Defects-driven appearance of half-metallic ferrimagnetism in Co-Mn-based Heusler alloys

Half-metallic ferromagnetic full-Heusler alloys containing Co and Mn, having the formula Co₂MnZ where Z is a sp element, are among the most studied Heusler alloys due to their stable ferromagnetism and the high Curie temperatures which they present. Using state-of-the-art electronic structure calculations we show that when Mn atoms migrate to sites occupied in the perfect alloys by Co, these Mn atoms have spin moments antiparallel to the other transition metal atoms. The ferrimagnetic compounds, which result from this procedure, keep the half-metallic character of the parent compounds and the large exchange-splitting of the Mn impurities atoms only marginally affects the width of the gap in the minority-spin band. The case of [Co_1−xMn_x]₂MnSi is of particular interest since Mn₃Si is known to crystallize in the Heusler L2₁ lattice structure of Co₂MnZ compounds. Robust half-metallic ferrimagnets are highly desirable for realistic applications since they lead to smaller energy losses due to the lower external magnetic fields created with respect to their ferromagnetic counterparts.     

The radial dependence of the magnetic hyperfine field for Sn atoms in the ferromagnetic Heusler alloys

The magnetic hyperfine fields,B _hf, for impurity¹¹⁹Sn atoms in Z sites of ferromagnetic Heusler alloys Co₂MnZ (Z=Si, Ge) are measured by the Mössbauer effect. At 77 KB _hf=–1.43±0.04 T in Co₂MnSi andB _hf=+1.05±0.05 T in Co₂MnGe. From the comparison between the values ofB _hf for Sn atoms in Co₂MnZ (Z=Si, Ge, Sn), it follows that the negative contribution toB _hf drops as the interatomic distance begins to increase. This radial dependence also manifests itself in the anomalies of the temperature dependences of the hyperfine fields. The temperature anomaly is positive for Sn in Co₂MnGe and negative for Sn in Co₂MnSi.     

Magnetic properties of Co–Si alloy clusters

We have investigated the electronic structure and the magnetic properties of Co–Si alloy clusters using ab initio spin-polarized density functional calculations. The possible CoSi₂, CoSi, and Co₂Si phase clusters with oblique hexagon prism, icosahedron, and cuboctahedron structures are introduced. The CoSi phase cluster with icosahedron structure has the largest binding energy and amount of charge transfer. We found that HOMO-LUMO gap, magnetic moment, and spin polarization for the Co–Si alloy clusters with icosahedron structure increase with Co concentration. The Si atoms in the CoSi phase with icosahedron structure have negative magnetic moment.     

Structural stability,mechanical, electronic and thermal behaviour of Ru2CrZ (Z=Sb,Si, Pb,Ge) Heusler alloys

In this paper, the structural, elastic, electronic properties of Ru2CrZ (Z=Si, Ge, Pb, Sn) are explored using the generalized gradient approximation based on ab initio plane-wave pseudopotential density functional theory. With the help of the quasi-harmonic Debye model, we also investigate the variation of normalized volume V/V0, the heat capacities CV and CP, thermal expansivity, and Debye temperature of Ru₂CrZ (Z=Si, Ge, Pb, Sn). Results show that the Cu₂MnAl type structure is more stable then Hg₂CuTi type structure. The four compounds in the ground state are predicted to be nearly half-metal behavior with total magnetic moment near to the integer value. To provide a comparative and complementary study to future researches, we investigated the elastic and thermodynamic properties.     

Co doping effects on structural, electronic and magnetic properties in Mn2VGa

The electronic structure and magnetic properties of Co-doped Heusler alloys (Mn_1−xCo_x)₂ VGa (x=0.0, 0.25, 0.5, 0.75, 1.0) have been studied by first-principles calculations. The results show that the lattice constants decrease with increasing Co content except x=1.0. The spin polarization for x=0.5 is only 34%, much lower than the other concentrations. The compounds of x=0.0, 0.25 show nearly half-metallicity because the Fermi level slightly touches the valence bands. And the compounds of x=0.75, 1.0 exhibit the half-metallic character with 100% spin polarization. It is found the local moments of Mn(Co) basically show a linear increasing trend while the moments of V show a linear decreasing trend with increasing doping concentration. However, the local moments for x=0.5 quite depart from the linear trend. The majority-spin component at the Fermi level increases while the minority-spin component at the Fermi level decreases with the substitution of Co atoms for Mn atoms when x≤0.75. For x≥0.75, the majority-spin component remains more or less the same and the gap in the minority DOS increases with Co doping. The majority spin states are shifted to valence bands and the majority spin states around E_F increase due to a leakage of charge from the unoccupied spin-up states to the occupied majority states with increasing Co content.     

