S. A. Gavrilov

Efficient second-harmonic generation by scattering from porous gallium phosphide

We experimentally study second-harmonic generation by femtosecond Cr: forsterite-laser radiation scattered on the surface of porous gallium phosphide with characteristic pore sizes and distances between the pores comparable with the second-harmonic wavelength. The intensity of the second-harmonic signal from samples with initial crystallographic surface orientations (110) and (111) is more than an order of magnitude higher than the intensity of the second harmonic generated in reflection from single-crystal gallium phosphide. The efficiency of second-harmonic generation by macroporous gallium phosphide substantially increases as the pump wave-length becomes shorter. The influence of light localization and scattering effects on the enhancement of second-harmonic generation and polarization properties of the second-harmonic is discussed.

Thermodynamic potential and phase diagram for multiferroic bismuth ferrite (BiFeO 3 )

We construct a Landau–Ginzburg thermodynamic potential, and the corresponding phase diagram for pristine and slightly doped bismuth ferrite, a ferroelectric antiferromagnet at room temperature. The potential is developed based on new X-ray and neutron diffraction experiments complementing available data. We demonstrate that a strong biquadratic antiferrodistortive-type coupling is the key to a quantitative description of Bi1−xLaxFeO3 multiferroic phase diagram including the temperature stability of the antiferromagnetic, ferroelectric, and antiferrodistortive phases, as well as for the prediction of novel intermediate structural phases. Furthermore, we show that “rotomagnetic” antiferrodistortive–antiferromagnetic coupling is very important to describe the ferroelectric polarization and antiferrodistortive tilt behavior in the R3c phase of BiFeO3. The Landau–Ginzburg thermodynamic potential is able to describe the sequence of serial and trigger-type phase transitions, the temperature-dependent behavior of the order parameters, and the corresponding susceptibilities to external stimuli. It can also be employed to predict the corresponding ferroelectric and antiferrodistortive properties of Bi1−xLaxFeO3 thin films and nanoparticles by incorporating the gradient and surface energy terms that are strongly dependent on the shape, size, and preparation method.Multiferroics: The potential of bismuth ferriteA theoretical approach for describing the complex phase diagram of the multiferroic bismuth ferrite has been developed. Multiferroics are materials that exhibit multiple types of ferroic ordering, such as ferromagnetism and ferroelectricity, simultaneously. Bismuth ferrite is perhaps one of the best known of these as it exhibits multiferroicity at room temperature, making it useful for a range of applications, but the underlying physical mechanisms responsible for its multiferroic properties remains somewhat unclear. An international team of researchers led by Dmitry Karpinsky from the Scientific-Practical Materials Research Centre of NAS of Belarus and the Moscow Institute of Electronic Technology use existing experimental data to construct a Landau-Ginzbur-like thermodynamic potential that can not only provide a quantitatively description of bismuth ferrites known behavior, but also predicts new intermediate phases.

Magnetic properties and magnetoresistance in La0.5Sr0.5Co1−xMexO3 (Me = Cr, Ga, Ti, Fe) cobaltites

Magnetic and magnetotransport properties of stoichiometric cobaltites La0.5Sr0.5Co1−xMexO3 (Me = Cr, Ga, Ti, Fe, x ≤ 0.25) have been investigated by neutron powder diffraction, magnetization and electrical measurements in fields up to 14 T. It is shown that doping with Fe up to x = 0.2 slightly increases magnetization herewith the Curie point decreases. The chromium doping leads to dramatic decrease of magnetization and the Curie point, associated coherent magnetic contribution in NPD patterns quickly decreases with doping and becomes nearly negligible for x = 0.2 compound. The substitution with diamagnetic Ga and Ti ions decreases the magnetization to a lesser extent than in the chromium doped compound. The Cr, Ti and Ga substitution leads to a strong increase in magnetoresistance at low temperature as compared with undoped cobaltite. The obtained results indicate that the magnetic interactions between Co and Fe are positive whereas those between Co and Cr ions are negative. Enhancement of magnetoresistance is attributed to the magnetic field induced transition from local antiferromagnetic order to ferromagnetic one.

