Defect clusters in titanium-substituted spinel-related lithium ferrite

The cation distribution in spinel-related titanium-substituted lithium ferrite, Li_0.5+0.5xFe_2.5−1.5xTi_xO₄ has been explored using interatomic potential and ab initio calculations. The results suggest that the cation distribution with Ti⁴⁺ substituting for Fe³⁺ on octahedral B sites and excess Li⁺ substituting for Fe³⁺ on tetrahedral A sites is stabilised by the formation of clusters of two octahedrally coordinated Ti⁴⁺ ions and one tetrahedrally coordinated Li⁺ ion linked through a common oxygen.     

The Influence of Mechanical Milling and Subsequent Calcination on the Formation of Lithium Ferrites

The influence of ball milling and subsequent calcination of a 2.5:1 molar mixture of α-Fe₂O₃ and Li₂CO₃ on the formation of lithium ferrites has been investigated. Premilling was found to considerably lower the temperature at which the lithium ferrites LiFeO₂ and LiFe₅O₈ are formed. A β-to-α phase transition in LiFe₅O₈ was found to take place on cooling from ca. 1000°C depending on the milling history and cooling regime.     

Magnetic Properties of Iron Clusters in Silver

Time differential perturbed γ-γ angular correlation technique was used to measure the magnetic hyperfine field (MHF) at Tb sites in the intermetallic compound Tb₃In₅ using the ¹⁴⁰La → ¹⁴⁰Ce nuclear probe. The measurements were carried out in the temperature range of 8 to 295 K. Two different temperature dependent magnetic frequencies were observed below 30 K, which were assigned as ¹⁴⁰Ce substituting the two inequivalent Tb sites in the orthorhombic structure of Tb₃In₅. The temperature dependence of MHF also shows a possible deviation from an expected Brillouin-like behavior for temperatures below 18 K. A Néel transition at 27 K was observed from magnetization measurements in the samples. The magnetization as a function of the applied magnetic field was measured at two temperatures, 5 and 40 K, and the results show antiferromagnetic and a typical paramagnetic behavior, respectively. In both cases it was not observed saturation under high magnetic field.     

On the Generalized Continuity Equation

A generalized continuity equation extending the ordinary continuity equation is found using quanternions to show it is compatible with Dirac, Schrǒdinger, Klein-Gordon and diffusion equations. This generalized equation is Lorentz invariant. The transport properties of electrons are found to be governed by the Schr6dinger-like equation and not by the diffusion equation.     

Characterization of Maghsail meteorite from Oman by Mössbauer spectroscopy, X-ray diffraction and petrographic microscopy

The meteorite found at Maghsail (16 55 70 N–53 46 69 E) west of Salalah Oman, has been studied by ⁵⁷Fe Mössbauer spectroscopy, X-diffractometry and petrographic microscopy. In the polished section the meteorite exhibits a porphyritic texture consisting of pyroxene and olivine phenocrysts in a fine to medium grained ground mass in addition to minor phases possibly skeletal chromite, troilite and minute amount of iron oxides. X-ray diffraction supports the existence of these compounds. The Mössbauer spectra of powdered material from the core of the rock at 298 K and 78 K exhibit a mixture of magnetic and paramagnetic components. The paramagnetic components are assigned to the silicate minerals olivine and pyroxene. On the other hand, the magnetic spectra reveal the presence of troilite and iron oxides. The petrographic analyses indicate that the iron oxides are terrestrial alteration products.     

Synthesis and cation distribution of copper-substituted spinel-related lithium ferrite

Single-phased Cu²⁺-substituted spinel-related Li_0.5Fe_2.5O₄ was synthesized by sintering a mixture of Cu²⁺-substituted corundum-related α-Fe₂O₃ and Li₂CO₃ at 700 °C which is ∼325-400 °C lower than the temperature at which the material is prepared by the conventional ceramic methods. X-ray powder diffraction, X-ray photoelectron spectroscopy, Mössbauer spectroscopy and magnetic measurements were used to characterize the material. In contrast to high-temperature synthetic routes, the present one leads to a Cu⁺-and Fe²⁺-cation free material, thereby optimizing its technological value. Rietveld refinement of the XRD data favors a structural model in which Cu²⁺ substitutes for both Fe³⁺ and Li⁺ at the octahedral sites. Mössbauer and magnetic data are consistent with this model if spin thermal reversal and/or spin canting are taken into account for the later.     

