Anisotropy of magnetocaloric effects in easy-axis antiferromagnets

Magnetocaloric effects (MCEs) in two-sublattice anisotropic antiferromagnets with single-ion anisotropy of easy-axis type have been investigated upon orientation of a magnetic field both along and transversely to the easy axis of magnetization. It has been shown that MCEs that arise in a longitudinal magnetic field in the paramagnetic range above the Néel temperature T_N are of a normal character, whereas in low fields in the magnetically ordered range below T_N, they are anomalous: isothermal magnetization increases magnetic anisotropy and adiabatic magnetization reduces the temperature of the antiferromagnet. Upon the magnetization of anisotropic antiferromagnets perpendicular to the easy axis of magnetization above the Néel point T_N, we observe normal (direct) MCEs that are weaker than in longitudinal fields and decrease with the relative growth of the single-ion anisotropy parameter D > 0. In low fields, upon magnetization by transverse fields below T_N, as in the case of longitudinal magnetization, anomalous (inverse) MCEs arise, but they are several orders of magnitude weaker than analogous effects in longitudinal fields and disappear completely upon passage to the limiting case of isothermal antiferromagnet.     

Ground state of the one-dimensional half-filled Hubbard model

We investigate the ground state (T = 0 K) of the one-dimensional symmetrical (n = 1) Hubbard model formalized in terms of the system of integral equations, which we previously obtained using the method of the generating functional of Green’s functions with the subsequent Legendre transformation. In a wide range of variations in the parameter of Coulomb interaction U, the following characteristics of the system have been calculated: the electron density of states, the electron band spectrum, the number of doubly occupied lattice sites, the localized magnetic moment, the correlator of the square of the longitudinal component of spin at a site,<S_Z²>, and the internal energy of the system. It has been shown that, for all U > 0, the model yields two solutions, i.e., an antiferromagnetic insulator and a paramagnetic insulator, in which there are no single-electron quasi-particles at the Fermi level. The energy of the paramagnetic solution in the region of U < 1.1 is considerably less than that of the antiferromagnetic solution for the case of U > 1.1, we have the opposite situation.     

Magnetic and structural properties of rapidly quenched Nd-Fe-Co-Ge-B alloys

We have studied the magneto-resistive effect in bulk Y_3/4Lu_1/4Ba₂Cu₃O₇ + CuO composites prepared by the fast-sintering technique. It has been found that the composites exhibit large magnetoresistance in low magnetic fields (<100 Oe) in a broad temperature range (tens of kelvins below the critical temperature T _c). The HTSC-based composites exhibit a much higher sensitivity to weak magnetic fields at liquid-nitrogen temperature as compared to pure HTSC ceramics. By choosing a proper bias current j, it is possible to control the shape of the resistivity-magnetic field ρ(H) characteristic of the composites and to vary the parameter R ₀ = {R(H = 0) ? R(H)}/R(H = 0). Under the condition j > j _c (where j _c is the critical-current density), large values of the magnetoresistance R ₀, up to thousands percent, are obtained in the range of weak magnetic fields (tens of oersteds) at 77 K. This effect is attractive for practical applications of these composite materials as active elements of magnetic-field sensors. The sign of the magneto-resistive effect is positive in contrast to that of manganese oxides. This may be important for some devices.     

Optimum Combination of Thermoplastic Formability and Electrical Conductivity in Al–Ni–Y Metallic Glass

Both thermoplastic formability and electrical conductivity of Al–Ni–Y metallic glass with 12 different compositions have been investigated in the present study with an aim to apply as a functional material, i.e. as a binder of Ag powders in Ag paste for silicon solar cell. The thermoplastic formability is basically influenced by thermal stability and fragility of supercooled liquid which can be reflected by the temperature range for the supercooled liquid region (ΔT_x) and the difference in specific heat between the frozen glass state and the supercooled liquid state (ΔC_p). The measured ΔT_x and ΔC_p values show a strong composition dependence. However, the composition showing the highest ΔT_x and ΔC_p does not correspond to the composition with the highest amount of Ni and Y. It is considered that higher ΔT_x and ΔC_p may be related to enhancement of icosahedral SRO near T_g during cooling. On the other hand, electrical resistivity varies with the change of Al contents as well as with the change of the volume fraction of each phase after crystallization. The composition range with the optimum combination of thermoplastic formability and electrical conductivity in Al–Ni–Y system located inside the composition triangle whose vertices compositions are Al₈₇Ni₃Y₁₀, Al₈₅Ni₅Y₁₀, and Al₈₆Ni₅Y₉.     

