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.     

First principle study of the magnetism of Sr2CoMoO6-δ (δ = 0, 1/2) double perovskites

We investigate the electronic structure of bulk Sr₂CoMoO_6-δ double perovskites using the ab initio Full Potential Linearized Augmented Plane Wave method in order to study their magnetic properties within the GGA and GGA+U methods. We discuss the relative stability of ferromagnetic (FM) and antiferromagnetic (AFM) orders (i) without and with taking into account the observed tilting of the oxygen octahedra and (ii) by introducing oxygen vacancies. We show that a very good agreement with experimental results — AFM order for δ= 0 and FM order for δ= 1/2 — is obtained only when the tilting of the oxygen tetrahedra is taking into account and when the GGA+U method is used.     

Dehydrogenation at the Fe/Lu2O3 interface upon rapid thermal annealing

The interfaces between ferromagnetic electrodes and tunnel oxides play a crucial role in determining the performances of spin-based electronic devices, such as magnetic tunnel junctions. Therefore, a deep knowledge of the structural, chemical, and magnetic properties of the buried interfaces is required. We study the influence of rapid thermal annealing treatments up to 500 °C on the interfacial properties of the Fe/Lu₂O₃ system. As-grown stacks reveal the presence of hydrogenated Fe-Lu-H intermetallic phases at the Fe/Lu₂O₃ interface most likely due to the H absorption on the Lu₂O₃ surface upon exposure to air and/or to the oxide growth. The annealing treatments induce remarkable changes of the structural, chemical, and magnetic properties at the interface, as evidenced at the atomic scale by the sub-monolayer sensitivity of conversion electron Mössbauer spectroscopy. The use of complementary techniques such as X-ray diffraction, time-of-flight secondary ion mass spectrometry, and in situ X-ray photoelectron spectroscopy, confirms that the main effect of the annealing is to gradually promote the dehydrogenation at the Fe/Lu₂O₃ interface.     

Structural and magnetic properties of La/Fe multilayers

The structural and magnetic properties of La/Fe multilayers were investigated by X-ray diffraction, RHEED, magnetometry and⁵⁷Fe Mössbauer spectroscopy. Comparison is made with previous results obtained for Ce/Fe multilayers. Remarkably sharp interfaces are found, with roughness between 2 and 2.5 Å. The magnetic interface in the Fe sublayers resulting from the distribution of magnetic hyperfine fields distinctly exceeds the extension of the structural interface and points to a magnetic proximity effect. This is discussed in relation to a strong 3d-5d hybridization recently found in measurements of magnetic circular X-ray dichroism. Both the structural and magnetic La/Fe interface is less extended than the interface in Ce/Fe multilayers. Below a thickness of about 25 Å, the individual Fe layers grow in an amorphous structure on the La layers. In this case, Curie temperatures are below 200 K and the Fe-layer saturation magnetization is reduced up to 50%, and there is evidence of a non-collinear spin structure. It is argued that this mainly reflects the properties of pure amorphous Fe.     

The effect of 4d states based full Heusler alloy on the electronic and magnetic properties of new half metallic ferromagnetism: DFT+U study

A full heusler alloy Ru₂MoSb is studied using the full potential linearized augmented plane wave (FP-LAPW) method based on density functional theory (DFT). Structural, electronic and magnetic properties are investigated for this material. We have used the generalized gradient approximation of Perdew, Burke, and Ernzerh (GGA-PBE), the GGA+U and the modified Becke–Johnson potential of GGA+U (mBJ-GGA+U) to model exchange correlation potential. The Hubbard on-site Coulomb interaction correction U is calculated by constraint local density approximation for both 4d elements Ru and Mo.For both approximations GGA+U and mBJ- GGA+U, density of states and band structure reveal that our compound has a half-metallic character. Magnetic properties show that Ru₂MoSb is ferromagnetic with an integer magnetic moment of 3 µB which is in good agreement with the Slater–Pauling rule. The half-metallicity of Ru₂MoSb is stable under lattice constant changes which makes it a potential contender for spintronic applications.The negative values of the cohesion energy and the formation energy indicate that our Heusler alloy can be synthesized and stabilized experimentally.     

