Mariana P. Proenca

Nanoporous alumina as templates for multifunctional applications

Due to its manufacturing and size tailoring ease, porous anodic alumina (PAA) templates are an elegant physical-chemical nanopatterning approach and an emergent alternative to more sophisticated and expensive methods currently used in nanofabrication. In this review, we will describe the ground work on the fabrication methods of PAA membranes and PAA-based nanostructures. We will present the specificities of the electrochemical growth processes of multifunctional nanomaterials with diversified shapes (e.g., nanowires and nanotubes), and the fabrication techniques used to grow ordered nanohole arrays. We will then focus on the fabrication, properties and applications of magnetic nanostructures grown on PAA and illustrate their dependence on internal (diameter, interpore distance, length, composition) and external (temperature and applied magnetic field intensity and direction) parameters. Finally, the most outstanding experimental findings on PAA-grown nanostructures and their trends for technological appl...

Size and surface effects on the magnetic properties of NiO nanoparticles

NiO nanoparticles (NPs) were prepared by a sol-gel process using the citrate route. The sol-gel parameters were tuned to obtain samples with different average particle sizes, ranging from 12 to 70 nm. Magnetic characterization revealed an increase in the blocking temperature with the diameter of the NPs and an increase in the effective magnetic anisotropy (K(eff)) with decreasing particle size. The magnetic moment per particle was calculated for all samples using the susceptibility value at T = 300 K. The number of uncompensated spins per NP was found to be proportional to n (n(S)≡ total number of spins), indicating that they are randomly distributed on the NP surface. For small diameters (<30 nm) the surface anisotropy constant was estimated, using, for NiO NPs, a recent model describing the evolution of K(eff) with particle size. Hysteretic loops performed at low temperatures after field cooling displayed loop shifts (∼6.5 kOe in the field axis and ∼0.18 emu g(-1) vertically), coercive field enhancement (H(C)≈ 4.8 kOe) and training effects for the smaller NPs. The sample with NPs of larger diameters presented magnetic properties close to those of bulk NiO.

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 array...

Magnetic properties of Co nanopillar arrays prepared from alumina templates

The preparation of magnetic nanopillars from anodic alumina templates represents a cheap way to obtain extensive ordered arrays, and thus is very appealing for nanotechnology applications. In this paper we report the preparation of arrays of Co nanopillars with 120 nm height and varying diameter. The high anisotropy of Co offers an additional possibility to control their magnetic properties. The magnetic properties of arrays of Co nanopillars are studied both experimentally and by micromagnetic simulations. Experiment and modeling show crucial changes of hysteresis loops when the diameter is increased. Magnetic data are interpreted considering the change of crystalline structure as well as the influence of geometry. The micromagnetic simulations explain the measured magnetic properties by the role of magnetocrystalline anisotropy and the combined influence of the shape anisotropy and the interactions. They also show the change in the reversal mode with the increased diameter from vortex propagation to curling when the field is applied parallel to the nanopillar axis, and from coherent rotation to curling when it is applied perpendicular.

Tunning pore filling of anodic alumina templates by accurate control of the bottom barrier layer thickness

The role of the alumina barrier layer thickness (δ(b)) on the growth of Ni nanowires (NWs) in porous anodic alumina (PAA) has been revealed. By varying the final anodization voltage to form dendrites at the bottom of the nanoporous structure, we are able to optimize δ(b) (in the 2-16 nm range), allowing us to obtain a Ni pore filling percentage (f(p)) of almost 100% for δ(b) = 10 nm. However, deviations from this optimal δ(b)-value led to a strong decrease of f(p). Moreover, an increase of the electrodeposition efficiency (EE) and NW homogeneity was also verified for δ(b) up to 10 nm. Such increase in nominal δ(b) leads to a consistent growth rate in all pores and consequently a complete and uniform nanopore filling. On the other hand, the decrease in electrodeposition efficiency visible for δ(b) > 10 nm is related with hydrogen evolution and dielectric breakdown of the insulator layer due to the required high deposition voltages. Non-uniform NW growth is then visible, with the consequent decrease in f(p). The control of the pore filling and length homogeneity of the fabricated 1D metallic nanostructures, combined with the ability to adjust the pore dimensions of PAA, can bring novel approaches for the fabrication of nano-objects and thus exciting new applications.

