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Densely packed arrays of magnetic nanowires have been synthesized by electrodeposition filling of nanopores in alumina and titania membranes formed by self-assembling during anodization process. Emphasis is made on the control of the production parameters leading to ordering degree and lattice parameter of the array as well as nanowires diameter and length. Structural, morphological and magnetic properties exhibited by nanowire arrays have been studied for several nanowire compositions, different ordering degree and for different nanowire aspect ratios. The magnetic behaviour of nanowires array is governed by the balance between different energy contributions: shape anisotropy of individual nanowires, the magnetostatic interaction of dipolar origin among nanowires, and magnetocrystalline and magnetoelastic anisotropies induced by the pattern templates. These novel nanocomposites, based on ferromagnetic nanowires embedded in anodic nanoporous templates, are becoming promising candidates for technological applications such as functionalised arrays for magnetic sensing, ultrahigh density magnetic storage media or spin-based electronic devices.

High-quality densely packed hexagonal arrays of Ni nanowires have been prepared by filling self-ordered nanopores in alumina membranes. Nanowires with different diameter d (18–83 nm) and lattice parameter D (65 and 105 nm) have been studied by atomic force, high resolution scanning electron microscopies, Rutherford backscattering, and vibrating sample magnetometer techniques. Axial loops coercivity and remanence decrease with increasing ratio diameter to lattice parameter, r, until nanowires start to interconnect locally. Additionally, hysteresis of in-plane loops increases with packing factor. In order to interpret the experimental results, multipolar magnetostatic interactions among nanowires with increasing ratio r are considered. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The synthesis of templates with modulated pore channels by combined mild and hard anodization processes is described. The hard anodization pulses, implemented during anodization, are controlled not only in time length and amplitude, but also in shape: square and exponential signals have been applied. Electrodeposition of Co is subsequently performed to obtain uniform and modulated diameter nanowire arrays. Square and exponential modulated diameter nanowires are imaged by scanning electron microscopy and hcp hexagonal polycrystalline structure is confirmed in all Co nanowires. Magnetic behavior strongly depends on nanowire shape and is interpreted considering the modification of magnetostatic interactions between wires induced by local stray fields from magnetic charges at the ends of the wider segments in modulated wires. As a consequence, magnetization processes under parallel and perpendicular field configurations denote the contribution of both thin and wide segments. [ABSTRACT FROM AUTHOR]

In this study we report the application of synchrotron X-ray fluorescence, photoluminescence and Raman scattering techniques to the analysis of the incorporation of impurities in unintentionally doped ZnO nanowires. Highly ordered one-dimensional ZnO arrays were fabricated by an oxidation process of Zn metal electrodeposited in nanoporous anodic alumina template. X-ray fluorescence data show the contribution of residual elements into the ZnO nanowires growth. A rough analytical quantification of the main light and heavy chemical contents derives impurity concentrations below 1%. The optical efficiency of ZnO nanowires is strongly affected by non-radiative centers up to temperatures as low as 100 K. The photoluminescence was found to be totally dominated by optical transitions associated with the anodic alumina template. Finally, the Raman scattering provides no evidence of local vibrational modes or secondary phases, but it shows the unambiguous signature of the ZnO hexagonal phase. [ABSTRACT FROM AUTHOR]

Abstract: In the present work, we introduce our latest achievements in the development of novel highly ordered composite magnetic nanostructures employing anodized nanoporous membranes as precursor templates where long-range hexagonal symmetry is induced by self-assembling during anodization process. Subsequent processing as electroplating, sputtering or pressing are employed to prepare arrays of metallic, semiconductor or polymeric nanowires embedded in oxide or metallic membranes. Particular attention is paid to recent results on controlling the magnetic anisotropy in arrays of metallic nanowires, particularly Co, and nanohole arrays in Ni membranes. [Copyright &y& Elsevier]

Abstract: Since the discovery of M41S materials family in 1992, some special features like aligned pores perpendicularly to the substrate surface and long range order, have been looked for with great interest for many applications of these kind of nanomaterials. The growth of thin films displaying meso- and nano-porous structures have attracted the attention of many research groups in the last decade and, with that aim several techniques such as: MBE, CVD, AFM, ion beam lithography, etc., have been used. On the other hand, a lot of down–top techniques, particularly those in which, self-assembly processes play a relevant role in the growth mechanisms of that nanostructures have been reported. Among them, electrochemical techniques constitute one of the most used to fabricate highly ordered nanostructures to be used as templates for replicating other nanostructured materials and for growing functionalized material arrays. In this paper, a brief overview on the nanofabrication techniques is done mainly of those related with the nanowires and, in general, 1D nanostructures fabrication. In addition, we show some results on ordered and disordered nanoporous anodic alumina membranes (AAM) and anodic titania membranes (ATM), respectively. Besides some functionalized systems based on these membranes used as templates are presented such as, magnetic nanowire arrays, biosensors, and carbon nanotubes. The potentiality of these systems for applications on diverse field, such as, nanoelectronic, magneto-optic, biotechnology and optoelectronic is demonstrated. [Copyright &y& Elsevier]

