Characterisation of the reduction properties of La0.5Sr0.5Fe0.5Co0.5O3

We report here on the characterisation by temperature programmed reduction, ⁵⁷Fe Mössbauer spectroscopy and X-ray absorption spectroscopy of the phases resulting from treatment of the perovskite-related material La_0.5Sr_0.5Fe_0.5Co_0.5O₃ in a flowing 90% hydrogen/10% nitrogen atmosphere. The results show that treatment of La_0.5Sr_0.5Fe_0.5Co_0.5O₃ (which contains approximately 50% Fe⁴⁺ and 50% Fe³⁺) in the flowing 90% hydrogen/10% nitrogen atmosphere at 600°C does not result in the reduction of any of the constituent elements of the material and that the perovskite structure is still retained. The Mössbauer spectrum recorded following heating in the gaseous reducing environment at 1,000°C shows the presence of metallic iron, an Fe³⁺-containing phase with parameters compatible with the presence of SrLaFeO₄ which has a K₂NiF₄-type structure, and a paramagnetic Fe³⁺ phase. The X-ray absorption spectroscopy results show the presence of metallic cobalt. The Mössbauer spectrum recorded following heating at 1,200°C continues to show the Fe³⁺-containing components plus a larger contribution from metallic iron. The X-ray absorption spectroscopy results show the presence of metallic cobalt, SrLaFeO₄, La₂O₃ and SrO.     

Magnetic behavior of the oxide spinels: Li0.5Fe2.5−2x Al x Cr x O4

In order to study the effect of substitution of Fe³⁺ by Al³⁺ and Cr³⁺ in Li_0.5Fe_2.5O₄ on its structural and magnetic properties, the spinel system Li_0.5Al _x Cr _x Fe_2.5?2x O₄ (x=0.0, 0.2, 0.4, 0.5, 0.6 and 0.8) has been characterized by X-ray diffraction, high field magnetization, low field ac susceptibility and ⁵⁷Fe Mossbauer spectroscopy. Contrary to the earlier reports, about 50% of Al³⁺ is found to occupy the tetrahedral sites. The system exhibits canted spin structure and a central paramagnetic doublet was found superimposed on magnetic sextet in the Mössbauer spectra (x>0.5).     

Synthesis and magnetic properties of InM2+CrS4(M2+ = Mn,Fe,Co, Ni)

The crystallographic and magnetic properties of the compounds InM²⁺CrS₄(M = Mn, Fe, Co, Ni) were studied. Three of these compounds except MnInCrS₄ were antiferromagnetic. The Néel termperature and paramagnetic Curie temperature were determined to be 20 and 82K for InFeCrS₄, 36 and 115K for InCoCrS₄ and 22 and 26K for InNiCrS₄, respectively.     

The superexchange interactions in mixed Co-Fe ludwigite

The crystal structure, cation distribution and exchange interactions in the Co_2.25Fe_0.75O₂BO₃ 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 Co_2.25Fe_0.75O₂BO₃ 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.     

Crystal structure and the magnetic state of Pr0.5Sr0.5Co0.5Fe0.5O3

The magnetic properties and crystal structure of the Pr_0.5Sr_0.5Co_0.5Fe_0.5O₃ compound are studied by neutron and x-ray diffractions using synchrotron radiation. These measurements show that this compound is a dielectric spin glass with a magnetic moment freezing temperature of about 70 K. As temperature decreases in the range 30–95 K, a structure phase transition of the first order occurs with an increase in the symmetry from orthorhombic (space group Imma) to tetragonal (space group I4/mcm). It is assumed that the transition is caused by a change in the 4f electron configuration of the Pr³⁺ ions.     

