V. Ya. Mitrofanov

Random fields in disordered magnetics with Jahn-Teller ions

Abstract The character of distribution of random fields acting on a two-fold degenerate orbital E-term is investigated. The distribution functions for deformational and Coulomb fields caused by point defects are found. Random fields of exchange type on Jahn-Teller ions in crystals with a spinel structure are also examined. Estimates of distribution width for different fields are performed. The influence of random fields on the structure phase transition temperature and also on the behaviour of specific heat, elastic module and magnetic anisotropy of crystals is analysed.

Orientationally degenerate (OH)− centers in perovskite-type proton-conducting oxides: Low-temperature thermodynamics and H/D isotope effects

It is shown that proton-associated dipole centers (OH)− can provide significant, glasslike contributions to the low-temperature properties of perovskite-type proton conducting oxides, such as ABO3−y, doped with cations of lower valence. These contributions result from the splitting of the orientationally degenerate states of dipole centers by random crystal fields and proton tunneling. It is demonstrated that H/D (or H/T) substitution leads to large and abnormal isotope effects for the properties caused by (OH)− degenerate states.

The energy spectrum of quadrupole impurity centers of different types in antiferromagnets

We study the energy spectrum and some properties of various quadrupole centers (magnetic impurities or magnetic impurity complexes that are symmetric with respect to the magnetic sublattices of the antiferromagnet). We allow for the effect of spin-phonon coupling on the quadrupole splitting parameter and show that such coupling can lead to a considerable decrease in the value of this parameter and even change its sign. We investigate the behavior of quadrupole centers with an orbitally degenerate ground state and of quadrupole impurity complexes formed by mixed-valence ions. We demonstrate that such centers may greatly affect the resonant, magnetic, and thermodynamic properties of antiferromagnets. Finally, we analyze the existing experimental data and show that several new effects can be observed in systems with such centers (in particular, a magnetic analog of the Jahn-Teller effect and a strong magnetoelectric effect).

Magnetoelastic properties of crystals with Jahn-Teller centers

Abstract A theory describing the contribution of Jahn-Teller (JT) ions of the iron group to the magnetoelastic properties of crystals has been developed. It is shown that at low temperatures the magnitude of the effect is determined by the dispersion of random low-symmetry fields in the system. A comparison is made of theoretical results with experimental data (for ferrites with Fe 2+ , Mn 3+ , Cu 2+ , Co 2+ and Ni 2+ ions) and their qualitative correspondence is demonstrated.

Cu2+−Mn2+ Pair Spectra in KZnF3

Abstract The specific features of energy spectra in the ground and excited orbital degenerated states of Mn2+−Cu2+ pair in KZnF3 are considered.

Synthesis and properties of magnetic superhydrophobic mesoporous Fe2O3–SiO2 composites

Ways to obtain Fe2O3–SiO2 iron-containing silica composites with organized mesoporous structure (MCM-41) and large specific surface area (up to 1476 m2 g–1) were considered. The influence exerted by the method used to synthesize the materials on their structure, texture characteristics, particle size, and magnetic properties were studied. The aggregative stability of samples was examined. It was shown that treatment of the resulting composites with compounds from the chlorosilane group affects their hydrophobic properties.

Preparation and magnetic characteristics of mesoporous nickel oxide–silica composites

Mesoporous NiO–SiO2 (MCM-41) silica-matrix composites with various nickel oxide concentrations (NiO : SiO2 = 0.025 : 1 to 0.2 : 1) have been produced by oxide cocondensation under hydrothermal synthesis conditions in the presence of cetyltrimethylammonium bromide as a template and (2-cyanoethyl) triethoxysilane as an organosubstituted trialkoxysilane additive. X-ray diffraction data have been used to evaluate the maximum nickel(II) oxide concentration (NiO : SiO2 = 0.1 : 1) that allows the ordered mesopore structure of MCM-41 to persist in the silica-matrix composites. We have studied the magnetic properties of this material as functions of temperature and magnetic field. The results demonstrate that the magnetic properties of the nanocomposite with NiO : SiO2 = 0.1 : 1 at low temperatures (T < 20 K) are determined by incomplete spin compensation in the matrix and on the surface of the NiO nanoparticles.

Influence of oxygen non-stoichiometry on physical properties of NdSr2Mn2O7±δ

Abstract NdSr 2 Mn 2 O 7+δ compounds were synthesized by ceramic method under three different cooling conditions. The NdSr 2 Mn 2 O 7+δ samples were characterized by powder X-ray diffraction (XRD). Oxygen non-stoichiometry data for the studied powders were determined by using gravimetric and X-ray photoelectron spectroscopy (XPS) methods. The correlation of cooling method and oxygen as non-stoichiometry was established. The electroconductivity in samples was studied by using four-point probe method, and the strong correlation with non-stoichiometry was found out. Magnetization measurements were carried out. It was found that the magnetic and transport properties of the samples were also influenced by oxygen non-stoichiometry. The evolution of the magnetic properties could be explained by the formation of magnetic clusters in the vicinity of oxygen vacancies (OV) and strong competition between the superexchange and double exchange interactions.

Specific features of phase formation in initial and mechanically-activated perovskite-like Gd1 − xCaxMnO3 ± δ

The regularities of crystal structure formation and features of developed magnetic states of solid solution Gd1 − xCaxMnO3 ± δ (x = 0.0, 0.1, 0.2, 0.3, and 0.4) with temperature and partial pressure of oxygen has been established in coarse-grained and nanostructured states of samples.

Magnetic microphases in chrome-spinels from alpine-type ultramafic rocks, Central Urals

The ore and accessory chrome-spinels from metamorphosed dunites of the Cr-bearing Klyuchevskoi alpine-type ultramafic massif are studied. As a result of use of thermomagnetic analysis in the range of 4–900 K, magnetic resonance spectroscopy, and magnetic-force microscopy, secondary magnetic Fe3+-enriched microphases chaotically distributed in the primary nonmagnetic mineral were revealed for the first time in accessory chrome-spinels. It was established that the metamorphosed accessory chrome-spinels produce the magnetic properties of the host rocks and the primary nonmagnetic chrome-spinels forming ore bodies remains almost unaltered. This originates the contrast of magnetic properties between the ore body and host rocks and provides the geomagnetic anomaly in the ore-hosting zone.