Theoretical calculations on the adhesion, stability, electronic structure, and bonding of Fe/WC interface

The adhesion, stability, electronic structure, and bonding of Fe/WC interfaces were studied using first-principles calculations. The preferred stacking sequence is HCP structure that Fe atoms continue the natural stacking sequence of the bulk WC. For two different interfaces with HCP stacking geometry (C-HCP and W-HCP), the work of adhesion of the optimized Fe/WC interfaces are 9.7 J m⁻² for C-HCP and 5.1 J m⁻² for W-HCP, respectively. The effects of the interface on the electronic structures of both the metal Fe and ceramic WC are mainly localized within the first and second layers of the interface. C-HCP interface has strong covalency and W-HCP interface is dominated by metallic bonds. The magnetic moments of Fe atoms at interface are decreased in both interfaces. Calculations of the interfacial energies provide theoretical evidence for the excellent wear behaviors of Fe/WC composites. Besides, the chemical bonding properties for the interfacial atoms are also discussed in this paper based on Milliken population method.     

First-principles calculation of the structural,electronic,and magnetic properties of cubic perovskite RbXF_3(X = Mn,V,Co,Fe)

First-principles calculations by means of the full-potential linearized augmented plane wave method using the generalized gradient approximation with correlation effect correction(GGA+U) within the framework of spin polarized density functional theory(DFT+U) are used to study the structural,electronic,and magnetic properties of cubic perovskite compounds RbXF_3(X = Mn,V,Co,and Fe).It is found that the calculated structural parameters,i.e.,lattice constant,bulk modulus,and its pressure derivative are in good agreement with the previous results.Our results reveal that the strong spin polarization of the 3d states of the X atoms is the origin of ferromagnetism in RbXF_3.Cohesive energies and the magnetic moments of RbXF_3 have also been calculated.The calculated electronic properties show the half-metallic nature of RbCoF_3 and RbFeF_3,making these materials suitable for spintronic applications.     

Effect of Si substitution on electronic structure and magnetic properties of Heusler compounds Co2TiAl1−xSix

The electronic structures of Co-based Heusler compounds CoTiAl_1−xSi_x (x=0, 0.25, 0.5, 0.75 and 1) are calculated by first-principles using the full potential linearized augmented plane wave (FP-LAPW) method within GGA and LSDA+U scheme. Particular emphasis was put on the role of the main group elements. In recent years, the GGA calculations of Co₂TiAl (x=0) and Co₂TiSi (x=1) indicated that they are half-metallic, but the electronic structure of this compound with x=0.25, 0.5 and 0.75 has not been reported yet, neither theoretically nor experimentally. The calculated results reveal that these are half-metallic and exhibit an energy gap in the minority spin state and also show 100% spin polarization. The substitution of Al by Si leads to an increase in the number of valence electrons, with increasing x. Our calculated results clearly show that with the Si doping, the lattice parameter linearly decreases; bulk modulus increases, and the total magnetic moment increases. The calculated energy gap in the minority spin state, using GGA scheme, was smaller than that obtained by using LSDA+U scheme. The outcomes of this research also show that the Co-3d DOS and therefore, the magnetic properties of compounds are dependent on electron concentration of the main group elements and it will affect the degree of p-d orbital occupation.     

First-principles calculations and Bader analysis of oxygen-deficient induced magnetism in cubic BaTiO3−x and SrTiO3−x

The possibility of oxygen deficient induced magnetism in cubic BaTiO₃ (BTO) and SrTiO₃ (STO) perovskites is investigated by first-principles calculations, using the projector-augmented-wave method, within the generalised gradient approximation (GGA) and generalised gradient approximation with on-site effect (GGA + U), for the exchange correlation potential. For non-stoichiometric BaTiO_3?x and SrTiO_3?x, supercells were created in order to have two vacancy concentrations, i.e. x?=?0.125, 0.083. Spin charge distributions and magnetic moments associated with each ion, including local density of states projected in Bader atoms, were analysed by performing a full Bader charge analysis. Results show that oxygen vacancies could induce magnetism in BaTiO_3?x with x?=?0.125 and x?=?0.083 under GGA and GGA + U approximations. For SrTiO_3?x with x?=?0.125, ferromagnetism is induced with GGA, whereas with GGA + U the non-magnetic state is retained. On the other hand, with x?=?0.083, ferromagnetism is induced under GGA and GGA + U.     