EPR of cadmium ferric voltaite above the ferrimagnetic transition

A single isotropic EPR line of Fe³⁺ in synthetic cadmium ferric voltaite, (NH₄)₂Cd₅Fe₃Al(SO₄)₁₂ · 18H₂O, was observed in a wide temperature range from 295 to 1.57°K. The ferrimagnetic transition temperature of CdFe voltaite was determined to be ～ 0.7°K using the temperature dependence of the g-factor and the line width. The cubic crystal field parameter, a, for Fe³⁺ in CdFe voltaite is extracted from the EPR line width measurement using the exchange-narrowed line width model of Anderson and Weiss. The parameter a for Fe³⁺ in CdFe voltaite at 4.2°K is 157 × 10^-4cm^-1 which is consistent with the corresponding values for Fe³⁺ in other cubic structures.     

Valence and magnetic states of impurity iron ions in the La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>Co<Subscript>0.98</Subscript>Fe<Subscript>0.02</Subscript>O<Subscript>3</Subscript> perovskite

The local magnetic and valence states of impurity iron ions in the rhombohedral La_0.75Sr_0.25Co_0.98 ⁵⁷Fe_0.02O₃ perovskite were studied using Mössbauer spectroscopy in the temperature range 87–293 K. The Mössbauer spectra are described by a single doublet at 215–293 K. The spectra contained a paramagnetic and a ferromagnetic component at 180–212 K and only a broad ferromagnetic sextet at T < 180 K. The results of the studies showed that, over the temperature range 87–295 K, the iron ions are in a single (tetrahedral) state with a valence of +3. In the temperature range 180–212 K, two magnetic states of Fe³⁺ ions were observed, one of which is in magnetically ordered microregions and the other, in paramagnetic microregions; these states are due to atomic heterogeneity. In the magnetically ordered microregions in the temperature range 87–212 K, the magnetic state of the iron ions is described well by a single state with an average spin S = 1.4 ± 0.2 and a magnetic moment μ(Fe) = 2.6 ± 0.4μ _B .     

Causes of the Metamagnetism in a Disordered EuMn<Subscript>0.5</Subscript>Co<Subscript>0.5</Subscript>O<Subscript>3</Subscript> Perovskite

The magnetic properties of the EuMn_0.5Co_0.5O₃ perovskite synthesized under various conditions are studied in fields up to 140 kOe. The sample synthesized at T = 1500°C is shown to exhibit a metamagnetic phase transition, which is irreversible below T = 40 K, and the sample synthesized at T = 1200°C demonstrates the field dependence of magnetization that is typical of a ferromagnet. Both samples have T_C = 123 K and approximately the same magnetization in high magnetic fields. The metamagnetism is assumed to be related to a transition from a noncollinear ferromagnetic phase to a collinear phase, and the presence of clusters with ordered Co²⁺ and Mn⁴⁺ ions leads to ferromagnetism. The noncollinear phase is formed due to the competition between positive Co²⁺–Mn⁴⁺ and negative Mn⁴⁺–Mn⁴⁺ and Co²⁺–Co²⁺ interactions, which make almost the same contributions, and to the existence of a high magnetic anisotropy.     

Untersuchung eines Li-Cr-Ferrits mit Hilfe des Mößbauer-Effekts

The hyperfine-structure spectra of the 14.4 keVγ-ray of the nucleus⁵⁷Fe in the spinel type ferrite (Li ₀₅ ⁺ Fe _1.3 ³⁺ Cr _1.2 ³⁺ O ₄ ^2? ) have been measured through the use of the Mößbauer effect. These measurements were performed on powder samples at different temperatures between 88 °K and 633 °K. For the Fe³⁺ ions at both the tetrahedral (A) and octahedral (B) sites at 88 °K the effective magnetic fieldH at the Fe nuclei is the same and equal to 504±4 kG. The temperature dependence ofH _A however is different fromH _B . Application of an external magnetic field of 20 kG shows that the sign ofH _A is positive and the sign ofH _B is negative. As the temperature is increased a considerable broadening of the magnetic hfs-lines (and especially of the outer pair of lines) is observed. No theoretical discussion directly applicable to these measurements is as yet available. In the paramagnetic phase a well resolved doublet is obtained. It appears that this doublet must be interpreted as a quadrupole splitting.     

