The investigation of mixed Co–Fe ludwigite single crystals shows that their magnetic properties are close to the magnetic properties of Fe 3 BO 5 despite the predominance of cobalt ions. The magnetic properties of Co 3 – x Fe x BO 5 single crystals with x = 0.10 are studied in detail. Magnetometric measurements demonstrate a strong magnetic anisotropy with easy magnetization axis b , and the orbital magnetic moment of cobalt is in a frozen state. The detected temperature dependence of the absorption of Mössbauer spectra allowed us to determine the magnetic ordering temperature, which agrees with the results of magnetization measurements ( T C = 84 K).

Single-crystalline MgFeBO4, Mg0.5Co0.5FeBO4, and CoFeBO4 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 and Co K-edges over a wide temperature range (6.5-300K). The composition, the charge state, and local environment of both Fe and Co atoms have been determined. The effects of Co substitution for Mg on the local structural distortions have been revealed experimentally and the MO bond anisotropy has been found.

•The electrical resistance of the single crystalline warwickites has been measured.•The temperature-induced changes in the charge transfer mechanisms have been found.•The microscopic parameters of the electronic structure have been determined.•The studied warwickites have been classified as disordered correlated systems.•The Co substitution was found to lead to the weakening of the localization.

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 22 K, 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≈130 K. 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.

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 22 K, respectively. Using zero field ac susceptibility as entanglement witness we find that the low dimensional magnetic behavior above TSG show quantum entanglement behavior T   up to TE ≈ 130K. The  parameters have been deduced as a function of temperature and Co, 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 intraand 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 along the b axis, 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.

The Mn2BO4 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 TN=26K 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 Mn3+ ions. The discussion of magnetic properties is based on the superexchange interaction calculations.

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.

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 TN = 43 K 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.

A concentration series of single crystals of iron-cobalt ludwigites Co 3 − x Fe x O 2 BO 3 ( x = 0.0125, 0.025, 0.050, 0.10, 1.0) has been synthesized. The structure has been studied using X-ray diffraction and Mössbauer effect. A preferred occupation of nonequivalent crystallographic positions by iron in the ludwigite structure has been revealed. It has been found that the valence of substituting iron ions is three. It has been revealed that the structure of the γ-resonance spectrum of Co 2 FeO 2 BO 3 is complicated due to a composition disorder in the system.

The temperature dependences of the X-ray absorption spectra (XAS) and of the spectra of X-ray magnetic circular dichroism (XMCD) are measured near the L (3,2) absorption edges of Co and Fe in ludwigite Co2FeBO5 single crystals. The antiparallel orientation of the magnetic moments of cobalt and iron is demonstrated. The coercive fields related to cobalt and iron ions are determined. The orbital (m (l) ) and (m (s) ) spin contributions to the total magnetic moments of cobalt and iron ions are identified. The ratios and relative directions of m (l) and m (s) are found.

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 T 1 = 33 K and T 2 = 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 H sf = 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.

Single crystals of cobalt ludwigite Co 3 O 2 BO 3 with diamagnetic substitution of Ga 3+ ions for a part of the cobalt ions have been grown by the flux method. A detailed investigation of the crystal structure and magnetic properties of the compound has been carried out. A preferred character of the occupation of nonequivalent crystallographic positions by gallium has been revealed. It has been found that the effective magnetic moment and the magnetic ordering temperature are decreased compared to those in the original crystal of the Co 3 O 2 BO 3 ludwigite. It has been noted that the pronounced magnetic anisotropy observed in the crystallographic ab plane of the original material of the Co 3 O 2 BO 3 composition disappears in the presence of gallium.

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 Å3 for the parent compound Co3O2BO3 to 345.46 Å3 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.

