Susceptibility of the paraprocess in spinel ferrites with a frustrated magnetic structure

The temperature dependence of the susceptibility of the paraprocess χ_para (T) is investigated for samples in the CuGa_xAl_xFe_2?2xO₄ (x = 0.2, 0.3, 0.4, 0.5, 0.6, 0.7), CuGa_xAl_2xFe_2?3xO4 (x = 0.1, 0.2, 0.3, 0.4, 0.5), and Ga_xFe_{1? x}NiCrO₄ (x = 0.0, 0.2, 0.4, 0.6, 0.8) systems. It is found that long-range magnetic order arises in spinel ferrites at the temperature T_tran of the transition from a cluster spin-glass state to a frustrated magnetic structure with a maximum in the temperature dependence of the susceptibility of the paraprocess.     

Investigation of magnetic phase transitions in dilute copper ferrites

Mössbauer spectra of copper ferrites CuGa _x Al_2x Fe_{2 ? 3x} O₄ (x = 0.3, 0.4, 0.5) have been measured at temperatures in the range from 90 to 295 K. The degree of disturbance of long-range magnetic order has been determined as a function of the temperature. The Curie temperature has been found for the composition with x = 0.5, and the dependence of the Curie temperature on the concentration of the nonmagnetic impurity has been constructed.     

Mössbauer study of copper ferrite diluted by Ga<Superscript>3+</Superscript> and Al<Superscript>3+</Superscript> ions

Mössbauer spectra of the CuGa _x Al_2x Fe_{2 ? 3x} O₄ system (x = 0.1, 0.2, 0.3, 0.4, 0.5) have been studied at a temperature of 295 K. The results obtained are compared with Mössbauer data for the CuGa _x Al _x Fe_2?2x O₄ system. It is established that the hyperfine magnetic fields H _B (for octahedral sites) and H _A (for tetrahedral sites) for ferrites in both systems with ferrimagnetic ordering decrease linearly depending on the total number of nonmagnetic ions in octahedral and tetrahedral sites of the two systems.     

Magnetic State of Quasiordered Fe–Al Alloys Doped with Ga and B: Magnetic Phase Separation and Spin Order

Results of structural, magnetic, and Mössbauer studies of quasi ordered alloys Fe₆₅Al_{35 ? x}M _x (M _x = Ga, B; x = 0, 5 at %) are presented. The magnetic state of examined structurally–single-phase alloys at low temperatures is interpreted from the viewpoint of magnetic phase separation. An explanation is proposed for the observed behavior of magnetic characteristics of Fe₆₅Al₃₅ and Fe₆₅Al₃₀Ga₅ in the framework of the model of two magnetic phases, a ferromagnetic-type one and a spin density wave. The boron-doped alloy Fe₆₅Al₃₀B₅ is shown to demonstrate behavior that is typical of materials with the ferromagnetic type of ordering.     

Zn-substituted brownmillerite-type strontium ferrites

New La/Zn substituted strontium ferrites Sr_{(2 ?x)}La _x [Fe_{(2 ? x)}Zn _x ]O₅, (0 ≤ x ≤ 0.3) with brownmillerite- type structure are obtained and studied via X-ray phase analysis and Mössbauer spectroscopy. Depending on the conditions of synthesis, substituting Zn²⁺ cations can either mainly occupy tetrahedral positions in a brownmillerite structure, or be uniformly distributed between the tetrahedral and octahedral positions. It is shown that they are reversibly oxidized by atmospheric oxygen at elevated temperatures.     

Microwave Absorption Properties of Nickel–Zinc Ferrites Synthesized by Different Means

Ferromagnetic resonance (FMR) characteristics are investigated for Ni_xZn_{1 ? x}Fe₂O₄ ferrites (where x = 0.25, 0.5) synthesized using different organic complexation agents. The microstructure of the synthesized ferrites is studied, and the effect complexation agents have on their microstructure and FMR characteristics is determined.     

X-ray,Mössbauer,and magnetic research of system Y(Fe<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>2</Subscript> alloys

The structural transformations and magnetic phase transitions in the quasibinary system Y(Fe_{1 ? x} Al _x )₂ have been investigated by Mössbauer spectroscopy, X-ray diffractometry, and magnetic measurements of polycrystals.     