Micro Raman,Mossbauer and magnetic studies of manganese substituted zinc ferrite nanoparticles: Role of Mn

A series of Mn–Zn Ferrite nanoparticles (<15 nm) with formula Mn_xZn_1−xFe₂O₄ (where x=0.00, 0.35, 0.50, 0.65) were successfully prepared by citrate-gel method at low temperature (400 °C). X-ray diffraction analysis confirmed the formation of single cubic spinel phase in these nanoparticles. The FESEM and TEM micrographs revealed the nanoparticles to be nearly spherical in shape and of fairly uniform size. The fractions of Mn²⁺, Zn²⁺ and Fe³⁺ cations occupying tetrahedral sites along with Fe occupying octahedral sites within the unit cell of different ferrite samples are estimated by room temperature micro-Raman spectroscopy. Low temperature Mossbauer measurement on Mn_0.5Zn_0.5Fe₂O₄ has reconfirmed the mixed spinel phase of these nanoparticles. Room temperature magnetization studies (PPMS) of Mn substituted samples showed superparamagnetic behavior. Manganese substitution for Zn in the ferrite caused the magnetization to increase from 04 to18 emu/g and Lande's g factor (estimated from ferromagnetic resonance measurement) from 2.02 to 2.12 when x was increased up to 0.50. The FMR has shown that higher Mn cationic substitution leads to increase in dipolar interaction and decrease in super exchange interaction. Thermomagnetic (M–T) and magnetization (M–H) measurements have shown that the increase in Mn concentration (up to x=0.50) enhances the spin ordering temperature up to 150 K (blocking temperature). Magnetocrystalline anisotropy in the nanoparticles was established by Mossbauer, ferromagnetic resonance and thermomagnetic measurements. The optimized substitution of manganese for zinc improves the magnetic properties and makes these nanoparticles a potential candidate for their applications in microwave region and biomedical field.     

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 state of iron in andalusite

Iron in andalusite (Al₂SiO₅) with concentrations of 1.5 … 4.3 wt-% has been reported in the literature to occupy mainly Al1 sites as Fe³⁺, and only small portions occupy Al2 sites as Fe³⁺ and Fe²⁺. In no case a magnetic hyperfine splitting has been found at temperatures above 77 K. We have studied natural samples of andalusite containing 0.3 and 0.5 wt-% only. Mössbauer spectra recorded at 80 and 300 K show magnetically split spectra and, for the 0.5% sample, an additional doublet can be seen. The spectra are interpreted as being due to a slow paramagnetic relaxation resulting in Fe³⁺ and Fe²⁺ hyperfine multiplets. An increase in the Fe concentration reduces the paramagnetic relaxation time. It seems that the andalusite structure can contain a limited number of Fe²⁺ only, and a similar limitation does not exist for Fe³⁺ ions.     

EPR and optical absorption studies of Fe ions in sodium borophosphate glasses

Electron paramagnetic resonance (EPR) and optical absorption spectral investigations have been carried out on Fe³⁺ ions doped sodium borophosphate glasses (NaH₂PO₄-B₂O₃-Fe₂O₃). The EPR spectra exhibit resonance signals with effective g values at g=2.02, g=4.2 and g=6.4. The resonance signal at g=4.2 is due to isolated Fe³⁺ ions in site with rhombic symmetry whereas the g=2.02 resonance is due to Fe³⁺ ions coupled by exchange interaction in a distorted octahedral environment. The EPR spectra at different temperatures (123-295 K) have also been studied. The intensity of the resonance signals decreases with increase in temperature whereas linewidth is found to be independent of temperature. The paramagnetic susceptibility (χ) was calculated from the EPR data at various temperatures and the Curie constant (C) and paramagnetic Curie temperature (θ_p) have been evaluated from the 1/χ versus T graph. The optical absorption spectrum exhibits bands characteristic of Fe³⁺ ions in octahedral symmetry. The crystal field parameter (Dq) and the Racah interelectronic repulsion parameters (B and C) have also been evaluated and discussed.     