Influence of magnetic field on the tensoresistive effect in iron-based amorphous alloys

The effect of an applied magnetic field H on the tensoresistive effect in Fe-T-B (T = Fe, Cr, and Co) and Fe-B-Si amorphous metallic alloys has been studied. The applied magnetic field H is shown to substantially affect the coefficient of tensoresistance (CTR) π determined upon a longitudinal deformation of samples. The principal cause of such an effect is a change in Young’s modulus under the effect of a magnetic field, i.e., the ΔE effect. Based on the performed analysis, an equation for the determination of the ΔE effect from the measurement of the π(H) dependences is suggested. The results obtained indirectly confirm the assumption on the fact that the anomalous value of the coefficient of tensoresistance in amorphous alloys is due to a distortion of the spherical symmetry of the structure factor.     

Magnetic properties of <Emphasis Type="Italic">R</Emphasis>Co<Subscript>2</Subscript> compounds in the exchange-striction model of ferrimagnets

The original version of the exchange-striction model of a ferrimagnet has been employed for calculating a number of magnetic properties of RCo₂ ferrimagnets, where R = Er, Ho, Dy, Tb, and Gd are rareearth ions. The following magnetic properties are calculated: pressure dependence of the Curie temperature (Т _С), temperature dependences of magnetization in sublattices of cobalt and rare-earth atoms, and isotherms of magnetization of these lattices at Т > Т _С. For an ErСо₂ sample, the Н–Т phase diagram has been constructed and the magnetization in the magnetic fields Н = 0–70 Т has been calculated. The calculated and experimental results have been compared. Based on the exchange-striction model, the qualitative explanation of the difference in the type of the magnetic phase transformation in the intermetallic compounds with R = Tb and Gd and R = Er, Ho, and Dy is given.     

Pseudo-gap and the nonlinearity of the magnetization of textured polycrystals of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − <Emphasis Type="Italic">x</Emphasis></Subscript> above the temperature of transition into the superconducting state

Temperature dependences of the third and other higher harmonics of the magnetization of textured polycrystalline samples of YBa₂Cu₃O_{7 ? x} have been studied in the temperature range of 77–120 K. It has been revealed that the nonlinearity of magnetization of YBa₂Cu₃O_{7 ? x} is observed up to temperatures that considerably exceed the temperature of transition into the superconducting state. The observed nonlinearity of magnetization of YBa₂Cu₃O_{7 ? x} is ascribed to the appearance of a pseudogap state at T ～ 102 K in this compound. A method of determining the temperature of the appearance of the pseudogap T * in the high-T _c compounds is suggested, which is based on the measurement of magnetization harmonics.     

Investigation of the magnetocaloric effect in correlated metallic systems with Van Hove singularities in the electron spectrum

In this work, the magnetic contribution to the isothermal entropy change ΔS upon switching on a magnetic field has been investigated in correlated metallic ferromagnets within the Hubbard nondegenerate model. The analytical expression ΔS for obtained in the mean-field approximation depends substantially on the electronic structure (density of electron states), which presents new ways to increase the absolute value of ΔS relative to the known result obtained within the Heisenberg model. The temperature dependence of ΔS has been calculated at different values of the Coulomb interaction U and the number of electrons n for the Bethe infinite-dimensional lattice and square lattice with allowance for transfer integrals in the first (t) and the second (t') coordination shells. It has been found that the presence of Van Hove singularities in the electronic spectrum near the Fermi level makes it possible to considerably increase |ΔS| at a fixed magnetic field. The possibility of first-order magnetic phase transitions depending on the model parameters has been analyzed.     

Short-range atomic order in Fe<Subscript>2</Subscript>B powders

The possibility of application of a local atomic displacement (LAD) model for describing the mechanism of formation of a low-field part (H _hf ≤ 23.6 T) of a hyperfine-field distribution P(H) in a Fe₂B alloy at early stages of grain refinement has been investigated. It has been found that the appearance of Mössbauer contributions, which are not typical of the tetragonal C16 structure, in the P(H) function of the Fe₂B alloy is induced by a decrease in the relative interatomic distances Δr _Fe-B/r _Fe-B to ≤0.18 for some configurations of iron and boron atoms in the unit cell.     

