Magnetic and structural investigations of GdMn2-GdNi2 intermetallic compounds

Magnetic and structural properties of the intermetallic system Gd(Mn_1-xNi_x)₂ were investigated. The 0?x?0.4 and 0.7<x?1 compounds are Laves phases of the MgCu₂ type. The remaining compounds 0.4?x?0.7. crystallize with the hexagonal structure. For single crystal GdMn_0.8Ni_1.2 the P6₃/mmc space group and the lattice parameters a=5.175 Å and c=16.731 Å are obtained. The magnetic properties of investigated series are investigated in stationary and in pulsed magnetic fields in the temperature range 2.6–700 K. For 0.4?x?0.8 two magnetic transitions are observed. In the paramagnetic range the χ^-1_M (T) is described by a Néel type variation and the molecular field coefficients N_{i j} are calculated. The localized character of the magnetic moment of Mn is observed.     

Mössbauer study of fluorides: Ba2MFeF9 and NaBaFe2F9

The Mössbauer spectra of the compounds Ba₂NiFeF₉, Ba₂FeCrF₉ and NaBaFe₂F₉ have been studied as a function of temperature. Values of the Néel temperature are obtained and the effects of cationic inversion between the two sites of M^II and M^III in compounds Ni^IIFe^III and Fe^IICr^III are observed. In the latter compound, we observe broad lines at all temperatures and a smearing of the magnetic ordering temperature. However, the Fe^IIFe^III compound shows strict structural order. The two sites of Fe^III in NaBaFe₂F₉ have not been resolved.     

Etude par spectroscopie Mössbauer d'une serie de ferrites mixtes de terre rare et d'alcalinoterreux: SrLn2Fe2O7Mössbauer spectroscopy study of a series of mixed alkaline-earth and rare-earth ferrites: SrLn2Fe2O7

The room-temperature magnetic structure of some mixed alkaline-earth and rare-earth ferrites, with the formula Sr(Ba)Ln₂Fe₂O₇, consists of a pattern perpendicular to the c tetragonal axis, either along the [110] or the [1$\text{1}$0] direction. One of the G_x^? ± A_y^? magnetic modes is obtaine for SrTb₂Fe₂O₇ and one of the A_x^? ± G_y^? modes is obtained for SrNd₂Fe₂O₇ and for SrPr₂Fe₂O₇. This ambiguity is removed by the use of powder and single crystal Mössbauer spectroscopy, which also allows us to determine the magnetic structures of all other members of the series; at room temperature the magnetic moments are aligned along the [110] diagonal for all compounds. In lowering the temperature to 4.2 K, different magnetic moment reorientations have been observed. In SrTb₂Fe₂O₇ and SrNd₂Fe₂O₇ the magnetic moments go from the basal plane to along the c axis, whereas in BaLa₂Fe₂O₇ and SrGd₂Fe₂O₇ they rotate in the basal plane itself, the rotation angle being π/4 and π/2, respectively.     

Coexistence of paramagnetism and ferromagnetism in Fe-TiO2 nanoparticle

Magnetic properties of pure and Fe doped rutile TiO₂ and TiO_2-ε are investigated using the first principle density functional theory. The results show that the considered systems are ferromagnetic. Furthermore, the origin of ferromagnetism is discussed and it is found that the double exchange and super-exchange are the main interactions in these compounds. Based on the calculations, the magnitude of the magnetic moment depends on the concentration of impurities and oxygen vacancies and the largest magnetic moment corresponds to the Fe_xTi_1?xO_2?ε. Moreover, using a model based on the bound magnetic polarons, the coexistence of ferromagnetic and paramagnetic phases can occur in Fe_xTi_1?xO₂ containing different impurity ions such as Fe⁺² and Fe⁺³ with different Curie temperatures. The finding may presents the potential application of the considered system as diluted magnetic semiconductor.     

The spin glass-like behaviour of the glassy (Sb2S3)x(SbI3)yFez

Low field magnetic susceptibility of a glassy semiconductor (Sb₂S₃)_x(SbI₃)_yFe_z reveals a spin glass-like behaviour in spite of the sample not being metallic. The critical temperature is observed to depend on the concentration of the iron atoms (T_M = 0.179z^0.76).     