Influence of oxidized interlayers on magnetic properties of multilayer films based on amorphous ferromagnet–dielectric nanocomposites

Films of composites (Co₄₅Fe₄₅Zr₁₀)_x(Al₂O₃)_100–x, (Co₈₄Nb₁₄Ta₂)_x(SiO₂)_100–x, (Co₄₁Fe₃₉B₂₀)_x(SiO₂)_100–x and multilayer heterogeneous composite–composite structures {[(Co₄₅Fe₄₅Zr₁₀)_x(Al₂O₃)_100–x]/[(Co₄₅Fe₄₅Zr₁₀)_x(Al₂O₃)_100–x + N₂]}_n, {[(Co₄₅Fe₄₅Zr₁₀)_x(Al₂O₃)_100–x]/[(Co₄₅Fe₄₅Zr₁₀)_x(Al₂O₃)_100–x + O₂]}_n, {[(Co₄₁Fe₃₉B₂₀)_x(SiO₂)_100–x]/[(Co₄₁Fe₃₉B₂₀)_x(SiO₂)_100–x + O₂]}_n, and {[(Co₈₄Nb₁₄Ta₂)_x(SiO₂)_100–x]/[(Co₈₄Nb₁₄Ta₂)_x(SiO₂)_100–x + O₂]}_n have been deposited using the ionbeam sputtering method with a cyclic supply of reaction gases during deposition. The structure and magnetic properties of the films have been studied. It has been shown that the introduction of an oxidized interlayer makes it possible to suppress the perpendicular magnetic anisotropy in the (Co₄₅Fe₄₅Zr₁₀)_x(Al₂O₃)_100–x composite with the metallic phase concentration higher than the percolation threshold.     

Structural and magnetic phase transformations in the BiFeO<Subscript>3</Subscript>-CaFe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> system

The crystal structure and magnetic properties of the Bi_{1 ? x} Ca _x Fe_{1 ? x/2}Nb _x/2O₃ system were studied. It is shown that, at x ≤ 0.15, the unit-cell symmetry of solid solutions is rhombohedral (space group R3c). Solid solutions with x ≥ 0.3 have an orthorhombic unit cell (space group Pbnm). The rhombohedral compositions are antiferromagnetic, while the orthorhombic compositions exhibit a small spontaneous magnetization due to Dzyaloshinski?-Moriya interaction. In CaFe_0.5Nb_0.5O₃, the Fe³⁺ and Nb⁵⁺ ions are partially ordered and the unit cell is monoclinic (space group P2₁/n). In the concentration range 0.15 < x < 0.30, a two-phase state (R3c + Pbnm) is revealed.     

Magnetoresistance and the hall effect of the ordered alloys Fe<Subscript>100 −<Emphasis Type="Italic">x</Emphasis></Subscript>Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> (25 < <Emphasis Type="Italic">x</Emphasis> < 35 at. %)

The transverse magnetoresistance and the Hall effect of Fe_{100 ?x} Al _x ordered alloys with x = 26.5?34.1 at. % are studied at temperatures of 77 and 295 K. No saturation is found in the field dependences up to 17 kOe. The magnetoresistance is negative, depends nonmonotonically on the Al concentration, and reaches a maximum |ΔR/R| = 1.6% in the concentration range from 28 to 30 at. %. The coefficient of the anomalous Hall effect depends nonlinearly on the Al concentration and resistivity: it increases to a lesser extent as compared to the resistivity at x = 26–30 at. % and decreases with increasing resistivity at x > 30 at. %. An explanation is proposed for the observed anomalies in the magnetotransport properties in the framework of the model developed for the inhomogeneous magnetic microstructure of the Fe_{100 ?x} Al _x ordered alloys.     

Single-crystalline M-type Sr Hexaferrites studied by <Superscript>57</Superscript>Fe Mössbauer spectroscopy

The ⁵⁷Fe Mössbauer spectra of the single crystalline and the finely ground Sr_1?x La _x Fe_12?y Co _y O₁₉ (x = 0 : y = 0, x = 0.192 : y = 0.152 and x = 0.456 : y = 0.225) samples have been measured to investigate the La-Co substitution effects. All observed spectra at 150 K were well fitted using the five subspectra which correspond to the five crystallographical nonequivalent Fe sites in the M-type hexaferrite, indicating that the valence changes to Fe²⁺ ions in the Fe³⁺ ions were not observed in our Sr_1?x La _x Fe_12?y Co _y O₁₉ samples. In SrFe₁₂O₁₉, the relative absorption intensities in the five subspectra show the large anisotropies in the recoilless fractions at the five Fe sites whereas these anisotropies were not observed in Sr_0.544La_0.456Fe_11.775Co_0.225O₁₉. These results indicate the chemical compositional dependence on the anisotropies of the recoilless fractions at the five Fe sites. The substitution of a Co²⁺ ion for the Fe³⁺ ion changes the center shifts of the Fe³⁺ ions near the Co²⁺ ion by the perturbation of the Fe-O-Co hybridizations. Therefore, the Co²⁺ ions occupy the 4f ₁ and the 4f ₂ sites due to the chemical compositional dependences of the refined magnetic hyperfine field and center shifts of the Fe³⁺ ions.     

