Effect of the Structural Disorder and Short-Range Order on the Electronic Structure and Magnetic Properties of the Fe<Subscript>2</Subscript>VAl Heusler Alloy

The Fe₂VAl Heusler alloy is of great interest because ab initio calculations predict the absence of magnetization in it and a half-metal behavior with a pseudogap at the Fermi level. At the same time, experimental data (low-temperature specific heat, electrical resistivity, and magnetic properties) show that it is difficult to achieve such characteristics, and Fe₂VAl samples usually have the characteristics of a poor magnetic metal. Ab initio calculations have been performed for ordered and disordered (Fe_1–xV _x )₃Al Heusler alloys with x = 0.33. It has been shown that the alloy in a structurally ordered state (L2₁ structure) is a half-metal with a deep pseudogap at the Fermi level and does not have magnetization. At the same time, antisite defects in the iron and vanadium sublattices of the disordered alloy (D0₃ structure) lead to an increase in the conductivity and to the appearance of spin polarization and magnetization of (2.1±0.1)μ_B/f.u. The short-range order in the disordered phase has been generated by increasing the concentration of clusters characteristic of the bcc structure of α-Fe, which results in an increase in the magnetization to (2.5±0.1)μ_B/f.u.     

X-ray photoelectron and mössbauer spectroscopy studies of the valence state of transition metal ions in Co1–x Fe x Cr2O4 (x = 0.1, 0.2, 0.5) ceramics

The valence state of transition metal ions in the Co_1–x Fe _x Cr₂O₄ (x = 0.1, 0.2, 0.5) system has been investigated using X-ray photoelectron and Mössbauer spectroscopy. It has been shown that, in this system, there are Fe²⁺ and Fe³⁺ ions. The relative Fe³⁺/Fe²⁺ contents have been determined by fitting the experimental Fe 2p photoelectron spectra by a superposition of theoretical spectra of the Fe²⁺ and Fe³⁺ ions, as well as using Mössbauer spectroscopy.     

The proximity effect in an Fe-Cr-V-Cr-Fe system

The proximity effect was studied in a thin-film Fe-Cr-V-Cr-Fe layered system. As the chromium layer thickness (d_Cr) increases at a fixed thickness of iron layers (d_Fe), the dependence of the superconducting transition temperature (T_c) on d_Cr exhibits a maximum at d_Cr ? 40 Å followed by a sharp decrease. Investigation of the dependence of T_c on d_Fe at a fixed d_Cr showed that the depth of penetration of the Cooper pairs into a chromium layer does not exceed 40 Å. Analysis of the results obtained suggests that, at d_Cr ? 40 Å, chromium layers exhibit the transition from a nonmagnetic state to an incommensurate spin density wave state.     

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.     

Specific features of the electrical resistivity of half-metallic ferromagnets Co<Subscript>2</Subscript> <Emphasis Type="Italic">Me</Emphasis>Al (<Emphasis Type="Italic">Me</Emphasis> = Ti,V, Cr,Mn, Fe)

The transport properties of half-metallic ferromagnetic Heusler alloys Co₂MeAl (Me = Ti, V, Cr, Mn, Fe are transition 3d metals) have been measured in the temperature range of 4–900 K. The specific features of the behavior of the resistivity have been considered in the framework of the two-current model of conductivity that takes into account the existence of the energy gap in the electronic spectra of the alloys near the Fermi level of one of electron subbands that differs in the spin direction.     

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.     

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.     

Electrical conductivity and thermopower in Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Mn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript> S sulfides

This paper reports on the results of investigations into the structural, electrical, and thermoelectrical properties of sulfides Co _x Mn_{1 ? x} S (0 ≤ x ≤ 0.4) in the temperature range 80–950 K. It is established that the thermopower coefficient α decreases significantly with an increase in the cobalt concentration in the lattice of the α-MnS compound. The Co _x Mn_{1 ? x} S compounds with cobalt concentrations in the range 0 ≤ x ≤ 0.3 are semiconductors with hole conduction (α > 0), whereas the compound with x = 0.4 exhibits metallic conduction (α < 0). It is found that the band gap E _g of the compounds under investigation varies in the range from 1.46 eV for α-MnS (x = 0) to 0.26 eV for Co _x Mn_{1 ? x} S (x = 0.4).     

High-field magnetization of band ferromagnets Co<Subscript>2</Subscript><Emphasis Type="Italic">Y</Emphasis>Al (<Emphasis Type="Italic">Y</Emphasis> = Ti,V, Cr,Mn, Fe,Ni)

The temperature dependences of the magnetization of ferromagnetic Heusler alloys Co₂ YAl, where Y = Ti, V, Cr, Mn, Fe, and Ni have been studied at H = 50 kOe in the range 2 K < T < 1100 K. It is shown that the high-field (H ≥ 20 kOe) magnetization is described within the Stoner model.     

