The d-resonance scattering in amorphous Fe x Pd80−x Si20 alloy

The electrical resistivity of amorphous Fe _x Pd_80–x Si₂₀ alloy has been discussed in terms of extended Ziman theory. The increase in the electrical resistivity of Fe _x Pd_80–x Si₂₀ has been predicted due to the d-resonance scattering of the conduction electrons by Fe-atoms.Dedicated to Academician Vladimír Hajko on the occasion of his 65th birthday.     

Crystallisation of magnetron sputtered amorphous Si1− x C x films (x = 1/3) studied by grazing incidence X-ray diffractometry

The crystallisation of amorphous Si_{1? x} C _x films (x = 1/3) produced via magnetron sputtering on silicon substrates was investigated. Grazing incidence X-ray diffractometry was used to analyse the crystalline precipitates obtained after annealing at temperatures between 1200°C and 1350°C. After annealing times of 15 h at 1200°C and 15 h at 1350°C, crystallisation of SiC is complete. The average crystallite size, d, was determined using the Scherrer equation. The rate constants for the initial growth of the crystallites were determined by straight line fits in the d ? t diagrams (t being the annealing time), which obey the Arrhenius law. The activation enthalpy of 4.0 ± 0.7 eV is, within error limits, the same as that found for the growth of silicon carbide crystallites in magnetron sputtered Si_{1? x} C _x films (x = 1/2).     

Mössbauer study of amorphous (Fe1−x Ga x )84B16

Using the⁵⁷Fe Mössbauer effect the influence of the Ga content in amorphous (Fe_1?x Ga _x )₈₄B₁₆ on the average hyperfine fields \(\bar H\) and isomer shift has been studied. For the sample (Fe_0.98Ga_0.02)₈₄B₁₆ the \(\bar H\) , as well as the recoilless fraction,f _a were measured as functions of temperature ranging from 12 K to 300 K. The experimental results show a linear correlation between Inf _a and δ, and well as between δ andx. In the temperature range \(\bar H(T)\) can be described by the Brillouin function and the second-order Doppler shift is appreciable. The characteristic temperature for such an amorphous alloy is 372 K. the effective vibrating massM _eff=79 a.u.     

Electron paramagnetic resonance of Gd3+ ions in Ca1 − x − y Y x Gd y F2 + x + y crystals

Electron paramagnetic resonance of Ca_{1 ? x ? y} Y _x Gd _y F_{2 + x + y} single crystals has revealed spectra that are not typical of gadolinium-doped CaF₂ crystals. These spectra have a nearly tetragonal symmetry and are most probably caused by Gd³⁺ ions localized in yttrium clusters. Weak spectra of tetragonal Gd³⁺ centers, whose parameters are close to those of a cubic gadolinium center caused by an isolated Gd³⁺ ion, have been also detected. These centers are attributed to isolated Gd³⁺ ions localized near octahedral rare-earth clusters or their associations.     

Ferromagnetic resonance study of Y3−z Ca z Fe5−x−y Co x Ge y O12 films

Conclusions It has been found that the character of the magnetic anisotropy changes markedly when the Co + Ge YIG films are doped with calcium. For the induced uniaxial and in-plane anisotropy we observe an increasing departure from the predictions based on the two-parameter model and for the cubic anisotropy the constantK ₁ becomes dependent on the growth direction. As a source for the latter effect the octahedral or tetrahedral preference of Co ions depending on the growth direction may be considered. The measured linewidth proportional to frequency indicates the presence of a relaxation mechanism which is, probably, connected with octahedral Co³⁺ ions.The authors express their thanks to Dr. P. Görnert and M. Neviva for preparation of the LPE films and to Dr. P. Novák for valuable discussions.Dedicated to Jan Kaczér DrSc on the occasion of his 65th birthday.     