Magnetic properties of the mixed spinel Co1+xSnxFe2−2xO4

The structural and magnetic properties of the mixed spinel Co_1+xSn_xFe_2?2xO₄ system for 0.1≤x≤0.5 have been studied by means of X‐ray diffraction, magnetization, a.c. susceptibility and Mössbauer effect measurements. X‐ray intensity calculations indicate that Sn⁴⁺ ions occupy only octahedral (B) sites replacing Fe³⁺ ions and the added Co²⁺ ions substitute for A‐site Fe³⁺ ions. The lattice constants are determined and the applicability of Vegard's law has been tested. The Mössbauer spectra at 300 K have been fitted with two sextets in the ferrimagnetic state corresponding to Fe³⁺ at tetrahedral (A) and octahedral (B) sites for x≤0.4. The Mössbauer intensity data show that Sn possesses a preference for the B‐site of the spinel. As expected, the hyperfine field and Curie temperature determined from a.c. susceptibility decreases with increasing Sn content. The variation of the saturation magnetic moment per formula unit measured at 77 and 300 K with Sn content is satisfactorily explained on the basis of Néel's collinear spin ordering model for x=0.1–0.4.     

Synthesis and characterization of CaLaFe11O19

A new ferrimagnetic compound with the chemical formula CaLaFe₁₁O₁₉ has been synthesized by solid state reaction between the respective oxides and their structural, electrical and magnetic properties have been studied. One magnetic Fe³⁺ ion in CaLaFe₁₂O₁₉ is replaced by La³⁺ ion. The crystallographic results show the compound is hexagonal magnetoplumbite. The electrical conductivity has been measured from 300 to 800 K. The activation energy changes at Curie temperature (653 K). The compound is ferrimagnetic from 300 to 653 K and above T_c it acts as a paramagnetic. Variation of inverse molar susceptibility has been measured at various temperature in paramagnetic region and Curie molar constant (C_m) is calculated. AC susceptibility measurements are made at room temperature. The Seebeck coefficient (S) measurements show that the compound is n-type.     

Co57 and Fe57 mössbauer studies of the spinels FeCo2O4 and Fe0.5Co2.5O4

Mössbauer source and absorber spectra of FeCo₂O₄ and Fe_0.5Co_2.5O₄ have been obtained between 82 and 523 K. Interpretation of the spectra allow the cation distributions of the compounds to be determined. FeCo₂O₄ is Co²⁺_0.55Fe³⁺_0.45[Co²⁺_0.45Fe³⁺_0.55Co³⁺_1.0]O₄ and Fe_0.5Co_2.5O₄ is Co₁²⁺[Fe³⁺_0.5Co³⁺_1.5]O₄. Spinel tetrahedral site quadruple splitting is observed in both compounds.     

Preparation of substituted barium ferrite BaFe12−x(Ti0.5Co0.5)xO19 by citrate precursor method and compositional dependence of their magnetic properties

As a possible candidate for the left-handed metamaterial with negative permeability, a series of Ti, Co-substituted M-type barium hexaferrite BaFe_12−x(Ti_0.5Co_0.5)_xO₁₉ (x=0, 1, 2, 3, 4 and 5) was prepared by citrate precursor method. The formation processes of the substituted barium hexaferrite compounds from the precursors were followed by the measurements of powder X-ray diffraction (XRD), Infrared absorption spectra (FT-IR), and thermogravimetry and differential thermal analysis (TG/DTA) coupled with mass spectroscopy (MS). In the case of the non-substituted sample, the formation of the barium hexaferrite is regulated by the thermal decomposition of BaCO₃ and the solid-state reactions of BaO and Fe₂O₃ in the temperature range from 800 to 1100 K. The formation temperature of the substituted BaFe_12−x(Ti_0.5Co_0.5)_xO₁₉ is higher than that for the non-substituted sample and increases with the value of x, due to the effects of carbonate ions incorporated by the partial substitution of Fe³⁺ by (Ti_0.5Co_0.5)³⁺. On heating up to ca. 1200 K, all the substituted samples transform into the magenetoplumbite phase as is the non-substituted sample. The compositional dependence of the magnetic properties of the substituted barium hexaferrite was investigated by the magnetization measurement. The decrease in the magnetic anisotropy was confirmed by the change in the magnetization curve and coercivity H_C with the composition x. A negative permeability spectrum was observed in the BaFe₉(Ti_0.5Co_0.5)₃O₁₉ in the frequency range from 2 to 4 GHz.     

