A XPS study of Fe1-χCuχCr2S4

The X-ray photoelectron spectroscopy is applied for a chalcogen chromite system Fe_{1 ? x}Cu_xCr₂S₄ to obtain microscopic information about valence states of the constituting atoms. The features of the Cu 2p spectra show that copper is always in a monovalent state in this system. This supports a model Fe³⁺_{1 ? x}Cu¹⁺_xCr³⁺₂S^1-_{2x ? 1}S^2-_{5 ? 2x} for a compositional range 0.5 < x < 1.     

Electron hopping in Fe1−xCuxCr2S4 (0<x<0.5)

A theoretical expression for the line shape of the Mössbauer spectra in the presence of electron hopping between Fe²⁺ and Fe³⁺ is obtained by using a simple stochastic model. Analyses based upon this expression show that the origin of the complicated Mössbauer spectra observed in the magnetic semiconductors Fe_1?xCu_xCr₂S₄ (0<x<0.5) at 77 K is electron hopping between Fe⁺² and Fe³⁺ This hopping occurs at a rate of a few MHz. Quantitative estimates are given for some parameters; the isomer shifts, the internal magnetic fields, the quadrupole splittings and the proportions of Fe²⁺ and Fe³⁺. The valence distribution in this system is determined from the results. For example, the distribution Fe²⁺_0.69Fe³+_0.29Cu¹⁺_0.02Cr³⁺_1.72Cr²⁺_0.28S^2?₄ is obtained for x = 0.02. The existence of Cr²⁺ is concluded.     

Temperature dependent Mössbauer and neutron diffraction studies of Cu x Fe1−x Cr2S4 compounds

The cation distribution and magnetic structure of Cu _x Fe_1?x Cr₂S₄ (x?=?0.1, 0.2, 0.3, 0.4, and 0.5) has been studied by X-ray and neutron diffraction, vibrating sample magnetometer (VSM), and Mössbauer spectroscopy. The charge state of Fe is found to be ferrous (Fe²⁺) for the x?=?0.1 sample; ferric (Fe³⁺) for the x?=?0.5 sample; mixed state (Fe²⁺, Fe³⁺) for the x?=?0.2, 0.3, and 0.4 samples. The Mössbauer spectra of the x?=?0.1 sample show asymmetric line broadening, which is considered to be due to the Jahn–Teller effect of Cu²⁺ ions, and a symmetrical six-line pattern is shown for the x?=?0.5 sample. The valence state of the Cu ions for the x?=?0.1 and 0.5 samples is found to be divalent and monovalent, respectively. The magnetic structure of the samples was determined to be a ferrimagnetic structure with antiparallel alignment of the Fe and Cr ion magnetic moments.     

Mössbauer study of Fe-substituted orthorhombic and tetragonal YBa2(Cu1-xFex)3O7-σ system

Mössbauer study of orthorhombic and tetragonal YBa₂(Cu_1-xFe_x)₃O_7-σ;x=0.02, 0.04, 0.08 has been done to investigate the two inequivalent Cu-sites. Fe substituting Cu having pyramidal oxygen co-ordination is in Fe³⁺ state while Fe substituting Cu having square planar co-ordination is in Fe³⁺ and Fe⁴⁺ states in dynamic equilibrium.     

X-ray photoelectron and mössbauer spectroscopy studies of the valence state of transition metal ions in Co1–x Fe x Cr2O4 (x = 0.1, 0.2, 0.5) ceramics

The valence state of transition metal ions in the Co_1–x Fe _x Cr₂O₄ (x = 0.1, 0.2, 0.5) system has been investigated using X-ray photoelectron and Mössbauer spectroscopy. It has been shown that, in this system, there are Fe²⁺ and Fe³⁺ ions. The relative Fe³⁺/Fe²⁺ contents have been determined by fitting the experimental Fe 2p photoelectron spectra by a superposition of theoretical spectra of the Fe²⁺ and Fe³⁺ ions, as well as using Mössbauer spectroscopy.     