Structural and magnetic phase transformations in the diluted laves phases Pr(Fe<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>2</Subscript>

The Pr(Fe_{1 ? x} Al _x )₂ alloys with concentrations x = 0–1 have been synthesized under a high pressure. The phase composition and lattice parameters (a and c) have been determined as a function of x. The magnetic and Mössbauer measurements have been performed at T = 90–400 K. It has been established that the Curie temperatures of alloys linearly depend on their composition.     

Investigation of magnetic phase transitions in diluted ferrites: Nickel ferrite

The Mössbauer spectra of nickel ferrite NiGa _x Al _x Fe_{2 ? 2x} O₄ were measured with significant dilution by nonmagnetic ions (with x = 0.5, 0.7, and 0.8). In the temperature range of 90–295 K, the ratio of the near- and long-range forces was experimentally found and the Curie temperatures were determined.     

Influence of Si substitution on structural,electronic and magnetic properties of Fe<Subscript>2</Subscript>MnGa Heusler compound

We investigate the electronic and magnetic properties of Fe₂MnGa_1?x Si _x alloy (x = 0, 0.25, 0.5, 0.75, and 1) using first-principles density functional theory within the generalized gradient approximation method. The lattice constant decreases linearly whereas bulk modulus increases with increasing Si content. The total magnetic moment varies linearly with increasing Si content, which follows the Slater-Pauling rule. Electronic band structure calculations indicate that the Fe₂MnGa_1?x Si _x exhibits half-metallic character for all the concentrations studied and the spin polarization and the spin-down band gap both increase with the Si content. Based on the magnetic properties calculations, the Heisenberg exchange coupling parameters give Fe-Mn ferromagnetic coupling and Mn-Mn antiferromagnetic coupling. The T _C first decreases and then increases with Si content, which is in well agreement with the experimental results.     

Magnetorefractive effect in (Co<Subscript>50</Subscript>Fe<Subscript>50</Subscript>)<Subscript>x</Subscript>(Al<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> granular films

The reflectivity spectra and the magnetorefractive effect (MRE) of (Co₅₀Fe₅₀)_x(Al₂O₃)_1?x metal-dielectric granular films (0.07<x<0.52) are analyzed in the IR spectral range λ=2.5–25 µm. It is revealed that the specific features observed in the spectra at λ≈8.5 and 20 µm are associated with the excitation of phonon modes in the dielectric matrix. The magnetorefractive effect in the films is observed below the percolation thresh-old only in p-polarized light and above the percolation threshold for both the p and s polarizations. It is demonstrated that the optical properties of (Co₅₀Fe₅₀)_x(Al₂O₃)_1?x films in the IR spectral range, to a first approximation, can be interpreted in the framework of the effective-medium theory and the magnetorefractive effect can be explained in terms of the modified Hagen-Rubens relation.     

Redistribution of chromium atoms between the components of an intermetallic/oxide nanocomposite in the course of its production

Structural studies of nanocomposites produced by the method combining mechanical preactivation of the mixture comprising 8.1% Cr₂O₃, 65.9% Fe, and 25% Al by mass and self-propagating high-temperature synthesis (SPHTS) have been carried out by Mössbauer spectroscopy methods. It was found that a Fe/Al/Cr₂O₃ composite with a small Fe₂Al₅ intermetallic impurity is produced at the mechanical activation stage. At the SPHTS stage, interaction between the activated components of the mixture results in formation of the Fe_0.70?x Cr _x Al_0.3/Al₂O₃ (x = 0–0.2) composite.     

AC and DC properties of M-type SrCo<Subscript>x</Subscript>Ti<Subscript>x</Subscript>Fe<Subscript>(12−2x)</Subscript>O<Subscript>19</Subscript> hexagonal ferrite

Microwave characterization of SrCo _x Ti _x Fe_(12?2x)O₁₉ (x = 0.0, 0.2, 0.3, 0.5, 0.6, 0.7, 1.0) ferrites has been studied as a function of frequency, substitution and thickness, and static electrical current density-electric field characteristics have been investigated as a function of substitution. Microwave characteristics have been measured using power meter in the rectangular slotted waveguide and current density is measured using electrometer. The microwave absorption is evaluated using the standard available model. The results depict ?11.57 dB reflection loss at 10.38 GHz in composition x = 0.6. The electrical current density decreases at lower substitution and increases at higher substitution. The substitution of Co²⁺ and Ti⁴⁺ ions causes enhancement of electromagnetic and static electrical properties.     