Specific features of the properties of half-metallic ferromagnetic Heusler alloys Fe<Subscript>2</Subscript>MnAl,Fe<Subscript>2</Subscript>MnSi,and Co<Subscript>2</Subscript>MnAl

The structural, magnetic, and electrical properties of half-metallic Heusler alloys Fe₂MnAl, Fe₂MnSi, and Co₂MnAl have been investigated in the temperature range of 4–900 K. According to the X-ray diffraction analysis, these alloys have the B2 and L2₁ structures with different degrees of atomic order. The magnetic state of the alloys is considered as a two-sublattice ferrimagnet. The electrical resistivity and thermoelectric power have been discussed in the framework of the two-current conduction model taking into account the existence of an energy gap in the electronic spectrum of the alloys near the Fermi level for the subband with spin-down (minority) electrons.     

Ab initio calculations of the stability and structural defects of the <Emphasis Type="Italic">B</Emphasis>2 Cu<Subscript>x</Subscript>Fe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Al phases

The stability of the B2 Cu_xFe_1?x Al phases and the energy of defect formation are studied using ab initio band calculations. For B2 Cu_xFe_1?x Al alloys, vacancies in the 3d-metal sublattice and configurations with the minimum number of Fe-Cu bonds in the first coordination shell (including Fe antisite defects, which have a high local magnetic moment) are most stable. Complicated defect complexes with vacancies and displaced atoms, which are close to the atomic configurations of vacancy-ordered AlCu phases, can exist near the composition Cu_0.875Fe_0.125Al.     

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.     

Field dependence of orbital magnetic moments in the Heusler compounds Co<Subscript>2</Subscript>FeAl and Co<Subscript>2</Subscript>Cr<Subscript>0.6</Subscript>Fe<Subscript>0.4</Subscript>Al

Orbital and spin magnetic moments of the Heusler compounds Co₂FeAl and Co₂Cr_0.6Fe_0.4Al were measured by magnetic circular dichroism in X-ray absorption (XMCD). The orbital magnetic moments per spin are quite large (0.1–0.2) compared to bulk values of Fe and Co metals, indicating a considerable spin–orbit coupling in these Heusler compounds. A strong localization of the 3d electron states might be responsible for this observation. The Co and Fe orbital to spin moment ratio shows a distinct decrease of r(Fe)=0.04±0.02 and r(Co)=0.06±0.02 with increasing external field for the ternary compound Co₂FeAl, while the ratio is within error limits independent of the field for Co₂Cr_0.6Fe_0.4Al. This is discussed in terms of a relation to magnetocrystalline anisotropies. PACS 75.50.Cc; 71.20.Lp; 78.40.Kc     

Knight shift in superconducting oxides BaPb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Bi<Subscript>x</Subscript>O<Subscript>3</Subscript> (<Emphasis Type="Italic">x</Emphasis> < 0.35)

The Knight shift ²⁰⁷K_s for the ²⁰⁷Pb nuclei in the metal phase of the oxides BaPb_1?xBi_xO₃ (x < 0.35) has been analyzed as a function of the concentration. The shift, which is proportional to the density of states near the Fermi energy: ²⁰⁷K_s ～ N(E_F), reaches a maximum for an oxide with the maximum superconducting transition temperature T_c(x ≈ 0.25) = 12 K. A significant increase in the width of the shift distribution with the Bi concentration testifies to the formation of a nonuniform state of the electronic system in the conduction band of superconducting oxides, which is accompanied by an increase in short-wavelength contributions to the spin susceptibility. To detect the ²⁰⁷Pb NMR spectra in superconducting oxides with x > 0.2, the ¹⁷O-²⁰⁷Pb spin-echo double-resonance method is used, which provides successful detection of the ²⁰⁷Pb NMR signal with an anomalously high rate of spin-spin relaxation T ₂ ^?1 > 500 ms^?1. Thus, fundamental restrictions arising in investigations of rapidly relaxing ²⁰⁷Pb nuclei, which are “unobservable” in superconducting oxides BaPb_1?xBi_xO₃ when they are studied by traditional single-resonance methods of pulse NMR spectroscopy, have been overcome.     

Investigating the spectra of electron spin resonance in SiGe/Si heterolayers doped with phosphorous

Studies of the spin resonance spectra of electrons localized on donors and the conduction electrons in Si_{1 ? x} Ge _x layers grown on silicon show that the phosphorous atoms in a SiGe layer can have two positions in the lattice. The line with g ～ 1.998 refers to the electron localized in the phosphorous atom with a predominantly silicon environment; the line with g ～ 1.994 is observed when there is a substantial concentration of germanium in the phosphorous environment.     