Magnetic properties of U (Fe x Al1−x )2 and U(Fe y Ni1−y )2 compounds

The results of magnetic measurements and ferromagnetic resonance studies performed on U(Fe _x Al_1–x )₂ and U(Fe _y Ni_1–y )₂ compounds over a large temperature range are reported. The saturation magnetization decreases nearly linearly when substituting Fe by Al or Ni. In the composition range x<0.84 and y<0.81, the compounds are Pauli paramagnets, except in the region with y0.10. For UNi₂ two types of magnetic behaviours are shown. This compound can be both a ferromagnet withT _c =23.5 K and a Pauli paramagnet, depending on the crystal structure. Above the Curie temperatures, the reciprocal susceptibility for the compounds with x>0.84 and y>0.81 obeys a temperature dependence of the formX=X _o+C(T-) ^–1. The effective iron moments decrease when substituting iron by nickel or aluminium. The ferromagnetic resonance measurements show that theg values are not composition-dependent. A linear variation of the mean iron magnetization with the exchange field is observed. Finally, the magnetic behaviour of iron in these compounds is analysed.     

LDA’+DMFT investigation of electronic structure of K1 − x Fe2 − y Se2 superconductor

We investigate electronic structure of the new iron chalcogenide high temperature superconductor K_1?x Fe_2?y Se₂ (hole doped case with x = 0.24, y = 0.28) in the normal phase using the novel LDA’+DMFT computational approach. We show that this iron chalcogenide is more correlated in a sense of bandwidth renormalization (energy scale compression by factor about 5 in the interval ±1.5 eV), than typical iron pnictides (compression factor about 2), though the Coulomb interaction strength is almost the same in both families. Our results for spectral densities are in general agreement with recent ARPES data on this system. It is found that all Fe-3d(t _2g ) bands crossing the Fermi level have equal renormalization, in contrast to some previous interpretations. Electronic states at the Fermi level are of predominantly xy symmetry. Also we show that LDA’+DMFT results are in better agreement with experimental spectral function maps, than the results of conventional LDA+DMFT. Finally we make predictions for photoemission spectra lineshape for K_0.76Fe_1.72Se₂.     

X-ray diffraction studies of heterostructures based on solid solutions Al x Ga1 − x As y P1 − y : Si

The growth of MOCVD-hydride epitaxial heterostructures based on ternary solid solutions Al _x Ga_1?x As heavily doped with phosphorus and silicon has been studied using high-resolution X-ray diffraction and X-ray microanalysis. The prepared epitaxial films are five-component solid solutions (As _x Ga_1?x As _y P_{1 ? y} )_{1 ? z} Si _z .     

Investigating ionic mobility in solid solutions of Li x Na1 − x Ta y Nb1 − y O3 via Raman spectroscopy

The ionic mobility in Li _x Na_{1 ? x} Ta _y Nb_{1 ? y} O₃ solid solutions (SSes) is investigated by means of Raman spectroscopy. The average lifetime of an Li⁺ ion in the equilibrium position and the magnitude of the potential barrier are estimated from the temperature dependence of the line width corresponding to the vibrations of Li⁺ cations.     

Mössbauer Study of La1−x Ca x Mn1−y 57Fe y O3 with x=0,0.25; y=0.01

Temperature dependent Mössbauer measurements are done on the samples of La_1–x Ca _x Mn_1–y ⁵⁷Fe _y O₃ with x=0 and 0.25, and y=0.01. With decreasing temperature, the specimen with x=0.25 shows a paramagnetic to ferromagnetic transition around 175 K. In the specimen x=0.0, the temperature dependence of both the center shift () and the recoilless fraction (f) can be fitted very well with the Debye theory with a _D=320±50 K. But for the specimens with x=0.25, f and show distinct deviations from the Debye behavior in the temperature range in which the resistivity shows a sharp decrease. Dips observed in both the f and around the transition temperature suggest that the Jahn–Teller distortion observed in these systems is dynamic in nature.     

