Magnetoelastic waves in parametrically active nickel ferrite spinels

The effect of composition of polycrystalline ferrites based on nickel ferrite on the magnetoelastic coupling in the ultrasonic frequency range was experimentally investigated. The dependences of the velocities of magnetoelastic waves on the magnetizing field were measured. The maximum values of the magnetoelastic coupling coefficient (59%) and the sensitivity of transverse sound velocity to variations in magnetic field (0.1%Oe⁻¹) were obtained for ferrites of the Fe_2.026Ni_0.95Co_0.024O₄ composition.     

Raman fingerprint of chromate,aluminate and ferrite spinels

Synthetic and natural spinel single crystals having compositions closely approaching spinel end‐members ZnCr₂O₄, MgCr₂O₄, FeCr₂O₄, ZnAl₂O₄, MgAl₂O₄, CoAl₂O₄, FeAl₂O₄, MnAl₂O₄, MgFe₂O₄, and FeFe₂O₄ were investigated by Raman spectroscopy in the 100–900 cm⁻¹ range using both the red 632.8 nm line of a He‐Ne laser and the blue 473.1 nm line of a solid‐state Nd : YAG laser. Each end‐member exhibits a Raman fingerprint with at least one peculiar peak in terms of Raman shift and relative intensity. Chromates and ferrites exhibit the most intense A_1g mode at around 680 cm⁻¹, at lower wavenumbers than in the aluminates, in agreement with the heavier atomic mass of Cr and Fe with respect to Al. For aluminate spinels, the most intense and diagnostic peaks in the spectrum are as follows: F_2g(1) at 202 cm⁻¹ for MnAl₂O₄, E_g at 408 cm⁻¹ for MgAl₂O₄, F_2g(2) at 516 cm⁻¹ for CoAl₂O₄, F_2g(3) at 661 cm⁻¹ for ZnAl₂O₄, and A_1g at 748 cm⁻¹ for FeAl₂O₄. Noteworthy, analyzing the A_1g, F_2g(3), and, in particular, the E_g peak positions, it is possible to establish which subgroup a spinel belongs to; moreover, a careful inspection of both position and relative intensity of the same peaks allows the determination of the end‐member type. Copyright © 2015 John Wiley & Sons, Ltd.     

A molecular field theory for inverse spinel ferrite

A molecular field theory is developed for inverse spinel ferrites which takes into account the disorder arising from the random arrangement of two types of magnetic ions in the B sublattice. It is seen that the disorder gives rise to a local molecular field fluctuation acting on a spin in the A sublattice. The temperature behavior of the configuration average of the z component of the spin is presented.     

First-principles study of antiferromagnetic cobalt spinels

We report a comparative study of the electronic structure and magnetic properties of two cobalt compounds Co₃O₄ and Co₃S₄, through first-principles Hubbard-U calculations. Our results indicate that Co₃O₄ and Co₃S₄ have similarities in crystal structure (normal spinel), magnetic order (antiferromagnetism), Co spin configuration (high spin Co²⁺ and low spin Co³⁺), and comparable band-gap energy. However, the U-dependence on electronic structure in two materials are different from each other. With a change in the applied U values, the band dispersion and the type of band gap are significantly changed in Co₃O₄, while the band-gap energy only is affected in Co₃S₄.     

Effect of Cu2+-O2− covalency on the magnetostriction of ferrite spinels

The magnetostriction of a number of copper-containing ferrite spinels was measured. In all the ferrites studied, below room temperature, |λ_‖| was found to decrease with temperature. It is conjectured that this decrease in |λ_‖| originates from an increase in the degree of covalency among the Cu²⁺-O^2? ions, which, in turn, reduces their spin-orbit coupling.     

Mössbauer study of iron-cobalt-rhodium spinels

Mössbauer source and absorber studies have been carried out on the spinel system CoFe_xRh_2?xO₄ for x = 0.005, 0.3, 0.5, 1.0, 1.2 and 1.5. For 0.005 ? x ? 1.2, the cation distribution is normal with Co²⁺ on Asites. At x = 1.5, the distribution is nearly inverse. In the cases x = 0.005 and 0.3, iron on the B sites does not produce a quadrupole doublet indicating that the B sites are cubic which is contrary to the usual case in spinels.     

