Crystal structure and magnetic anisotropy of ludwigite Co<Subscript>2</Subscript>FeO<Subscript>2</Subscript>BO<Subscript>3</Subscript>

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 (T _N1 ≈ 110 K, T _N2 ≈ 70 K) rather than Co₃O₂BO₃ (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.     

Effect of the diamagnetic dilution on the magnetic ordering and electrical conductivity in the Co3O2BO3: Ga ludwigite

Single crystals of cobalt ludwigite Co₃O₂BO₃ with diamagnetic substitution of Ga³⁺ 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 Co₃O₂BO₃ ludwigite. It has been noted that the pronounced magnetic anisotropy observed in the crystallographic ab plane of the original material of the Co₃O₂BO₃ composition disappears in the presence of gallium.     

Influence of Electron Delocalization on the Magnetic Properties of Iron Ludwigite Fe3O2BO3

⁵⁷Fe Mössbauer spectroscopic studies of iron ludwigite Fe₃O₂BO₃ performed between 4 and 450 K allow the discussion of magnetic spin arrangements and the dynamics of electronic configurations of iron. The observed magnetic transitions are related to charge ordering.     

Crystal structure and magnetic properties of Mn substituted ludwigite Co3O2BO3

The needle shape single crystals Co_3−x Mn_xO₂BO₃ 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=−100 K points out the predominance of antiferromagnetic interactions. Spin-glass magnetic ordering takes the onset at T_N=41 K. The crystallographic and magnetic properties of Co₃O₂BO₃:Mn are compared with the same for the isostructural analogs Co₃O₂BO₃ and CoO₂BO₃:Fe.     

Evolution of the mössbauer spectra of ludwigite Co3 − x Fe x O2BO3 with substitution of iron for cobalt

A concentration series of single crystals of iron-cobalt ludwigites Co_{3 ? x} Fe _x O₂BO₃ (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 Co₂FeO₂BO₃ is complicated due to a composition disorder in the system.     

Magnetic Ordering Dependence on Iron Ions Distribution in Cu<Subscript>2</Subscript>FeBO<Subscript>5</Subscript> Ludwigite

A comparative analysis of the copper and iron ions bonds exchange energies was conducted for various variants of orderings and distributions of iron ions among crystallographic positions in ludwigite Cu₂FeBO₅. Analysis showed that the exchange bonds of iron ions play a key role in the formation of magnetic order. The magnetic ordering strongly depends on the distribution of iron ions among the positions. In the case when the Fe³⁺ is in the same position as in Fe₃BO₅, the most favorable magnetic structure is similar to the magnetic structure of ludwigite Fe₃BO₅. In other cases, the type of magnetic ordering is different.     

Magnetoelectric characterization of xNi0.75Co0.25Fe2O4-(1−x)BiFeO3 nanocomposites

Magnetoelectric (ME) nanocomposites containing Ni_0.75Co_0.25Fe₂O₄-BiFeO₃ phases were prepared by citrate sol-gel process. X-ray diffraction (XRD) analysis showed phase formation of xNi_0.75Co_0.25Fe₂O₄-(1−x)BiFeO₃ (x=0.1, 0.2, 0.3 and 0.4) composites on heating at 700 °C. Transmission electron microscopy revealed the formation of powders of nano order size and the crystal size was found to vary from 30 to 85 nm. Dispersion in dielectric constant (ε) and dielectric loss (tan δ) in the low-frequency range have been observed. It is seen that nanocomposites exhibit strong magnetic properties and a large ME effect. On increasing Ni_0.75Co_0.25Fe₂O₄ contents in the nanocomposites, the saturation magnetization (M_S) and coercivity (H_C) increased after annealing at 700 °C. The large ME output in the nanocomposites exhibits strong dependence on magnetic bias and magnetic field frequency. The large value of ME output can be attributed to small grain size of ferrite phase of nanocomposite being prepared by citrate precursor process.     

An investigation of FeCoCrO4 by Mossbauer spectroscopy

The spinel FeCoCrO₄ has been studied between 4.2 and 538°K. Characteristic Mossbauer spectra of paramagnetic, magnetic and electronic relaxation types have been observed. The Mossbauer parameters for Fe³⁺ ions situated at tetrahedral (A) and octahedral (B) sites have been calculated. The cation distribution in magnetic and paramagnetic phases is found to be approximately Fe_0.5³⁺Co_0.5²⁺[Co_0.5²⁺Fe_0.5³⁺Cr³⁺]O₄. The Neel temperature been determined by the temperature scanning method to be 310±5°K.     