Distinguishing nanowire and nanotube formation by the deposition current transients

High aspect ratio Ni nanowires (NWs) and nanotubes (NTs) were electrodeposited inside ordered arrays of self-assembled pores (approximately 50 nm in diameter and approximately 50 μm in length) in anodic alumina templates by a potentiostatic method. The current transients monitored during each process allowed us to distinguish between NW and NT formation. The depositions were long enough for the deposited metal to reach the top of the template and form a continuous Ni film. The overfilling process was found to occur in two steps when depositing NWs and in a single step in the case of NTs. A comparative study of the morphological, structural, and magnetic properties of the Ni NWs and NTs was performed using scanning electron microscopy, X-ray diffraction, and vibrating sample magnetometry, respectively.

CROSSOVER BETWEEN MAGNETIC REVERSAL MODES IN ORDERED Ni AND Co NANOTUBE ARRAYS

Ordered arrays of Ni and Co nanowires and nanotubes, with diameters between 30 nm and 60 nm, were prepared by electrodeposition into nanoporous alumina templates. The study of the corresponding magnetization reversal processes was performed by analyzing the angular dependence of coercivity (Hc) and using a simple analytical model. The agreement between experimental and theoretical data shows that magnetization in nanowire arrays reverses by means of nucleation and propagation of a transverse domain wall, independently of the diameter. However, a critical diameter of ~ 50 nm was found in the case of nanotubes, above which a nonmonotonic angular dependent Hc was observed, evidencing a transition between vortex and transverse reversal modes.

Cobalt ferrite thin films deposited by electrophoresis on p-doped Si substrates

The structural and magnetic properties of cobalt ferrite (CoFe2O4) thin films deposited by electrophoresis on p-doped Si(001) substrates have been characterized. The films were polycrystalline and composed by cobalt ferrite with the cubic spinnel structure. The observed decrease of the coercive field with the sixth power of the grain size was indicative of a competition between the magnetocrystalline anisotropy and the exchange coupling energy, on these randomly oriented nanosized grained films.

Temperature dependence of the training effect in electrodeposited Co/CoO nanotubes

High aspect ratio Co/CoO nanotubes (NTs) were obtained by potentiostatic electrodeposition of Co inside nanoporous alumina templates followed by the natural oxidation of their inner walls. Magnetic measurements performed at low temperatures after field cooling the samples from above its blocking temperature (TB∼220 K), evidenced the existence of exchange bias (EB) coupling between the Co ferromagnetic outer wall and the CoO antiferromagnetic inner wall of the NTs. A decrease in the magnitude of the EB field was measured at T<TB when cycling the Co/CoO NT arrays through consecutive hysteresis loops. This decrease is known as the training effect (TE) and is here studied in the 6 K≤T<TB temperature range. The TE was fitted using the recursive Binek formula, giving small values for the characteristic decay rate of the training behavior, and evidencing a decrease of EB with increasing antiferromagnetic layer thickness. A phenomenological theory for the temperature dependence of the TE in exchange biased system...

Influence of the Electrodeposition Cathodic Potential on the Composition and Magnetic Properties of CoNi Nanowires

Ordered hexagonal arrays of Co (x)Ni (1-x) nanowires (NWs) with different content of cobalt (0.35 < x < 0.75), were successfully fabricated by a potentiostatic electrodeposition technique inside anodic aluminum oxide (AAO) templates. The CoNi NWs were electrodeposited using only one electrolyte bath and tuning the Co% by the applied deposition potential. The systematic study of the morphology, chemical composition and magnetic properties of the CoNi NW arrays was performed. The obtained results unambiguously demonstrated the influence of the Co content on the magnetic properties of CoNi NWs. The Co% was found to decrease when increasing the applied cathodic potential and the coercivity of the CoNi NWs linearly increased with the Co%. The magnetic texture of the fabricated NW arrays was found lower for smaller Co concentrations, which was in good agreement with the squareness results.