Magnetic properties of (Ge,Pb)1−xMnxTe

Pseudoternary alloys {(GeTe)_1?y(PbTe)_y}_1?x(MnTe)_x have been prepared and the magnetic and electrical properties of these alloys have been studied. The magnetic interaction in (PbTe)_1?x(MnTe)_x is very weak, but it has been found that the {(GeTe)_1?y(PbTe)_y}_1?x (MnTe)_x alloys (y ≦ 0.1) are ferromagnetic. In the latter, the increase of the amount of Pb substitution makes both the paramagnetic Curie temperature and the carrier density lower. These experimental results are discussed by using the RKKY interaction.     

Thermal destruction of antiferromagnetic Fe-Fe exchange bonds and mechanism of the transition to the spin-glass state in (Fe0.65Ni0.35)1−x Mnx systems with antiferromagnetic competing exchange interactions

The thermal evolution of the competition between the ferro-and antiferromagnetic exchange interactions in (Fe_0.65Ni_0.35)_1?x Mn_x alloys, which display different magnetic properties, depending on composition and temperature, is investigated. The distribution functions of the magnetic hyperfine fields P(B _hf) for ⁵⁷Fe are determined by Mössbauer spectroscopy in the temperature range 5–300 K for the alloys with x=0, 0.024, 0.082, 0.136, 0.195, and 0.252. The temperature dependence of the integrated intensity I _s (T) is analyzed for the low-and high-field portions of P(B _hf). The features found in the behavior of I _s (T) are interpreted as results of variation of the ratio between the competing exchange interactions of different signs as a result of the thermal destruction of antiferromagnetic Fe-Fe exchange bonds. It is shown that the changes in the spin structure in the low-temperature range are due to the thermal destruction of Fe-Fe exchange bonds. One of the consequences of this destruction is “reentrance” (an increase in the hyperfine field with increasing temperature for some of the Fe atoms). The relationship between the thermal destruction of Fe-Fe exchange bonds and the magnetic transitions of the Fe-Ni-Mn system to the spin-glass state is considered.     

Brillouin zone effects on the critical temperature of superconductors and some related normal metal properties

A Fermi surface passing through a set of Bragg planes gives rise to variations of several normal metal properties, e.g. the electronic density of states N(0), the magnetic susceptibility, the Seebeck coefficient, the Debye temperature and the electron-phonon interaction strength V_ph, and consequently also to variation of the critical temperature for superconductivity, T_c.This behaviour is analysed on the basis of a nearly free electron model and a comparison is made with experimental results in various alloy systems. These systems include alloys of non-transition metals and pseudo-binary alloyed compounds with Cu₃Au- and CuAl₂-type structure.The observed variations of normal metal properties are in reasonable agreement with the theoretical estimates. The variations in T_c indicate that enhancements not only of N(0) but probably also of V_ph occur near Bragg planes.     

Magnetic transitions and magnetostrictive properties of Laves compounds Sm0.88Nd0.12(Fe1−xCox)1.93

The structure, magnetic and magnetostrictive properties of Sm_0.88Nd_0.12(Fe_1−xCo_x)_1.93 (0≤x≤1.0) alloys have been investigated. The alloys have the cubic MgCu₂ structure over the whole composition range and the lattice parameter a decreases with increasing x. For 0≤x≤0.2, substitution of Co for Fe slightly increases the saturation magnetization M_s and Curie temperature T_c, while further substitution causes a decrease in both M_s and T_c. The spin reorientation is observed, and a phase diagram for the spin configurations of the Sm_0.88Nd_0.12(Fe_1−xCo_x)_1.93 system is determined. The spontaneous magnetostriction λ₁₁₁ increases as x is increased, while a monotonic decrease of the saturation magnetostriction λ_s with x originates from the increase of λ₁₀₀ with opposite sign to that of λ₁₁₁, which may be caused by the filling of the d band due to Co substitution.     

The magnetic entropy changes in Gd1−xBx alloys

A series of Gd_(1−x)B_x alloys have been prepared by arc melting method. After introducing small quantity of B atom in Gd, the Curie temperature of these alloys increase while the magnetic entropy changes are almost same as that of Gd. The refrigerant capacities of these alloys are also greater than that of Gd. These results suggest that Gd_(1−x)B_x alloys may be utilized as refrigerant in household magnetic refrigeration.     