Mossbauer, infrared and magnetic susceptibility studies of iron sodium borate glasses doped by sulphur

The affect of sulphur on the structural properties of iron sodium diborate glasses having the composition {(100−x)Na₂B₄O₇+xFe₂O₃}+yS, where x=0.05, 0.15 and 0.25 mol% and Y=0, 2.5 and 5 wt% was studied by infrared, Mossbauer spectroscopy and magnetic susceptibility measurements. It was found that, for samples having 5 mol% Fe₂O₃ and free from sulphur, the iron ions are present in both Fe²⁺ and Fe³⁺ states and also 92% of the total iron enters the glass network as a glass former. The ratio of Fe³⁺/Fe²⁺ increases with increasing the iron content for sulphur-free samples and others containing sulphur. This ratio also decreases with increasing the sulphur content. The magnetic susceptibility was found to decrease with increasing the sulphur content. Also, the increase of Fe₂O₃ content led to a less symmetrical environment of Fe³⁺ ions and vice versa for the Fe²⁺ environment.     

Mössbauer effect study in Fe<Subscript>1− <Emphasis Type="Italic">x</Emphasis></Subscript>V<Subscript>x</Subscript>BO<Subscript>3</Subscript> solid solutions

The Mössbauer effect in the Fe_{1? x}V_xBO₃ solid solutions has been measured at 130 and 300 K. The Fe_0.05V_0.95BO₃ 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 H_hf 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.     

Specific features of magnetic states of impurity iron ions in the perovskite La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>Co<Subscript>0.98</Subscript> <Superscript>57</Superscript>Fe<Subscript>0.02</Subscript>O<Subscript>3</Subscript>

Single-phase polycrystalline La_0.75Sr_0.25Co_0.98⁵⁷Fe_0.02O₃ samples have been prepared by solidstate ceramic technology. The samples have the rhombohedral structure (space group \(R\bar 3c\)). The studies of perovskite La_0.75Sr_0.25Co_0.98⁵⁷Fe_0.02O₃ by Mössbauer spectroscopy on impurity ⁵⁷Fe nuclei in the temperature range of 5–293 K have revealed the existence of a superparamagnetic relaxation in the temperature range of 100–210 K. The parameters of hyperfine interactions (hyperfine magnetic fields, line shifts, and quadrupole shifts) and the anisotropy energy have been measured, and the frequencies of magnetic moment relaxation of iron ions have been estimated.     

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

Ludwigite single crystals of compositions Co₃O₂BO₃ and Co_{3 − x} Fe _x O₂BO₃ (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 T _C1 = 43 K and T _C2 = 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: T _C1 = 83 K and T _C2 = 74 K. A ferromagnetic ordering of the P type is found in the Co_{3 − x} Fe _x O₂BO₃ (x ≈ 0.14) compound. Original Russian Text ? N.V. Kazak, N.B. Ivanova, V.V. Rudenko, A.D. Vasil’ev, D.A. Velikanov, S.G. Ovchinnikov, 2009, published in Fizika Tverdogo Tela, 2009, Vol. 51, No. 5, pp. 916–919.     

Current controlled negative differential resistance behavior in Co2FeO2BO3 and Fe3O2BO3 single crystals

I–V curves showing negative differential resistance (NDR) are reported for single crystals of Co₂FeO₂BO₃ at 315 K and 290 K and for Fe₃O₂BO₃ at 300 K, 260 K and 220 K. Resistivity measurements are presented for both systems, parallel and perpendicular to the c axis, in the range 315–120 K. The high hysteretic behavior of the I–V curves in Co₂FeO₂BO₃ around room temperature is discussed and the heat dissipated is estimated, suggesting an increase in the sample temperature of almost 22 K for the I–V curve at 315 K and a dominant contribution of Joule self-heating for the observed NDR. In contrast, insignificant hysteresis is observed on the I–V curves of Fe₃O₂BO₃ around room temperature. The depinning of charge order domains is suggested as the main contribution to the NDR phenomenon for Fe₃O₂BO₃. The high reproducibility of the NDR in the Fe₃O₂BO₃ single crystal allows its use as a low frequency oscillator, as it is demonstrated.     

Magnetic resonance study of phases in FeVO4-Co3V2O8 system

Multicomponent vanadates Co_3+xFe_4−xV₆O₂₄ have been synthesized using the solid state reaction method from Co₃V₂O₈ and FeVO₄.oxides. The electron paramagnetic resonance/ferromagnetic resonance (EPR/FMR) spectra of 20 samples containing solid state phases formed in the FeVO₄-Co₃V₂O₈ system have been recorded at room temperature. The howardevansite structure (H-type phase) is produced, which corresponds to the Co_2.616Fe_4.256V₆O₂₄ formula while a homogeneity range of lyonsite (L-type phase) type structure could be described by the Co_3+1.5xFe_4−xV₆O₂₄ formula (0.476<x<1.667). Considering the values of g-factor and linewidth of each registered spectrum the existence of three types of magnetic centers was inferred and correlated with phases detected by XRD method.     

Mössbauer and optical investigation of Co3−x Fe x O4 thin films grown by sol–gel process

Structural transformation and the related variation in magnetic and optical properties of Co_3?x Fe _x O₄ thin films grown by a sol–gel method have been investigated as the Fe composition varies up to x?=?2. The normal spinel phase is dominant below x?=?0.55 and the inverse spinel phase grows as x increases further. Conversion electron Mössbauer spectroscopy (CEMS) measurements indicate that the normal spinel phase have octahedral Fe³⁺ ions mostly while the inverse spinel phase contain octahedral Fe²⁺ and tetrahedral Fe³⁺ ions. For higher Fe composition (x?>?1.22), Co²⁺ ions are found to substitute the octahedral Fe²⁺ sites. The measured optical absorption spectra for the Co_3?x Fe _x O₄ films by spectroscopic ellipsometry support the CEMS interpretation.     

