Ab-initio investigation of electronic properties and magnetism of half-Heusler alloys XCrAl (X=Fe, Co, Ni) and NiCrZ (Z=Al, Ga, In)

The electronic structures and magnetism of the half-Heusler alloys XCrAl (X=Fe, Co, Ni) and NiCrZ (Z=Al, Ga, In) have been investigated to search for new candidate half-metallic materials. Here, we predict that NiCrAl, and NiCrGa and NiCrIn are possible half-metals with an energy gap in the minority spin and a completely spin polarization at the Fermi level. The energy gap can be attributed to the covalent hybridization between the d states of the Ni and Cr atoms, which leads to the formation of bonding and antibonding peaks with a gap in between them. Their total magnetic moments are 1μ_B per unit cell; agree with the Slater-Pauling rule. The partial moment of Cr is largest in NiCrZ alloys and moments of Ni and Al are in antiferromagnetic alignment with Cr. Meanwhile, it is also found that FeCrAl is a normal ferromagnetic metal with a magnetic moment of 0.25μ_B per unit cell and CoCrAl is a semi-metal and non-magnetic.     

A first-principles study on the lower-valence coexisting Cr2TiX (X=Al, Ga, Si, Ge, Sn, Sb) Heusler alloys

The electronic, magnetic, and bonding properties of the Cr₂TiX (X=Al, Ga, Si, Ge, Sn, Sb) Heusler alloys have been investigated using first-principles calculations. The results show that Cr₂TiSb exhibits a half-metallic nature and Cr₂TiGa and Cr₂TiSn exhibit a nearly half-metallic nature. From analysis of the density of states and the electron density difference along the Ga→Sn→Sb series for sp atoms, we found that the Cr-Ti bond demonstrates covalent character with more or less the ionic and metallic nature. In addition, the Cr-Ti bonding strength increases along this series. All the compounds have a negative total magnetic moment, most of which are confined to the Cr atoms. There exists a 1.0μ_B increasing trend of the total moment along the III→IV→V main group for sp atoms, and only the total moment of Cr₂TiSb coincides well with the Slater-Pauling behavior.     

Effect of Cr on the electronic structure of Co3Al intermetallic compound: A first-principles study

The effect of doping with Cr on the electronic structure and magnetism of Co₃Al has been studied by density functional calculations. It has been found that the Cr atom has a strong site preference for the B-site in Co₃Al. With the substitution of Cr for Co, the total densities of states (DOS) change obviously: A DOS peak appears at E_F in the majority spin states and an energy gap is opened in the minority spin states. The effect of Cr in Co₃Al is mainly to push the antibonding peak of the Co (A,C) atoms high on the energy scale and to form the energy gap around E_F, and also to contribute to the large DOS peak at E_F in the majority spin direction. The calculations indicate a ferromagnetic alignment between the Co and Cr spin moments. The calculated total magnetic moment decreases and becomes closer to the Slater–Pauling curve with increasing Cr content. This is mainly due to the decrease of the Co (A,C) spin moments. At the same time, the moments of Co (B) and Cr (B) only change slightly.     

Effect of Fe substitution on the magnetic properties of half-Heusler alloy CoCrAl

The effect of Fe substitution for the vacant site in half-Heusler alloy CoCrAl is studied. A series of single phase CoFe_xCrAl (x=0.0, 0.25, 0.5, 0.75 and 1.0) alloys has been successfully synthesized. The lattice constant is found to increase almost linearly with increasing Fe content, indicating Fe atoms enter the lattice of CoCrAl instead of existing as a secondary phase. When Fe entering the vacant site, spin polarization occurs and the alloy turns from a semimetal in CoCrAl to a half-metallic ferromagnet (HMF) in CoFeCrAl. This is due to the reconstruction of the energy band with Fe substitution. The Curie temperature and saturation magnetic moments are enhanced and increase monotonically with increasing Fe content. The variation of the spin moment follows the Slater-Pauling curve and agrees with the theoretical calculation as well.     