A mechanism of oxygen-induced passivation of porous silicon in the HF: HCl: C2H5OH solutions

The problem of stabilizing the properties of porous silicon films was studied. A thermodynamical analysis of electrochemical processes occurring in the course of anodic Si dissolution is performed. A new description of electrode reaction of silicon interaction with hydrofluoric acid is suggested. It is shown that hydrogen-induced passivation of the Si surface governs the equilibrium dissolution potential of the silicon electrode. Thermodynamic calculations indicate that it is possible to substitute the chemically and thermally unstable surface groups (of the SiHx type) for more stable silicon-oxygen compounds directly in the course of formation of porous Si in electrolytes with the addition of strong hydrohalic acids. The results obtained made it possible to explain thermodynamically the stabilizing effect of an HCl additive in electrolytes used for formation of porous Si on its chemical and physical properties.

Optical properties of porous nanosized GaAs

The optical properties of porous GaAs layers obtained by electrochemical etching of single-crystal n-and p-GaAs(100) wafers are studied. It is shown that the shape of the nanocrystals, their mean diameter, and their surface states depend on the conductivity type of the initial crystal. A shift of the peaks corresponding to the main optical phonons to lower frequencies in the Raman spectra is observed. Surface-phonon frequencies determined from the Raman spectra coincide with those determined from the reflection spectra in the infrared spectral region. The forms of the spectral dependences of the reflection coefficient in the phonon-resonance region in bulk GaAs differ from those in porous GaAs. This is caused by the appearance of additional oscillators related to spatially confined lattice vibrations in GaAs nanocrystals. Atomic-force microscopy is used to study the surface morphology of porous GaAs samples formed on the n-GaAs substrates, and a nanosized surface profile is observed. Estimations made for the mean diameter of GaAs nanocrystals based on data from the Raman scattering, infrared spectroscopy, photoluminescence, and atomic-force microscopy yield results that are in good agreement with each other.

Role of Ag+ Ion Concentration on Metal-Assisted Chemical Etching of Silicon

Abstract. Metal-assisted silicon etching in the HF/H2O2/H2O solution with silver ions as a catalyst was investigated. It is found that geometric parameters of layers of nanostructured silicon are determined by the silver-catalyst concentration. A spontaneous stop of the etching process at low Ag+ ion concentration is explained by formation of insoluble Ag2SiO3.

Effect of the structure and thermoelastic properties of components on the average stress in anodic aluminum oxide having pores filled with metallic nanowires

We have assessed the average stress induced by the thermal expansion mismatch between the components of composites consisting of metallic nanowires and an anodic aluminum oxide matrix and performed model calculations for composites containing tin, zinc, and indium nanocrystals.

Sequence of phase transformations and inhomogeneous magnetic state in nanosized Sr2FeMoO6 – δ

We have studied the effect of synthesis conditions on the phase composition, grain size and morphology, degree of superstructural ordering, and magnetic properties of the Sr2FeMoO6–δ double perovskite. The results demonstrate that its magnetic state, dependent on the nonuniformity of grain morphology and the degree of superstructural ordering of the iron and molybdenum cations, correlates with the initial solution pH. Analysis of the sequence of phase transformations during strontium ferromolybdate crystallization in the citrate gel process from a pH 4 starting solution allowed us to propose combined conditions that ensure the preparation of single-phase Sr2FeMoO6–δ powder with an average grain size in the range 50–120 nm and the highest degree of superstructural ordering of the iron and molybdenum cations: P = 88%.

Magnetic properties of cobaltites doped with chromium, gallium, and iron ions

The magnetic and magnetotransport properties of cobaltites La0.5Sr0.5Co1–xMexO3 (Me = Cr, Ga, Fe) have been studied. The initial compound (x = 0) is a ferromagnet with TC = 247 K and a saturation magnetization close to 2μB per formula unit. It has ben shown that chromium substitution (x = 0.2) decreases the spontaneous magnetization to 0.3μB, while the iron substitution (x = 0.2) does not change the magnetization. The obtained data have been interpreted in a model of positive superexchange interactions between cobalt and iron and negative superexchange interactions between cobalt and chromium.

Effect of the activity of fluorine-containing electrolytes on reaching the maximum thickness of porous anodic titanium oxide

Results obtained in a study of the effect of the activity of fluorine-containing electrolytes on the thickness of an anodic layer of porous titanium dioxide are presented. It is shown that the process of chemical dissolution of the oxide during the course of the formation of porous anodic titanium oxide is affected by the dissociation of fluoride compounds in electrolytes. It is demonstrated that nonaqueous solutions with a water content not exceeding 10% should be used to obtain thick films.