Bandgap engineering of Cd1−xSrxO

Structural, electronic and optical properties of Cd_1−xSr_xO (0≤x≤1) are calculated for the first time using density functional theory. Our results show that these properties are strongly dependent on x. The bond between Cd and O is partially covalent and the covalent nature of the bond decreases as the concentration of Sr increases from 0% to 100%. It is found that Cd_1−xSr_xO is an indirect bandgap compound for the entire range of x and the bandgap of the alloy increases from 0.85 to 6.00 eV with the increase in Sr concentration. Frequency dependent dielectric functions ε₁(ω), ε₂(ω), refractive index n(ω) and absorption coefficient α(ω) are also calculated and discussed in detail. The peak value of refractive indices shifts to higher energy regions with the increase in Sr. The striking feature of these alloys is that Cd_0.5Sr_0.5O is an anisotropic material. The larger value of the extraordinary refractive index confirms that the material is positive birefringence crystal. The present comprehensive theoretical study of the optoelectronic properties of the material predicts that it can be effectively used in optoelectronic applications in the wide range of spectrum; IR, visible and UV.     

Symmetry energy of the nucleus in the relativistic Thomas–Fermi approach with density-dependent parameters

We introduce an inhomogeneous term, f(t,x), into the right-hand side of the usual Burgers equation and examine the resulting equation for those functions which admit at least one Lie point symmetry. For those functions f(t,x) which depend nontrivially on both t and x, we find that there is just one symmetry. If f is a function of only x, there are three symmetries with the algebra s l(2,R). When f is a function of only t, there are five symmetries with the algebra s l(2,R) ⊕ _s 2A ₁. In all the cases, the Burgers equation is reduced to the equation for a linear oscillator with nonconstant coefficient.     

First-principles calculations of structural,electronic and thermal properties of Zn1−x Mg x S ternary alloys

Structural, electronic and thermal properties of Zn_1?x Mg _x S ternary alloys are studied by using the full potential-linearized augmented plane wave method (FP-LAPW) within the density functional theory (DFT). The Wu-Cohen generalized gradient approximation (WC-GGA) is used in this approach for the exchangecorrelation potential. Moreover, the modified Becke-Johnson approximation (mBJ) is adopted for band structure calculations. The dependence of the lattice constant, bulk modulus and band gap on the composition x showed that the first exhibits a small deviation from the Vegard’s law, whereas, a marginal deviation of the second from linear concentration dependence (LCD). The bowing of the fundamental gap versus composition predicted by our calculations agrees well with the available theoretical data. The microscopic origins of the gap bowing are explained by using the approach of Zunger and co-workers. Thermal effects on some macroscopic properties of Zn_1?x Mg _x S alloys are also investigated using the quasi-harmonic Debye model, in which the phononic effects are considered. As, this is the first quantitative theoretical prediction of the thermal properties of Zn_1?x Mg _x S alloys, no other calculated results and furthermore no experimental studies are available for comparison.     

Global properties of radial wave functions in Schwarzschild's space-time

The radial factorR(x, x _s ) of a scalar field in Schwarzschild's space-time satisfies a second order ordinary differential equation with two regular singular points atx=0 andx=x _s and one irregular singular point atx=∞. The analytical properties of four solutions ?₁, ?₂, ?₃, and ?₄ (defined by their power series expansions aboutx=0 andx=x _s ) with respect tox _s are studied. An analytical continuation is given for each solution outside its circle of convergence. Relations to the flat-space solutions are established. Finally the coefficients relating linearly any three of these solutions are determined and studied as functions of the parameterx _s .     

