Magnetic and structural phase transitions in the shape-memory ferromagnetic alloys Ni2+x Mn1−x Ga

The results of an experimental investigation of the temperature dependences of the magnetic susceptibility and resistivity in the shape-memory ferromagnetic alloys Ni_2+x Mn_1−x Ga (x=0–0.20) are reported. A T−x phase diagram is constructed on the basis of these data. It is shown that partial substitution of Ni for Mn causes the temperatures of the structural (martensitic) T _M and magnetic T _C (Curie point) phase transitions to converge. In the region where T _C =T _M the transition temperature increases linearly with magnetic field in the range from 0 to 10 kOe. The kinetics of a magnetic-field-induced martensitic phase transition is investigated, and the velocities of the martensite-austenite interphase boundary during direct and reverse transitions are measured. A theoretical model is proposed and the T−x phase diagram is calculated. It is shown that there exist concentration ranges where the magnetic and martensitic transitions merge into a first-order phase transition. The theoretical results are in qualitative agreement with experiment. Zh. éksp. Teor. Fiz. 115, 1740–1755 (May 1999)     

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.     

Crystal structure and physical properties of magnetic shape memory alloys Ni50 − x Cu x Mn29Ga21

The phase composition, crystal structure, and physical properties (magnetization, electrical resistivity, thermoelectric power, relative elongation, and thermal expansion coefficient) of the stoichiometric alloy Ni₅₀Mn₂₅Ga₂₅ and nonstoichiometric alloys Ni_{50 ? x} Cu _x Mn₂₉Ga₂₁ (x = 0, 1, 2) with the thermoelastic martensitic transformation have been investigated. The influence of the chemical composition on the transformations and physical properties of the alloys has been determined.     

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.     

Magnetic and electric properties of Yb x Mn1 − x S alloys

Results from investigating the structural, magnetic, and electrical properties of Yb _x Mn_{1 ? x} S alloys (0 ≤ x ≤ 0.2) synthesized on the basis of manganese monosulfide are presented. Substituting manganese for ytterbium increases the concentration of charge carriers and lowers the activation energy. The observed anomalies in the temperature dependence of resistivity are explained by an impurity semiconductor model with donor 4f levels.     

Structure and dielectric properties of Bi6 − x Sr x Ti2 − x Nb2 + x O18 (x = 0–2) solid solutions

The structural and electrophysical characteristics of a series of solid solutions of layered perovs-kite-like oxides Bi_{6 ? x} Sr _x Ti_{2 ? x} Nb_{2 + x} O₁₈ (x = 0, 0.25, 0.5, 1.0, 1.5, 2.0) have been studied. The temperature dependences of the relative permittivity ?/?₀(T) and dielectric loss tangent tanδ have been measured. The dependences of the maximum of the permittivity ?/?₀, Curie temperature T _C, lattice parameters, and the unit cell volume on x have been obtained. The structural parameter a, which corresponds to the polar direction, and the value of the orthorhombic distortion of the unit cell have been found to demonstrate noticeable negative deviations from the Vegard’s law. It has been established that the variations of the orthorhombic distortion correlate with the variations of the permittivity maximum; however, they do not markedly influence the Curie temperature that varies linearly over entire range of changes in x.     

Magnetic properties of RNi1−xIn1+x (R=Gd–Er) compounds

AC and DC bulk magnetic measurements were performed for RNi_1−xIn_1+x (R=Gd–Er and x=0,0.1, 0.25) compounds. These compounds crystallize in the hexagonal ZrNiAl-type structure. The lattice parameters a and c for the RNiIn series decrease linearly with increasing number of 4f electrons. For nonstoichiometric RNi_1−xIn_1+x additional indium atoms occupy the 2(c) crystallographic site and the a parameter increases while the c parameter decreases with increasing indium content. The stoichiometric samples show ferromagnetic behavior with the critical temperature changing from 96 K for R=Gd to 9 K for R=Er. In the nonstoichiometric RNi_1−xIn_1+x compounds increase in the indium content leads to decrease in the ferromagnetic critical temperatures and to a change of the antiferromagnetic ordering for x=0.25 in the case of R=Dy, Ho and Er.     

