Yu. A. Koksharov

Preparation, Structure, and Properties of Magnetic Materials Based on Co-Containing Nanoparticles

The recent experimental data on the preparation and properties of materials containing Co-based magnetic nanoparticles are summarized. Particular attention is focused on the synthesis of cobalt nanoparticles in “rigid” matrices (polymers, metals, and solid surfaces) and their static magnetic properties, which are of great importance for practical applications. The conclusion is made that surface effects play an important role in determining the magnetic properties of nanoparticles.

Low temperature electron paramagnetic resonance anomalies in Fe-based nanoparticles

A study of the electron paramagnetic resonance of Fe-based nanoparticles embedded in polyethylene matrix was performed as a function of temperature ranging from 3.5 to 500 K. Nanoparticles with a narrow size distribution were prepared by the high-velocity thermodestruction of iron-containing compounds. A temperature-driven transition from superparamagnetic to ferromagnetic resonance was observed for samples with different Fe content. The unusual behavior of the spectra at about 25 K is considered evidence of a spin-glass state in iron oxide nanoparticles.

Electron paramagnetic resonance of ferrite nanoparticles

Three types of iron-based oxide nanoparticles (weight compositions Fe2O3, BaFe2O4, and BaFe12O19) embedded in a polyethylene matrix are studied using the electron paramagnetic resonance technique. All nanoparticles are found to be multiphase. Thermal variations of electron paramagnetic resonance spectra reveal the presence of two phases in the Fe2O3 nanoparticles. One such phase undergoes an antiferromagnetic-like transition near 6 K. Nanoparticles of BaFe2O4 demonstrate a resonance anomaly near 125 K that could indicate the presence of a magnetic phase. Reduced magnetic anisotropy in BaFe12O19 nanoparticles may be related to either structural imperfection or particle smallness (effective diameter of less than 10 nm). Our data clearly show that low temperature experiments are desirable for the correct identification of nanoparticles by means of the electron paramagnetic resonance technique.

Electrical and Magnetic Properties of Nanomaterials Containing Iron or Cobalt Nanoparticles

We have prepared nanocomposites consisting of narrowly sized metal-containing nanoparticles embedded in a polyethylene matrix and have established conditions for the fabrication of thick films and bulk materials from the synthesized polymer powders. Dielectric permittivity and resistivity measurements demonstrate that the electrical properties of the nanocomposites depend significantly on the nanoparticle size and content.The microwave absorption and permittivity of the materials are shown to vary little in a broad frequency range. The magnetization (including the remanent one) of the cobalt-containing nanomaterials is higher than that of the iron-containing samples.

Iron(III) Oxide Nanoparticles in a Polyethylene Matrix

A method is proposed for the preparation of iron(III) oxide nanoparticles via thermal decomposition of iron(III) acetate in a high-temperature solution of polyethylene. The nanoparticles were characterized by EXAFS, EPR, and Mössbauer spectroscopy. The nearest neighbor environment of Fe in the nanoparticles was shown to be similar to that in the structure of γ-Fe2O3 . According to the Mössbauer results, the material contains iron(III) oxide in superparamagnetic and ferromagnetic states similar to γ-Fe2O3 . The particle size determined by high-resolution transmission electron microscopy is consistent with x-ray diffraction data. Experimental data are presented on the field-dependent magnetization of the material.

Properties of three-dimensional composites based on opal matrices and magnetic nanoparticles

Three-dimensional nanocomposites consisting of an opal matrix and a metal have been prepared by the interaction of salts and oxides of different elements (Ni, Co, Fe, etc.) embedded in an opal matrix with isopropanol in the range of supercritical state parameters of the alcohol. According to X-ray powder diffraction analysis and transmission electron microscopy data, the composites consist of an X-ray amorphous opal matrix with pores filled by nanoparticles of Co (or CoOx), metallic Ni, or Fe3O4 with a magnetite structure of various morphology. The sizes of the nanoparticles do not exceed the diameter of the pores in the opal matrix. A complex investigation of the nanocomposites has been performed using the electron magnetic resonance and vibrating magnetometry methods. All the studied samples at room temperature exhibit a ferromagnetic behavior. The coercive force of the samples lies in the range from 150 Oe for iron-containing nanocomposites to 565 Oe for cobalt-containing nanocomposites.

Cobalt-Containing Core-Shell Nanoparticles on the Surface of Poly(tetrafluoroethylene) Microgranules

Cobalt-containing nanoparticles have been prepared via thermal decomposition of cobalt acetate on the surface of poly(tetrafluoroethylene) (PTFE) microgranules forming a fluidized bed over the surface of hot mineral oil. Using transmission electron microscopy, the average size of the cobalt-containing nanoparticles has been determined to be 3.6 nm. The composition and structure of the nanoparticles have been determined by x-ray diffraction, extended x-ray absorption fine structure spectroscopy, and electron paramagnetic resonance, and the magnetic properties of the synthesized nanomaterial have been studied. The results indicate that the nanoparticles have a core-shell structure, with a metallic cobalt core (≃10 vol%) and a shell consisting of three phases: Co3O4 (≃80%) and small amounts of CoO and CoF2 (≃10%). The fluoride phase results from the interaction of the nanoparticles with surface fluorine atoms of the PTFE microgranules.

Radicals as EPR probes of magnetization of gadolinium stearate Langmuir–Blodgett film

Abstract In the present work we have applied the method of the EPR spin probes which allows performing simultaneously EPR and magnetization measurements to the investigation of magnetism of the Gd stearate Langmuir–Blodgett (LB) films. For this purpose we have prepared and studied by the EPR technique the Gd and Y stearate LB films. Placing the small BDPA crystal on the film surface we have found that for the Gd LB sample the effective g -value of the radicals resonance depends on the film orientation in respect to the external magnetic field direction. The relative shift of the EPR signal corresponded to the magnetization of the film along the field direction. Such effect has not been observed for the Y stearate LB film. The data obtained give an experimental proof for the room temperature magnetic ordering in the gadolinium stearate LB film.

Fe-containing nanoparticles in siloxane rubber matrices

Polymer-matrix composites consisting of Fe3O4-Fe2O3 and FeCoB nanoparticles in SKTN and SIEL siloxane rubber matrices are synthesized. The introduction of nanoparticles into siloxane oligomers leads to their polymerization, resulting in polymer-matrix nanomaterials. Transmission electron microscopy and small-angle x-ray scattering are used to determine the size of the Fe-containing nanoparticles. The magnetic properties of the nanocomposites are studied by electron magnetic resonance.

Fe-containing nanoparticles on the surface of silica microgranules

We have prepared nanomaterials consisting of SiO2 microgranules and Fe-containing nanoparticles on their surface and determined the size and composition of the nanoparticles. The nanoparticles are shown to have a core-shell structure (α-Fe core and γ-Fe2O3 shell). The α-Fe core and Fe2O3 shell are responsible, respectively, for the broad and narrow lines in the EMR spectrum of the nanoparticles. The magnetic properties of larger nanoparticles are dominated by the core, while those of smaller nanoparticles are dominated by the oxide shell.