Properties of the Ti<Subscript>40</Subscript>Zr<Subscript>10</Subscript>Cu<Subscript>36</Subscript>Pd<Subscript>14</Subscript> BMG Modified by Sn and Nb Additions

The results of investigation of the influence of additions of 2 and 3 at.% of Sn and simultaneously of Sn and 3 at.% Nb on microstructure and properties of the bulk metallic glasses of composition (Ti₄₀Cu_36?x Zr₁₀Pd₁₄Sn _x )_100?y Nb _y are reported. It was found that the additions of Sn increased the temperatures of glass transition (T _g), primary crystallization (T _x ), melting, and liquidus as well as supercooled liquid range (ΔT) and glass forming ability (GFA). The nanohardness and elastic modulus decreased in alloys with 2 and 3 at.% Sn additions, revealing similar values. The 3 at.% Nb addition to the Sn-containing amorphous phase decreased as well all the T _g, T _x , T _L, and T _m temperatures as ΔT and GFA; however, relatively larger values of this parameters in alloys containing larger Sn content were preserved. In difference to the previously published results, in the case of the amorphous alloys containing small Nb and Sn additions, a noticeable amount of the quenched-in crystalline phases was not confirmed, at least of the micrometric sizes. In the case of the alloys containing Sn or both Sn and Nb, two slightly different amorphous phase compositions were detected, suggesting separation in the liquid phase. Phase composition of the alloys determined after amorphous phase crystallization was similar for all compositions. The phases Cu₈Zr_3, CuTiZr, and Pd₃Zr were mainly identified in the proportions dependent on the alloy compositions.     

Creep correlations of metals at elevated temperatures

Creep data for pure metals at temperatures above those at which rapid recovery occurs (above about 0.45 the melting temperature) are correctable by means of the equations ? = f (te^?ΔH/RT, σ) and σ = f (?&#x0307 _s e^ΔH/RT). These correlations were applied successfully to data for aluminum, iron, nickel, copper, zinc, platinum, gold, and lead as well as for simple alloys. For a given metal, ΔH is a constant about equal to the activation energy for self-diffusion.     

Size dependence of the melting temperature of metallic nanoclusters from the viewpoint of the thermodynamic theory of similarity

The generalized Thomson formula T_m = T_m^(∞)(1-δ)R for the melting point of small objects T_m has been analyzed from the viewpoint of the thermodynamic theory of similarity, where R is the radius of the particle and T_m^(∞) is the melting point of the corresponding large crystal. According to this formula, the parameter δ corresponds to the value of the radius of the T_m(R^-1) particle obtained by the linear extrapolation of the dependence to the melting point of the particle equal to 0 K. It has been shown that δ = αδ₀, where α is the factor of the asphericity of the particle (shape factor). In turn, the redefined characteristic length δ₀ is expressed through the interphase tension σ_sl at the boundary of the crystal with its own melt, the specific volume of the solid phase v_s and the macroscopic value of the heat of fusion λ_∞:δ₀ = 2σ_slv_s/λ_∞. If we go from the reduced radius of the particle R/δ to the redefined reduced radius R/r₁ or R/d, where r₁ is the radius of the first coordination shell and d ≈r₁ is the effective atomic diameter, then the simplex δ/r₁ or δ/d will play the role of the characteristic criterion of thermodynamic similarity. At a given value of α, this role will be played by the simplex Estimates of the parameters δ₀ and δ₀/d have been carried out for ten metals with different lattice types. It has been shown that the values of the characteristic length δ₀ are close to 1 nm and that the simplex δ₀/d is close to unity. In turn, the calculated values of the parameter δ agree on the order of magnitude with existing experimental data.     

Anisotropy of the electric transport properties of decagonal Al–Cu–Co(Fe) quasicrystals

The method of growth from a melt solution was used to obtain iron-alloyed (0.08 at %) Al–Cu–Co single crystals with a decagonal symmetry. The temperature dependences of the electrical resistivity in magnetic fields of 0–18 T were measured using samples oriented in the periodic direction (ρ_p(T)) and in the quasi-periodic plane (ρ_q(T)). A strong anisotropy of the resistivity was observed; the ρ_p(T) curve is linear, whereas the ρ_q(T) curve is approximated well by a second-order polynomial. A strong anisotropy of the magnetoresistance was also observed; a positive magnetoresistance Δρ/ρ ~ 10^–3 for the current flowing in the quasiperiodic plane; and a weak (close to zero) negative magnetoresistance for the current flowing along the periodic direction.     

Normal state of metallic hydrogen sulfide

A generalized theory of the normal properties of metals in the case of electron–phonon (EP) systems with a nonconstant density of electron states has been used to study the normal state of the SH₃ and SH₂ phases of hydrogen sulfide at different pressures. The frequency dependence of the real Re Σ (ω) and imaginary ImΣ (ω) parts of the self-energy Σ (ω) part (SEP) of the Green’s function of the electron Σ (ω), real part Re Z (ω), and imaginary part Im Z (ω) of the complex renormalization of the mass of the electron; the real part Re χ (ω) and the imaginary part Imχ (ω) of the complex renormalization of the chemical potential; and the density of electron states N (ε) renormalized by strong electron–phonon interaction have been calculated. Calculations have been carried out for the stable orthorhombic structure (space group Im3?m) of the hydrogen sulfide SH₃ for three values of the pressure P = 170, 180, and 225 GPa; and for an SH₂ structure with a symmetry of I4/mmm (D4h1?7) for three values of pressure P = 150, 180, and 225 GP at temperature T = 200 K.     

