On the role of Fe2+ ions and cation vacancies in the disaccommodation of ferrites

A study was made of the disaccommodation spectrum of ferrites Mn _x Fe _3–x O _4+y , x>1, in a temperature range above 0°C. A comparison with analogous spectra of ferrites rich in iron (x<1), which contain Fe²⁺ ions in addition to cation vacancies, shows that the disaccommodation maximum around room temperature, observed in iron rich ferrites, is subject to the presence of Fe²⁺ ions. On the other hand, the processes taking place at higher temperatures seem to be substantially common to both types of ferrites and are dependent only on the presence of vacancies.     

The pyroelectric effect in magnetoelectric 0.8 PZT-0.2 MZF and 0.8 PZT-0.2 NZF composites

The pyroelectric response is measured using the dynamic method in preliminarily polarized magnetoelectric 0.8PbZr_0.53Ti_0.47O₃-0.2Mn_0.4Zn_0.6Fe₂O₄(0.8 PZT-0.2 MZF) and 0.8PbZr_0.53Ti_0.47O₃-Ni_0.4Zn_0.6Fe₂O₄ (0.8 PZT-0.2 NZF) composites, where PZT is a ferroelectric, and MZF and NZF are manganese-zinc and nickel-zinc ferrites, respectively. Values of the pyroelectric coefficient Γ for the materials under study vary over the range of 1.5–4 × 10⁻⁸ C cm⁻² K⁻¹. It is shown that the polarized samples of the composites have a heterogeneous distribution of polarization across the sample thickness. The heterogeneity is more pronounced in the near-surface layers of the investigated materials.     

Nonequivalent positions of Fe3+ ions and relaxation effects in Mn-Zn-ferrites

A study is made of manganese-zinc ferrites Mn_1–xZn_xFe₂O₄ where x varies from 0.3–0.9. It is established that the variation of the form of the spectra with variation of x is of a relaxation nature. In this system the nonequivalent nature of the neighborhood of the ions has an effect on the relaxation processes and on the character of the magnetic ordering.Translated from Izvestiya VUZ, Fizika, No. 3, pp. 77–81, March, 1973.     

Structural and Mössbauer studies of nanocrystalline Mn<Superscript>4+</Superscript>-doped Li<Subscript>0.5</Subscript>Fe<Subscript>2.5</Subscript>O<Subscript>4</Subscript> particles prepared by mechanical milling

The structure and magnetic properties of spinel-related Mn⁴⁺-doped Li_0.5Fe_2.5O₄ nanocrystalline particles of the composition Li_0.5Fe_2.25Mn_0.1875O₄, prepared by milling a pristine sample for different times, were investigated. The average crystallite and particle size, respectively, decreased form ～40 nm to ～10 nm and ～2.5 μm to ～10 nm with increasing milling time from 0 h to 70 h. Rietveld refinement of the XRD data of the non-milled sample show the Mn⁴⁺ dopant ions to substitute for Fe³⁺ at the octahedral B-sites of the spinel-related structure. The Mössbauer spectra of the milled ferrites indicate that more particles turn superparamagnetic with increasing milling time. The Mössbauer data collected at 78 K suggest that while in the non-milled sample the Mn⁴⁺ ions substitute for Fe³⁺ at the octahedral B-sites, this is reversed as milling proceeds with doped Mn⁴⁺ ions, balancing Fe³⁺ vacancies and possibly Li⁺ ions progressively migrate to the tetrahedral A-sites. This is supported by the slight increase observed in the magnetization of the milled samples relative to that of the non-milled one. The magnetic data suggest that in addition to the increasing superparamagentic component of the milled particles, thermal spin reversal and/or spin canting effects are possible at the surface layers of the nanoparticles.     

Electromagnetic properties of manganese-zinc ferrite with lithium substitution

Polycrystalline manganese-zinc ferrite with lithium substitution of composition Li_0.5xMn_0.4Zn_0.6−xFe_2+0.5xO₄ (0.0≤x≤0.4) was prepared by the usual ceramic method. X-ray diffraction analysis confirmed that the samples have a spinel structure and are of single phase for some values of Li content. Lithium doping considerably modifies saturation magnetization since its value increases from 57.5 emu/g for x=0.0 to 82.9 emu/g for x=0.4. Lithium inclusion increases the real permeability (over 1 MHz) while the natural resonance frequency shifts to lower values as the fraction of Li increases. These ferrites show good electromagnetic properties as absorbers in the microwave range of 1 MHz - 1 GHz.     

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.     

