Ab-initio calculations of the electronic structure of impurities and alloys of ferromagnetic transition metals

We review recent theoretical work on the electronic structure and the magnetic properties of ferromagnetic transition-metal alloys. All calculations are based on density-functional theory in the local-spin-density approximation. We report about calculations for dilute alloys using the KKR-Green's function method and for concentrated disordered alloys using the charge-self-consistent KKR-CPA method.     

Electric field gradients in cubic alloys

The theoretical investigations of electric field gradient (EFG) and asymmetry parameter (ν) for cubic dilute alloys are reviewed. The valence and size EFG's and the Sternheimer antishielding factor are discussed in detail. The calculations of EFG and ν for simple and transition metal (TM) dilute alloys of aluminium and copper, TM dilute alloys of vanadium and trinary dilute alloys of noble metals are summarized. It is emphasized that the size EFG is as important as the valence EFG to calculate the total EFG and ν, which are compared with the experimental values. An epilogue for further investigations is added.     

Attractive colloidal rods in shear flow

The effect of shear flow on the isotropic-nematic phase transition of attractive colloidal rods is investigated by a combination of simulations and experiments. The isotropic phase aligns with the flow, while the nematic phase undergoes a collective rotational motion which frustrates the merging of the coexisting regions. The location of binodals, spinodals, and the tumbling-to-aligning transition line in the shear-rate versus concentration plane are investigated. The phase diagrams in the shear-concentration plane for the various strengths of attractions can be mapped onto a master curve by appropriate scaling.     

On the accuracy of various statistical methods for describing ordering phase transitions in BCC alloys

The accuracy of various statistical methods for describing B2-or D0₃-type (CuZn or Fe₃Al) ordering phase transitions characteristic of BCC alloys is examined. The temperature-concentration phase diagrams were calculated for several models of alloys discussed in the literature with the use of three methods, namely, the mean-field and pair cluster approximations and the tetrahedron cluster field method developed in this work. The calculation results were compared with each other and with Monte Carlo calculations. The accuracy of various methods was found to sharply depend on the type of interactions in the system, primarily, on the presence of strong competing interactions and short-range correlations that hinder ordering. In the absence of such correlations, in particular, in Fe-Al type alloys with extended interactions, the use of the pair cluster approximation allows the results obtained in the mean-field approximation to be substantially refined without any noticeable complication of calculations. At the same time, for systems with short-range and competing interactions, the use of this approximation could drastically distort the form of phase diagrams, whereas the tetrahedron cluster field method allows the phase diagrams of these systems to be calculated fairly accurately for temperatures and concentrations of interest to physics.     

Ab initio structural parameters and transition pressures of GaN x As1− x dilute alloys

The results are presented of an ab initio total energy study of the high-pressure phases of GaN _x As_{1? x} dilute alloys calculated within the full-potential linearized augmented plane wave method in the generalized gradient approximation. Three candidate structures, namely zinc-blende, rocksalt and CsCl, have been considered. In each case, the structural parameters and transition pressures of dilute alloys of interest are calculated. In agreement with earlier calculations of Stenuit and Fahy [Phys. Rev. B 76 (2007) 035201], the deviation of the lattice constants from Vegard's law for zinc-blende GaN _x As_{1? x} alloys is found to be negligible.     

The magnetoresistivity of dilute AlMn alloys

A set of accurate measurements of the low temperature magnetoresistivity of dilute AlMn alloys is presented. By using an AlV alloys as a baseline, the resulting negative magnetoresistivity is obtained and compared with theoretical calculations.     

Calculation of hyperfine interaction parameters in metals-friedel oscillations in dilute Fe and Cu alloys

We review some recent calculations on hyperfine interaction parameters in metals based on density functional theory in the local density approximation. We restrict ourselves to the calculation of Friedel oscillations of the charge and magnetization density in dilute Fe and Cu alloys. In particular, we estimate hyperfine fields, isomer shifts and electric field gradients for a few shells around the impurities. The calculations are performed using the KKR-Green's function method. Especially, we discuss which valence properties determine the hyperfine interaction parameters and what information about the electronic structure can be obtained from their measurement.     

