N. V. Kazak

Crystal structure and magnetic anisotropy of ludwigite Co2FeO2BO3

Co3O2BO3 and Co2FeO2BO3 single crystals with a ludwigite structure are fabricated, and their crystal structure and magnetic properties are studied in detail. Substituted ludwigite Co2FeO2BO3 undergoes two-stage magnetic ordering at the temperatures characteristic of Fe3O2BO3 (TN1 ≈ 110 K, TN2 ≈ 70 K) rather than Co3O2BO3 (TN = 42 K). This effect is explained in terms of preferred occupation of nonequivalent crystallographic positions by iron, which was detected by X-ray diffraction. Both materials exhibit a pronounced uniaxial magnetic anisotropy. Crystallographic direction b is an easy magnetization axis. Upon iron substitution, the cobalt ludwigite acquires a very high magnetic hardness.

Low-temperature magnetic behavior of the rare-earth cobaltites GdCoO3 and SmCoO3

The temperature and magnetic field dependences of the static magnetization of the polycrystalline rare-earth cobaltites GdCoO3 and SmCoO3 have been measured. It is shown that, below room temperature, the magnetization of both compounds derives primarily from the rare-earth ion paramagnetism. The GdCoO3 and SmCoO3 compounds have been found to differ substantially in magnetic behavior, which can be traced to differences in their electronic shell structures. The magnetic behavior of GdCoO3 is close to that of an array of free Gd3+ ions, whereas in SmCoO3 the deviation from the free-ion properties is very large because of the Sm3+ ground state being crystal-field split. Van Vleck magnetic susceptibility measurements of SmCoO3 suggest that the splitting is ∼10 K.

Effect of strontium and barium doping on the magnetic state and electrical conductivity of GdCoO3

A coordinated investigation of the magnetic and electrical properties of polycrystalline cobalt oxide compounds CdCoO3, Gd0.9Ba0.1CoO3, and Gd0.9Sr0.1CoO3 is carried out. Undoped GdCoO3 reveals a low conductivity; a magnetic moment of 7.4 μB per molecule, which is less than the theoretical value for the Gd3+ ion; and an asymptotic Curie temperature of −6 K. Doping GdCoO3 with barium and strontium to substitution of 10 at. % Gd brings about an increase in the conductivity and magnetic transitions at T = 300 K for Gd0.9Ba0.1CoO3 and T = 170 K for Gd0.9Sr0.1CoO3. The magnetization anomalies imply the formation of magnetic clusters. The behavior of the electrical conductivity at high temperatures suggests a variable activation energy. At low temperatures, Mott hopping conduction sets in.

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.

Spin-glass magnetic ordering in CoMgGaO2BO3 ludwigite

Single crystal, needle shaped samples of diamagnetically diluted cobalt ludwigite CoMgGaO2BO3 have been grown by the flux method. X-ray diffraction and both dc and ac magnetic measurements are described. The unit cell volume changes significantly with dilution, from 328.31 A3 for the parent compound Co3O2BO3 to 345.46 A3 for CoMgGaO2BO3. The magnetic transition temperature is considerably lower for the latter compound (25 K against 43 K for Co2O2BO3). The dc magnetization temperature dependences are split between the field cooled and zero-field cooled regimes and the ac magnetic susceptibility temperature curves are frequency dependent, which indicates possible spin-glass freezing in the magnetic system.

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.

Crystal structure and magnetization of a Co3B2O6 single crystal

The crystal structure and magnetic properties of Co3B2O6 single crystals are studied. Orthorhombic symmetry with space group Pnnm is detected at room temperature. The measurements of static magnetization and dynamic magnetic susceptibility reveal two magnetic anomalies at T1 = 33 K and T2 = 10 K and an easy-axis magnetic anisotropy. The effective magnetic moment indicates a high-spin state of the Co2+ ion. A spin-flop transition is found at low temperatures and Hsf = 23 kOe. EXAFS spectra of the K-edge absorption of Co are recorded at various temperatures, the temperature-induced changes in the parameters of the local environment of cobalt are analyzed, and the effective Co-Co and Co-O distances are determined. The magnetic interactions in the crystal are analyzed in terms of an indirect coupling model.

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.

Antiferromagnetic ordering in REM cobaltite GdCoO 3

Temperature and magnetic-field dependences of the static magnetization of polycrystalline cobaltite GdCoO3 have been measured. The magnetic properties of the GdCoO3 sample have been studied in the paramagnetic and antiferromagnetic states. The magnetic phase diagram has been constructed. The exchange field between the Gd-Gd sublattices and the anisotropy field have been estimated.

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.