Microstructure characterization and magnetic behavior of NiAlxFe2−xO4 and Ni1−yMnyAl0.2Fe1.8O4 ferrites

NiAl_xFe_2−xO₄ and Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites were prepared by the conventional ceramic method and were characterized by X-ray diffraction, scanning electron microscopy, and magnetic measurements. The single spinel phase was confirmed for all prepared samples. A proper explanation of data is possible if the Al³⁺ ions are assumed to replace Fe³⁺ ions in the A and B sites simultaneously for NiAl_xFe_2−xO₄ ferrites, and if the Mn²⁺ ions are assumed to replace Ni²⁺ ions in the B sites for Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites. Microstructural factors play an important role in the magnetic behavior of Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites with large Mn²⁺ content.     

Distortion of a matrix structure and the cluster formation in MnxZnyFezO4 ferrite single crystals

The evolution of the hard and soft modes in the crystal structure of Mn_xZn_yFe_zO₄ manganese-zinc spinel ferrites is investigated theoretically and experimentally (x-ray structure analysis and magnetic measurements). It is shown that tetragonal and rhombohedral distortions of the cubic structure and fluctuations of the soft mode are accompanied by the formation of clusters in the form of quasi-two-dimensional fragments (50–200 and 400–1200 Å), which involve families of γ-Mn₂O₃, γ-Mn₃O₄, and α-Fe₂O₃ oxide planes inherently bound to the spinel matrix.     

P2-type Na<Subscript>0.67</Subscript>Mn<Subscript>0.6</Subscript>Fe<Subscript>0.4-x-y</Subscript>Zn<Subscript>x</Subscript>Ni<Subscript>y</Subscript>O<Subscript>2</Subscript> cathode material with high-capacity for sodium-ion battery

The P2-type Na_0.67Mn_0.6Fe_0.4O₂ (NaMnFe), Na_0.67Mn_0.6Fe_0.3Zn_0.1O₂ (NaMnFeZn), and Na_0.67Mn_0.6Fe_0.2Zn_0.1Ni_0.1O₂ (NaMnFeZnNi) are prepared using an acetate decomposition reaction and developed as promising cathode materials for high-capacity sodium-ion batteries. The XRD patterns show that Zn²⁺ and Ni²⁺ ions are successfully incorporated into the lattice of the Na-Mn-Fe-O system, and the P2-type structure remains unchanged after substitution. The charging/discharging tests exhibit that the Na_0.67Mn_0.6Fe_0.4O₂, Na_0.67Mn_0.6Fe_0.3Zn_0.1O₂, and Na_0.67Mn_0.6Fe_0.2Zn_0.1Ni_0.1O₂ electrodes have the capacities of 200.4, 182.0, and 202.2 mAhg^?1, respectively. The Na_0.67Mn_0.6Fe_0.4O₂ electrode has a higher initial capacity but faster capacity decay. When partially substituting Zn and Ni for Fe, the Na_0.67Mn_0.6Fe_0.3Zn_0.1O₂ and Na_0.67Mn_0.6Fe_0.2Zn_0.1Ni_0.1O₂ electrodes exhibit lower reversible capacity but improved cycling stability (88.3 and 93.4% capacity retention over 100 cycles). The greatly improved electrochemical performance of the Na_0.67Mn_0.6Fe_0.2Zn_0.1Ni_0.1O₂ electrode apparently belongs to the contribution of the Zn²⁺ and Ni²⁺ substitution, which facilitates to alleviate the Jahn-Teller distortion of Mn and suppresses the polarization.     

Thermoelectric power and magnetic properties of Cu doped NiZn ferrite for technological application

A series of samples in the system Ni_0.65Zn_0.35Cu_xFe_2-xO₄ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) were prepared by the usual ceramic technique. The thermoelectric power and the magnetic susceptibility were measured. The transition from the ferrimagnetic to the paramagnetic state is accompanied by an increase in the thermo EMF. NiZn ferrite shows n-type conductivity due to the presence of Fe²⁺ ions. The addition of Cu²⁺ ions creates lattice vacancies which give rise to p-type conductivity.

The Tawfik coefficient was determined for NiZn ferrite in the paramagnetic state. This coefficient was reduced by addition of Cu up to x < 0.5.     