X-ray study of some spinels containing gallium and manganese

The ternary oxides CrMnGaO₄, NiMnGaO₄, CuMnGaO₄ and ZnMnGaO₄, crystallize in the cubic spinel structure with lattice parametera=8.41±0.02 Å, 8.34±0.02 Å, 8.36±0.02 Å and 8.32±0.02 Å, respectively. The oxidation state of manganese in these spinels was determined x-ray spectroscopically. The site distribution was determined from the structural properties and calculated site preference energies of cations in the lattice. The ionic structures were found to be Ga³⁺ [Mn²⁺ Cr³⁺] O ₄ ^2? . Ga³⁺ [Cu²⁺ Mn³⁺] O ₄ ^2? , Mn²⁺ [Ga³⁺ Ni³⁺] O ₄ ^2? and Zn²⁺ [Mn³⁺ Ga³⁺] O ₄ ^2? .     

High-resolution67Zn-Mössbauer study of oxide spinels

We report on first experiments to investigate the electronic structure in the normal spinels ZnAl₂O₄ and ZnFe₂O₄ and in the inverse spinels Zn₂SnO₄ and Zn₂TiO₄ using the high-resolution⁶⁷Zn-Mössbauer spectroscopy. The electric field gradient for⁶⁷Zn at the B (octahedral) site in ZnAl₂O₄ is negative, whereas the A (tetrahedral) site remains essentially cubic, however, with a more positive center shift. ZnFe₂O₄ orders antiferromagnetically at ≈10K. Due to superexchange a magnetic field is observed at⁶⁷Zn. In the inverse spinels short range order leads to only few (Zn,Sn) and (Zn,Ti) configurations at the octahedral sites. The s-electron densities at the⁶⁷Zn sites are distinct and cover a surprisingly broad range for Zn₂TiO₄. This strongly suggests that d-electrons of Ti play an essential role in the chemical bond of this compound.     

Special electronic structures of inverse spinels LiMVO<Subscript>4</Subscript>(M = Ni and Cu): a first-principles study

We use the Ulam method to study spectral properties of the Perron-Frobenius operators of dynamical maps in a chaotic regime. For maps with absorption we show numerically that the spectrum is characterized by the fractal Weyl law recently established for nonunitary operators describing poles of quantum chaotic scattering with the Weyl exponent ν = d-1, where d is the fractal dimension of corresponding strange set of trajectories nonescaping in future times. In contrast, for dissipative maps we numerically find the Weyl exponent ν = d/2 where d is the fractal dimension of strange attractor. The Weyl exponent can be also expressed via the relation ν = d₀/2 where d₀ is the fractal dimension of the invariant sets. We also discuss the properties of eigenvalues and eigenvectors of such operators characterized by the fractal Weyl law.     

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.     

Electrochemical and structural comparison of zinc-cobalt doped spinels with other doped lithium manganese spinels

Doping of lithium manganese spinels by zinc and cobalt of the type (Li_1xZn_x)(Mn_2–2x Co_2x)O₄ may stabilize the delithiated spinels and may offer some indications with respect to the validity of capacity fade models. There are structural chemical arguments for this cation distribution. These and other doped lithium manganese spinels were prepared by heat treatment (in the range between 500 and 800 °C for 12 h) of solution precipitated precursors. Samples were characterised structurally and electrochemically by XRD and galvanostatic cycling. Extended cycling, storage in the charged state and storage in the discharged state were investigated. Pure phases of Zn-Co doped samples were obtained only for quenching the fired samples. Otherwise separation into a tetragonal spinel and a cubic spinel occurred. XRD results prove the occupation of tetrahedral coordinated cation positions by zinc ions, in contradiction to the results of other authors. XRD profiles show an anisotropic line broadening, which is attributed to an anisotropic microstrain, maybe induced by a non-cooperative Jahn-Teller distortion. Capacity retention was better for extended cycling and worse for both kinds of storage, compared to a purely cobalt doped spinel. Therefore, thermodynamic stabilisation of the delithiated spinels could not be confirmed. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998     

TEM study of NbC heterogeneous precipitation in ferrite

The heterogeneous precipitation of NbC in ferrite has been quantitatively characterized by transmission electron microscopy in a Fe–C–Nb model alloy for different isothermal heat treatments. The elongation and size distribution of precipitates were derived using dark field imaging. For each precipitation state, the precipitation of NbC occurs on dislocations due to the as-quenched state. This precipitation mechanism leads to characteristic arrays of precipitates in which precipitates grow in a self-similar manner. A detailed study of these arrays has shown that most dislocations decorated by these arrays are edge dislocations with ?112? type line vectors. There is only one variant on a given dislocation. This selection can be interpreted by the interaction between dislocation and precipitate strain fields.     

Low temperature NMR study of cadmium-substituted copper ferrite

NMR spectra of Cd_XCu_1−xFe₂O₄ /0≤x≤0.30/ at liquid helium temperature have been obtained in the frequency range of 50 to 110 MHz using the spin-echo technique. Influence of substitution of Cu²⁺ ions by Cd²⁺ ions on effective magnetic fields at⁵⁷Fe nuclei has been found only at octahedral sites.     