Systematic study of magnetic nanowire arrays grown in anodic alumina membranes (AAM) has been done by means of Rutherford backscattering spectroscopy (RBS). The AAM used as templates were morphologically characterized by using high resolution scanning electron microscopy (HRSEM), fast Fourier transform (FFT) and atomic force microscopy (AFM). The highly ordered templates with a mean pore diameter size of 30 nanometers, a mean inter-pore spacing of 100 nm and lengths ranging from 4 to 180 microns were obtained through two-steps anodization process, and the Ni and Co nanowire arrays were grown by electrodeposition techniques. The main attention is addressed to Ni nanowire arrays. RBS results allowed us to determine the real depth profile of atomic composition of the obtained nanowire arrays. In addition, the RBS spectra fitting showed that the porosity increased from the top to the bottom of the samples. Two phenomenological models are proposed to understand the apparition of that secondary porosity and a linear relation between the total amount of electrodeposited Ni and the electrodeposition time was obtained. As an example, it is also reported the relation between RBS results and magnetic properties, such as coercive field and remanence/saturation magnetization ratio of the samples. Particularly, for Ni nanowires arrays obtained by using voltage pulses, it is demonstrated that the larger the nanowires, the higher the definition for easy axis parallel to the nanowire length is possible. [ABSTRACT FROM AUTHOR]

Magnetic characteristics of arrays of Ni nanowires embedded in porous alumina are reviewed as a function of their spatial ordering. The different steps for the controlled production of highly-ordered nanowires is firstly described. Nanopores are formed into an hexagonal symmetry arrangement by self-organized process during anodization of pure Al. Parameters of the anodization allow us to control their diameter, hexagonal lattice parameter and size of crystalline domains. Subsequently, Ni nanowires are grown inside the pores by electrodeposition. Control of the pores filling and of geometrical ordering characteristics has been performed by SEM, HRSEM, RBS and AFM techniques. The magnetic characterisation of the arrays has been achieved by SQUID and VSM magnetometers, while information on the magnetic state of individual nanowires is obtained by MFM. Experimental studies are presented, particularly coercivity and remanence, for arrays with different degree of ordering (crystalline domains up to around 1 µm), and for ratio diameter to lattice parameter (diameter ranging between 20 and 180 nm, and distance between 35 and 500 nm). FMR studies have allows us to obtain complementary information of the anisotropy and magnetic characteristics. A modelling of multipolar interacting nanowires is introduced to account for the influence of short and long range ordering degree of the arrays. [ABSTRACT FROM AUTHOR]

Abstract: Arrays of Ni nanowires electrodeposited into self-aligned and randomly disordered titania nanotube arrays grown by anodization process are investigated by X-ray diffraction, SEM, rf-GDOES and VSM magnetometry. The titania nanotube outer diameter is about 160nm, wall thickness ranging from 60 to 70nm and 300nm in depth. The so-obtained Ni nanowires reach above 100nm diameter and 240nm length, giving rise to coercive fields of 98 and 200Oe in the perpendicular or parallel to the nanowires axis hysteresis loops, respectively. The formation of magnetic vortex domain states is also discussed. [Copyright &y& Elsevier]

Abstract: Densely packed arrays of magnetic nanowires with hexagonal symmetry have been prepared by electrodeposition filling of the nanopores in alumina membranes previously formed by self-assembling induced by anodization. The influence of geometrical characteristics of arrays of Ni nanowires on their hysteresis loops have been studied. These characteristics are controlled by suitable choosing of preparation parameters: nanowires diameter ranges between 18 and 80nm for lattice parameter of hexagonal symmetry of 65 and 105nm, while length of nanowires is taken between 500 and 2000nm. Additionally, the temperature dependence of coercivity when applying the field parallel to the nanowires or in-plane of the membrane has been measured. All these results allows us to conclude that magnetic behaviour is determined by the balance between different energy contributions, namely, the shape anisotropy of individual nanowires, the magnetostatic interaction among nanowires (confirmed to play a decisive rôle), and seemingly the magnetoelastic anisotropy induced in the nanowires by the alumina matrix through temperature changes as a consequence of their different thermal expansion coefficients. [Copyright &y& Elsevier]

Abstract: Arrays of magnetic nanowires with composition Ni x Fe(100− x ) (65⩽x⩽100), 35nm in diameter and 2000nm long have been prepared by electrodeposition filling of highly ordered nanoporous alumina membranes. Long-range order of nanopores, with crystalline domain size of around 2μm2 and 105nm lattice parameter of hexagonal symmetry, is achieved by self-ordering process, and characterized by SEM and AFM. Magnetic behavior of the arrays has been determined by VSM. Maximum coercivity of around 1.23kOe and reduced remanence (about 0.8 saturation magnetization) is observed for , while minimum values are observed for . Detailed AFM and MFM studies allow us to gain additional information of the filling degree of pores which can result in a distributed nanowires length that finally correlates with a deterioration of macroscopic magnetic behavior of the array. [Copyright &y& Elsevier]

Hysteresis loops of two-dimensional arrays of magnetic nanowires have been micromagnetically modelled. The calculations focus on magnetostatic interactions in addition to Zeeman and magnetic anisotropy energy terms. Starting from an ideally ordered hexagonal array, the generation of local disorder is first modelled and subsequently, hysteresis loops are calculated by Monte Carlo and iterative methods considering dipolar and higher-order multipole effects. The main conclusions of the modelling are: (i) distortion of the hexagonal ordering results in an increasing effective field to reach magnetic saturation while roughly maintaining coercivity, and (ii) an increase of the average distributed coercivity of individual nanowires gives rise to enhanced coercivity and anisotropy field.The results of these simulations are analysed in view of experimental loops of arrays of Ni nanowires filling nanoporous alumina membranes with different degrees of 2D-polycrystalline arrangement. After a careful analysis by image processing, it is concluded that fluctuations in the diameter and cross-section of individual nanowires play an important role in a deep correlation between modelled and experimental hysteresis loops. [Copyright &y& Elsevier]