Analysis of magnetic disaccommodation in La-Co-substituted strontium ferrites

M-type strontium ferrites substituted by La³⁺-Co²⁺(Sr_1−xLa_xFe_12−xCo_xO₁₉) were prepared by ceramic process. Effects of the substituted amount of La³⁺ and Co²⁺ on structure and magnetic properties of Sr_1−xLa_xFe_12−xCo_xO₁₉ compounds have systematically been investigated by X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and magnetic disaccommodation. In the measurement range from 80 to 500 K, the magnetic disaccommodation is represented by means of isochronal curves. It is well known that magnetic disaccommodation cannot be obviously found in the M-type of pure strontium ferrites. However, three peaks were observed in Sr_1−xLa_xFe_12−xCo_xO₁₉, and this behavior is explained in terms of the presence of Fe²⁺ cation and to the site occupation by the magnetic Co²⁺ ionic within the hexagonal structure.     

Mössbauer spectrometric study of ternary Sm2Fe17Nx(x>3)

Among the ternary rare earth-iron-nitrogen compounds, Sm₂Fe₁₇N_x, invented and refined by Asahi Chemical Industry Co., Ltd., shows high magnetic anisotropy, intrinsic coercivity and high saturation magnetization. Powders of Sm₂Fe₁₇, prepared by are melting method, were nitrided in a mixed gas of hydrogen and ammonia to get x=0≈8 samples /1.2/. These samples have crystal structure of Th₂Zn₁₇ and Fe atoms are estimated to locate in 6c, 9d, 18f, and 18h sites /2.3/. The Mossbauer spectrum of Sm₂Fe₁₇N_{3. z} showed several unique magnetic properties. For example, Q.S. due to Fe occupying 18h site shifted to the direction of minus velocity. The result indicates that the nitrogen sites are not only 9e but 3b. CEM spectra of Sm₂Fe₁₇N_x powders showed the doublet due to iron oxide indicating that the surface of Sm₂Fe₁₇N_x powders were oxidised /4/. but even if they were exposed to strongly oxidative atmosphere, the thickness of the oxidised layers increased little, resulting no effect to the magnetic properties and to the Mossbauer magnetic parameters of the bulk.     

Magnetic properties of rare earth iron borates: Spectroscopic investigation by the method of rare earth probe

The magnetic phase transitions and magnetic structures in RFe₃(BO₃)₄ (R = Y, Gd-Er) iron borates have been investigated by the method of erbium spectroscopic probe. The magnetic ordering temperatures have been determined. On the basis of the comparison of the character of splitting of the spectral lines of the probe Er³⁺ ion in RFe₃(BO₃)₄(R = Y, Dy-Er) iron borates and in GdFe₃(BO₃)₄, a complicated whose magnetic structure is known, a conclusion is drawn about the orientation of the magnetic moments of iron: in dysprosium and terbium iron borates, an easy-axis magnetic structure is implemented, whereas an easy-plane structure occurs in holmium, erbium, and yttrium iron borates.     

Magnetic and Magnetodielectric Properties of Ho<Subscript>0.5</Subscript>Nd<Subscript>0.5</Subscript>Fe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

The magnetic and magnetodielectric properties of Ho_0.5Nd_0.5Fe₃(BO₃)₄ ferroborate with the competing Ho–Fe and Nd–Fe exchange couplings have been experimentally and theoretically investigated. Step anomalies in the magnetization curves at the spin-reorientation transition induced by the magnetic field B ║ c have been found. The spontaneous spin-reorientation transition temperature T_SR ≈ 8 K has been refined. The measured magnetic properties and observed features are interpreted using a single theoretical approach based on the molecular field approximation and calculations within the crystal field model of the rare-earth ion. Interpretation of the experimental data includes determination of the crystal field parameters for Ho³⁺ and Nd³⁺ ions in Ho_0.5Nd_0.5Fe₃(BO₃)₄ and parameters of the Ho–Fe and Nd–Fe exchange couplings.     

57Fe and 151Eu Mössbauer studies of magnetoresistive Europium based cobalt perovskites

Eu_0.8Sr_0.2Fe _x Co_1?x O_3?z CMR perovskites with different iron concentrations (x?=?0, 0.025, 0.075, 0.15, 0.3) were investigated by X-ray diffraction, AC magnetic susceptibility, magnetotransport, as well as ⁵⁷Fe and ¹⁵¹Eu Mössbauer spectrometry. The valence state of europium ions was found to be trivalent, independently of the iron concentration. ⁵⁷Fe Mössbauer spectra and magnetic susceptibility of the investigated perovskites presented complementary results for the magnetic transitions.