting due to its large magnetocrystalline anisotropy that favors in many cases (we have shown [3] that the texture of electrodeposited Co nanowire arrays can be tailored by the synthesis process, i.e., pH of the electrolytical bath and plating time) a perpendicular-to-the-axis easy magnetization axis for the hcp equilibrium phase. Thus, the effective anisotropy energy is determined by competition between shape and magnetocrystalline anisotropies, it being possible to tune the preferred magnetization direction (easy magnetization axis) of the system between the longitudinal and perpendicular directions with respect to the nanowire axis. For example, CoNi alloy nanowires can be grown where longitudinal anisotropy is promoted while still retaining significantly large saturation magnetization.

 

On the other hand, the nucleation and propagation of a magnetic domain wall between opposing magnetic domains in the magnetization reversal process of magnetic nanowires is a topic of growing interest. For instance, by equating the direction of a domain’s magnetization with a binary 0 or 1, a DW also becomes a mobile edge between data bits: the nanowire can thus be thought of as a physical means of transporting information in magnetic form. This is an appealing development because computers currently recorded information onto their hard disks in magnetic form. For isolated magnetic nanowires, the magnetization reversal can occur by one of only two idealized mechanisms, the Vortex mode (V), whereby spins in rotation remain tangent to the tube wall, or the Transverse mode (T), in which a net magnetization component in the (x,y) plane appears. Starting from the equations presented by Landeros et al. [4], we can calculate the zero-field energy barrier as well as the width of the domain wall for each reversal mode as a function of the geometrical parameters. From this, we have proposed simple equations to calculate the switching field of an isolated magnetic wires assuming that the magnetization reversal is driven by means of one of the two previously presented modes. This model, presented has been published in several previous publications and applied to different experimental systems (nanowires and nanotubes based on Ni, Co, CoNi, and Fe3O4) in the past [6]. Some of these publications also address the aspect of angle resolved measurements [1, 3, 7].

 

Moreover, spin-polarized currents provide a powerful means of manipulating the magnetization of nanodevices, and give rise to spin transfer torques that can drive magnetic domain walls along nanowires. These developments give promise of a nonvolatile memory device with the high performance and reliability of conventional solid-state memory but at the low cost of conventional magnetic disk drive storage. This nonvolatile memory device is the racetrack memory that comprises an array of magnetic nanowires arranged horizontally or vertically on a silicon chip. The idea is to use individual spintronic reading and writing nanodevices to modify or read a train of ~10 to 100 domain walls, which store a series of data bits in each nanowire. Thus, we investigate the motion of domain walls in magnetic nanowires.

 

[1] L. G. Vivas, M. Vázquez, J. Escrig, S. Allende, D. Altbir, D. C. Leitao, J. P. Araujo, Magnetic anisotropy in CoNi nanowire arrays: Analytical calculations and experiments, Phys. Rev. B 85, 035439 (2012).

[2] A. Pereira, C. Gallardo, A. P. Espejo, J. Briones, L. G. Vivas, M. Vázquez, J. C. Denardin, J. Escrig, Tailoring the magnetic properties of ordered 50-nm-diameter CoNi nanowire arrays, J. Nanopart. Res. 15, 2041 (2013).

[3] L. G. Vivas, J. Escrig, D. G. Trabada, G. A. Badini-Confalonieri, M. Vázquez, Magnetic anisotropy in ordered textured Co nanowires, Appl. Phys. Lett. 100, 252405 (2012).

[4] P. Landeros, S. Allende, J. Escrig, E. Salcedo, D. Altbir, E. E. Vogel, Reversal modes in magnetic nanotubes, Appl. Phys. Lett. 90, 102501 (2007).

[5] J. Escrig, J. Bachmann, J. Jing, M. Daub, D. Altbir, K. Nielsch, Crossover between two different magnetization reversal modes in arrays of iron oxide nanotubes, Phys. Rev. B 77, 214421 (2008).

[6] J. Escrig, R. Lavín, J. L. Palma, J. C. Denardin, D. Altbir, A. Cortés, H. Gómez, Geometry dependence of coercivity in Ni nanowire arrays, Nanotechnology 19, 075713 (2008).

[7] R. Lavín, J. C. Denardin, J. Escrig, D. Altbir, A. Cortés, Gómez, Angular dependence of magnetic properties in Ni nanowire arrays, J. Appl. Phys. 106, 103903 (2009).