Magnetic and mechanical properties of Ni–Mn–Ga/Fe–Ga ferromagnetic shape memory composite

A ferromagnetic shape memory composite of Ni–Mn–Ga and Fe–Ga was fabricated by using spark plasma sintering method. The magnetic and mechanical properties of the composite were investigated. Compared to the Ni–Mn–Ga alloy,the threshold field for magnetic-field-induced strain in the composite is clearly reduced owing to the assistance of internal stress generated from Fe–Ga. Meanwhile, the ductility has been significantly improved in the composite. A fracture strain of 26% and a compressive strength of 1600 MPa were achieved.     

Iron content and temperature dependences of diffuse scattering in Ti–(50–x)Ni–xFe (6 ≤ x ≤ 10) alloys

Diffuse scatterings appearing in electron diffraction patterns of Ti–(50???x)Ni–xFe (x?=?6, 7, 8, 10, in at.%) alloys were investigated. In the alloys, martensitic (R-phase) transformation is suppressed down to 4.2 K, but the electrical resistivity exhibits a local minimum at T _min (210 K, 195 K, 180 K and 140 K for x?=?6, 7, 8, 10, respectively). The following results were obtained for all the alloys. Diffuse scattering appears below T _min and its intensity maximum is located at an incommensurate position of g?+??ζζ0?*, where g is a reciprocal lattice vector of the B2-phase. The value?ζ?at T _min is significantly smaller than 1/3 and increases with decreasing temperature; it decreases with increasing Fe content. The value of?ζ?at T _min agrees with the length of nesting vector obtained by a band calculation, suggesting that the diffuse scattering is caused by the nesting effect of the Fermi surface in the B2-type structure.     

Phase transitions in ferromagnetic Ni2+x Mn1−x Ga alloys with regard for the modulation order parameter

The phase diagram of ferromagnetic alloys Ni_2+x Mn_1?x Ga is reconstructed on the basis of temperature dependences of the resistance. It is seen from this diagram that for small x, structural transitions from the cubic to the tetragonal phase are preceded by structural transformations in the cubic phase. In the framework of the phenomenological Landau theory of phase transitions, phase diagrams of the structural and magnetic phase transitions in these alloys are analyzed with regard for the modulation order parameter. It is shown that premartensitic and postmartensitic phase transitions related to the appearance of the modulated structure can occur along with martensitic transformations. The strain and modulation order parameters substantially affect the magnetic phase transitions via the interaction with the magnetic order parameter.     

Effect of cobalt doping on thermoelastic martensitic transformations and physical properties of magnetic shape memory alloys Ni50 − x Co x Mn29Ga21

The magnetic and thermoelastic martensitic transformations and physical properties (magnetization, electrical resistivity, thermoelectric power, relative elongation, and thermal expansion coefficient) of multicomponent magnetic shape memory alloys Ni_{50 ? x} Co _x Mn₂₉Ga₂₁ (x = 0, 1, 2, 3, 10 at %) have been investigated. The critical temperatures of thermoelastic martensitic transformation and magnetic transitions have been determined. It has been found that the alloy with 10 at % Co undergoes a martensitic transformation in the temperature range of 6–10 K.     

Magnetic behaviour of U(MnxAl1−x)2 compounds

The results of magnetic measurements performed on U(Mn_xAl_1−x)₂ compounds in the temperature range 4.2K < T < 800K are reported. In the low temperature range (T < 200K), UMn₂ shows a Pauli-type paramagnetism. Above 420K a Curie-Weiss behaviour is evidenced. The magnetic properties of U(Mn_xAl_1−x)₂ compounds were analysed assuming a superposition of a temperature dependent term on a Pauli-type contribution, χ_O. The effective moments as well as the χ_O values were determined both in the low (T < 200K) and high (T > 420K) temperature range. The experimental data were discussed considering changes in the band structure and/or quenching of spin fluctuations.     