Theoretical investigations of the (110) interface between the full Heusler alloys and GaAs

The ab initio calculations of the electronic structure and magnetic properties of the (110) interface between Co₂YZ (Y = Cr or Mn and Z = Al, Si, or Ge) and GaAs are carried out by means of the density functional theory depending on the contact configuration. It is revealed that two of four possible atomic interface configurations have high spin polarization. For Co₂MnSi/GaAs(110), one of the contacts has almost 100% spin polarization. Calculations of the adhesion energy on the interfaces allow the most stable contacts to be established.     

Hyperfine magnetic field on Cd-111 in heusler alloys Co2MnZ (Z=Si,Ga, Ge,Sn)

The time differential perturbed angular correlation method has been used to measure, as a function of temperature, the hyperfine magnetic field at Cd sites in the Heusler alloys Co₂MnZ (Z=Si, Ga, Ge, Sn). The hyperfine fields, normalized to the total magnetic moment per formula unit, show an approximately linear trend toward more positive values with increasing lattice parameter.     

Effect of the main-group elements on the electronic structures and magnetic properties of Heusler alloys Mn2NiZ (Z=In, Sn, Sb)

The magnetic properties and electronic structure of Mn₂NiZ (Z=In, Sn, Sb) have been studied. The magnetic structure of these alloys is mainly determined by the main-group element Z instead of the distance between the Mn atoms. Electronic structure calculations suggest that Mn₂NiIn and Mn₂NiSn are both ferrimagnets with antiparallel alignment between the Mn moments. But this antiferromagnetic coupling is weakened by the increasing number of valence electrons of the Z atoms. When it comes to Mn₂NiSb, a ferromagnetic coupling between the Mn atoms is observed. Mn₂NiSn and Mn₂NiSb have been synthesized successfully. Their M_s at 5 K agree well with the theoretical value.     

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.     

Pressure and disorder effects on the half-metallic character and magnetic properties of the full-Heusler alloy Co2FeSi

We investigate the pressure and site disorder effects on the half-metallicity and magnetic properties of the full-Heusler alloy Co₂FeSi using first-principles density functional theory within the GGA and GGA+U schemes. The calculated lattice constant, bulk modulus and total magnetic moments are in excellent agreement with recent experiments. The volume compression leads to a slight increase of the minority band gap, i.e., the half-metallic properties of Co₂FeSi can maintain under pressure. The disorder calculations reveal that Fe–Co type disorder significantly destroys the half-metallic character and reduces the spin polarization of Co₂FeSi while disorder between Fe and Si can maintain half-metallic properties. Our results also show that the Fe–Co type disorder leads to degradation of the magnetism while the Fe–Si type disorder affects hardly the magnetism as observed in Co₂FeSi.     

Magnetic and chemical order in Heusler alloys containing cobalt and titanium

Saturation magnetization, X-ray and neutron diffraction measurements have been made on alloys at the compositions Co₂TiAl, Co₂TiSi, Co₂TiGa, Co₂TiGe, Co₂TiSn, Co₂TiSb and CoTiSb. The alloys containing Ga and Sn have fully ordered Heusler, L2₁, chemical structures. Co₂TiAl is similarly ordered but with some partial Ti-Al disorder. The alloys Co₂TiSi and Co₂TiGe each contain a secondary phase in addition to the primary Heusler phase. Co₂TiSb contains the two phases Co₁₂₂TiSb and Co and CoTiSb is ordered in the Cl_b structure.The alloys containing the group IIIB or IVB elements Al, Si, Ga, Ge or Sn are ferromagnetically ordered, with the magnetic moments associated with the ordered Co sites. The two alloys containing the group VB element Sb have vacant chemically ordered ‘Co’ sites but are paramagnetic.     

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.     