EELS study of niobium carbo-nitride nano-precipitates in ferrite

Micro-alloying steels allow higher strength to be achieved, with lower carbon contents, without a loss in toughness, weldability or formability through the generation of a fine ferrite grain size with additional strengthening being provided by the fine scale precipitation of complex carbo-nitride particles. Niobium is reported to be the most efficient micro-alloying element to achieve refinement of the final grain structure. A detailed microscopic investigation is one of the keys for understanding the first stages of the precipitation sequence, thus transmission electron microscopy (TEM) is required. Model Fe-(Nb,C) and Fe-(Nb,C,N) ferritic alloys have been studied after annealing under isothermal conditions. However the nanometre scale dimensions of the particles makes their detection, structural and chemical characterization delicate. Various imaging techniques have then been employed. Conventional TEM (CTEM) and high resolution TEM (HRTEM) were used to characterise the morphology, nature and repartition of precipitates. Volume fractions and a statistical approach to particle size distributions of precipitates have been investigated by energy filtered TEM (EFTEM) and high angle annular dark field (HAADF) imaging. Great attention was paid to the chemical analysis of precipitates; their composition has been quantified by electron energy loss spectroscopy (EELS), on the basis of calibrated 'jump-ratios' of C-K and N-K edges over the Nb-M edge, using standards of well-defined compositions. It is shown that a significant addition of nitrogen in the alloy leads to a complex precipitation sequence, with the co-existence of two populations of particles: pure nitrides and homogeneous carbo-nitrides respectively.     

Iron-rich spinels from Brazilian soils

Tropical soils often contain high amounts of iron oxides. Hematite (αFe₂O₃) and goethite (αFeOOH) are the most widespread iron oxides, but magnetite (Fe₃O₄) and maghemite (γFe₂O₃) occur in magnetic pedons. A wide range of spinel compositions in the Fe₃O₄-γFe₂O₃ series has been identified in magnetic Brazilian soils. Isomorphic substitution of mainly Ti⁴⁺, Al³⁺ and Mg²⁺, but also of Cr³⁺ and Mn²⁺ and other minor elements for iron are related to changes in their structural stability and magnetic properties. Magnetic iron oxides of selected Brazilian pedodomains are discussed, distinguishing those produced from mafic rocks (tuffite, basalt), where primary magnetite transforms to maghemite, from those produced in non-mafic lithologies (such as steatite), where inherited magnetite may be exceptionally stable in the soil. This revised version was published online in August 2006 with corrections to the Cover Date.     

ZnO-based spinels grown by electrodeposition

We report on the synthesis of thin films of ZnCo₂O₄ and ZnMn₂O₄ spinels, as well as pure Co₃O₄ and Mn₃O₄ spinels, by means of electrodeposition. Spinel thin films have been analyzed by energy dispersive spectroscopy, X-ray diffraction, and Raman spectroscopy. We show that under determined deposition conditions the initial wurtzite structure of Co- and Mn-doped ZnO develops into spinel structures when the Co and Mn concentration in the films is above the solubility limit of these ions in the typical ZnO-wurtzite structure.     

Mg-rich iron oxide spinels from tuffite

Powdered crystals of an iron-rich spinel separated from tuffite were examined by chemical analysis, X-ray diffractometry (XRD), magnetometry, scanning electron microscopy and Mössbauer spectroscopy. They are found to contain mainly 48 mass% Fe₂O₃; 16.9% FeO; 14.3% TiO₂; 9% MgO and 3% Al₂O₃. The magnetization is 29 J T^-1 kg^-1 at B₀ ~ 0.9 T. XRD pattern reveals two cubic phases with α₀ = 0.8382(5) and 0.8412(5) nm, respectively. Mössbauer spectra are very complex, but the hyperfine field distribution patterns can be decomposed in two relatively well-resolved Gaussian distributions for the Fe³⁺ block (averaged isomer shift relative to α-Fe, δ = 0.27 mm s^-1) with B_hf = 43.6 T (14.6% of the whole spectral area, WSA) and B_hf = 47.3 (27.7% WSA). A second less resolved distribution block (δ = 0.63 mm s^-1) shows two main maxima in the P(B_hf) curve at 35.3 T (25.7% WSA) and 45.2 T (19.7% WSA), respectively. The Mg-rich maghemite (lower α₀) appears to be a direct alteration product of a Mg-rich magnetite (higher α₀), via an oxidation process of structural Fe²⁺ to Fe³⁺ proceeding from the outer to the inner part of the crystal.