Crystal structure and structural transition caused by charge-transfer phase transition for iron mixed-valence complex (n-C3H7)4N[FeFe(dto)3] (dto=C2O2S2)

(n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] shows a new type of first order phase transition called charge-transfer phase transition around 120 K, where the charge transfer between Fe^II and Fe^III occurs reversibly. Recently, we have succeeded in obtaining single crystals of the title complex and determined the crystal structure at room temperature. Crystal data: space group P6₃, Z=2. Moreover, we have investigated the structural transition caused by the charge-transfer phase transition by means of powder X-ray diffraction measurement. When the temperature is decreased, the a-axis, which corresponds to the hexagonal ring size in two-dimensional honeycomb network structure of [Fe^IIFe^III(dto)₃]_∞, contracts by 0.1 Å at the charge-transfer transition temperature (T_CT), while the c-axis, perpendicular to the honeycomb network layer, elongates by 0.1 Å at T_CT. Consequently, when the temperature is decreased, the unit cell volume decreases without noticeable anomaly around T_CT, which is responsible for the quite small vibrational contribution to the entropy change, compared with usual spin crossover transition. Thus, the charge-transfer phase transition around 120 K for (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] is regarded as spin entropy driven phase transition.     

Multiferroic properties of solben gel derived Bi5Fe1-xCoxTi3O15 thin films

Co-doped Bi₅FeTi₃O₁₅ thin films (BFCT-x, Bi₅Fe_1-xCo_xTi₃O₁₅) were prepared using a sol—gel technique. XRD patterns confirm their single phase Aurivillius structure, and the corresponding powder Rietveld analysis indicates the change of space group around x=0.12. The magnetic hysteresis loops are obtained and ferromagnetism is therefore confirmed in BFCT-x thin films. The remanent magnetization (M_r) first increases and reaches the maximum value of 0.42 emu/cm³ at x=0.12 due to the possible Fe³⁺—O—Co³⁺ ferromagnetic coupling. When x = 0.25, the M_r increases again because of the dominant Fe³⁺—O—Co³⁺ ferromagnetic coupling. The remanent polarization (2P_r) of BFCT-0.25 was measured to be as high as 62 μC/cm², a 75% increase when compared with the non-doped BFCT-0 films. The 2P_r remains almost unchanged after being subjected to 5.2 × 10⁹ read/write cycles. Greatly enhanced ferroelectric properties are considered to be associated with decreased leakage current density.     

Magnetoelectric and Mössbauer studies of Fe2 TeO6

The magnetic symmetry of antiferromagnetic Fe₂TeO₆ indicates that this material should exhibit magnetoelectricity. This prediction has been confirmed by the observation of the electrically induced magnetoelectric (ME) effect in powder samples. Both parallel and perpendicular ME susceptibilities were measured as a function of increasing temperature. The ME effect vanishes at 209°K which is identified as the Néel point of the compound. The single crystal ME susceptibilities are derived from the powder results. The maximum value of the axial ME susceptibility α₃₃ = M₃/E₃ is (in Gaussian units) 3 × 10⁻⁵ at T = 175°K. In addition to magnetoelectricity the magnetic symmetry of Fe₂ TeO₆ indicates that no hyperfine field should exist at the Te⁶⁺ sites. This prediction was confirmed by Mössbauer studies on I¹²⁹ produced in Te⁶⁺ sites by irradiating samples of Fe₂ TeO₆ in a reactor.     

Far infrared absorption of Fe2+ in KMgF3

Far infrared absorption in cubic KMgF₃ doped with Fe²⁺ is reported . A line is observed at 87cm^?1, which is assigned to the (Γ_5g → Γ_3g, Γ_4g) transition in the Fe²⁺. The reduction in the spin-orbit coupling from the free ion value has been predicted by Ham, Schwarz and O'Brien.     

Mössbauer study of amorphous BaO−Fe2O3−B2O3 system

HomogeneousBaO?Fe₂O₃?B₂O₃ glasses containing maximum Fe₂O₃ content of 63 mol.% are prepared by splat cooling technique. Mössbauer study reveals that the glass mainly consists of tetrahedral network of Fe³⁺O₄. Mössbauer spectrum of the glass shows a well defined hyperfine structure at low temperatures. Magnetic ordering temperature estimated is about 130 K for the most iron-rich specimen, being much lower than that of the corresponding crystalline phases, BaO;·;Fe₂O₃ and BaO·;2Fe₂O₃. The magnetic structure is suggested to be of a short-range antiferromagnetic ordering.     

High temperature ferromagnetism of the vacuum-annealed (In1−xFex)2O3 powders

(In_1−xFe_x)₂O₃ (x = 0.02, 0.05, 0.2) powders were prepared by a solid state reaction method and a vacuum annealing process. A systematic study was done on the structural and magnetic properties of (In_1−xFe_x)₂O₃ powders as a function of Fe concentration and annealing temperature. The X-ray diffraction and high-resolution transmission electron microscopy results confirmed that there were not any Fe or Fe oxide secondary phases in vacuum-annealed (In_1−xFe_x)₂O₃ samples and the Fe element was incorporated into the indium oxide lattice by substituting the position of indium atoms. The X-ray photoelectron spectroscopy revealed that both Fe²⁺ and Fe³⁺ ions existed in the samples. Magnetic measurements indicated that all samples were ferromagnetic with the magnetic moment of 0.49-1.73 μ_B/Fe and the Curie temperature around 783 K. The appearance of ferromagnetism was attributed to the ferromagnetic coupling of Fe²⁺ and Fe³⁺ ions via an electron trapped in a bridging oxygen vacancy.     