Morphology and magnetic properties of Fe x Co1−x /Co y Fe3−y O4 nanocomposites prepared by surfactants-assisted-hydrothermal process

Recently, we have demonstrated the successful synthesis of Fe _x Co_1−x /Co _y Fe_3−y O₄ nanocomposites with various alkaline solutions by using surfactants-assisted-hydrothermal (SAH) process. In this article, the synthesis of Fe _x Co_1−x /Co_yFe_3−y O₄ nanocomposites with their sizes varying between 20 nm and 2 μm was reported. X-ray powder diffraction (XRD) analyses showed that the surfactants, pH, precipitator, and temperature of the system play important roles in the nucleation and growth processes. The magnetic properties tested by vibrating sample magnetometer (VSM) at room temperature exhibit ferromagnetic behavior of the nanocomposites. These Fe _x Co_1−x /Co _y Fe_3−y O₄ nanocomposites may have a potential application as magnetic carriers for drug targeting because of their excellent soft-magnetic properties.     

Photoluminescence properties of heavily doped heterostructures based on (Al x Ga1 − x As)1 − y Si y solid solutions

It has been established that the photoluminescence spectra of heavily doped heterostructures based on Al _x Ga_{1 ? x} As)_{1 ? y} Si _y solid solutions exhibit quenching of the main exciton bands of Al _x Ga_{1 ? x} As ternary solid solutions and appearance of other maxima. The quenching of the main exciton bands can be associated both with the DX-center formation and with the change in the character of the band structure of (Al _x Ga_{1 ? x} As)_{1 ? y} Si _y quaternary solid solutions.     

Pressure Studies on Fe 1−x Si 1+x Single Crystals (−0.003≤ x ≤0.025)

A study of the pressure response of single crystalline Fe 1 m x Si 1+x samples revealed beyond an initial increase a pressure driven reduction of the band gap E g . Concomitantly, there is a well resolvable change of the variable range hopping conductivity, presumably caused by an increase of the characteristic temperature of the system due to band broadening effects in the electronic density of states.     

57Fe Mössbauer study of amorphous and nanocrystalline Fe73.5Nb3Cu1Si13.5B9 after neutron irradiation

⁵⁷Fe Mössbauer spectroscopy is used to study neutron irradiation induced changes in the short-range order of Fe_73.5Nb₃Cu₁Si_13.5B₉ alloy. The samples are investigated in both amorphous and nanocrystalline states. Neutron irradiation leads to an increase of the standard deviation of a hyperfine field distribution (HFD), implying rearrangement of the atoms towards disordering. Simultaneously, changes in the average value of the hyperfine field and a net magnetic moment position occur as a consequence of a spin reorientation, atom mixing and microscopic stress centres which are introduced by neutron irradiation.     

On the magnetic susceptibility of the liquid Pd1−x Si x alloy system at 1825 K

The isothermal magnetic susceptibility (x) of the completely miscible liquid Pd_1–x Si _x alloy system shows a rapid monotonous decrease withx from strong paramagnetism to weaklyx-dependent diamagnetism. The measured susceptibility isotherm at 1825 K is analysed within 0x1 by using a semiphenomenological method of decomposing the magnetic susceptibility into its constituent parts. Because of the empirical similarity between liquid and glassy metals this interpretation also provides assertions about the magnetism of glassy Pd_1–x Si _x aroundx=0.2.     

Calculation of the electronic structure and hyperfine fields for Fe1 − x Co x B and (Fe1 − x Co x )2B compounds by the Korringa-Kohn-Rostoker method

The magnetic properties of Fe_{1 ? x} Co _x B and (Fe_{1 ? x} Co _x )₂B disordered compounds were investigated using first-principles calculations of the electronic structure in the framework of the density functional theory with the Korringa-Kohn-Rostoker method. The concentration dependences of the magnetic moments and the electron density were calculated for the Fe_{1 ? x} Co _x B solid solutions. The results obtained were used to analyze in detail and to interpret the transition from a magnetic phase to a nonmagnetic phase, which was previously revealed from the experiments in the compounds under investigation. The performed analysis of the calculated hyperfine fields induced by the electronic shells at the iron and cobalt atoms in the (Fe_{1 ? x} Co _x )₂B borides made it possible to explain the experimentally observed magnetic anisotropy.     