Wetting of grain boundaries by the second solid phase in Al-based alloys

This work presents data on changes in the structure and properties of aluminum-based alloys subjected to severe plastic deformation by high-pressure torsion. Microstructure and the composition and change in temperature for phases are studied for double (Al-Zn, Al-Mg) and ternary (Al-Mg-Zn) alloys. Tie-lines of the liquid phase wetting of grain boundaries and grain boundary solvus lines are constructed on phase diagrams of our Al-Zn and Al-Mg systems. The shifting of the phase composition in the bulk of aluminum grains from the Al + τ region to the Al + η region of the Al-Mg-Zn phase diagram with an increase in the specific area of grain boundaries is described.     

Charge transfer in binary alloys and to impurities in iron

The energy density formalism in the Thomas-Fermi-Dirac approximation was used to study charge transfer in equiatomic binary ordered alloys and in dilute iron alloys. This method was applied outside of a core region around each nucleus; the inner region was treated by taking the electron density to be that given by free atom wave functions.The results for the equivolume charge transfer in binary alloys agreed satisfactorily with existing KKR-selfconsislent field calculations.For metalloid impurities in iron, the charge transfer to the impurity, and the variation of lattice parameter with concentration was computed. The latter compared fairly well with experiments.The theory explicitly displays the role of the electron density at the cell boundary in alloy formation and charge transfer effects. Alloy formation is visualized as a process in which atomic cells of the pure metals are expanded or compressed to the equilibrium cell volume of the alloy, bringing the cells together to form the alloy, and then allowing the electron distribution to relax.The general conclusion of this work is that modifying the TFD theory by using it only in regions where the electron density does not vary too rapidly, and using quantum theory results for the density close to the nuclei. provides a satisfactory method for studying charge transfer.     

Processing pathway effects in CoCrCuNi+X (Fe,Mn) high-entropy alloys

A parallel study of mechanical alloying and solidification was carried out on FCC high-entropy alloys (HEAs) CoCrCuNi, CoCrCuFeNi and CoCrCuMnNi to investigate the effects of each processing methods on the resulting microstructure, crystal structure and microhardness. Elemental powders were mechanically alloyed followed by spark plasma sintering (SPS) at 800°C and 900°C to achieve densified discs, while arc melting was carried out from bulk pieces of the elemental metals followed by furnace annealing at 800°C and 900°C for 5?h. Both processing routes resulted in a primary FCC phase with secondary Cu-rich FCC segregation as interdendrites for the solidified alloys and particle boundaries for the SPS alloys, with the exception of a small amount of σ phase present in the SPS processed alloys. The solidification of the CoCrCuNi, CoCrCuFeNi and CoCrCuMnNi HEAs resulted in typical dendritic microstructure, followed by the precipitation of a small Cr-rich phase in the CoCrCuMnNi alloy after annealing. The grain size of the mechanically alloyed powder was approximately 20?nm from the Scherrers equation and the SPS processed HEAs consisted of a Cu-rich phase in the particle boundaries, forming cobblestone-like microstructure. The microhardness was examined in the as-cast, annealed and SPS states. It was found that the SPS processed samples had an increased microhardness by a factor of 2.5.     

理论研究VTiTaNbAlx高熵合金的结构与弹性力学性质

实验已证明VTiTaNbAlx高熵合金(HEAs)为单相固溶体,它采用了体心立方结构. 在这篇文章中,我们使用基于密度泛函理论的饼模轨道(EMTO)结合相干势近似(CPA)方法,计算并分析了此高熵合金体系的平衡体性质,弹性常数及多晶弹性模量. 结果表明：此系列高熵合金符合单相高熵合金的理论判据,具有较好的内在塑性,等摩尔比的VTiTaNb高熵合金趋于弹性各向同性. 随着Al含量的增加,此系列高熵合金的弹性各向异性趋于增大,但对多晶弹性模量几乎没有影响;同时讨论了基于弹性常数计算的德拜温度.     

理论研究VTiTaNbAl_x高熵合金的结构与弹性力学性质

实验已证明VTiTaNbAl_x高熵合金(HEAs)为单相固溶体,它采用了体心立方结构.在本文中,我们使用基于密度泛函理论的饼模轨道(EMTO)结合相干势近似(CPA)方法,计算并分析了此高熵合金体系的平衡体性质,弹性常数及多晶弹性模量.结果表明:此系列高熵合金符合单相高熵合金的理论判据,具有较好的内在塑性,等摩尔比的VTiTaNb高熵合金趋于弹性各向同性.随着Al含量的增加,此系列高熵合金的弹性各向异性趋于增大,但对多晶弹性模量几乎没有影响;同时讨论了基于弹性常数计算的德拜温度.     