Spin-orbit effects in carbon nanotubes – Analytical results

We report a study of the normal and superconducting state properties of the Ti _x V_1?x alloys for x = 0.4, 0.6, 0.7 and 0.8 with the help of dc magnetization, electrical resistivity and heat capacity measurements along with the electronic structure calculation. The superconducting transition temperature T _c of these alloys is higher than that of elemental Ti and is also higher than elemental V for x ≤ 0.7. The roles of electron density of states, electron-phonon coupling and spin fluctuations in the normal and superconducting state properties of these alloys have been investigated in detail. The experimentally observed value of T _c is found to be considerably lower than that estimated on the basis of electron density of states and electron-phonon coupling in the x = 0.4, 0.6 and 0.7 alloys. There is some evidence as well for the preformed Cooper pair in all these Ti-V alloys in the temperature regime well above T _c . Similar to x = 0.6 [Md. Matin, L.S. Sharath Chandra, R.K. Meena, M.K. Chattopadhyay, A.K. Sinha, M.N. Singh, S.B. Roy, Physica B 436, 20 (2014)], the normal state properties of the x = 0.4 alloy showed the signature of the presence of spin fluctuations. The difference between the experimentally observed T _c and that estimated by considering electron density of states and electron-phonon coupling in the x = 0.4, 0.6 and 0.7 alloys is attributed to the possible influence of these spin fluctuations. We show that the non-monotonous variation of T _c as a function of x in the Ti _x V_1?x alloys is due to the combined effects of the electron-phonon coupling and the spin fluctuations.     

Zur Supraleitung von Schichtpaketen aus Normal- und Supraleitern

Superimposed films of a super- and a normalconducting metal are produced on a quartz plate by evaporation. In most cases lead and occasionally also tin is used as superconductor. The normal metals are copper, manganese and chromium. For more than 50 samples of different combinations the transitiontemperatures are measured. The mean free path of the electrons for the metals can be varied by the conditions of condensation. By means of a phenomenological theory, which we owe in principle to a discussion with ProfessorFröhlich, Liverpool, we succeed in formulating expressions fort=f(D _s ,D _n ,l _s ,l _n ,ξ₀).D,l andξ ₀ are the thickness, mean free path and the coherence length of the supra- or the normal metals andt is the reduced transitiontemperature. The developed equations can be well fitted to the measured data also to those of other authors.     

D.C. electrical,optical and photoelectrical properties of GexTe1−x amorphous thin films

We give the main results of measurements on Ge_xTe_1?x(x = 0.5, 0.4, 0.3, 0.2, 0.1) amorphous alloys: variation of activation energy, conductivity, optical absorption, and μτ with composition. The striking features are that quantum efficiency is independent of photon energy and that shallow traps exist whose density increases with germanium content.     

Magnetic anisotropy,magnetostriction and intermediate range order in Co-P alloys

Coercivity, magnetostriction and saturation field as a measure of magnetic ansitropy energy have been investigated in Co_100-xP_x with 5 ? x ? 26. According to their magnetization curves and coercivities crystalline alloys (5 ? x ? 11) contain hcp-inclusions whereas amorphous material (12 ? x ? 26) is magnetically soft. The saturation magnetization is mainly determined by the nearest neighbour shell but magnetostriction is also correlated with the further environment of the cobalt atoms. The abrupt breakdown of the value |λ_s| of the saturation magnetostriction with increasing phosphorus content of the crystalline alloys points to a growing structural disorder. λ_s of the amorphous material depends on the saturation magnetization alone (λ_s ≈ σ₀²) which decreases with increasing phosphorus content. The small value of λ_s suggests that angular correlations do not exceed the second coordination shell.     

First-principles study of optical properties in Ca-doped ZnO alloys

Various electronic and optical properties of Zn_1?x Ca _x O ternary alloys of wurtzite structure are calculated using a first-principles approach based on the framework of the generalized gradient approximation to density-functional theory. In particular, on-site Coulomb interactions are introduced, which can reasonably well predict the electronic properties and band gaps of the Zn_1?x Ca _x O (0??x??0.25) system. The imaginary part of the calculated dielectric function indicates that the optical transition between O 2p states in the valence band and Zn 4s states in the conduction band shifts to the high-energy range as the Ca concentration increases. The calculated band gap shows a significant increase with increasing Ca concentration. Therefore, Zn_1?x Ca _x O ternary alloys may be a potential candidate alloy for optoelectronic materials, and especially for light-emitters and detectors.     