Mössbauer studies of magnetic behavior of 3d-doped REBa2Cu3−x Fe x O7+δ (RE=Y,Er, Dy,Gd)

A Mössbauer study has been made on⁵⁷Fe ions substituted into the Cu(1) site of REBa₂Cu_3?x Fe _x O_7+δ (RE=Y, Er, Dy, Gd;x=0.15, 0.30). At low temperature, the iron atoms antiferromagnetically order with a transition temperature which is dependent on the Fe concentration. The temperature dependence of the magnetic subspectra representing Fe ions with various local oxygen environments in YBa₂Cu_3?x Fe _x O_7+δ and ErBa₂Cu_3?x Fe _x O_7+δ fit a 2D-Ising model with a ratio of the anisotropic exchange between the two directions on the order of 0.5–1.0(10^?3) for the Y-compounds and on the order of 1 for the Er-compounds. The magnitude of the local dopant magnetization is related to a short-range chemical order which determines the magnetic chain size and defines the correlation lengths. For the Y-compound, the order is quasi-1D with strong intrachain but very weak interchain coupling. For the Er-compounds, the magnetic coupling is Ising 2D. The strong fluctuation behavior expected in low dimensional systems above and belowT _N is observed via characteristic relaxation in the Mössbauer linewidth nearT _N. For both the Dy- and Gd-compounds, the magnetic order is 3D. The magnitude of the rare-earth magnetic moments appears to affect the character of the magnetic interaction in the Cu(1)-site. However, a Mössbauer effect measurement at¹⁵⁵Gd nuclei in GdBa₂Cu_2.85Fe_0.15O_7+δ (T _N(Fe)～14 K) shows paramagnetic behavior at 4.9 K.     

Jump rate and linear distance of vacancy on the structure and electrical conductivity of Ni0.65Zn0.35CuxFe2−xO4 ferrites

Ferrite compositions of Ni_0.65Zn_0.35Cu_xFe_2−xO₄ (0⩽x<1) were examined using X-ray analysis. The effect of the linear distance of vacancy jumping on the lattice parameter was studied. The jump rate of vacancy increased with increasing Cu concentration. The increase of jump rate of vacancy enhanced the linear distance which increased the conductivity and mobility of the charge carriers. The majority of charge carriers of our systems are holes. The estimated linear distance of each jump was 2.86×10⁻⁷ m. The decrease of thermal conductivity was attributed to the increase of the jump rate and also the linear distance. The formation of oxygen vacancies during the substitution of Cu²⁺ ions for Fe³⁺ ions helped the internal stress to decrease the lattice parameter. Because the ionic radius of O²⁻ (0.136 nm) is larger than that of Fe³⁺ (0.067 nm) ion.     

Etude en fonction du temps de l'oxydation des magnetites substituees a l'aluminium et au chrome au moyen de la conductivite electrique

The oxidation of substituted magnetites (Fe²⁺Fe_2?x³⁺M_x³⁺O₄^2?(O < x < 2) into the lacunar phase γ(Fe_1?z³⁺M_z³⁺)O₃^{2 ?} (x = 3z) was followed over time by electrical conductivity over the temperature r Curves σ = f(t) are different according to whether we deal with an n or p-type spinel. However, for intermediate substitution rates (1,1 < x < 1,8), n-type ferrichromites are oxidized after an initial period in the same way as p-type semi-conductors.For the initial period, the chemisorption kinetics of oxygen upon these n-type samples, reduced under vacuum, was found to follow Elovich's law with an activation energy depending on the degree of coverage.     

Synthesis and characterization of Cu2+ substituted magnetite

Samples of magnetite, both pure and doped with divalent copper, Fe_3???xCu_xO₄, with x?=?0, 0.05, 0.10 and 0.20 atm.%, were synthesized hydrothermally. The samples were characterized by Atomic Absorption Spectroscopy, Mössbauer Spectroscopy, X-ray diffraction, Scanning Electron Microscopy and SQUID magnetometry. The analyses made by the above techniques showed that as the Cu²⁺ concentration increases, a simultaneous reduction in the magnetic and structural parameters takes place, namely: magnetic hyperfine interactions at octahedral sites, particle size and lattice constant. Degradation in the particles morphology as well as a distribution of their size were also observed. Our study points two important effects of Cu²⁺ in magnetite, the first one is its incorporation within the structure, replacing Fe²⁺ ions and decreasing both the magnetic hyperfine interactions at octahedral sites and the bulk magnetization, the second one is the contraction of the crystalline lattice of magnetite, because incorporation of Cu²⁺ within the structure, generation of vacancies or both simultaneous effects.     

Etude De La Conductivite Electrique Des Ferrites Oxyfluores A Structure Spinelle

An investigation of the electrical conductivity of some oxyfluoride spinels of formula Zn_x²⁺Fe_1?x³⁺[M²⁺ Fe³⁺]O_4?xF_x (M = Fe, Co, Ni) and Fe³⁺[N_x²⁺Fe²⁺Fe_1?x³⁺]O_4?xF_x (N = Fe, Ni) shows that the conduction depends on the composition of the B sites: the activation energy increases, the conductivity and the Fe₃O₄ transition temperature decrease as the substitution rate of Fe³⁺ by N²⁺ in the B sites increases. The authors conclude to a hopping mechanism between the B cations; the anionic sublattice and the cationic A sublattice do not participate in the conduction.     