Magnetic and ferroelectric properties of exchange-frustrated multiferroics (Ni1 ? xTx)3V2O8 (T = Co, Mn, Zn)

The effect of the substitution of Co²⁺, Mn²⁺, and Zn²⁺ ions for Ni²⁺ ions on the magnetic, dielectric, and ferroelectric properties of vanadate single crystals (Ni_{1 − x} T _x )₃V₂O₈ has been analyzed. It has been found that the low-level (x ≤ 0.1) substitution of both magnetic and nonmagnetic ions stabilizes the ferroelectric state with a cycloidal magnetic structure. The existence region of this state is expanded to low temperatures down to 3 K for Zn²⁺ and below 1.8 K for Co²⁺ and Mn²⁺ owing to the suppression of a low-temperature weak ferromagnetic phase. At the same time, the ferroelectric phase disappears completely at large concentrations of Co and Mn. The effect of magnetic fields on the magnetic and ferroelectric states has been analyzed. It has been shown that the magnetic field along the c axis suppresses the ferroelectric state, whereas the magnetization along the antiferromagnetism axis (a axis) induces the reentrant phase transition from a paraelectric weak ferromagnetic structure to a ferroelectric structure. The corresponding H-T phase diagrams have been drawn.     

Effects of MnO2 on the electromagnetic properties of NiCuZn ferrites prepared by sol-gel auto-combustion

Mn-doped NiCuZn ferrites with compositions of (Ni_0.2Cu_0.2Zn_0.6)O(Fe_2−x,Mn_xO₃)_0.98 (x=0, 0.02, 0.04, 0.06) were prepared by a novel sol–gel auto-combustion process. The synthesized nano-sized ferrite powders can be sintered at 900°C, and the sintered ferrites are characterized by fine-grained microstructural feature and high permeability. Mn content in formulations largely affects the grain size and main electromagnetic properties of sintered NiCuZn ferrites. With increasing Mn content, the initial permeability is significantly increased, while the electrical resistivity and quality factor are decreased. The dielectric constant and dissipation factor are also affected by the incorporation of MnO₂. The possible mechanism for the influence of MnO₂ on the electromagnetic properties was discussed.     

NMR study of local magnetic properties of beryllium substituted copper ferrite

The spin echo NMR spectra of⁵⁷Fe have been taken at 77 K to describe the local magnetic properties of beryllum substituted copper ferrite Cu_1−xBe_xFe₂O₄ for 0≤x≤0.2. From the spectra the concentration dependence of hyperfine magnetic fields for tetrahedral and octahedral sites have been derived. The results show that statistical distribution of beryllium atoms over magnetic sublattices takes place and suggest that only below x=0.2 solid solutions of beryllium copper ferrites can exist.     

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.     

Synthesis of (Mg0.476Mn0.448Zn0.007)(Fe1.997Ti0.002)O4 powder and sintered ferrites by high energy ball-milling process

(Mg_0.476Mn_0.448Zn_0.007)(Fe_1.997Ti_0.002)O₄ nanocrystalline powder prepared by high energy ball-milling process were consolidated by microwave and conventional sintering processes. Phases, microstructure and magnetic properties of the ferrites prepared by different processes were investigated. The (Mg_0.476Mn_0.448Zn_0.007)(Fe_1.997Ti_0.002)O₄ nanocrystalline powder could be prepared by high energy ball-milling process of raw Fe₃O₄, MnO₂, ZnO, TiO₂ and MgO powders. Prefired and microwave sintered ferrites could achieve the maximum density (4.86 g/cm⁻³), the average grain size (15 μm) was larger than that (10 μm) prepared by prefired and conventionally sintered ferrites with pure ferrite phase, and the saturation magnetization (66.77 emu/g) was lower than that of prefired and conventionally sintered ferrites (88.25 emu/g), the remanent magnetization (0.7367 emu/g) was higher than that of prefired and conventionally sintered ferrites (0.0731 emu/g). Although the microwave sintering process could increase the density of ferrites, the saturation magnetization of ferrites was decreased and the remanent magnetization of ferrites was also increased.     

EPR and optical spectroscopy of impurities in two synthetic beryls

Two synthetic beryls (Al₂Be₃Si₆O₁₈) of different color (purple and blue-green) were studied with electron paramagnetic resonance (EPR) and optical spectroscopy. In both crystals, the known spectra of Cu²⁺ and Fe³⁺ were observed with the same relative intensity. In the purple sample heated at 700°C in hydrogen atmosphere, two different kinds of Mn²⁺ EPR spectra were observed. The main one is pseudoaxial, it arises from ions substituted for Al³⁺ at position 4c of the structure. The weaker one is more complex, it has orthorhombic symmetry and is characterized by an unusually large zero-field splitting (B ₂⁰ = 741 · 10⁻⁴ cm⁻¹) and an isotropic hyperfine constantA = 70 G. This spectrum arises from Mn²⁺ at position 6f in the structure, normally occupied by Be. From optics, the blue-green color arises from Cu²⁺, while the purple one is due to Mn³⁺.     