Electronic and magnetic properties of Mn-doped ZnO: Total-energy calculations

Based on the spin generalized gradient approximation (σGGA) of the density functional theory (DFT), the structural, magnetic, and electronic properties of Mn-doped ZnO structure have thoroughly been investigated. It is found that the Mn atom prefers to substitute one of the Zn atoms, producing the energetically most stable configuration for the Mn-doped ZnO structure. Employing the Hubbard potential within the calculations suggests various changes and modifications to the structural, magnetic and electronic properties of the Mn-doped ZnO. Our calculations reveal that the local magnetic moment at the Mn site using the ordinary σGGA functional is 4.84 μ_B/Mn, which is smaller than that evaluated by including the Hubbard potential of 5.04 μ_B/Mn. Overall, the electronic band structure of the system, within the σGGA+U, is half-metallic, with metallic nature for the majority state and semiconducting nature for the minority state. Simulated scanning tunneling microscopy (STM) images for both unoccupied and occupied states indicate siginficant brightness on both Zn and Mn atoms and much brighter protrusions around the O atoms, respectively.     

Effect on alloying at the Fe/Ni(001) interfaces on the interlayer exchange coupling

We investigate the stability of various ordered FeNi alloys at the interfaces of Fe/Ni superlattices by using ab initio density functional calculation. We consider an Fe_0.5Ni_0.5 ordered alloy of one or two monolayers thick at different positions beyond the interface and the possibility of an interdiffusion of a complete monolayer of Ni(Fe) in Fe(Ni) slab. An interfacial atomic layer of Fe_0.5Ni_0.5 exchanged with its adjacent Ni monolayers, leading to a buffer zone of Ni₃Fe composition is found to be the most stable structural configuration. For this atomic arrangement we investigate the magnetic profile and the resulting interlayer exchange coupling between the Ni slabs for Fe spacer thickness of 0 to 4 monolayers.     

Phase stability,electronic, magnetic and elastic properties of Ni2CoZ(Z = Ga,Sn): A first principles study with GGA method and GGA+U approach

First principles calculations based on density functional theory are used to investigate the phase stability, electronic, magnetic and elastic properties of ferromagnetic metallic full-Heusler Ni₂CoZ(Z = Ga, Sn) alloys via the FP-LAPW method by the generalized gradient GGA and GGA+U approximations for the exchange and correlation energy, within the Perdew–Burke–Ernzerhof (PBE 96) parameterization. The results of calculating electronic structures and magnetic properties reveal that the both Ni₂CoGa and Ni₂CoSn crystallize in L2₁ phase with regular cubic structure. The two investigated compounds exhibit metallic ferromagnetic behaviors for the GGA+U calculation. The computation of elastic constants with GGA+U approach shows that our compounds are mechanically stable.     

A better ferrimagnetic half-metal LuCu3Mn4O12: Predicted from first-principles investigation

Electronic structure calculations based on density functional theory (DFT) within the generalized gradient approximation (GGA) and GGA+U for manganite cuprate compound LuCu₃Mn₄O₁₂ have been performed, using the full-potential linearized augmented plane wave method. The calculated results indicate that LuCu₃Mn₄O₁₂ is ferrimagnetic and half-metallic in both GGA and GGA+U calculations. The minority-spin band gap is 0.7 eV within GGA, which is larger than that of LaCu₃Mn₄O₁₂ (0.3 eV), indicating its better half-metallicity. Further, the minority-spin gap enlarges from 0.7 to 2.8 eV with U taken into account, and simultaneously the Fermi level being shifted to the middle of the gap, making the half-metallic energy gap to be 1.21 eV. These results demonstrate that electronic correlation effect enhances the stability of half-metallic property. These facts make this system interesting candidates for applications in spintronic devices.     

Electronic structure and magnetic coupling properties of EuFe2P2: First-principles calculations

We have investigated the electronic structure and magnetic properties of EuFe₂P₂ using first-principles density functional theory within the generalized gradient approximation (GGA)+U schemes. Our calculated ground state magnetic configurations of EuFe₂P₂ is ferromagnetic which Eu²⁺ spins order along c axis. We argue that this kind of magnetic structure of Eu is determined by the indirect RKKY interactions between Eu and direct coupling interaction between Eu 4f with Fe 3d state by our spin-polarized density of states calculations. From the charge density and the Laplace charge density of EuFe₂P₂, we believe that the magnetic moment of Fe is determined by not only Fe-P coupling interactions but also Fe-Fe directly exchange interactions.     

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.     

Polarized neutron reflectometry of the influence of Fe/Si,Fe/FeSi2 and Au/Fe interfaces on the magnetic properties of epitaxial Fe films

The magnetic and structural properties of epitaxial Fe films grown on Si(1 1 1) are investigated by polarized neutron reflectometry (PNR) at room temperature. The influence of different types of interfaces, Fe/Si, Fe/FeSi₂ and Au/Fe on the magnetic properties of Fe films deposited by molecular beam epitaxy onto Si(1 1 1) are characterized. We observe a drastic reduction of the magnetic moment in the entire Fe film deposited directly on the silicon substrate essentially due to strong Si interdiffusion throughout the whole Fe layer thickness. The use of a silicide FeSi₂ template layer stops the interdiffusion and the value of the magnetic moment of the deposited Fe layer is close to its bulk value. We also evidence the asymmetric nature of the interfaces, Si/Fe and Fe/Si interfaces are magnetically very different. Finally, we show that the use of Au leads to an enhancement of the magnetization at the interface.     