ConAl (n= 1–8)合金团簇结构和磁性质研究

采用密度泛函理论中的广义梯度近似(DFT-GGA)对Co_nAl (n= 1–8)合金团簇进行了系统的几何、 电子结构和磁性质研究. 研究结果表明Al原子倾向于与Co原子形成最大的成键数, 即Al原子均处在团簇原子拥有最大配位数的位置上. Al掺杂后Co_nAl团簇的稳定性减弱, 磁性降低. 磁性降低的幅度与实验上对较大Co_nAl_M团簇的磁性检测结果获得了很好地符合. 在所有Co_nAl团簇的最稳定结构中, 除Co₄Al外, Al与近邻Co原子均呈现反铁磁性耦合. 相对于纯Co团簇,非磁性Al元素的掺入以及Al掺杂后Co原子整体自旋极化的减弱 是导致Co_nAl团簇磁性的降低主要原因. 关键词： nAl合金团簇')" href="#">Co_nAl合金团簇 几何结构 磁性 自旋极化     

The role of Co impurities and oxygen vacancies in the ferromagnetism of Co-doped SnO2: GGA and GGA+U studies

The electronic structure and ferromagnetic stability of Co-doped SnO₂ are studied using the first-principle density functional method within the generalized gradient approximation (GGA) and GGA+U schemes. The addition of effective U_Co transforms the ground state of Co-doped SnO₂ to insulating from half-metallic and the coupling between the nearest neighbor Co spins to weak antimagnetic from strong ferromagnetic. GGA+U_Co calculations show that the pure substitutional Co defects in SnO₂ cannot induce the ferromagnetism. Oxygen vacancies tend to locate near Co atoms. Their presence increases the magnetic moment of Co and induces the ferromagnetic coupling between two Co spins with large Co-Co distance. The calculated density of state and spin density distribution calculated by GGA+U_Co show that the long-range ferromagnetic coupling between two Co spins is mediated by spin-split impurity band induced by oxygen vacancies. More charge transfer from impurity to Co-3d states and larger spin split of Co-3d and impurity states induced by the addition of U_Co enhance the ferromagnetic stability of the system with oxygen vacancies. By applying a Coulomb U_O on O 2 s orbital, the band gap is corrected for all calculations and the conclusions derived from GGA+U_Co calculations are not changed by the correction of band gap.     

Ab initio calculation and analysis of the properties of digital magnetic heterostructures and diluted magnetic semiconductors of IV and III-V groups

We present the first-principles calculations of digital magnetic heterostructures Si/M, Ge/M. GaAs/M, GaSb/M, GaN/M and GaN/M (50%) with M=Cr, Mn, Fe, and Co. The interaction between magnetic dopants results in a wide spin-polarized two-dimensional band inside the gap. It is found that beginning occupation of the minority-spin band greatly increases the energy of the ferromagnetic (FM) state and leads, as a rule, to the antiferromagnetic (AFM) spin ordering. This mechanism causes transition to the AFM state, when interaction between magnetic atoms is too strong, and defines the optimum of Curie temperature as a function of transition element concentration in magnetic layers.     

A first principles study on the full-Heusler compound Mn2CuSi

Using a state-of-the-art full-potential electronic structure method within the generalized gradient approximation (GGA), we study the electronic structure and magnetic properties of the Mn₂CuSi full-Heusler alloy. Calculations show that CuHg₂Ti-type structure alloy is a half-metallic ferrimagnet with the Fermi level (ε_F) being located within a tiny gap of the minority-spin density of states. The conduction electron at ε_F keeps a 100% spin polarization. A total spin moment, which is mainly due to the antiparallel configurations of the Mn partial moments, is −1.00μ_B for a wide range of equilibrium lattice parameters. Simultaneously, the small spin magnetic moments of Cu and Si atoms are antiparallel. The gap mainly originates from the hybridization of the d states of the two Mn atoms. Thus, Mn₂CuSi may be the compound of choice for further experimental investigations.     

Electronic structure and magnetic properties of RCo5−xMx (R=Y,Pr and M=Al,Si) system

Detailed theoretical and experimental investigations of the electronic and magnetic properties of the RCo_5?xM_x compounds (R=Y, Pr and M=Si, Al) have been performed. All theoretical investigations of the electronic and magnetic properties of the system have been done using the Korringa–Kohn–Rostoker (KKR) band structure method. The Si for Co substitution in RCo₅ does not change the magnetic ordering: the RCo_5?xSi_x with R=Y, Nd and Pr is ferromagnetic, whilst the heavy rare-earth containing compounds are ferrimagnetic. The important modifications induced by this substitution concerns the magnetic properties of the system: the Curie temperature and the magnetic moments of Co decrease with Si content, indicating the weakening of the Co–Co exchange interaction. The band structure calculations evidence the hybridization between the 3d electronic states of Co and the 3p states of Si as possible reason for the diminishing of Co–Co exchange interaction. Also, the volume effect on the magnetic properties of the YCo₄Si was investigated using theoretical methods. The results are compared with the experimental measurements in order to distinguish the origin of magnetization reduction in YCo₄Si compared with YCo₄Al.     