Characterization and magnetic properties of electrospun Co1−x Zn x Fe2O4 nanofibers

By the electrospinning and calcination techniques, we have prepared uniform nanofibers of Co_1−x Zn _x Fe₂O₄ (0.0≤x≤0.5) ferrites with diameters of 110–130 nm. The Co_1−x Zn _x Fe₂O₄ nanofibers are single-phase spinels and the lattice constant with Zn content deviates from the Vegard’s law for these Co_1−x Zn _x Fe₂O₄ nanofibers. The Co_1−x Zn _x Fe₂O₄ nanocrystal grains by which are built nanofibers increase with calcination temperature. Variations of coercivity and saturation magnetization with calcination temperature can be explained in terms of the grain-size (D) effect. The coercivity (H _c) of Co_0.5Zn_0.5Fe₂O₄ nanofibers varies as D ^0.65 and basically follows the predicted D ^2/3 dependence based on the random anisotropy model in a D range below the single-domain size around 40 nm. The saturation magnetization of Co_1−x Zn _x Fe₂O₄ nanofibers initially increases with increasing Zn content, reaches a maximum value at x=0.3 and then decreases with further increase of Zn content, while the coercivity exhibits a continuous reduction with the increase of Zn content.     

EPR studies of Cu2+ and V4+ ions in phosphate glasses

Two lead-phosphate glass systems doped with both copper and vanadium ions in different ratios were studied by EPR (electron paramagnetic resonance) method. EPR spectra and parameters (g_‖ = 2.44, g_⊥ = 2.08 andA _‖ = 117.6 · 10⁻⁴ cm⁻¹) obtained for x(CuO · V₂O₅)(l−x)[2P₂O₅ · PbO] glasses withx ≤ 10 mol% suggest a tetrahedral (Td) coordination of Cu²⁺ ions and not a tetragonally elongated octahedron as has been assumed in previous works. The ground state of the paramagnetic electron is thed _xy copper orbital with a 4p_z contribution of 6%. For 20 ≤x ≤ 40 mol% a broad line (ΔB = 307 G) characteristic for clustered ions appears atg = 2.18. The V⁴⁺ ions are evidenced only in the spectra of x(CuO · 2V₂O₅)(1 −x)[2P₂O₅ · PbO] glasses and the resonance parameters suggest a pentacoordinated C_4v local symmetry for these ions. The hyperfine structures characteristic for Cu²⁺ and V⁴⁺ ions disappear for 10 ≤x ≤ 40 mol% due to the mixed exchange Cu²⁺−V⁴⁺ pair formation in these glasses.     

Effect of Nd ion on the magnetic properties of Ni–Mn ferrite nanocrystal

The nanocrystalline Ni_0.7Mn_0.3Nd_0.1Fe_1.9O₄ ferrites were prepared by the emulsion method. X-ray diffractometer (XRD), transmission electron microscope (TEM), Mössbauer spectra and vibrating samples magnetometer (VSM) were used to study the structure, morphology and magnetic properties. The magnetic properties of Nd³⁺-doped Ni_0.7Mn_0.3Fe₂O₄ nanocrystal ferrites were investigated in detail. The Ni_0.7Mn_0.3Nd_0.1Fe_1.9O₄ nanocrystal ferrite with particle size of 10.7 nm shows superparamagnetism.     

119Sn and 57Fe M?ssbauer study of the local structure of perovskite-type ferrites CaFe2 ? x N x O5 ( N = Sc, Al) and manganite CaMn7O12

Anion-deficient substituted ferrites Ca₂Fe_{2 − x} N _x O₅ (N = Sc³⁺, Al³⁺) and mixed manganite CaMn₇O₁₂ have been investigated by ¹¹⁹Sn and ⁵⁷Fe probe M?ssbauer spectroscopy. The mechanism of charge compensation for heterovalent impurity Sn⁴⁺ ions in the structure of the ferrite Ca₂Fe₂O₅ has been established. The presence of nonequivalent crystallographic positions of manganese cations, caused by their charge ordering in the structure of the manganite CaMn₇O₁₂, is shown. Magnetic ordering of Mn³⁺ and Mn⁴⁺ cations in the octahedral sublattice of CaMn₇O₁₂ at T < T _M2 ≈ 90 K is established. Original Russian Text ? A.V. Sobolev, I.A. Presnyakov, K.V. Pokholok, V.S. Rusakov, T.V. Gubaidulina, A.V. Baranov, G. Demazeau, 2007, published in Izvestiya Rossiiskoi Akademii Nauk Seriya Fizicheskaya, 2007, Vol. 71, No. 9, pp. 1347–1354.