Band structure calculations for Heusler phase Co2YBi and half-Heusler phase CoYBi (Y=Mn, Cr)

We have studied the electron structure and magnetic properties of Heusler phase Co₂YBi and half-Heusler phase CoYBi (Y=Mn, Cr) by using the full-potential linearized-augmented plane-wave (FLAPW) method. Co₂MnBi and Co₂CrBi are predicted to be half-metallic magnetism with a total magnetic moment of 6 and 5 μ_B, respectively, well consistent with the Slater-Pauling rule. We also predict CoMnBi to be half-metallic magnetism with a slight compression. The gap origin for Co₂MnBi and Co₂CrBi is due to the 3d electron splitting of Mn (Cr) and Co atoms, and the gap width depends on Co electron splitting. The atom coordination surroundings have a great influence on the electron structure, and consequently the Y site in the X₂YZ structure has a more remarkable electron splitting than the X site due to the more symmetric surroundings. The investigation regarding the lattice constant dependence of magnetic moment shows that the Co magnetic moment exhibits an opposite behavior with the change of the lattice constant for Heusler and half-Heusler alloys, consequently leading to the different variation trends for total magnetic moment. The variation of total and atom magnetic moment versus lattice constant can be explained by the extent of 3d electron splitting and localization of Mn (Cr) and Co atoms for both the series of alloys.     

Heat capacity of Heusler alloys: Ferromagnetic Ni2MnSb,Ni2MnSn,NiMnSb and antiferromagnetic CuMnSb

We report the results of the investigation of the specific heat of the ferromagnetic Heusler Ni₂MnSn, Ni₂MnSb, NiMnSb and antiferromagnetic CuMnSb alloys. The low-temperature behaviour of the specific heat may be described as C=γT+βT³ for ferromagnetic compounds and as C=γT+δ T²+βT³ for antiferromagnetic CuMnSb. The values of the density of states from the heat capacity measurements are higher than those from electronic band structure calculations. Debye temperatures are in a good agreement with those obtained from thermal expansion measurements. The Grüneisen parameter is calculated for Ni₂MnSn and CuMnSb from the magnetic contribution to the specific heat in the vicinity of T_C or T_N.     

High spin polarization in ordered Cr3Co with the DO3 structure: A first-principles study

The electronic structure of the highly ordered alloy Cr₃Co with the DO₃ structure has been studied by FLAPW calculations. It is found that the ferrimagnetic state is stable and that the equilibrium lattice constant of Cr₃Co equals 5.77 Å. A large peak in majority spin density of states (DOS) and an energy gap in minority spin DOS are observed at the Fermi level, which results in a high spin polarization of 90% in the ordered alloy Cr₃Co. The total magnetic moment of Cr₃Co is 3.12μ_B, which is close to the ideal value of 3μ_B derived from the Slater-Pauling curve. An antiparallel alignment between the moments on the Cr (A, C) sites and the Cr (B) sites is observed. Finally, the effect of lattice distortion on the electronic structure and on magnetic properties of Cr₃Co compound is studied. A spin polarization higher than 80% can be obtained between 5.55 and 5.90 Å. With increasing lattice constant, the magnetic moments on the (A, C) sites increase and the moments on the (B, D) sites decrease. They compensate each other and make the total magnetic moment change only slightly.     

Magnetism and Hall effect of the Heusler alloy Co2ZrSn synthesized by melt-spinning process

Magnetization and Hall resistivity have been measured for the Heusler alloy Co₂ZrSn synthesized by the melt-spinning process. The temperature dependence of magnetization follows the spin-wave theory at a low temperature. Abnormal behaviors are observed both in resistance and Hall effect below 8 K. The present Hall resistivity measurement shows that the anomalous Hall effects coexist with normal Hall effects. The negative value of normal Hall coefficient over the whole temperature range reveals that the major charge carriers are electrons. The anomalous Hall coefficient is proportional to the zero-field resistivity, suggesting that magnetic skew scattering is the dominant mechanism in the ferromagnetic regime. The reason for the abnormity below 8 K during transport is discussed.     

Influence of carbon concentration on structural, magnetic and electrical transport properties for antiperovskite compounds AlCxMn3

The structural, magnetic and transport properties of the antiperovskite AlC_xMn₃ (1.0≤x≤1.4) are investigated. It is found that the lattice parameter a increases monotonously with nominal carbon concentration x. The Curie temperature T_C increases with increasing x from 1.0 to 1.1 and then decreases with further increasing x. The highest T_C value is 364 K, about 70 K higher than that of stoichiometric AlCMn₃ reported previously. This may be attributed to a competition between the lattice expansion and the strong Mn 3d-C 2p hybridization. Below 100 K, the resistivity can be well described as ρ(T)=ρ₀+AT², corresponding to the electron-electron scattering. A increases with x, suggesting certain changes in the electronic structure, e.g. carrier density. Above 250 K, all ρ(T) curves depart from the linear dependence on temperature and seem to take on a tendency towards saturation.     