Simulation of the structural,electronic, and magnetic properties of Fe<Subscript>3</Subscript>C<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>B<Subscript>x</Subscript> borocementites

The structural, electronic, and magnetic properties and the enthalpy of formation of iron borocementites Fe₃C_1?x B_x (x= 0, 0.25, 0.50, 0.75, 1.00) are analyzed using ab initio calculations in the framework of the electron density functional theory. It is found that the unit cell parameter a of the orthorhombic lattice increases linearly and the parameters b and c decrease as the boron concentration increases. The density of states at the Fermi level changes only slightly, and the main variations in the band structure occur in the region of the bottom of the valence bands. The magnetic moment of the iron atoms and the total magnetization and stability of the Fe₃C_1?x B_x phases increase linearly with an increase in the boron concentration.     

Magnetic ordering in La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>x</Subscript>MnO<Subscript>3−<Emphasis Type="Italic">x</Emphasis>/2</Subscript> anion-deficient manganites

The structural, magnetic, and electrotransport properties of La_1?xSr_xMnO_{3? x/2}(0≤x≤0.30) manganites with perovskite structure are investigated experimentally as a function of oxygen deficiency. In the solid solutions La_1?xSr_xMnO₃, a change in the type of symmetry of the unit cell is observed at x=0.125. Samples with x≤0.125 are characterized by an O′-orthorhombic unit cell, whereas samples with x>0.125 are characterized by a rhombohedral unit cell. The structural properties of the anion-deficient solid solutions La_1?xSr_xMnO_3?x/2 are analogous to those of the stoichiometric system. It is assumed that, as the oxygen content decreases, La_{1? x}Sr_xMnO_3?x/2 anion-deficient solid solutions experience a series of successive magnetic phase transformations in the ground state: from an A-type (x=0) antiferromagnet to a cluster spin-glass-type inhomogeneous magnetic state (0.175>x≤0.30) through a two-phase (antiferromagnetic and ferromagnetic) state (0>x≤0.175). The anion-deficient solid solution with x=0.175 has the maximal value of the ferromagnetic component. As the oxygen deficiency increases, the resistivity of La_{1? x}Sr_xMnO_3?x/2 samples first decreases (up to a value of x=0.175), acquiring an activation character, and then increases (up to a value of x=0.30). In this case, none of the anion-deficient solid solutions exhibits a metal-semiconductor transition in the whole range of concentrations considered. A peak of magnetoresistance at a temperature below the point of magnetic ordering is observed only in the sample with x=0.175. The results of experiments carried out with a series of La_1?xSr_xMnO_3?x/2 anion-deficient solid solutions are summarized in the concentration diagrams of the spontaneous magnetic moment and the critical temperature of magnetic phase transitions. Hypothetical magnetic phase states are pointed out. The experimental results obtained can be interpreted in terms of the phase-separation model and the competition between ferromagnetic and antiferromagnetic indirect superex-change interactions. It is assumed that Mn³⁺-O-Mn³⁺ indirect superexchange interactions in the orbitally disordered phase are positive in the case of octahedral coordination of manganese ions and are negative when the coordination of at least one Mn³⁺ ion is pentahedral.     

Investigation of the cation distribution in perovskite-like oxides with Mössbauer spectroscopy

Complex perovskite-like oxides, such as LnFe_2/3Mo_1/3O₃ orthoferrites, Ln_8?ySr_yCu_8?xFe_xO₂₀ (8-8-20), Pr₄BaCu₄FeO_13-δ(4-1-5), YBa_2-yLa _y Cu_3-xFe _x O_7-δ and Y_1-yCa _y Ba _y La _y Cu_3-xFe _x O_7-δ (1-2-3), are studied by means of Mössbauer spectroscopy. At room temperature, the spectra of the orthoferrites contain only magnetic components. The spectra of the 1-2-3 compounds contain only magnetically disordered components: iron atoms substitute for copper at Cu(1) sites, taking various configurations: planar squares, quadratic pyramids, and octahedra. Cuprates 8-8-20 and 4-1-5 have a wide diversity of spectra. In the 8-8-20 oxides, a phase related to the pyramidal environment of the iron cations is present at any iron concentration. In all the perovskites, iron cations become magnetically ordered only at octahedral sites of the structure.