Effect of severe plastic deformation on the properties of the Pt<Subscript>3</Subscript>Fe antiferromagnet

The effect of atomic disordering on the magnetic, electrical, and optical properties of the Pt_74.1Fe_25.9alloy close in composition to the stoichiometric Pt₃Fe alloy has been studied. It has been shown that, as a result of severe plastic torsional deformation under high pressure, the alloy transforms from the antiferromagnetic state (T _N=164 K) into the ferromagnetic state (T _C≈400 K). In this case, the residual electrical resistivity increases by a factor of more than two and the thermopower changes its sign from positive to negative. The results of the studies of the optical conductivity agree with the previously calculated electronic spectra of the atomically ordered and disordered Pt₃Fe alloys in the range of interband transitions and with the obtained data on the electrical properties in the infrared range.     

Electronic properties of strain-disordered Ni<Subscript>2.16</Subscript>Mn<Subscript>0.84</Subscript>Ga alloy

The effect of ultrarapid quenching from the melt and severe plastic torsional deformation under high pressure on the crystalline structure and the electrical, optical, and magnetic properties of a Ni_2.16Mn_0.84Ga alloy was studied. The electrical properties are discussed in terms of the Mott two-band model. The peculiarities of the magnetic properties are associated with the magnetism of itinerant electrons. The optical properties correlate with the variations in the electronic spectrum upon disordering of the alloy that follow from the results of the available energy-band-structure calculations.     

Suppression of hole doping in fine-grained HTSC YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> due to structural disordering

It was proven experimentally that the structural disordering inherent to fine-grained high-temper- ature YBa₂Cu₃O _y superconductors (with an average grain size of 〈D〉 < 2 μm) leads to a reduction of the level of hole doping and the creation of features inherent to the pseudogap state (antiferromagnetic correlations and the lowered density of states at the Fermi level) even in samples with optimum oxygen content y ≈ 6.92.     

Atomic and magnetic structures of Fe<Subscript>70</Subscript>Al<Subscript>5</Subscript>B<Subscript>25</Subscript> amorphous alloys

The short-range order around boron, aluminum, and iron atoms in Fe₇₅B₂₅ and Fe₇₀Al₅B₂₅ amorphous alloys has been studied by ¹¹B and ²⁷Al nuclear magnetic resonance at 4.2 K and ⁵⁷Fe Mössbauer spectroscopy at 87 and 295 K. The average magnetic moment of iron atoms μ(Fe) in these alloys has been measured by a vibrating sample magnetometer. It has been revealed that the substitution of aluminum atoms for iron atoms does not disturb μ(Fe) in the Fe₇₀Al₅B₂₅ alloy, gives rise to an additional contribution to the ¹¹B NMR spectrum in the low-frequency range, and shifts maxima of the distribution of hyperfine fields at the ⁵⁷Fe nuclei. In the Fe₇₀Al₅B₂₅ amorphous alloy, the aluminum atoms substitute for iron atoms in the nearest coordination shells of boron and iron atoms. This alloy consists of nanoclusters in which boron and iron atoms have a short-range order of the tetragonal Fe₃B phase type.     

Effect of change in structural and magnetic states of Pt<Subscript>74.1</Subscript>Fe<Subscript>25.9</Subscript> alloy on its optical properties

The effect of atomic disordering on the optical properties of Pt_74.1Fe_25.9 alloy, whose stoichiometry is close to that of Pt₃Fe, has been investigated. The optical constants of ordered and plastically deformed alloys, which are, respectively, in the paramagnetic and ferromagnetic states, have been measured by the polarimetric method. The frequency dispersions of the permittivity, optical conductivity, and reflectivity, as well as the microscopic characteristics of conduction electrons (plasma and relaxation frequencies), have been calculated. The energy dependences of the optical conductivity are compared to the calculated energy-band structure of atomically ordered and disordered Pt₃Fe compounds.     

High-temperature transition in SrFeO<Subscript>2.5</Subscript>: LSDA+U simulation

The effects of oxygen vacancy disordering in structural features and magnetic characteristics of SrFeO_2.5 are studied by the LMTO method in frameworks of the LSDA+U formalism in supercell approximation. Results clearly show that the high-temperature pseudocubic phase of SrFeO_2.5 may contain iron ions in five-fold oxygen coordination.     

Structural disordering in Cr-doped Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript> spinels

The processes of lithium redistribution in the structure of cubic Li₄Ti₅O₁₂ spinel, caused by both chromium doping and thermal activation, have been investigated by nuclear magnetic resonance. It is shown that Li ions migrate from tetra- to octahedral crystallographic positions with an increase in temperature. This process becomes more pronounced at temperatures above 400 K. In contrast, the fraction of tetrahedrally coordinated Li increases as a result of doping with chromium.     