Structural and Magnetic Properties of NiFe2−2xSnxCuxO4

The substituted nickel ferrite (NiFe_2−2xSn_xCu_xO₄, x=0, 0.1, 0.2, 0.3) was prepared by the conventional ceramic method. The effect of substitution of Fe³⁺ ions by Sn⁴⁺ and Cu²⁺ cations on the structural and magnetic properties of the ferrite was studied by means of ⁵⁷Fe Mössbauer spectroscopy, alternating gradient force magnetometry (AGFM) and Faraday balance. Whereas undoped NiFe₂O₄ adopts a fully inverse spinel structure of the type (Fe)[NiFe]O₄, Sn⁴⁺ and Cu²⁺ cations tend to occupy octahedral positions in the structure of the substituted ferrite. Based on the results of Mössbauer spectroscopic measurements, the crystal-chemical formula of the substituted ferrite may be written as (Fe)[NiFe_1−2xSn_xCu_x]O₄, where parentheses and square brackets enclose cations in tetrahedral (A) and octahedral [B] coordination, respectively. The Néel temperature and the saturation magnetization values of the NiFe_2−2xSn_xCu_xO₄ samples were found to decrease with increasing degree of substitution (x). The variation of the saturation magnetization with x measured using the AGFM method and that calculated on the basis of the Mössbauer spectroscopic measurements are in qualitative agreement.     

Interaction of dopant cations with 4:1 defect clusters in non-stoichiometric 3d transition metal monoxides: A theoretical study

The ASED-MO theory and a normalized ion energy model have been used to study the energetics of substitutional dopants in Mn, Fe, Co, and Ni non-stoichiometric monoxides; we find that certain dopants stabilize 4:1 defect clusters (4 octahedral M²⁺ vacancies surrounding an M³⁺ tetrahedral interstitial). For example, Fe³⁺-based 4:1 clusters in Mn_1−xO and Mn³⁺-based 4:1 clusters in Fe_1−xO have comparable stabilities to those in the undoped material, while Mn³⁺- and Fe³⁺-based 4:1 clusters in Co_1−xO and Ni_1−xO are significantly more stable than those in the undoped oxides. Cr³⁺ has a strong octahedral site preference energy in all cases, which prevents formation of doped 4:1 defect clusters. Despite a strong tetrahedral site preference energy, Zn²⁺ does not cause formation of 4:1 clusters because they would destabilize oxygen 2p orbitals.     

Ti4+ and Ge4+ ionic substitution in Cu-ferrite,electrical conductivity and thermoelectric power

Both the DC conductivity and the thermoelectric power of Cu_1+xGe_xFe_2−2xO₄ and Cu_1+xTi_xFe_2−2xO₄ ferrites, for 0⩽x⩽0.4, were measured in a wide range of temperature from RT up to 773 K. The measurements showed that the substitution of both tetravalent ions (Ge⁴⁺ and Ti⁴⁺) tend to convert Cu-ferrite from n- to p-type semiconductor. The results were analyzed on the basis of the small polaron model. The cation distribution for each system was proposed. The activation energy ΔE, Fermi energy E_F, the density of charge carriers n or p and the carriers mobility μ were determined for both systems. In addition, an energy band schemes of Cu–Ge and Cu–Ti ferrites were schematically represented.     

M?ssbauer study of compounds of Cu3 ? xFexSnS4 and Cu2Fe1 ?xZnxSnS4 systems

A M?ssbauer study of the structural and charge states of ⁵⁷Fe and ¹¹⁹Sn atoms in the compounds of Cu_{3 −x} Fe _x SnS₄ and Cu₂Fe_{1 − x} Zn _x SnS₄ systems was performed. It was shown that the iron atoms in the compounds of both systems were in the divalent and trivalent states occupying the tetrahedral positions of the structure. The character of the changes of the degree of covalency of the Fe²⁺-S, Fe³⁺-S and Sn⁴⁺-S bonds during the isomorphic substitution in the systems was established.     

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.     