Influence of 50 MeV Lithium Ion Irradiation on Hyperfine Interactions of Cr0.5 Li0.5 Fe2O4

Spinel oxide Cr_0.5 Li_0.5 Fe₂O₄ has been irradiated at Nuclear Science Centre, New Delhi, by 50 MeV lithium ions of fluence 5*10¹³ ions/cm² and irradiation effect on hyperfine interactions has been investigated by Mossbauer spectroscopy. The Mossbauer spectrum of irradiated sample shows no paramagnetic doublet contribution and the hyperfine fields corresponding to the Fe³⁺ in the octahedral (B) and the tetrahedral (A) sites are very well separated. That is the observed superimposed A and B sites in unirradiated sample are split into separate lines after Li irradiation. Further an increase of the intensity of the lines (2)–(5) with respect to (1)–(6) signals an orientation of the hyperfine magnetic field towards a direction perpendicular to the ion path due to the irradiation induced strain by the latent tracks. The computer simulation of Mossbauer spectra indicated that the irradiated Fe³⁺-site occupancy of the A-site hyperfine field increased from 43% to 55% whereas the B-site hyperfine field decreased from 57% to 45% compared to unirradiated sample.     

Magnetization and Mössbauer studies of 57Fe doped SrCoO3

⁵⁷Fe (1%) doped SrCoO₃ obtained by high-pressure method, has been investigated by magnetization and Mössbauer spectroscopy studies (MS) in the temperature range 4.2 K to 300 K. The ferromagnetic ordering temperature T _C obtained is 272(2) K. Isothermal magnetization curves have been measured at various temperatures, from which the saturation moments (M _sat) have been deduced. The ⁵⁷Fe MS spectra display standard six-line patterns with an isomer shift typical of Fe^3?+? and a very small quadrupole splitting (QS = 0.14(1) mm/s above T _C). The magnetic hyperfine field at 4.2 K is 276(1) kOe. The temperature dependencies of the iron hyperfine field and M _sat (1.83 µ _B at 5 K) are almost identical. This shows that the Fe^3?+? is replacing Co^4?+?, both of the same electronic configuration. They also interact similarly, namely the Fe–Co exchange is almost identical to the Co–Co exchange.     

Spectral studies of Co substituted Ni-Zn ferrites

The spinel ferrites Zn_0.35Ni_0.65−xCo_xFe₂O₄, 0≤x≤1, have been prepared using the standard ceramic technique. Room temperature Mössbauer, X-ray and infrared IR spectra were used for carrying out this study. X-ray patterns reveal that all the samples have single-phase cubic spinel structure. The Mössbauer spectra of the samples show a paramagnetic phase for x=0 and a six-line magnetic pattern and a central paramagnetic phase for x≥0.1. They are analyzed and attributed to two magnetic subpatterns and two quadrupole doublets due to Fe³⁺ ions at the tetrahedral A-sites and octahedral B-sites. Four absorption bands are observed in IR spectra. They confirm the spinel structure of the samples and existence of Fe³⁺ ions in the sample sublattices. The deduced hyperfine interactions, lattice parameters, absorption band positions and intensities and force constant are found to be dependent on the substitution factor x, where the cation distribution is estimated. The hyperfine magnetic fields, magnetization and lattice resonant frequency are found to be dependent on the interionic distance.