The crystal structure, cation distribution and exchange interactions in the Co2.25Fe0.75O2BO3 ludwigite have been explored through X-ray diffraction, electrical resistivity, ac-susceptibility and Mossbauer effect measurements. The crystal data have shown that iron atoms occupy the most symmetric crystallographic sites Fe4 and Fe2. The complex magnetic behavior with two magnetic transitions near 70 and 115 K at low temperatures was found. The Mossbauer data have displayed the trivalent iron states only. The values of superexchange energies have been estimated for Co3O2BO3 and Co2.25Fe0.75O2BO3 yielding a significant role of frustrations in the ludwigite magnetic system. Variable range Mott hopping conductivity law was proved to be valid in the wide temperature region, pointing out a localized character of charge carriers rather than collective.

Co 3 O 2 BO 3 and Co 2 FeO 2 BO 3 single crystals with a ludwigite structure are fabricated, and their crystal structure and magnetic properties are studied in detail. Substituted ludwigite Co 2 FeO 2 BO 3 undergoes two-stage magnetic ordering at the temperatures characteristic of Fe 3 O 2 BO 3 ( T N 1 ≈ 110 K, T N 2 ≈ 70 K) rather than Co 3 O 2 BO 3 ( T N = 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.

The needle shape single crystals Co3−x MnxO2BO3 with ludwigite structure have been prepared. According to the X-ray diffraction data the preferable character of distinct crystallographic positions occupation by Mn ions is established. Magnetization field and temperature dependencies are measured. Paramagnetic Curie temperature value Θ=−100K points out the predominance of antiferromagnetic interactions. Spin-glass magnetic ordering takes the onset at TN=41K. The crystallographic and magnetic properties of Co3O2BO3:Mn are compared with the same for the isostructural analogs Co3O2BO3 and CoO2BO3:Fe.

J. Bartolomé,1 A. Arauzo,2 N. V. Kazak,3 N. B. Ivanova,4,5 S. G. Ovchinnikov,3,6 Yu. V. Knyazev,4 and I. S. Lyubutin7 1Instituto de Ciencia de Materiales de Aragón and Departamento de Fı́sica de la Materia Condensada, CSIC–Universidad de Zaragoza, 50009 Zaragoza, Spain 2Servicio de Instrumentación Cientı́fica–Área de Medidas Fı́sicas, Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain 3L.V. Kirensky Institute of Physics, SB of RAS, 660036, Akademgorodok, Krasnoyarsk, Russia 4Siberian Federal University, 660074, Kirensky street 26, Krasnoyarsk, Russia 5Krasnoyarsk State Agrarian University, Mira street 90, Krasnoyarsk, Russia 6Siberian State Aerospace University, Krasnoyarskiy Rabochiy street 31, Krasnoyarsk, Russia 7Shubnikov Institute of Crystallography, RAS, 119333, Leninskiy prospect 59, Moscow, Russia (Received 14 December 2010; published 26 April 2011)

Magnetic and Mossbauer spectroscopy (MS) measurements have been performed on a single crystal of Co2.25Fe0.75O2BO3 with ludwigite structure. Two magnetic transitions (T-N = 115 K and T-C = 70 K) were traced from the ac susceptibility temperature dependence. The MS spectra as a function of temperature clearly show the onset of magnetic ordering below 115 K. Magnetization measurements on the parent Co3O2BO3 and Fe3O2BO3 compounds have been done for comparison. In Fe3O2BO3 the anisotropy of the different phases has been determined, showing that the anisotropy axis changes from the a to the b axis in the low-temperature antiferromagnetic phase. High magnetic uniaxial anisotropy has been detected for both Co3O2BO3 and Co2.25Fe0.75O2BO3. From the angle-dependent magnetization measurements it is found that in both compounds the easy axis of magnetization is the b [010] axis, where an antiferromagnetic component is superimposed on the main ferromagnetic component. In the c direction the behavior is purely antiferromagnetic. In Co2.25Fe0.75O2BO3 a strong reduction of the remanent magnetization and a very strong increase in coercive field along the b axis with respect to those found in Co3O2BO3 were observed from magnetic hysteresis cycles measured below T-C. The increase of coercive field is caused by the increase of defects upon Co substitution by Fe.