Ilya V. Roslyakov

Long-range ordering in anodic alumina films: A microradian X-ray diffraction study

A quantitative analysis of long-range order in the self-organized porous structure of anodic alumina films has been performed on the basis of a microradian X-ray diffraction study. The structure is shown to possess orientational order over macroscopic distances larger than 1 mm. At the same time, the interpore positional order is only short-range and does not extend over more than � 10 interpore distances. These positional correlations are mostly lost gradually rather than at the domain boundaries, as suggested by the divergence of the peak width for the higher-order reflections. In the direction of the film growth the pores have a very long longitudinal self-correlation length of the order of tens of micrometres.

Origin of long-range orientational pore ordering in anodic films on aluminium

Porous anodic aluminium oxide has a long history of practical application for corrosion protection and coloring. In the last few decades a lot of hi-tech applications of this material have been found owing to the discovery of anodization conditions leading to the formation of highly ordered porous structures with a narrow pore size distribution. Here we show that in-plane orientation of the porous system in anodic films on aluminium is fully determined by the intrinsic crystallographic orientation of the Al substrate. The anisotropy of aluminium oxidation rates on a scalloped metal–oxide interface leads to reorientation of Al spikes in certain directions, which builds up an in-plane orientational order on a macroscopic scale restricted by a crystallite size. This is a unique example of the inheritance of the substrate crystal structure by an amorphous film through a size difference of three orders of magnitude.

Arrays of interacting ferromagnetic nanofilaments: Small-angle neutron diffraction study

Magnetic properties of spatially ordered arrays of interacting nanofilaments have been studied by means of small-angle diffraction of polarized neutrons. Several diffraction maxima or rings that correspond to the scattering of the highly ordered structure of pores/filaments with hexagonal packing have been observed in neutron scattering intensity maps. The interference (nuclear-magnetic) and pure magnetic contributions to the scattering have been analyzed during the magnetic reversal of the nanofilament array in a field applied perpendicular to the nanofilament axis. The average magnetization and the interference contribution proportional to it increase with the field and are saturated at H = HS. The magnetic reversal process occurs almost without hysteresis. The intensity of the magnetic contribution has hysteresis behavior in the magnetic reversal process for both the positive and negative fields that form the field dependence of the intensity in a butterfly shape. It has been shown that this dependence is due to the magnetostatic interaction between the filaments in the field range of H ≤ HS. A theory for describing the magnetic properties of the arrays of interacting ferromagnetic nanofilaments in the magnetic field has been proposed.

Preparing magnetic nanoparticles with controllable anisotropy of functional properties within a porous matrix of alumina

To prepare magnetic anisotropic nanostructures, the films of porous alumina obtained by the two-step anodization technique were used as the matrix in this work. The electrocrystallization of metals in porous alumina allowed us to form threadlike nanoparticles with narrow size distributions and controllable geometric anisotropy. It was shown that the magnetic properties of Co anisotropic nanoparticles largely depend on the crystal structure of the sample, while the behavior of Ni nanowires in the magnetic field is fully determined by the form-factor nanostructures.

Magnetic properties of a two-dimensional spatially ordered array of nickel nanowires

The structural and magnetic characteristics of two-dimensional spatially ordered arrays of magnetic nickel nanowires embedded in the anodized alumina template have been investigated. It has been shown using small-angle polarized-neutron diffraction that, there exists the samples under investigation, in a highly ordered hexagonal structure of pores and magnetic nanowires separated by a characteristic distance d = 106 ± 2 nm. An analysis has been made of different contributions to neutron scattering, such as the nonmagnetic (nuclear) contribution, the magnetic contribution dependent on the magnetic field, and the interference contribution indicating a correlation between the magnetic and nuclear structures. The performed analysis of the results obtained has demonstrated that, when the magnetic field is applied perpendicular to the longitudinal axis of the nanowire in a completely magnetized sample, there arise demagnetizing fields around each nanowire that form a regular hexagonal lattice.

Longitudinal pore alignment in anodic alumina films grown on polycrystalline metal substrates

A quantitative analysis of longitudinal pore alignment in anodic alumina films grown on polycrystalline metal substrates was performed on the basis of small-angle X-ray diffraction mapping. The very high sensitivity of the diffraction pattern to the orientation of the anodic alumina film allowed the average pore alignment within the irradiated area to be determined, with an accuracy better than 0.1°. It is shown that pores deviate from the orientation orthogonal to the metal surface by a small angle that is constant within a single-crystal grain. Strong correlation between the longitudinal pore alignment within the anodic alumina film and the grain structure of the aluminium substrate indicates the important role of the crystallographic orientation of the metal in the pore growth process.

Morphology of anodic alumina films obtained by hard anodization: Influence of the rate of anodization voltage increase

The influence of the anodization voltage ramp on the morphology and thickness homogeneity of porous anodic alumina films was studied. The samples were prepared in the oxalic acid at 120 V during the hard anodization process. As a nondestructive characterization method, the smallangle Xray scattering technique was used. The analysis of diffraction patterns allows determining the mean value and dispersion of interpore distance and the channel tortuosity with high accuracy. The increase of voltage ramp at the initial stage of hard anodization process was shown to lead to reduction of mechanical deformation (tortuosity) of anodic alumina film during crystallization.

Effect of double nuclear scattering on nuclear-magnetic interference in experiment with small-angle diffraction of polarized neutrons

An experiment on small-angle polarized-neutron diffraction by a two-dimensional spatially ordered array of nickel nanowires embedded in a porous anodic alumina matrix is discussed. The contributions of nonmagnetic (nuclear) structures and nuclear magnetic interference indicating the correlation between magnetic and nuclear structures are discussed. Magnetic scattering is two orders of magnitude smaller than nuclear scattering and, hence, turns out to be weakly distinguishable. The ordered magnetic composite nanostructure of a sample leads to strong interaction between the neutron wave and the structure itself, which, in turn, implies a twofold (miltiple scattering) nuclear scattering process. Nuclear magnetic interference scattering must be analyzed allowing for twofold scattering conditions, which substantially distorts the intensity distribution of the interference contribution of first-order diffraction peaks.

Influence of substrate microstructure on longitudinal correlation length of porous system of anodic alumina: Small-angle scattering study

Three series of anodic alumina membranes have been studied using the small-angle diffraction of neutrons and synchrotron radiation. The samples are obtained by the oxidation of aluminum wafers by sulfuric and oxalic acids at various anodization voltages and differences in the distance between the pores. Using experiments on small-angle diffraction, a linear dependence between the average grain size of an initial aluminum wafer and the average rectilinearity of pores of the synthesized membranes is established. We suggest that the observed correlation is caused by the influence of the crystallographic orientation of grains of an initial aluminum wafer on the growth of a porous oxide film.

Two-dimensional spatially ordered arrays of cobalt nanowires: polarized SANS study

Structural and magnetic properties of two-dimensional spatially ordered system of ferromagnetic cobalt nanowires embedded into Al2O3 matrix have been studied using polarized small-angle neutron scattering. The small-angle scattering pattern exhibits many diffraction peaks (up to the third reflection order), which corresponds to the scattering from highly correlated hexagonal structure of pores and magnetic nanowires. A comprehensive analysis of contributions to the scattering intensity, including nonmagnetic (nuclear) contribution, magnetic contribution depending on the magnetic field, and nuclear-magnetic interference indicating the correlation between the magnetic and nuclear structures was carried out. A detailed pattern of the remagnetization of an ordered array of the magnetic nanowires has been obtained. It has been illustrated that polarized neutron scattering provides unique information inaccessible by the standard magnetometry techniques.