Intracenter luminescence of Mn2+ in Cd1−x MnxTe and Cd1−x−y MnxMgyTe under intense optical pumping

A comparative analysis of the kinetic properties of intracenter 3d luminescence of Mn²⁺ ions in the dilute magnetic superconductors Cd_1?x Mn_xTe and Cd_1?x?y Mn_xMg_yTe is carried out. The influence of relative concentrations of the cation components on the position of the intracenter luminescence peak indicates that the introduction of magnesium enhances crystal field fluctuations. As a result, the processes facilitating nonlinear quenching of luminescence are suppressed. The kinetics of 3d-luminescence quenching in Cd_1?x Mn_xTe are accelerated considerably upon elevation of optical excitation level due to the evolution of cooperative processes in the system of excited manganese ions.     

Doping dependence of correlation effects in K1 − x Fe2 − y Se2 superconductors: LDA’ + DMFT investigation

We present a detailed LDA’ + DMFT investigation of the doping dependence of correlation effects in the novel K_{1 ? x} Fe_{2 ? y} Se₂ superconductor. Calculations are performed at four different hole doping levels, starting from a hypothetical stoichiometric composition with the total number of electrons equal to 29 per unit cell through 28 and 27.2 electrons toward the case of 26.52, which corresponds to the chemical composition K_0.76Fe_1.72Se₂ studied in recent ARPES experiments. In the general case, the increase in hole doping leads to quasiparticle bands in a wide energy window ±2 eV around the Fermi level becoming more broadened by lifetime effects, while correlation-induced compression of Fe-3d LDA’ bandwidths stays almost the same, of the order of 1.3 for all hole concentrations. However, close to the Fermi level, the situation is more complicated. In the energy interval from ?1.0 eV to 0.4 eV, the bare Fe-3d LDA’ bands are compressed by significantly larger renormalization factors up to 5 with increased hole doping, while the value of Coulomb interaction remains the same. This fact manifests the increase in correlation effects with hole doping in the K_{1 ? x} Fe_{2 ? y} Se₂ system. Moreover, in contrast to typical pnictides, K_{1 ? x} Fe_{2 ? y} Se₂ does not have well-defined quasiparticle bands on the Fermi levels, but has a “pseudogap”-like dark region instead. We also find that with the growth of hole doping, Fe-3d orbitals of various symmetries are affected by correlations differently in different parts of the Brillouin zone. To illustrate this, we determine the quasiparticle mass renormalization factors and energy shifts that transform the bare Fe-3d LDA’ bands of various symmetries into LDA’ + DMFT quasiparticle bands. These renormalization factors effectively mimic more complicated energy-dependent self-energy effects and can be used to analyze the available ARPES data.     

Rapid stress annealing dependence of structural and magnetic properties of Fe75 − x Co x Cu1Nb3Si15B6 alloys

Structural and magnetic properties of nanocrystalline Fe_75???x Co _x Cu₁ Nb₃Si₁₅B₆ (x?=?0, 2, 5) alloys are reported using magnetic measurements X-ray diffraction, Mössbauer spectroscopy. Results show that: (1) for the specimens with x?=?0 reveal that the volume fraction of the nanograins and their grain diameter ranges between 56% and 80% and 10 and 18 nm, (2) annealing above 700°C apart from Fe₃Si type nanocrystals, magnetically hard Fe₃B, Fe₂₃B phases also appear, leading to a sharp increase of the coercive field, (3) Co content and applied stress during annealing has considerable effect on relative permeability and stress induced anisotropy, which is perpendicular to the ribbon axis, Mössbauer spectroscopy also suggests changes in spin texture.