Description of methane adsorption on microporous carbon adsorbents on the range of supercritical temperatures on the basis of the Dubinin–Astakhov equation

Temperature dependences of parameters n and E are calculated according to the adsorption equation of Dubinin–Astakhov for methane adsorption on eight active carbons in the range of supercritical temperatures of 170–340 K and pressures of 0–20 MPa. At temperatures above ~240 K, characteristic adsorption energy E of methane grows linearly at an increase in temperature. The temperature coefficients of characteristic energy of methane adsorption on active carbon tend to decrease at an increase in standard characteristic adsorption energy E₀. The average value of parameter <n> for the studied adsorbents tends to grow at an increase in standard characteristic adsorption energy E₀.     

Creep of Copper at Intermediate Temperatures

Activation energies for creep of copper at intermediate temperatures, where crystal recovery was negligible, were determined by the simple technique of rapidly alternating the test temperature between T₁ and T₂ (T₂= T₁ + about 10°K) throughout a constant stress creep test. The activation energy for creep ΔH was found to be 37,000 ± 3,000 cal per mol, independent of stress and strain. The same creep laws as have been previously established for high temperature creep were found to be valid for creep at intermediate temperatures. But the ΔH was found to be lower than that for self-diffusion in the intermediate temperature range whereas it is known to be equal to that for self-diffusion at high temperatures.     

Effect of electrospark dispersion on the martensitic transformation in Co—Fe—Ni—Ti—(Cu) alloys

The effect of electrospark dispersion on the γ ai α matensitic transformation in Co-Fe-Ni-Ti-(Cu) alloyshas been studied. A relationship between the lattice parameters of the initial γ phase (austenite), the structure of the arising α martensite, the temperature of the onset of the martensitic transformation M _s , and the temperature hysteresis of the martensitic transformation ΔT has been established.     

Magnetostatic field in a thin plate with a stripe-domain structure

Magnetostatic fields inside and outside of a plate with a stripe-domain structure have been calculated based on the formulas derived depending on the domain and domain-wall parameters and the plate thickness. In a plate with equal-size domains, the induction B _z perpendicular to the plate surface nearly repeats the magnetization (without extremes) in thick samples, while in a thin plate, the maximum B _z values correspond to domain-wall edges. It has been shown that the maximum value of the field component H _x perpendicular to the domain-wall plane is, on the order of magnitude, close to the logarithm of the ratio of the wall thickness to the period of the domain structure; for thin domain walls, it can exceed the saturation magnetization by several times.     

Specific features of magnetic order in a multiferroic compound CuCrO<Subscript>2</Subscript> determined using NMR and NQR data for <Superscript>63, 65</Superscript>Cu nuclei

Results of studying the paramagnetic and ordered phases of a CuCrO₂ single crystal using nuclear magnetic and nuclear quadrupole resonances on ^63,65Cu nuclei are presented. The measurements have been carried out in wide ranges of temperature (T = 4.2–300 K) and magnetic-field strength (Н = 0–94 kOe), with the magnetic fields being directed along a and c axes of the crystal. The components of the electric-field gradient tensor and the magnetic-shift tensor (K _a,c) have been determined. The temperature dependences K _a(H || a) and K _c(H || c) for the paramagnetic phase are described by the Curie–Weiss law and reproduce the behavior of the magnetic susceptibility (χ_a,c). The hyperfine field on a copper nucleus has been determined, which is equal to h _hf ^a,c = 33 kOe/μB. Below the temperature Т _N = 23.6 K, nuclear magnetic resonance and nuclear quadrupole resonance spectra for ^63,65Cu nuclei have been recorded typical of helical magnetic structures, which are incommensurable with the lattice period.     

Superconducting cellular MgB<Subscript>2</Subscript>

A new composite superconducting material in the form of a cellular structure consisting of large Mg grains surrounded by thin MgB₂ layers has been prepared. The superconducting properties of such a cellular structure were found to depend on the thickness of the superconducting layer d _s. As d _s decreases from ～30 to 1 μm, the critical temperature decreases by more than 10 K. The derivative of the upper critical field with respect to the temperature (dH _c2/dT) near T _c increases from ～0.2 T/K at high d _s to ～0.35 T/K at d _s ～6 μm. The critical current density of the cellular samples calculated for the area of the superconductor section is above 10⁵ A/cm² (T = 4.2 K and H = 1 T).