Wing profile measurements of the Na-doublet lines in C2H2/O2/N2, H2/O2/N2 and H2/O2/Ar frames at 1 atm

Fluorescence-excitation (wing) profiles of the Na-D doublet lines were measured over a wavelength range extending from 0.3 to 200 Å from the line center for the red D₁ and blue D₂ wings and from 0.3 to 3 Å for the red D₂ and the blue D₁ wings, respectively. The line profiles were determined with the aid of a tunable CW dye-laser as a background source by measuring the total fluorescence intensity observed on detuning the laser wavelength. The flames were premixed, laminar, shielded flames at 1 atm, with temperatures ranging from 1860 to 2270 K; N₂ and Ar served as diluent gases. The line core and near-wing profiles (i.e. the region covering 0.3<Δλ<7 Å for the outer wings and 0.3<Δλ<3 Å for the inner ones) in all of the flames studied appeared to have the same frequency dependence, regardless of the nature and concentrations of the gases used. The blue D₂-line profile followed an unexpected (-2.2) law, while the other three profiles obeyed the theoretically expected (-2) law (the dispersion profile function). The line profile in the Δλ range between the impact and quasistatic regions was found to depend on the main perturbers involved. We found that the far blue D₂- and red D₁-wings in the Ar-diluted H₂/O₂ flame obeyed the (-$\text{5}\text{4}$) and (-$\text{3}\text{2}$) laws, respectively, as predicted by the quasi-static theory for the Lennard-Jones interaction. For the N₂-diluted C₂H₂/O₂ and H₂/O₂ flames, we did not find these wing dependences in the Δλ range investigated.     

The magnetization of GdAl2

The magnetization of a single crystal of GdAl₂ has been measured parallel to the easy direction as a function of field (maximum field 1.7 T) within the temperature range 4.2–300 K. The main emphasis was placed on the results obtained for the ferromagnetic phase. From an analysis based on molecular field theory it is deduced that the magnetic moment at 0 K is 7.2 μ_B per Gd ion and that the molecular field cannot be represented by a simpler polynomial than λ₁M + λ₂M³ + λ₃M⁵. The same data is analysed using spin-wave theory from which it is deduced that the spin-wave stiffness is 18 meV Ų and that the conduction band susceptibility is approximately 2.6 x 10^-6 emu g^-1. The conduction electron polarization, parallel to the Gd ion moment, amounting to 0.2 μ_B per Gd ion implies the presence of an internal field acting on the conduction electrons of approximately 200 T at 0 K.     

ESR study of Fe3+ and Mn2+ ions on trigonal sites in ilmenite structure MgTiO3

Electron spin resonance has been observed for Fe³⁺ and Mn²⁺ ions occupying sites with trigonal symmetry in undoped and doped Verneuil-grown crystals of the ilmenite type compound MgTiO₃. At 300 K, the fine structure parameters in the spin Hamiltonian are (in 10^?4cm^?1) D = +844 (± 1), (a? F) = +118 (± 1), a = 69 (± 7) for Fe³⁺ and D = +164 (± 1), (a ? F) = +10.2 (± l), a = 7.0 (± 1) for Mn²⁺. These values are compared with literature data for Fe³⁺ and Mn²⁺ in other oxides, especially Al₂o₃, with particular reference to the recent “superposition” theory of the effect of a trigonal distortion. From the orientation of the axes of cubic pseudosymmetry of the spin Hamiltonian, and with the assumption that a has the same sign for both ions, it is proposed that Fe³⁺ and Mn²⁺ occupy the same octahedral site, namely the Mg²⁺ site. Anomalous line splittings observed for one sample were attributed to twinning on (0001) or {1120} planes.     