Electrical resistivity of microinhomogeneous PdMnxFe1−x alloys

The electronic band-structure calculations of the PdFe ferromagnet and the PdMn antiferromagnet performed in this work permit one to conclude that the specific features of the electrical resistivity observed in the ternary PdMn_xFe_1−x alloy system [the deviation from the Nordheim-Kurnakov rule ρ₀(x)∼x(1−x), which is accompanied by a high maximum of residual resistivity (not typical of metals) ρ ₀ ^m ∼220 μΩ cm at x _C∼0.8 and a negative temperature resistivity coefficient in the interval 0.5≤x≤1] are due to the microinhomogeneous (multiphase) state of the alloys and a variation in the band-gap parameter d spectrum caused by antiferromagnetic ordering of a PdMn-type phase. __________ Translated from Fizika Tverdogo Tela, Vol. 44, No. 2, 2002, pp. 193–197. Original Russian Text Copyright ? 2002 by Kourov, Korotin, Volkova.     

First-principles study of single-walled armchair Cx(BN)y nanotubes

The structural and electronic properties of the armchair C_x(BN)_y nanotubes are studied using the density functional theory with a generalized gradient approximation. The results show that the properties of the C_x(BN)_y nanotubes are intermediate between those of carbon nanotubes and BN nanotubes, and also adjustable by their radius, ratio of carbon component, and configurations.     

Co-Si-B and Fe-Co-B amorphous alloys: Induced anisotropy and various magnetic properties

The dependence on the metalloid content of some magnetic properties of Co_100−x(Si_0.6B_0.4)_x (22.5 ⩽ x ⩽ 30) and Co₇₅Si_25−xB_x (10 ⩽ x ⩽ 25) amorphous alloys has been studied.Ribbons were subjected to different kinds of heating treatments: field annealing, stress annealing and stress-field annealing (tensile stress and longitudinal magnetic field applied simultaneously). While the anisotropies induced by simple field annealings are of the order of magnitude of 0.1 kJm^-3, the anisotropy induced by stress-field annealing can reach values up to 0.5 kJm^-3. The preferred axis is longitudinal for most of the annealing conditions. The temperature and composition dependence of the magnetostriction have been studied too.Stress, field and stress-field induced anisotropies have also been measured in Co₆₆Fe₉B₂₅ samples (λ_s > 0). In this case the preferred axis is transverse to the ribbon axis.     

Higher order field equations II; Limit properties of green's functions

In a previous paper wave propagation was studied according to a sixth-order partial differential equation involving a complex mass M. The corresponding Yang-Feldman integral equations (indicated as SM-YF-equations), were formulated using modified Green's functions G^M_R(x) and G^M_A(x), which then incorporate the partial differential equation together with certain boundary conditions. In this paper certain limit properties of these modified Green's functions are derived: (a) It is shown that for |M| → ∞ the Green's functions G^M_R(x) and G^M_A(x) approach the Green's functions Δ_R(x) and Δ_A(x) of the corresponding KG-equation (Klein-Gordon equation). (b) It is further shown that the asymptotic behaviour of G^M_A(x) and G^M_A(x) is the same as of Δ_R(x) and Δ_A(x) - and also the same as for D_R(x) and D_A(x) for t→ ± ∞, where D_R and D_A are the Green n's functions for the KG-equation with mass zero. It is essential to take limits in the sense of distribution theory in both cases (a) and (b). The property (b) indicates that the wave propagation properties of the SM-YF-equations, the KG-equation with finite mass and the KG-equation with mass zero are closely related in an asymptotic sense.     

Influence of Al and Ni on the magnetic properties of Finemet alloys

Crystallization behavior and soft magnetic properties of the FeSiBCuNbM (M=Al or Ni) Finemet alloys are investigated by X-ray diffraction, differential scanning calorimetry, hysteresis loop tracer, and vibrating sample magnetometry. The nanocrystalline alloys are prepared by annealing melt-spun amorphous ribbons at different temperatures. Results indicate that the partial substitution of Ni or Al for Nb results in the increase of saturation magnetic induction density (B_s) of the alloys. The alloys with Al or Ni show favorable combination of soft magnetic properties. The partial substitution of Ni for Nb enhances the B_s value, while Al decreases coercivity. The mechanism underlining the magnetic behavior is discussed.