Atomic configuration of antiphase boundaries in alloys

Computer simulation methods in the approximation of pairwise interatomic Morse potentials are used to calculate the atomic configurations and the energy of formation of 1/2 <110> {111} antiphase boundaries in ordered Cu₃Au and Ni₃Fe alloys. Atomic displacements result in considerable smearing of the first five planes closest to an antiphase boundary and a slip of atomic planes. These effects substantially reduce the energy of formation of antiphase boundaries. The influence of the size factor on the atomic configuration on the antiphase boundaries is discussed.Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 2, pp. 43–47, February, 1985.In conclusion, the authors express their thanks to A. I. Tsaregorodtsev for useful discussion of this work.     

Theoretical study of the structural,electronic and magnetic properties of equiatomic quaternary CoTcCrZ (Z = Si,Ge, P) Heusler alloys

In this study, full potential linearized augmented plane wave (FP-LAPW) method has been used to calculate structural, electronic and magnetic characteristics of CoTcCrZ (Z = Si, Ge, P) Equiatomic Quaternary Heusler alloys (EQHAs). Furthermore, Generalized Gradient Approximation (GGA) and Hubbard potential (GGA+U) are adopted in the parametrization of Perdew-Burke-Ernzerhof (PBE). Structural stability calculations confirmed that the Type II is the most stable structure of all considered alloys. Further investigations were carried out for the most stable Type (i.e. Type II). Comparison of electronic structure calculations performed by GGA and GGA+U methods concluded that considered alloys show half-metallic nature for GGA+U method. Magnetic moments for all these alloys are determined which are found in accordance to the Slater-Pauling rule. Half-metallicity has been verified in all these considered Heusler alloys (HAs) from the calculations of spin-polarization at the Fermi level (E_F). Moreover, the Curie temperature (Tc) is estimated by employing mean field approximation (MFA).     

Metastable surface alloys produced by ion implantation,laser and electron beam treatment

Ion Implantation, Laser and Electron-beam Treatment (LET) of metals have been employed extensively to produce metastable surface alloys. Recent published work on implanted alloys is reviewed first. The dilute implanted alloys (solute concentration <10 at. %) are shown to lead to crystalline metastable solid solutions. At higher solute concentrations, an amorphous phase has been observed for several binary systems and recently for a ternary system. The physical mechanisms at play, are discussed in detail. A review of the surface alloys produced by LET of metals is then presented—with an emphasis on the mechanisms involved. In particular, general criteria governing formation of metastable solid solutions under LET are proposed and shown to have excellent agreement with available data on metals and Si.     

On accuracy of different cluster models used in describing ordering phase transitions in fcc alloys

A general formulation of cluster methods applied to calculations of thermodynamic quantities of alloys in terms of renormalizing fields describing interaction between a cluster and its environment is given. We have shown that the well-known cluster variation method and the cluster field method, which was suggested earlier, are special cases of our approach. These methods have been used in calculations of phase diagrams of fcc alloys with L1₂ and L1₀ ordering transitions with several realistic interaction models. It turns out that, for all these models, the simple tetrahedron version of the cluster field method suggested in this paper describes the phase diagrams almost as accurately as more complicated cluster variation techniques. Possible applications of the tetrahedron version of the cluster field method to inhomogeneous states and kinetics of phase transitions in fcc alloys are discussed. Zh. éksp. Teor. Fiz. 115, 158–179 (January 1999)     

Ordering in binary FCC alloys

A new method of approximation for studying the order-disorder transition in binary alloys is presented. The formulation is in terms of face-centered cubic alloys, although the method can also be applied to other structures. The technique is essentially a generalization of the constant-coupling approximation. We have applied the principles of that method to derive the free energy of the alloy using a tetrahedral cluster of nearest neighbouring sites as the basic unit of the calculation. We only consider the case of nearest neighbour pair interactions, but show how the method can be generalized to include many body and second neighbour interactions. Numerical results are presented and comparison of these results is made with results of cluster variation calculations on the same system and with experimental results on the copper-gold alloy system.     

Spin-wave impurity states in metallic ferromagnets

It is shown that spin waves in dilute ferromagnetic transition metal alloys can be described in terms of effective matrix-matrix and impurity-matrix exchange integrals. Such a parametrization is exact within the random phase approximation for long-wavelength spin waves. The effective impurity-matrix exchange integral is determined in the tight-binding approximation in terms of the impurity potential and local density of states. The present theory is applied qualitatively to NiFe alloys.