Mossbauer study of the Cu-Cd ferrite system

The magnetic properties of the Cd_xCu_1?xFe₂O₄ ferrite system (x = 0 to 1) have been investigated by means of Mossbauer Spectroscopy. Mossbauer Spectra for x = 0.0 to 0.6 suggest the existence of two hyperfine fields, one due to the Fe³⁺ tetrahedral ions (A-sites) and the other due to Fe³⁺ octahedral ions (B-sites), while for x = 0.7 it shows relaxation behaviour and for x ? 0.8 it exhibits a paramagnetic quadrupole doublet. The systematic dependence of the isomer shift, quadrupole interactions and nuclear fields of ⁵⁷Fe³⁺ ions in both A- and B-sites has been determined as a function of cadmium content. The variation of nuclear magnetic fields at the A- and B-sites are explained on the basis of A-B and B-B supertransferred hyperfine interactions. Analysis of the relaxation spectrum observed at x = 0.7 (300 K) suggests that the relaxation mechanism is due to domain wall oscillations. It has been found here that the QS increases from CuFe₂O₄ as the cadmium concentration is increased.     

The effect of zinc concentration on vacancies jump rate, diffusion coefficient and jump length in Cu-Zn ferrite

Samples with the chemical formula Cu_1−xZn_xFe₂O₄ (x=0.2, 0.4, 0.6, 0.8 and 1) were prepared by the standard ceramic method. The dielectric constant and dielectric loss tangent were studied as a function of vacancy jump rate. The results show that the dielectric constant and dielectric loss tangent decrease with increasing vacancy jump rate. In addition, the electron jump length in the octahedral sites was studied as a function of zinc concentration. The increase in jump length with Zn concentration has been attributed to the substitution of Fe⁺³ for Zn²⁺ at the A-sites, which increases the B-B interaction. The increase of diffusion coefficient with increasing Zn concentration was reinforced by the increase of jump rate.     

Magnetic phase present in YBa2(Cu1−x Fe x )3O6+δ oxides

The Mössbauer study of⁵⁷Fe: YBa₂Cu₃O_6+δ oxides was very important to establish the preferential occupation of Cu(1) site by Fe at very low concentrations. Recent determination of antiferromagnetic ordering for Cu moments in Cu(2) sites (T≈450 K) and our early observation of a small proportion of a magnetic phase at room temperature for Fe:YBa₂Cu₃O₆ lead us to perform systematic studies of YBa₂(Cu_1?x Fe _x )₃O₆ withx=0.005, 0.03, 0.05, 0.10 and 0.15 in order to obtain information about the iron occupation of Cu(2) sites.     

Effect of Cu-Cr co-substitution on magnetic properties of nanocrystalline magnesium ferrite

This study deals with the temperature and composition dependence of magnetization and magnetic anisotropy of Cu²⁺-Cr³⁺ co-substituted magnesium ferrite, Mg_1−xCu_xCr_xFe_2−xO₄ (x=0.0-0.5). The synthesized materials are characterized using thermo gravimetric analysis, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray fluorescence, Mössbauer spectrometer, superconducting quantum interference device magnetometer and vibrating sample magnetometer. The M-H loops measured up to 50 kOe at 300, 200 and 100 K, revealed narrow hysteresis curves with a coercive field and saturation magnetization varying for different compositions. The high field regimes of these loops are modeled using the Law of Approach to saturation to extract anisotropy information and saturation magnetization. Both the saturation magnetization and the anisotropy constant are observed to increase with the decrease in temperature while decrease with the Cu-Cr co-substituents for all the samples. Explanation of the observed behavior is proposed in terms of the preference of the co-substituent ions of Cu²⁺ and Cr³⁺ and their predominant choice to substitute into the octahedral sites of the cubic spinel lattice.     

Magnetic properties of CuZn ferrite investigated by Mössbauer spectroscopy

The magnetic properties of the Zn_xCu_1?xFe₂O₄ ferrite system (x = 0 to 1) have been investigated by means of Mössbauer Spectroscopy. Mössbauer spectra of the CuZn ferrite system, taken at room temperature for x = 0.0 to 0.4 suggest the existence of two hyperfine fields, one due to the Fe³⁺ tetrahedral ions (A-sites) and the other due to the Fe³⁺ octahedral ions (B-sites), while for x = 0.5 it shows relaxation behaviour and for x ? 0.6 it exhibits a paramagnetic quadrupole doublet. The systematic dependence of the isomer shifts, quadrupole interactions and nuclear magnetic fields of ⁵⁷Fe³⁺ ions in both tetrahedral and octahedral sites has been determined as a function of zinc content. The variation of nuclear magnetic fields at the A and B sites are explained on the basis of A-B and B-B supertransferred hyperfine interactions. Analysis of the relaxation spectrum observed at x = 0.5 (300 K) suggests that the relaxation mechanism is due to domain wall oscillations.