EPR and magnetic susceptibility studies of iron ions in ZnO-Fe2O3-SiO2-CaO-P2O5-Na2O glasses

Room temperature electron paramagnetic resonance (EPR) spectra and temperature dependent magnetic susceptibility data have been obtained on bulk x(ZnO,Fe₂O₃)(65−x)SiO₂20(CaO, P₂O₅)15Na₂O (6≤x≤21 mole%) glasses prepared by melt quenching method. EPR spectra of the glasses revealed absorptions centered at g≈2.1 and 4.3. The variations of the intensity and line width of these absorption lines with composition have been interpreted in terms of the variation in the concentration of the Fe²⁺ and Fe³⁺ ions in the glass and the interaction between the iron ions. EPR and magnetic susceptibility data of the glasses reveal that both Fe²⁺ and Fe³⁺ ions are present in the glasses, with their relative concentration being dependent on the glass composition. The studies reveal superexchange type interactions in these glasses, which are strongly dependent on their iron content.     

Short range order in (Ni65Fe35)77B23 metallic glass by neutron diffraction

The S(Q) structure factor of (Ni₆₅Fe₃₅)₇₇B₂₃ metallic glass was measured by time-of-flight neutron diffraction up to 24 Å^?1 momentum transfer. The distribution of transition metal-boron and transition metal-transition metal first neighbour atom pairs is resolved in the distribution function obtained from S(Q) by Fourier transformation. The distances of these first neighbours pairs, the width of their distribution, the partial coordination numbers and the short range order parameter are given. The results on (Ni₆₅Fe₃₅)₇₇B₂₃ and on the previously studies Fe₈₁B₁₉ amorphous alloys are compared.     

Crystal structure and ferromagnetism of (n-C3H7)4N[CoFe(dto)3] (dto=C2O2S2)

Recently, we have discovered a new type of first order phase transition around 120 K for (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] (dto=C₂O₂S₂), where the charge transfer transition between Fe^II and Fe^III occurs reversibly. In order to elucidate the origin of this peculiar first order phase transition. Detailed information about the crystal structure is indispensable. We have synthesized the single crystal of (n-C₃H₇)₄N[Co^IIFe^III(dto)₃] whose crystal structure is isomorphous to that of (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃], and determined its detailed crystal structure. Crystal data: space group P6₃, a=b=10.044(2) Å, c=15.960(6) Å, α=β=90°, γ=120°, Z=2 (C₁₈H₂₈NS₆O₆FeCo). In this complex, we found a ferromagnetic transition at T_c=3.5 K. Moreover, on the basis of the crystal data of (n-C₃H₇)₄N[Co^IIFe^III(dto)₃], we determined the crystal structure of (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] by simulation of powder X-ray diffraction results.     

The doping effects of BiFe1-xCoxO3 (x=0.0-0.8) in layered perovskite Bi4Ti3O12 ceramics

Bi5Fe1-xCoxTi3O15(x = 0.0, 0.2, 0.4, 0.5, 0.6, and 0.8) multiferroic ceramics are synthesized in two steps using the solid state reaction technique. X-ray diffraction patterns show that the samples have four-layer Aurivillius phases. At room temperature (RT), the samples each present a remarkable coexistence of ferromagnetism (FM) and ferroelectricity (FE). The remnant polarization (2P r ) reaches its greatest value of 14 μC/cm 2 at x = 0.6. Remnant magnetization (2M r ) first increases and then decreases, and the greatest 2M r is 7.8 menu/g when x = 0.5. The magnetic properties for x = 0.4 are similar to those for x = 0.6, indicating that the magnetic properties originate mainly from the coupling between Fe 3+ and Co 3+ ions, rather than from their own magnetic moments.     

Infrared spectrum and vibrational potential function of the chlorite anion in the matrix-isolated M+ClO2− species

The chlorite anion, ClO₂^?, was prepared for spectral investigation by depositing alkali metal atoms and chlorine dioxide at high dilution in argon onto a 15°K optical surface. Intense infrared absorptions near 820, 790, and 420 cm^?1, which exhibited small alkali metal effects, are due to the three intraionic modes of the chlorite anion in the M⁺ClO₂^? species. The ClO force constant for ClO₂^? (4.11 mdyn/Å) is less than the value for ClO₂ (6.61 mdyn/Å), which is consistent with the antibonding character of the additional electron on ClO₂^?.     

First-principles electronic-structure calculation on the spin distribution and the half-metallic properties of the mixed valence iron compound Ba2F2Fe1.5S3

The first principles within the full potential linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation (GGA) approach were applied to study the new mixed valence compound Ba₂F₂Fe_1.5S₃. The density of states, the electronic band structure and the spin magnetic moment are calculated. The calculations reveal that the compound has an antiferromagnetic interaction between the Fe^III and Fe^II ions arising from the bridging S atoms, which validate the experimental assumptions that there is a low-dimensional antiferromagnetic interaction in Ba₂F₂Fe_1.5S₃. The spin magnetic moment mainly comes from the Fe^III and Fe^II ions with smaller contribution from S anion. By analysis of the band structure, we find that the compound has half-metallic property.