Infrared and structural studies of Mg1-xZnxFe2O4 ferrites

Compositions of polycrystalline Mg-Zn mixed ferrites with the general formula Mg_1−xZn_xFe₂O₄ (0≤x≤1) were prepared by the standard double sintering ceramic method. The structural properties of these ferrites have been investigated using X-ray diffraction and infrared absorption spectroscopy. The lattice parameter, particle size, bonds length, force constants, density, porosity, shrinkage and cation distribution of these samples have been estimated and compared with those predicted theoretically. Most of these values were found to increase with increasing Zn content. The energy dispersive (EDS) analysis confirmed the proposed sample composition. The scanning electron microscope (SEM) and transmission electron microscope (TEM) micrographs showed aggregates of stacked crystallites of about 200-800 nm in diameter. Far infrared absorption spectra showed two significant absorption bands. The wave number of the first band, ν₁, decreases with increasing Zn content, while the band, ν₂ shifts linearly towards higher wave numbers with Zn contents, over the whole composition range. The room temperature electrical resistivity was found to decrease as Zn-content increases. Values of the vacancy model parameters showed that the packing factors P_a and P_b decrease, the fulfillment coefficient, α, remains almost constant and the vacancy parameter, β, strongly increases with increasing Zn content in the sample. The small values of P_a, P_b, α and the strong increase of the vacancy parameter, β, indicate the presence of cation or anion vacancies and the partial participation of the Zn²⁺ vacancies in the improvement of the electrical conductivity in the Mg-Zn ferrites.     

Structural and magnetic studies of the Zn-substituted magnesium ferrite chromate

The formation of iron oxide nanoparticles in course of a sol-gel preparation process was traced by UV/Vis and ⁵⁷Fe Mössbauer absorption spectroscopy. Samples were extracted at different stages of the reaction. While spectra measured on samples extracted at low reactor temperatures showed the starting materials Fe(acac)₃ diluted in benzyl alcohol undergoing slow paramagnetic relaxation, a sample extracted at a reactor temperature of 180 °C gave clear evidence for emerging iron oxide nanoparticles. A prolonged stay at 200 °C results in a complete transformation from Fe(acac)₃ to maghemite nanoparticles.     

Mechanosynthesis,magnetic and Mössbauer characterization of pure and Ti4+-doped cubic phase BiFeO3 nanocrystalline particles

The mechanosynthesis of cubic γ-phase pure BiFeO₃ and Ti⁴⁺-doped BiFeO₃ nanocrystalline particles and their preliminary characterization with magnetic measurements and Mössbauer spectroscopy are reported. The BiFeO₃ nanoparticles (5–40 nm) were prepared by heating a 48 h pre-milled 1:1 molar mixture of α-Bi₂O₃ and α-Fe₂O₃ at 400 °C for (1 h). Doping α-Fe₂O₃ in the initial mixture of reactants with Ti⁴⁺ was found to lead to the formation of Ti⁴⁺-doped BiFeO₃ nanoparticles by milling the reactants for 32 h. The magnetization of the BiFeO₃ nanoparticles is found to be tripled under a maximum external field of 1.35 T and their magnetic hardness increases by ～15 times relative to those of the corresponding bulk. The Ti⁴⁺-doped BiFeO₃ nanoparticles exhibit higher magnetization relative to the pure ones. These observations are related to the spiral modulated spin structure of the compound. The Mössbauer data show ～12 % of the BiFeO₃ nanocrystalline particles to be superparamagnetic having blocking temperatures of less than 78 K. The quadrupole shift values of the magnetic spectral component favor the cubic structural symmetry. These observations were mainly associated with possible collective magnetic excitations as well as transverse relaxation of canted surface spins. The Ti-doped BiFeO₃ nanoparticles gave statistically-poor Mössbauer spectra with no signs of a superparamagnetic behavior.     

The Electronic and Magnetic Properties of FCC Iron Clusters in FCC 4D Metals

The electronic and magnetic structures of small FCC iron clusters in FCC Rh, Pd and Ag were calculated using the discrete variational method as a function of cluster size and lattice relaxation. It was found that unrelaxed iron clusters, remain ferromagnetic as the cluster sizes increase, while for relaxed clusters antiferromagnetism develops as the size increases depending on the host metal. For iron in Rh the magnetic structure changes from ferromagnetic to antiferromagnetic for clusters as small as 13 Fe atoms, whereas for Fe in Ag antiferromagnetism is exhibited for clusters of 24 Fe atoms. On the hand, for Fe in Pd the transition from ferromagnetism to antiferromagnetism occurs for clusters as large as 42 Fe atoms. The difference in the magnetic trends of these Fe clusters is related to the electronic properties of the underlying metallic matrix. The local d densities of states, the magnetic moments and hyperfine parameters are calculated in the ferromagnetic and the antiferromagnetic regions. In addition, the average local moment in iron-palladium alloys is calculated and compared to experimental results.