Modeling of a structural phase transition in La2−x SrxCuO4

The structural, vibrational, and elastic properties of La₂CuO₄ are calculated using a model for calculating the energy of the crystal based on interionic potentials with the multiparticle Jahn-Teller contribution included explicitly. The microscopic reasons for the structural instability of the La₂CuO₄ lattice relative to rotations of the oxygen octahedra are investigated. A structural phase transition from the orthorhombic phase (space group D _2h ¹⁸ ) into the tetragonal phase (space group D _4h ¹⁷ ) under hydrostatic compression of an La_2−x Sr_xCuO₄ crystal is modeled. The (P,x) structural phase diagram for La_2−x Sr_xCuO₄ is constructed. Fiz. Tverd. Tela (St. Petersburg) 41, 1096–1102 (June 1999)     

A computational NICS and 13C NMR characterization of the substitution patterns of C70−2x(BN)x fullerenes (x=1–25)

A density functional study has been performed to investigate the electronic and magnetic properties of BN substituted fullerenes C_70?2x(BN)_x (x=1, 2, 3, 6, 9, 12, 15, 17, 19, 21, 23 and 25) based on the NMR parameters and NICS index. The calculated ¹³C chemical shielding (CS) tensors are found to be perturbed at the first and second neighbors of the doped atoms while the other distant carbon atoms not to be influenced significantly. ¹³C Chemical shifts (δ_iso) of the second neighbors of nitrogen and boron are significantly shifted to upfield and downfield (the second neighboring effects), respectively. Besides, chemical shifts are also affected by the curvature of the corresponding sites; for example, the perturbed sites at the caps yield smaller absolute values of chemical shifts than those located at the equator. Nucleus independent chemical shifts (NICS) at the cage centers of heterofullerenes (from ?25.29 to ?8.80) demonstrate that all the substituted species are aromatic, but less than C₇₀ (?27.29). The predicted NICS values may be useful for identification of the heterofullerenes through their endohedral ³He NMR chemical shifts.     

First-principles study of the structural and elastic properties of AuxV1–x and AuxNb1–x alloys

Ab initio total energy calculations, based on the Exact Muffin-Tin Orbitals (EMTO) method in combination with the coherent potential approximation (CPA), are used to calculate the total energy of Au_xV_1–x and Au_xNb_1–x random alloys along the Bain path that connects the body-centred cubic (bcc) and face-centred cubic (fcc) structures as a function of composition x (0 ≤ x ≤ 1). The equilibrium Wigner–Seitz radius and the elastic properties of both systems are determined as a function of composition. Our theoretical prediction in case of pure elements (x = 0 or x = 1) are in good agreement with the available experimental data. For the Au–V system, the equilibrium Wigner–Seitz radius increase as x increases, while for the Au–Nb system, the equilibrium Wigner–Seitz radius is almost constant. The bulk modulus B and C₄₄ for both alloys exhibit nearly parabolic trend. On the other hand, the tetragonal shear elastic constant C′ decreases as x increases and correlates reasonably well with the structural energy difference between fcc and bcc structures. Our results offer a consistent starting point for further theoretical and experimental studies of the elastic and micromechanical properties of Au–V and Au–Nb systems.     

Theory of the electronic structure of Ga1-yInyNxAs1−x and related alloys

Abstract

We present a quantitative k.P Hamiltonian which describes analytically the composition dependence of the energy gap, interband momentum matrix element, band edge effective masses and conduction band dispersion of GaN_XAs_1?x alloys for low N concentrations (x < ～ 0.05). The model has been confirmed using an sp³s? tight-binding Hamiltonian whose results agree well both with experiment and with previous pseudopotential calculations. The model should be of wide use to guide the future development of this material system and its applications.     

Structure,magnetic properties and magnetocaloric effects of Fe50Mn15−x Co x Ni35 alloys

Fe50Mn15-xCoxNi35(x=0,1,3,5,7)alloys were prepared by arc melting under purified argon atmosphere.The ingots were homogenized at 930°C for 90h followed by water quenching.The crystal structure,magnetic properties and magnetocaloric effects of the alloys were studied by X-ray diffraction(XRD)and MPMS-7-type SQUID.The results show that all samples still maintained a single-(Fe,Ni)-type phase structure.With the increase of the content of Co,the Curie temperatures of these alloys increased and exhibited a second-order magnetic transition from ferromagnetic(FM)to paramagnetic(PM)state near Curie temperature.The maximum magnetic entropy change and the relative cooling power of Fe50Mn10Co5Ni35alloy was 2.55 J/kg·K and 181 J/kg,respectively,for an external field change of 5T.Compared with rare earth metal Gd,Fe50Mn15-xCoxNi35 series of alloys have obvious advantage in resource price;their Curie temperatures can be tuned to near room temperature,maintain a relatively large magnetic entropy change at the same time and they are a type of potential magnetic refrigeration materials near room temperature.