N. B. Ivanova

Spin-glass behavior in single crystals of hetero-metallic magnetic warwickites MgFeBO4, Mg0.5Co0.5FeBO4, and CoFeBO4

Abstract Magnetic properties of heterometallic warwickites MgFeBO4, Mg0.5Co0.5FeBO4, and CoFeBO4 are presented, highlighting the effect of Co substitution on the magnetic properties of these compounds. The analysis of magnetization and heat capacity data has shown that these compounds exhibit a spin-glass transition below TSG=10, 20 and 22xa0K, respectively. Using zero field ac susceptibility as entanglement witness we find that the low dimensional magnetic behavior above TSG show quantum entanglement behavior χ(Τ)∝T−α(Τ) up to TE≈130xa0K. The α parameters have been deduced as a function of temperature and Co content, indicating the existence of random singlet phase in this temperature region. Above TE the paramagnetism is interpreted in terms of non-entangled spins giving rise to Curie–Weiss paramagnetism. The different intra- and inter-ribbon exchange interaction pathways have been calculated within a simple indirect coupling model. It is determined that the triangular motifs in the warwickite structure, together with the competing interactions, induce frustration. The spin-glass character is explained in terms of the substitutional disorder of the Mg, Fe and Co atoms at the two available crystallographic sites, and the frustration induced by the competing interactions. The Co substitution induces uniaxial anisotropy, increases the absolute magnetization and increases the spin-glass freezing temperature. The entanglement behavior is supported in the intermediate phase irrespective of the introduction of anisotropy by the Co substitution.

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 (25u2009K against 43u2009K 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.

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.

Uniaxial anisotropy and low-temperature antiferromagnetism of Mn2BO4 single crystal

Abstract The Mn 2 BO 4 single crystals have been grown by the flux technique. A careful study of the crystal structure and magnetic properties have been carried out. The antiferromagnetic transition at T N =26xa0K has been traced through the dc magnetization and specific heat temperature dependences. Magnetic uniaxial anisotropy has been found with easy axis magnetization lying in the ab-plane. The obtained value of effective magnetic moment is assigned to the non-quenched orbital moment of Jahn–Teller Mn 3+ ions. The discussion of magnetic properties is based on the superexchange interaction calculations.

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.

Structure and magnetism of copper-substituted cobalt ludwigite Co3O2BO3

Single crystals of Co3O2BO3 with partial (4%) substitution of Co by Cu ions were synthesized by the flux technique. X-ray diffraction and magnetic studies were carried out. The x-ray diffraction measurements show clearly that Cu preferentially occupies only one of the four nonequivalent crystallographic positions, the one with the smallest electric field gradient. The ferrimagnetic ordering near TNu2009=u200943u2009K and the high magnetic hardness in this magnetic phase are similar to those of Co3O2BO3. Copper substitution causes a small reduction in the effective magnetic moment.

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.

Crystal and local atomic structure of MgFeBO4, Mg0.5Co0.5FeBO4 and CoFeBO4: Effects of Co substitution

Single crystalline MgFeBO_4, Mg_0.5Co_0.5FeBO_4 and CoFeBO_4 have been grown by the flux method. The samples have been characterized by X-ray spectral analysis, X-ray diffraction and X-ray absorption spectroscopy. The X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra have been measured at the Fe andCoK-edges over a wide temperature range (6.5 - 300 K). The composition, the charge state and local environment of both Fe and Co atoms have been determined. The effects of Mg substitution by Co on the local structural distortions have been revealed experimentally and the M-O bond anisotropy has been found.