Effect of severe plastic deformation on the electronic properties of the Cu<Subscript>72</Subscript>Au<Subscript>24</Subscript>Ag<Subscript>4</Subscript> alloy

The electrical, magnetic, and optical properties of the Cu₇₂Au₂₄Ag₄ ternary alloy in the ordered and disordered states have been studied by the method of severe torsional plastic deformation under pressure in Bridgman anvils. It has been shown that, as a result of this deformation, the residual electrical resistivity of the alloy increases by approximately 11% and the magnitude of the negative thermopower decreases. The high diamagnetic susceptibility of the alloy has been explained by a significant role of charge carriers with the effective mass considerably smaller than the free-electron mass. The behavior of the optical conductivity has been discussed with due regard for the results of energy-band calculations. The experimental data obtained for the Cu₇₂Au₂₄Ag₄ alloy have been compared with the results of similar studies of the Cu₃Au binary alloy.     

Dependence of the optical properties of Fe<Subscript>78</Subscript>Si<Subscript>10</Subscript>B<Subscript>12</Subscript> amorphous alloy on its structural state

The optical properties of Fe₇₈Si₁₀B₁₂ ferromagnetic alloy in amorphous, crystalline, and intermediate structural states have been investigated by ellipsometry in the spectral range of 0.22–18 μm. It is established that alloy crystallization leads to a significant change in the optical constants and the frequency dependences of the dielectric functions calculated based on these optical constants. The structural reconstruction under heat treatment leads to an increase in the intensity and shift of interband absorption bands. The plasma and relaxation frequencies of conduction electrons are determined; their numerical values also depend on the degree of atomic ordering.     

Effect of alloying additions and atomic disordering on the physical properties of magnetic Ni<Subscript>2</Subscript>MnGa-based shape memory alloys

The effect of atomic disordering induced by melt quenching or severe plastic deformation via high-pressure torsion on the physical properties (thermal expansion coefficient, electrical resistivity, thermoelectric power, magnetization) of a stoichiometric Ni₅₀Mn₂₅Ga₂₅ alloy and nonstoichiometric Ni₅₀Mn_28.5Ga_21.5 alloys with 2 at % Cu or Co is studied in the temperature range 2 K ≤ T ≤ 900 K and the magnetic field range H ≤ 7 MA/m.     

Investigation of changes in the order parameters near antiphase boundaries in the Cu<Subscript>3</Subscript>Au alloy

Investigation of the specific features of order-disorder structural phase transitions in ordering alloys in the presence of antiphase boundaries of conservative and nonconservative types has been performed in a computer experiment by the example of the model Cu₃Au alloy. The distributions of the long-range and short-range order parameters in the planes parallel to antiphase boundaries have been obtained by the Monte Carlo method for different temperatures. The mechanisms of disordering near conservative and nonconservative boundaries in ordering alloys are revealed. It is shown that the effect of diffusion of superstructural parameters is significant near nonconservative antiphase boundaries in comparison with conservative boundaries. Obviously, nonconservative boundaries can make a relatively large contribution to the effect of positive temperature dependence of the yield strength of ordering alloys with the L1₂ superstructure.     

Chemical modification of the surface of Fe<Subscript>75</Subscript>Si<Subscript>25</Subscript> alloy particles with stearic acid under high-energy ball-milling conditions

The morphology, phase composition and surface structure of Fe₇₅Si₂₅-alloy particles are studied by electron microscopy, X-ray diffraction analysis, and Mössbauer, Auger, IR (infrared) and X-ray photoelectron spectroscopy. The alloy particles used as fillers for the polyethylene matrix are produced by high-energy ball milling in an organic medium with the addition of stearic acid. The addition of stearic acid is shown to promote plasticization of the brittle Fe₇₅Si₂₅ alloy and the formation of a surface layer of no more than 1.5 nm thick, consisting of oxides based on iron and silicon, responsible for the chemisorption of stearic acid on the surface. Chemical modification of the surface of filler particles with an amphiphilic surfactant is carried out to enhance their adhesion in the polymer matrix.     

Iron-induced magnetic,transport and magnetoresistance behavior in Nd<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> epitaxial films

The effects of Fe doping on Mn site in the colossal magnetoresistive film, Nd_0.67Sr_0.33MnO₃ have been studied by preparing the series Nd_0.67Sr_0.33Mn_1-xFe_xO₃ (x=0,0.05 and 0.1). Upon doping, no structural changes have been found. However, the Curie temperature, the associated metal-to-insulator transition temperature and the magnetization decrease drastically with Fe doping. The resistivity in the paramagnetic regime for all the samples follows Emin–Holsteins theory of small polaron. The polaron activation energy, W_p and resistivity coefficient, A increase with Fe doping. This effect may be ascribed to the fact that upon Fe doping, the long-range ferromagnetic order is destroyed and, therefore, W_p is enhanced in the system. As compared to the La-based system, Fe doping has a stronger tendency to destabilize the long-range ferromagnetic order in the Nd-based system. Large MR (as high as 90%) observed in the epitaxial NSMFO film may be attributed to the good lattice-matching between the grown film and substrate. PACS 75.47.Gk; 75.47.Lx; 75.70.-i     

