V. V. Rudenko

Magnetic, optical, and electrical properties of solid solutions VxFe1−xBO3

Complex studies of magnetic, electrical, and optical properties of Vx Fe1−xBO3 solid solutions are carried out in the entire range of concentrations between the extreme compounds VBO3 and FeBO3. A concentration semiconductor-insulator transition accompanied by a change in the magnetic structure is observed. It is found that the physical properties of the solid solution under investigation differ from those predicted in the model of a virtual crystal in the form of an aggregate of V and Fe centers taken with the weight of x and 1−x, respectively. The systems of electron energy levels of the VB6O6 and FeB6O6 clusters are calculated from first principles using the Hartree-Fock method. The calculated electron structure forms the basis for simulating the optical absorption spectra, which are in good agreement with experimental results. A qualitative explanation is given for the entire body of data on electrical conductivity and magnetization.

Magnetic and electrical properties of cobalt oxyborate Co3BO5

Single crystals of cobalt oxyborate Co3BO5 are synthesized. The results of investigations into the structural, magnetic, and electrical properties are reported. It is found that Co3BO5 crystals exhibit two magnetic anomalies at T1 = 17 K and T2 = 45 K. The temperature dependence of the electrical resistivity is investigated. Deviations both from the activation law of variation in the electrical resistance and from the Mott variable-range hopping conduction are revealed.

Conductivity Study of Co3O2BO3 and Co3-xFexO2BO3 Oxyborates

Single crystals of cobalt oxyborates Co3O2BO3 and Co3-xFexO2BO3 were synthesized. The crystal structure and electric properties were investigated. The difference in the electrical resistivity behaviors was found. For parent Co3O2BO3 nor simple activation law, nor Mott variable range hopping (VRH) are acquirable to describe the experimental data in wide temperature region. In contrast for Co3-xFex O2BO3 Mott’s variable-range hopping conductivity clearly dominates.

Magnetic anisotropy of the VBO3 and CrBO3 transition-metal borates

Temperature and field dependences of the magnetization of VBO3 and CrBO3 single crystals with the magnetic field applied parallel and perpendicular to the (111) basal plane were measured. VBO3 was found to have a considerable uniaxial anisotropy with a field Ha≈6.25 T. CrBO3 was shown to exhibit not only uniaxial but also hexagonal anisotropy. The experimental anisotropy constants were estimated, and their temperature dependences are presented.

Temperature dependence of the uniaxial magnetic anisotropy of rhombohedral antiferromagnetic crystals with ions in the S state

The contributions of the anisotropic exchange to the firstand second-order uniaxial anisotropy constants have been calculated for hematite at T = 0 K and arbitrary temperatures. The results of the calculations and the most significant mechanisms are taken into account in interpreting the temperature dependence of the anisotropy in rhombohedral antiferromagnetic crystals. The first-order anisotropy constant for hematite is described by the dipole interaction, contributions of the “single-ion” nature, and relatively small contributions of the anisotropic exchange. The second-order constant for hematite includes the “single-ion” contribution and the contribution of the anisotropic exchange. For FeBO3 and MnCO3 crystals, the main contributions to the first-order anisotropy constant come from the dipole and single-ion mechanisms.

Mössbauer effect study in Fe1− xVxBO3 solid solutions

The Mössbauer effect in the Fe1− xVxBO3 solid solutions has been measured at 130 and 300 K. The Fe0.05V0.95BO3 composition was studied in the interval 4.2–300 K. The experimental data obtained are described in terms of the model of a dilute magnetic insulator in which atoms of the first coordination sphere provide a major contribution to the hyperfine field at iron nucleus sites. It was found that, at low temperatures, the field Hhf is generated primarily by the iron ion itself and depends only weakly on substitution. The hyperfine interaction parameters in the discrete configuration series 6Fe, 5Fe1V, 4Fe2V, 3Fe3V, and 2Fe4V were deter-mined. The magnitude of the isomer shift suggests that iron in the crystal resides in the trivalent state.

Magnetic and electrical properties of Fe1.91V0.09BO4 warwickite

We have performed a complex investigation of the structure and the magnetic and electrical properties of a warwickite single crystal with the composition Fe1.91V0.09BO4. The results of Mössbauer measurements at T=300 K indicate that there exist “localized” (Fe2+, Fe3+) and “delocalized” (Fe2.5+) states distributed over two crystallographically nonequivalent positions. The results of magnetic measurements show that warwickite is a P-type ferrimagnet below T=130 K. The material exhibits hopping conductivity involving strongly interacting electrons. The experimental data are analyzed in comparison to the properties of the initial (unsubstituted) Fe2BO4 warwickite. The entire body of data on the electric conductivity and magnetization are interpreted on a qualitative basis.

Low-field magnetization of ludwigites Co3O2BO3 and Co3 − xFexO2BO3 (x ≈ 0.14)

Ludwigite single crystals of compositions Co3O2BO3 and Co3 − xFexO2BO3 (x ≈ 0.14) have been synthesized. The crystal structure is investigated at room temperature, and the magnetization is studied in the temperature range T = 4.2–100 K in magnetic fields of up to 600 Oe. The orthorhombic symmetry is revealed, and the unit cell parameters are determined. A number of features are established for the temperature dependence of the magnetization. In unsubstituted Co3O2BO3, two magnetic transitions are found at TC1 = 43 K and TC2 = 15 K. At temperatures below 40 K, spin-glass state is revealed. Substitution of iron ions for cobalt ions leads to a noticeable shift in the magnetic transitions toward the high-temperature range: TC1 = 83 K and TC2 = 74 K. A ferromagnetic ordering of the P type is found in the Co3 − xFexO2BO3 (x ≈ 0.14) compound.

Analysis of the electrical and optical properties of VBO3 single crystals and Fe1−xVxBO3 solid solutions on the basis of a many-electron model of energy band structure

A many-electron model of the energy band structure of VBO3 and of Fe1−xVxBO3 solid solutions is proposed with strong electron correlations taken into account. Experimental optical absorption spectra and data on the resistivity are discussed in the framework of the suggested model. Variation in the magnetic and electronic properties of VBO3 and Fe1−xVxBO3 under high pressure is predicted. For VBO3, a Mott-Hubbard (insulator-metal) transition is expected in the high-pressure phase. In Fe1−xVxBO3 solid solutions, a nontrivial variation in the properties is predicted, leading to the appearance of a different magnetic state.

Experimental observation of the virtual electronic states of a mott-Hubbard insulator FeBO3 in infrared absorption spectra

A number of new peaks close to the previously predicted energies of virtual electronic states [S.G. Ovchinnikov and V.N. Zabluda, Zh. Eksp. Teor. Fiz. 125, 150 (2004) [JETP 98, 135 (2004)]] have been observed in the absorption spectra of FeBO3 single crystals in the infrared range. The 0.27-eV peak was absent in the scheme presented in the work cited above and, to explain it, a multielectron theory taking into account the 2T2 low-spin term of the Fe3+ ion has been developed.