Crystal growth, structure and ferromagnetic properties of a Ce3Pt23Si11 single crystal

A high-quality single crystal of Ce₃Pt₂₃Si₁₁ has been grown using the Czochralski method. The crystal structure is presented and the chemical composition has been checked using an electron microprobe analyzer. Measurements of the electrical resistivity and magnetic susceptibility performed at low temperature show a ferromagnetic transition at T_c=0.44 K.     

Synthesis and physical properties of antiperovskite-type compound In0.95CNi3

The compound In_0.95CNi₃ has been synthesized and the basic properties have been investigated. It has the typical antiperovskite structure (space group Pm3m, lattice parameter 3.7836 Å). The electronic specific coefficient γ and Debye temperature Θ_D are found to be 14.1 mJ/mol K² and 372 K, respectively. It behaves as a ferromagnetic metal below the Curie temperature (577 K). The emergence of ferromagnetism is suggested to originate from the deviation of the Ni/In atomic ratio from the ideal stoichiometry. The possible mechanisms have been discussed in detail in terms of this deviation.     

Magnetic and electronic structures of zinc-blende FeX (X=P,As, Sb) by first principles calculations

Magnetic and electronic structure calculations are carried out for hypothetical zinc-blende (zb) phase of FeX (X=P, As, Sb) by using the full-potential linearized augmented plane wave (FLAPW) method. For zb FeSb, the total energy has been calculated as a function of lattice constant in ferromagnetic (FM) and antiferromagnetic (AFM) states. We found that the ground state of zb FeSb is very stable with respect to compression and expansion of the unit cell. The magnetic moment of zb FeSb in the AFM state is increasing with the lattice constant. The magnetic and electronic structures calculations of FeAs (FeP) are carried out for the lattice constants of GaAs (GaP), InAs (InP), and Si. Our finding shows that AFM is the ground state for all of our calculated zb FeX compounds and do not belong to the class of zb half metallic ferromagnets.     

Magnetic and optical properties of Fe2VAl and Fe3Al

Full-potential linearized augmented plane wave plus local orbital method (FPLAPW + lo) calculations were performed for Fe₂VAl and Fe₃Al in order to investigate magnetic and optical properties and to show the origin of various optical transitions. It was found that the lattice constant and spin magnetic moments with the GGA method differ more from the respective experimental values than those calculated with the LSDA method. Furthermore, our calculated lattice constant and spin magnetic moments with the LSDA method were in overall better agreement with experiment. Our predictions agreed well with recent experimental reflectivity spectra. Meanwhile, the spectral peaks at the transitions were analyzed from the imaginary part of the dielectric function.     

Magnetic properties of ternary gallides of type RNi4Ga (R=rare earths)

The magnetic properties of RNi₄Ga (R=La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm and Lu) compounds have been investigated. These compounds form in a hexagonal CaCu₅ type structure with a space group P6/mmm. Compounds with the magnetic rare earths, R= Nd, Sm, Gd, Tb, Dy, Ho, Er and Tm, undergo a ferromagnetic transition at 5, 17, 20, 19, 12, 3.5, 8 and 6.5 K, respectively. The transition temperatures are smaller compared to their respective parent compounds RNi₅. PrNi₄Ga is paramagnetic down to 2 K. LaNi₄Ga and LuNi₄Ga are Pauli paramagnets. All the compounds show thermomagnetic irreversibility in the magnetically ordered state except GdNi₄Ga.     

Oscillations in magnetocaloric effect and magnetic properties of RE2Al3Si2 (for RE=Dy,Ho and Er) and REAlSi (for RE=Ce and Pr)

We present the magnetic and thermal properties of a series of compounds RE₂Al₃Si₂ for RE=Dy, Ho, Er, and REAlSi for RE=Pr, Ce. The 2–3–2 family crystallizes with themonoclinic Y₂Al₃Si₂-type structure while the 1–1–1 family crystallizes in the body-centered tetragonal α-ThSi₂-type structure. The measurements were done on single crystals, grown using high-temperature flux technique and molten Al as a solvent . Susceptibility and heat capacity data were taken from 1.8 to 200 K, from the heat capacity data, the isothermal magnetic entropy change was obtained. Our results indicate signal oscillations in magnetocaloric properties for those compounds, in particular, Dy₂Al₃Si₂ that shows an anomaly that can be associated with a spin reorientation. Similar results are known for some Dy discilicides and dialluminades.     