Effects of lithium excess amount on the microstructure and electrochemical properties of LiNi<Subscript>0.5</Subscript>Mn<Subscript>1.5</Subscript>O<Subscript>4</Subscript> cathode material

Spinel LiNi_0.5Mn_1.5O₄ cathode materials with different lithium excess amount (0, 2%, 6%, 10%) were synthesized by a facile solid-state method. The effect of lithium excess amount on the microstructure, morphology, and electrochemical properties of LiNi_0.5Mn_1.5O₄ materials was systematically investigated. The results show that the lithium excess amount does not change the particle morphology and size obviously; thus, the electrochemical properties of LiNi_0.5Mn_1.5O₄ are mainly determined by structural characteristics. With the increase of lithium excess amount, the cation disordering degree (Mn³⁺ content) and phase purity first increase and then decrease, while the cation mixing extent has the opposite trend. Among them, the LiNi_0.5Mn_1.5O₄ material with 6% lithium excess amount exhibits higher disordering degree and lower impurity content and cation mixing extent, thus leading to the optimum electrochemical properties, with discharge capacities of 125.0, 126.1, 124.2, and 118.9 mAh/g at 0.2-, 1-, 5-, and 10-C rates and capacity retention rate of 96.49% after 100 cycles at 1-C rate.     

Effect of plastic deformation on the electronic properties of the Cu<Subscript>60</Subscript>Pd<Subscript>40</Subscript> alloy

The effect of severe plastic deformation by torsion under Bridgman anvil pressure (SPDT) on the electrical, magnetic, and optical properties of the Cu₆₀Pd₄₀ alloy was studied. It is shown that, after the alloy is disordered, the Curie-Weiss constants of the paramagnetic component are changed insignificantly. In this case, the temperature-independent negative component of the magnetic susceptibility decreases more than fivefold. The electrical resistance and negative thermopower, on the contrary, increase severalfold as a result of SPDT. The character of the optical conductivity is discussed using the band structure calculation results.     

Mechanism of thermal decomposition of the nonstoichiometric compound YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.8</Subscript>

Thermal decomposition of the nonstoichiometric high-temperature superconductor YBa₂Cu₃O_6.8 at a temperature of 200°C in air has been investigated using the full-profile analysis of X-ray diffraction lines. Two mechanisms of decomposition are revealed. The first mechanism, i.e., separation into two phases with a different oxygen content, occurs continuously. The second mechanism, i.e., disordering of the heavy atoms Y, Ba, Ba, Y along the crystallographic axis c, begins to occur after a 20- to 35-h annealing and progresses with a further annealing.     

Synthesis,Crystal Structure,and Magnetic Properties of the YbFeTi<Subscript>2</Subscript>O<Subscript>7</Subscript> Compound

We report on the synthesis conductions and results of experimental investigations of the crystal structure and magnetic properties of a new magnetic compound YbFeTi₂O₇. According to the X-ray diffractometry data, the crystal structure of the investigated compound is described by the rhombic space group Pcnb with unit cell parameters of a = 9.8115(1) Å, b = 13.5106(2) Å, and c = 7.31302(9) Å and atomic disordering in the distribution of iron ions Fe³⁺ over five structural sites. The magnetic measurements in the lowtemperature region revealed a kink in the temperature dependence of the magnetic moment and its dependence on the sample magnetic prehistory. The experimental results obtained suggest that with a decrease in temperature the sample passes from the paramagnetic state to the spin-glass-like magnetic state characterized by a freezing temperature of T _f = 4.5 K at the preferred antiferromagnetic exchange coupling in the sample spin system. The chemical pressure variation upon replacement of rare-earth ion R by Yb in the RFeTi₂O₇ system does not change the crystal lattice symmetry and magnetic state.     

Low-temperature electron properties of Heusler alloys Fe<Subscript>2</Subscript>VAl and Fe<Subscript>2</Subscript>CrAl: Effect of annealing

We present the results of measurements of low-temperature heat capacity, as well as electrical and magnetic properties of Heusler alloys Fe₂VAl and Fe₂CrAl prepared in different ways using various heat treatment regimes. The density of states at the Fermi level is estimated. A contribution of ferromagnetic clusters in the low-temperature heat capacity of the Fe₂VAl alloy is detected. The change in the number and volume of clusters as a result of annealing of an alloy affects the behavior of their low-temperature heat capacity, resistivity, and magnetic properties.     

FMR Studies of Ultra-Thin Epitaxial Pd<Subscript>0.92</Subscript>Fe<Subscript>0.08</Subscript> Film

Magnetic anisotropies of 20 nm epitaxial film of palladium–iron alloy Pd_0.92Fe_0.08 grown on the (001) MgO substrate were studied. Ferromagnetic resonance (FMR) spectroscopy and vibrating sample magnetometry (VSM) were exploited to determine magnetic parameters of the film. It was found that the synthesized film reveals cubic anisotropy with tetragonal distortion. The simulated magnetic hysteresis loops, obtained utilizing the magnetic anisotropy constants taken from the FMR spectra analysis, agree well with those measured by VSM.