后钙钛矿CaRhO3的电子结构和磁学性质的第一性原理研究

采用基于密度泛函理论的投影平面波方法,对后钙钛矿结构(Ppv)的CaRhO₃的电子结构和磁学性质进行了研究.广义梯度(GGA)近似下的计算表明,Ppv-CaRhO₃的基态为铁磁性半金属,Rh⁴⁺离子的磁矩大小为0.57μ_B,具有低自旋态构型;而考虑在位库仑作用修正的GGA+U计算,得到了与实验结果相符的反铁磁绝缘体基态,表明后钙钛矿结构中4d电子之间的关联效应对体 关键词： 电子结构 磁学性质 金属绝缘体转变     

Interface characteristics of Mo/Si and B4C/Mo/Si multilayers using non-destructive X-ray techniques

A methodology combining non-destructive X-ray techniques is proposed to study the interfacial zones of periodic multilayers. The used X-ray techniques are X-ray emission spectroscopy induced by electrons and X-ray reflectivity in the hard and soft X-ray ranges. The first technique evidences the presence of compounds at the interfaces and gives an estimation of the thickness of the interfacial zone. These informations are used to constrain the fit of the X-ray reflectivity curves that enables to determine the thickness and roughness of the various layers of the stacks. The results are validated in the soft X-ray range where the reflectivity curves are very sensitive to the chemical state of the elements present in the stack. The methodology is applied to characterize Mo/Si (1-4 nm/2 nm) and B₄C/Mo/Si (1 nm/2 nm/2 nm) multilayers. It is shown that the two interfacial zones of the Mo/Si multilayers are composed of the silicides MoSi₂ and Mo₅Si₃. It is found that the interface thickness is about to be 0.4-0.8 nm depending on the samples. The molybdenum silicides are also evidenced at the interfaces of the B₄C/Mo/Si multilayers. However, their interface thickness is 0.2 nm thinner than that of the same stack without the B₄C layers, these layers being at the Mo-on-Si side or at the Si-on-Mo side. Thus, the B₄C layers do not stop but only reduce the interdiffusion between the Mo and Si layers.     

Ab-initio investigation of Ni(Fe)/ZrO2(0 0 1) and Ni-Fe/ZrO2(0 0 1) interfaces

The atomic and electronic structures of Me/ZrO₂(0 0 1) interfaces, where Me is Ni, Fe or a Ni-Fe alloy, are investigated by the plane wave pseudopotential method within density-functional theory. The work of separation of metal films from oxide substrate for the O- and Zr-terminated Me/ZrO₂(0 0 1) interfaces is calculated. High adhesion at both Me/(ZrO₂)_O and Me/(ZrO₂)_Zr interfaces is found. The effect of oxygen vacancies on the adhesion at the metal-ceramic interfaces is also investigated. It is shown that Ni(Fe)-O interaction at the O-terminated interface weakens in the presence of interfacial oxygen vacancies. At interfaces with Ni-Fe alloys the adhesion depends strongly on the composition of the interfacial layers and their magnetic properties.     

A comparative study of a Heusler alloy Co2FeGe using LSDA and LSDA+U

We have calculated the on-site Coulomb repulsion (U) for the transition elements Co and Fe. To study the impact of Hubbard potential or on-site Coulomb repulsion (U) on structural and electronic properties the calculated values of U were added on GGA and LSDA. We performed the structure optimization of Co₂FeGe based on the generalized gradient approximation (GGA and GGA+U). The calculation of electronic structure was based on the full potential linear augmented plane wave (FP-LAPW) method and local spin density approximation (LSDA) as well as exchange correlation LSDA+U. The Heusler alloy Co₂FeGe fails to give the half-metallic ferromagnetism (HMF) when treated with LSDA. The LSDA+U gives a good result to prove that Co₂FeGe is a HMF with a large gap of 1.10 eV and the Fermi energy (E_F) lies at the middle of the gap of minority spin. The calculated density of states (DOS) and band structure show that Co₂FeGe is a HMF when treated with LSDA+U.