Ferromagnetism and antiferromagnetism in Ni2+x+yMn1−xAl1−y alloys

The magnetic behavior of Ni_2+xMn_1−xAl alloys around the stoichiometric 2:1:1 composition was investigated with several experimental techniques. The results of low-temperature magnetization measurements indicate that a competition mechanism between ferromagnetism and antiferromagnetism is expected in off-stoichiometric alloys. Although the Curie temperature is strongly dependent on the composition, the saturation magnetization has an unsystematic variation for deviations from the stoichiometric Ni₂MnAl alloy. A reentrant-spin-glass behavior is observed below 50 K.     

Magnetic properties of half-metallic semi Heusler Co1−xCuxMnSb compounds

A study of the half-metallic character of the semi Heusler alloys Co_1−xCu_xMnSb (0?x?0.9) is presented. We investigated the saturation magnetization M_S at temperatures from 5 K to room temperature and the temperature dependence of the DC magnetic susceptibility χ above Curie temperature T_C. The magnetic moments at 5 K, for most compositions are very close to the quantized value of 4 μ_B for Mn³⁺ ion, the compound with 90% Co substituted by Cu is still ferromagnetic with M_S (5 K)=3.78 μ_B/f.u. These results emphasize the role of Co atoms in maintaining the ferromagnetic order in the material. The Curie temperature is decreased from 476 K to about 300 K as the Cu content increases from 0% to 90%. Above T_C, the χ⁻¹ vs T curves follow very well the Curie–Weiss law. The effective moment μ_eff and paramagnetic Curie temperature θ are derived. A comparison between the values of M_S at 5 K and μ_eff shows a transition from localized to itinerant spin system in these compounds.     

Structural,magnetic and transport properties of half-metallic ferrimagnet Mn2VGa

Polycrystalline Mn₂VGa samples were synthesized using an arc furnace. X-ray diffraction (XRD) pattern was analyzed using General Structural Analysis System (GSAS) package and the refined lattice parameter was found to be 5.905 Å. We found magnetic ordering in the system below 783 K and the spontaneous magnetization was observed to be following the Bloch T^3/2 law below 80 K. The magnetic moment per formula unit at 5 K was observed to be 1.88 μ_B. The temperature variation of the electrical resistance was found to follow the relation R_n=R_0n+aT^α (α=1.616) and (R_n—normalized electrical resistance) in the temperature range of 25–300 K and we observed almost a temperature independent variation of the electrical resistance below 25 K indicating the absence of spin-flip scattering.     

Magnetic properties of R2WO6 (where R=Nd, Sm, Eu, Gd, Dy and Ho)

The magnetic and electrical measurements carried out on the R₂WO₆ tungstates showed a paramagnetic behaviour for samples with R=Nd, Gd, Dy and Ho and more complex one for samples with R=Sm and Eu in the temperature range 4.2-280 K and an insulating state at room temperature. With increasing atomic number of the R element the Curie-Weiss temperature increases from −43.5 K for Nd₂WO₆ to −2.7 K for Ho₂WO₆, excluding Sm₂WO₆ and Eu₂WO₆ compounds for that the Curie-Weiss region is not observed and the imaginary part of susceptibility is close to zero. The effective magnetic moment is close to the theoretical one for the free R ion and the magnetic moment measured in magnetic field of 14 T and at temperature of 4.2 K, generally, does not reach the saturation state. The temperature independent residual susceptibility is negative for Nd₂WO₆ and positive for the remaining compounds suggesting different proportions of the Landau, Pauli and van Vleck contributions to the total susceptibility. An increase of the orbital magnetic contribution to the total magnetic moment is suggested from the fitting of the Landé factor in the compounds under study.     

Magnetic properties of R2Fe17−xGax (R=Y and Gd) compounds

The magnetic properties of Y₂Fe_17−xGa_x for 3≤x≤7 and Gd₂Fe_17−xGa_x for 5≤x≤7 have been investigated using ⁵⁷Fe Mössbauer spectroscopy. These compounds have the rhombohedral Th₂Zn₁₇ structure. X-ray diffraction analysis of aligned powders shows that the easy direction of magnetization is parallel to the c-axis in Y₂Fe₁₀Ga₇ and Gd₂Fe₁₀Ga₇ and is perpendicular to the c-axis in Y₂Fe₁₄Ga₃, Y₂Fe₁₂Ga₅, Gd₂Fe₁₂Ga₅ and Gd₂Fe₁₁Ga₆. Mössbauer studies indicate that those samples are ordered ferromagnetically. The ⁵⁷Fe hyperfine field decreases with increasing Ga content. This decrease results from the decreased magnetic exchange interactions resulting from Ga substitution. The average isomer shift, δ, for R₂Fe_17−xGa_x (R=Y and Gd) at room temperature is positive and the magnitude of δ increases with increasing Ga content.