Positive magnetoresistance effect in rare earth cobaltites

The structure, magnetic, and magnetotransport properties of the Pr_0.5Sr_0.5Co_{1 ? x} Fe _x O₃ system have been studied. The ferromagnet-spin glass (x = 0.5)-G-type antiferromagnet (x = 0.7) transitions and the metal—insulator transitions (x = 0.25) have been revealed. It has been established that the magnetoresistance of the metallic ferromagnetic cobaltites changes sign from positive to negative as the external magnetic field increases. The positive component increases and the negative component decreases with decreasing temperature. The negative magnetoresistance increases sharply in the insulating spinglass phase. Possible causes of the low-magnetic-field positive magnetoresistance in the rare earth metallic cobaltites are discussed.     

Electronic Structure and Magnetic Phase Transition in Helicoidal Fe<Subscript>1 - <Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Si Ferromagnets

LSDA + U + SO calculations of the electronic structure of helicoidal Fe_{1 - x}Co _x Si ferromagnets within the virtual crystal approximation have been supplemented with the consideration of the Dzyaloshinski-Moriya interaction and ferromagnetic fluctuations of the spin density of collective d electrons with the Hubbard interactions at Fe and Co atoms randomly distributed over sites. The magnetic-state equation in the developed model describes helicoidal ferromagnetism and its disappearance accompanied by the occurrence of a maximum of uniform magnetic susceptibility at temperature T _C and chiral fluctuations of the local magnetization at T > T _C . The reasons why the magnetic contribution to the specific heat at the magnetic phase transition changes monotonically and the volume coefficient of thermal expansion (VCTE) at low temperatures is negative and has a wide minimum near T _C have been investigated. It is shown that the VCTE changes sign when passing to the paramagnetic state (at temperature T _S ).     

XANES study of interatomic interactions in (CoFeZr)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> (SiO<Subscript>2</Subscript>)<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript> nanocomposites

Analysis of the X-ray absorption near-edge structure of (Co₄₅Fe₄₅Zr₁₀)x(SiO₂)_1–x nanocomposites has shown the presence of interaction between atoms of the metallic and dielectric components in the nanocomposite. In this process, d-metal ions (Fe³⁺, Fe²⁺, Co²⁺) play the most active role. They interact with oxygen ions of the dielectric component and form not only Fe₂O₃ × CoO nanoferrites but also silicates of d metals.     

Crystal-structure and magnetic phase transformations in solid solutions of BiFeO<Subscript>3</Subscript>-AFe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> (A = Ca,Sr, Ba,Pb)

Solid solutions of Bi_{1 ? x} A _x (Fe_{1 ? x/2}Nb _x/2)O₃, where A = Ca, Ba, and Pb, are obtained and their crystal structure and magnetic properties are investigated. It is shown that for A = Ca and x ≈ 0.15, the symmetry of the unit cell changes from rhombohedral (space group R3c) to orthorhombic (Pbnm). The transformation leads to the emergence of spontaneous magnetization due to the Dzyaloshinskii-Moriya interaction. Solid solutions with A = Pb remain rhombohedral up to a concentration of x = 0.3. Spontaneous magnetization sharply increases in the compound with x ≈ 1 at low temperatures and is due to the formation of the spin-glass component.     

Mössbauer studies of multiferroics BiFe<Subscript>1 – <Emphasis Type="Italic">x</Emphasis></Subscript>Cr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3</Subscript> (<Emphasis Type="Italic">x</Emphasis> = 0–0.20)

The Mössbauer studies on ⁵⁷Fe nuclei in multiferroics BiFe_{1 – x} Cr _x O₃ (x = 0.05, 0.10, and 0.20) have been performed at room temperature. The multiferroics BiFe_{1 – x} Cr _x O₃ (x = 0.05, 0.10, and 0.20) with the rhombohedral R3c structure have been prepared by solid-state synthesis under high pressures. The effect of substitution of Cr cations for Fe cations on the spatial spin-modulated structure, and also hyperfine electrical and magnetic interactions of ⁵⁷Fe nuclei has been studied. The substituted ferrites demonstrate an anharmonic modulated spin structure of cycloid type, in which iron atoms with different cation environments take part. The anharmonism parameter of the cycloid linearly increases from m = 0.10 at x = 0 to m = 0.78 ± 0.02 at x = 0.20. The constants of magnetic uniaxial anisotropy K _u are estimated at room temperature: K _u ≈ 0.36 × 10⁶ erg/cm³ at x = 0 and K _u ≈ 4.22 × 10⁶ erg/cm³ at x = 0.20.