Sergey E. Kushnir

Synthesis of colloidal solutions of SrFe12O19 plate-like nanoparticles featuring extraordinary magnetic-field-dependent optical transmission

Hard-magnetic plate-like nanoparticles of SrFe12O19 were synthesised hydrothermally. The particles, fixed in a polymer, exhibited a coercivity in the range 0.9–1.8 kOe, and the saturation magnetization values reached 64 emu g−1. Electrostatically stabilized aqueous colloidal solutions of the plate-like particles with average thickness of 5–8 nm and average diameter of 30–80 nm were prepared. The nanoparticles readily oriented when applying a weak magnetic field as low as a few Oe, and due to the linear dichroism phenomena a strong change in the optical transmittance of the solution took place. The extinction coefficient ratio for the perpendicular and the parallel lights electric field vector relative to the magnetic field direction was found to be up to 4.5. The experimental results were compared with a theoretical model, in which the optical absorption of the nanoparticles was calculated from the polarizability of oblate ellipsoids.

Electric-Field-Assisted Self-Assembly of Colloidal Particles

A method for formation of photonic crystals has been proposed. The method is based on convective deposition of colloidal particles onto vertical substrates in the presence of a direct-current electric field directed perpendicular to the surface of the formed film and an alternating-current electric field applied parallel to the substrate plane. The structure and optical properties of the prepared colloidal crystals have been investigated using scanning electron microscopy, high resolution small-angle X-ray diffraction, and optical spectroscopy.

Hexaferrite submicron and nanoparticles with variable size and shape via glass-ceramic route

Strontium hexaferrite single domain particles embedded in the borate matrix were synthesized by the glass-ceramic method. The composites exhibit various morphologies depending on the initial glass composition and the thermal treatment conditions. The obtained materials contain magnetic particles with the average size ranging from tens to hundreds nanometers and with the magnetic properties changing from superparamagnetic to hard magnetic with record-high coercivities exceeding 10 kOe.

Anodizing with voltage versus optical path length modulation: a new tool for the preparation of photonic structures

One-dimensional photonic crystals – multilayered structures with periodic modulation of a refractive index – are of great practical importance in modern materials science and nanotechnology owing to a variety of their applications in optoelectronics, solar photovoltaics, and sensorics. Anodizing of valve metals under oscillating conditions, which allows one to produce porous oxide films with modulated porosity across the film thickness, is a low-cost, scalable, and reproducible method for the preparation of 1D photonic crystals. However, precise control of the refractive index and the optical path length of the single layers in anodic oxides is still a challenge, mainly due to the chemical widening of pores in the upper part of the oxide film during the anodizing and, thus destruction of the structural periodicity produced electrochemically. Here, a novel anodizing regime with feedback, which takes into account the rate of chemical etching as a parameter, is suggested for the precise control of morphology of the porous films of anodic oxides of valve metals. The suggested approach allows one to apply anodizing voltage as a function of the optical path length of an oxide layer. The potential of the suggested approach is demonstrated by the preparation of one-dimensional anodic alumina photonic crystals with an unprecedentedly high quality factor of the photonic band gaps. The possibility to tune the position of a photonic band gap within the range of 250–1400 nm with an inaccuracy of <1% is shown.

Simple phase transfer of nanoparticles from aqueous to organic media using polymer colloids as carriers

A new simple and versatile technique for the phase transfer of nanoparticles from water to water-immiscible organic solvents is presented. The proposed method is based on using charged colloids as carriers for the transfer of nanoparticles with opposite surface charge. The method has been successfully applied for the transfer of SrFe12O19 nanoplatelets and CdTe nanoparticles.

M-Zn (M = Sb, V, and Nb) Substituted Strontium Hexaferrites with Enhanced Saturation Magnetization for Permanent Magnet Applications

M-Zn (M = Sb, V, Nb) substituted M-type strontium hexaferrites were prepared by a ceramic method. The phase composition, morphology and magnetic properties were studied by x-ray diffractometry, scanning electron microscopy and vibrating sample magnetometry. Saturation magnetization increases with a substitution up to 75.0 emu/g (2.5 % higher compared to unsubstituted hexaferrite) and then decreases with a further substitution. A coercive field of substituted hexaferrite powders with highest saturation magnetization is more than 3 kOe. Substituted strontium hexaferrite powders prepared in this work are a rare example of high M S compositions without doping rare-earth elements and would be a promising candidate for a permanent magnet application.

Synthesis of magnetoresistive glass ceramics based on (La,Sr)MnO3 in the La2O3-SrO-MnOx-SiO2-B2O3 system

Magnetic glass-ceramic composites containing micron-sized particles of lanthanum strontium manganite in the matrix of borate-silicate phases were prepared by thermal treatment of amorphous samples of nominal composition 28La2O3-22SrO-25MnOx-17SiO2-8B2O3at 800–1150°C. The samples obtained exhibit high magnetization up to 25 A m2/kg at a magnetic field strength of 720 kA/m. The relative magnetoresistance amounts to 17% at 77 K and a magnetic field strength of 160 kA/m.