Thermomagnetic phenomena in hexagonal ferrimagnetic materials

The results of an investigation of the magnetocaloric effect in the CoTi-M and CoZn-W hexaferrite systems in the 150–500 K temperature range are presented. It is shown that the maxima of the magnetocaloric effect correspond to spontaneous spin-orientational transitions leading to transformation of the magnetic structure from easy-plane to uniaxial and to resultant changes in the values and signs of the magnetic anisotropy constant as a function of the composition and temperature. Estimates of the T(T) relation taking into account the magnetic anisotropy constants K₁, K₂, and K₃, which are known for the compounds considered, agree well with experiment. Hence, the main contribution to the magnetocaloric effect at temperatures far from the Curie point in the materials in question is governed by the rotation of the magnetization vector.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 63–68, October, 1993.     

Magnetic properties of nanosized hexaferrite powders

Hexagonal ultradispersed ferrites were obtained in a high energy planetary mill by mechanochemical treatment. Their structural characteristics — average size and internal stress — were studied. Fundamental magnetic characteristics — saturation magnetization and effective fields of crystal anisotropy — were examined in pulse magnetic fields with the law of approach to saturation, as well as by ferromagnetic resonance technique in the 26–53 GHz frequency range. It was demonstrated that size reduction of powders to a few tens of nanometers influenced significantly the effective anisotropy field. A model accounting for anisotropy contribution from the disturbed subsurface layer and magnetoelastic interactions was suggested. Mechanical activation was shown to be an alternative to chemical substitution for magnetic anisotropy control.     

Mechanochemical synthesis of nanosize tin dioxide powders

Structural parameters, dispersity, morphology, and magnetic properties of a tin dioxide-magnetite nanosize composite material mechanochemically synthesized from salt systems were studied. The possibility of using the composite nanopowder as a sorbent for nucleic acids was analyzed.     

Ferromagnetic resonance in hexaferrite nanopowders

Results obtained from experimental investigations into the effect of mechanical activation of a BaCo _x Ti _x Fe _12−2x O ₁₉ hexaferrite system (x = 1 and 1.2) with a low anisotropy field in a high-energy planetary mill on the structure and magnetic parameters of the powders are presented. Transformation of the ferromagnetic resonance spectra with increase in the mechanical activation time is interpreted on the basis of a core-shell model for nanoparticles composed of an unperturbed volume and a defect surface layer. Accordingly, the particle volume possesses properties of a bulk material, whereas the surface layer exhibits substantially differing characteristics. The surface layer anisotropy is shown to play an increasing role as the reduction ratio is increased. The type of the anisotropy and the anisotropy field (H_a ≈ 20 kOe) are determined from resonance measurements. The order of magnitude of the perturbed surface layer thickness is estimated from intensities of the ferromagnetic resonance curves to be 2–3 lattice constants. __________ Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 1, pp. 33–38, January 2008.     

Formation of silicon carbide and diamond nanoparticles in the surface layer of a silicon target during short-pulse carbon ion implantation

Synthesis of silicon carbide and diamond nanoparticles is studied during short-pulse implantation of carbon ions and protons into a silicon target. The experiments are carried out using a TEMP source of pulsed powerful ion beams based on a magnetically insulated diode with radial magnetic field B _r . The beam parameters are as follows: the ion energy is 300 keV, the pulse duration is 80 ns, the beam consists of carbon ions and protons, and the ion current density is 30 A/cm². Single-crystal silicon wafers serve as a target. SiC nanoparticles and nanodiamonds form in the surface layer of silicon subjected to more than 100 pulses. The average coherent domain sizes in the SiC particles and nanodiamonds are 12–16 and 8–9 nm, respectively.     

Magnetic permeability spectra of nanosized powders of hexaferrites

A study is made of the magnetic permeability spectra of ultrafine W-hexaferrites powders, obtained by mechanical activation. It is demonstrated that natural ferromagnetic resonance frequency can be controlled by mechanochemical treatment. The physical mechanisms of spectrum evolution are discussed.     

The microstructure and properties of thin WO3 films modified by gold

Direct current magnetron sputtering of a metallic W + Au target or high-frequency magnetron sputtering of WO₃ + Au oxide targets was used to prepare thin (about 100 nm) nanocrystalline WO₃ films with the addition of gold (disperse layers of catalytic gold were additionally deposited on the surface of films). The composition and micromorphology of the surface of films and the electrical and gas sensitive characteristics of nitrogen dioxide sensors were studied to determine the mechanism of the influence of gold on the properties of WO₃ films. The films were shown to contain the β-WO₃ orthorhombic and γ-WO_2.72 monoclinic phases and gold particles. The presence of the nonstoichiometric γ-WO_2.72 phase was shown to increase the concentration of oxygen vacancies in films and decrease the resistance of sensors to 1–2 MΩ. Gold nanoparticles 9–15 nm in size segregated on the surface of semiconductor crystallites and increased the response of sensors to NO₂. The conclusion was drawn that deposited catalytic gold layers increased the response to traces of nitrogen dioxide.     

Investigation of the thermodynamic efficiency of magnetic cooling cycles in the region of spinorientational transformations in hexagonal ferrimagnets

The feasibility of using hexagonal ferrimagnets with spin-orientational transitions as the working body of a magnetic refrigerator is studied. The temperature dependence of the magnetic part of entropy and its change in crystal magnetization along various directions are calculated. The thermodynamic efficiency of magnetic cooling cycles in the vicinity of orientational transitions is estimated. Optimal magnetization fields and working temperature interval are determined. The results of calculations are compared with experimental investigations of the magnetocaloric effect for theCo _0.62 Zn _1.38-W compound. V. D. Kuznetsov Siberian Physical-Technical Institute at Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 9–13, February, 2000.