Structural, electronic and magnetic properties of Mn, Co, Ni in Gen for (n=1-13)

The structural, electronic and magnetic properties of TMGe_n (TM=Mn, Co, Ni; n=1-13) have been investigated using spin polarized density functional theory. The transition metal (TM) atom prefers to occupy surface positions for n<9 and endohedral positions for n≥9. The critical size of the cluster to form endohedral complexes is at n=9, 10 and 11 for Mn, Co and Ni respectively. The binding energy of TMGe_n clusters increases with increase in cluster size. The Ni doped Ge_n clusters have shown higher stability as compared to Mn and Co doped Ge_n clusters. The HOMO-LUMO gap for spin up and down electronic states of Ge_n clusters is found to change significantly on TM doping. The magnetic moment in TMGe_n is introduced due to the presence of TM. The magnetic moment is mainly localized at the TM site and neighbouring Ge atoms. The magnetic moment is quenched in NiGe_n clusters for all n except for n=2, 4 and 8.     

Electronic structure and thermoelectric properties of (Mg<Subscript>2</Subscript>X)<Subscript>2</Subscript> / (Mg<Subscript>2</Subscript>Y)<Subscript>2</Subscript> (X,Y = Si,Ge, Sn) superlattices from first-principle calculations

To identify thermoelectric materials containing abundant, low-cost and non-toxicelements, we have studied the electronic structures and thermoelectric properties of(Mg₂X)₂/(Mg₂Y)₂ (X, Y = Si, Ge, Sn) superlattices withstate-of-the-art first-principles calculations using a modified Becke and Johnson (mBJ)exchange potential. Our results show that (Mg₂Ge)₂/ (Mg₂Sn)₂ and(Mg₂Si)₂/(Mg₂Sn)₂ are semi-metals using mBJ plusspin-orbit coupling (mBJ +SOC), while (Mg₂Si)₂/ (Mg₂Ge)₂ ispredicted to be a direct-gap semiconductor with a mBJ gap value of 0.46 eV andmBJ + SOC gap valueof 0.44 eV. Thermoelectric properties are predicted by through solving the Boltzmanntransport equations within the constant scattering time approximation. It is found that(Mg₂Si)₂/(Mg₂Ge)₂ has a larger Seebeck coefficient andpower factor than (Mg₂Ge)₂/ (Mg₂Sn)₂ and(Mg₂Si)₂/(Mg₂Sn)₂ for both p-type and n-type doping. Thedetrimental influence of SOC on the power factor of p-type (Mg₂X)₂/(Mg₂Y)₂ (X, Y = Si, Ge, Sn) is analyzed as afunction of the carrier concentration, but there is a negligible SOC effect for n-type.These results can be explained by the influence of SOC on their valence and conductionbands near the Fermi level.     

掺杂半导体β-FeSi2电子结构及几何结构第一性原理研究

采用基于第一性原理的密度泛函理论全势线性缀加平面波法，使用广义梯度近似处理交换相关势能，首先计算了β-FeSi₂的电子结构及其各元素各亚层电子的能态密度，β-FeSi₂的电子能态密度主要由Fe的d层电子和Si的p层电子的能态密度确定；其次通过计算不同掺杂系统的总能量确定了掺杂原子在β-FeSi₂中的置换位置，在β-FeSi₂中，Co置换Fe_Ⅱ位置的Fe原子，Al置换Si_Ⅰ位置的Si原子，这种择位置换与现有的计算结果完全一致；最后计算了Fe_1-xCo_xSi₂和Fe(Si_1-xAl_x)₂的电子结构，对它们的电子结构特征进行了分析，并探讨了电子结构对其热电性能(塞贝克系数、电传输及热传输性能)的影响. 关键词： 第一性原理 电子结构 热电性能 2')" href="#">FeSi₂