Xiaotian Wang

Recent advances in the Heusler based spin-gapless semiconductors

In recent years, spin-gapless semiconductors (SGSs) have received considerable interest in the fields of condensed matter physics and materials sciences due to their potential applications in novel spintronic devices. SGSs, with a zero gap at the Fermi level in one of the spin channels, can make electrons easy to excite from the valence band to the conduction band with a small input of energy and simultaneously produce electron and hole carriers with 100% spin polarization. Very recently, as a new member of SGSs family, fully-compensated ferrimagnetic spin-gapless semiconductors (FCF-SGSs) have been predicted. In addition to the properties of SGSs, FCF-SGSs possess zero magnetization, which is an added advantage for practical application. In this review article, we firstly review the progress on the Heusler-based materials with spin-gapless semiconducting behaviour, including half-Heusler compounds, full-Heusler compounds, DO3-type compounds, and LiMgPdSn-type quaternary Heusler compounds. Among these potential SGSs, some have been synthesized experimentally, while the others are just predicted by extensive first-principles calculations. Then, we explain the origin of the SGS characteristics in Heusler compounds based on the common Slater–Pauling curve and give a possible rule for making some on-demand designs of SGSs. Finally, we present a new spin injection scheme based on SGSs for practical applications and give a brief summary and outlook.

Origin of the half-metallic band-gap in newly designed quaternary Heusler compounds ZrVTiZ (Z = Al, Ga)

In this work, first-principles calculations have been used to investigate the electronic structures, magnetic properties, and half-metallic nature of the newly designed quaternary Heusler compounds ZrVTiAl and ZrVTiGa. The calculated results reveal that these two compounds are half-metallic ferrimagnets with a total magnetic moment (Mt) of 2 μB, and the Mt is in line with the Slater–Pauling curve of Mt = 18 − Zt, where Zt is the total number of valence electrons. Furthermore, via a schematic diagram of the possible d–d hybridization between the transition-metal elements Zr, V, and Ti, we discuss the origin of the half-metallic band gap in the majority spin channel. Also, we have investigated the half-metallic states versus the lattice parameter and the structural stability, i.e., the cohesion energy and formation energy of ZrVTiAl and ZrVTiGa compounds. We hope that our work may trigger Zr-based Heusler compounds for application in future spintronics devices.

A full spectrum of spintronic properties demonstrated by a C1b-type Heusler compound Mn2Sn subjected to strain engineering

Zero-gap half-metallic fully-compensated ferrimagnets (ZG-HM-FCFs) and fully-compensated ferrimagnetic spin-gapless semiconductors (FCF-SGSs) are promising candidates for spintronic applications due to the complete (100%) spin polarization of electrons around the Fermi level. Motivated by recent experimental and theoretical findings on binary Mn2-based C1b-type Heusler compounds, by means of first-principles calculations, we found that Mn2Sn exhibits metallic ferrimagnetism properties. Most interestingly, at a uniform strain, there is a novel transition in the physics from a metallic ferrimagnet (MFi) to true ZG-HM-FCF, HM-FCF, and FCF-SGS, and then to a fully-compensated ferrimagnetic semiconductor (FCF-S). The binary Mn2Sn compound remains as a MFi under tetragonal distortion, however. We also reveal that the structure of Mn2Sn is stable, according to its mechanical properties, calculated cohesion energy, and formation energy. Our work demonstrates that Mn2Sn is potentially an all-round candidate for spintronic applications because it shows a full spectrum of spintronic properties at a uniform strain.

Tuning the morphology of Co3O4 on Ni foam for supercapacitor application

In this work, NH4F was used as a vital additive to control the morphology of Co3O4 precursors on Ni foam in a conventional hydrothermal reaction, and then, via thermal decomposition, to obtain Co3O4 material. The amount of NH4F plays a pivotal role in the formed morphology of the Co3O4 precursors, and four morphologies of Co3O4 were obtained through close control of the amount of additive: nanowires, thin nanowire-clusters, thick nanowire-clusters, and fan-like bulks. The morphological evolution process of the Co3O4 precursors has been investigated according to their intermediates at different reaction stages, and some novel growth mechanisms are proposed: (1) the amount of NH4F in the solution system affects the chemical composition of the precursors; (2) with an increasing amount of NH4F in the solution system, the morphology will tend to form more ordered states and more distinct hierarchical structures; (3) with an increasing amount of NH4F in the solution system, the growth of products will tend to form denser structures; (4) the amount of NH4F in the solution system will affect the mass loading of products. The four different morphologies of Co3O4 were tested as free-standing electrode materials for supercapacitor application. Co3O4 with the thin-nanowire-cluster morphology exhibits the best electrochemical performance: the specific area capacitance is 1.92 F cm−2 at the current density of 5 mA cm−2 and goes up to 2.88 F cm−2 after 3000 charge–discharge cycles, while the rate capability is 72.91% at the current density of 30 mA cm−2.

Largest magnetic moments in the half-heusler alloys XCrZ (X = Li, K, Rb, Cs; Z = S, Se, Te): A first-principles study

A recent theoretical work indicates that intermetallic materials LiMnZ (Z = N, P) with a half-Heusler structure exhibit half-metallic (HM) behaviors at their strained lattice constants, and the magnetic moments of these alloys are expected to reach as high as 5 μB per formula unit. (Damewood et al. Phys. Rev. B 2015, 91, 064409). This work inspired us to find new Heusler-based half-metals with the largest magnetic moment. With the help of the first-principles calculation, we reveal that XCrZ (X = K, Rb, Cs; Z = S, Se, Te) alloys show a robust, half-metallic nature with a large magnetic moment of 5 μB at their equilibrium and strained lattice constants in their most stable phases, while the excellent HM nature of LiCrZ (Z = S, Se, Te) alloys can be observed in one of their metastable phases. Moreover, the effects of uniform strain in LiCrZ (Z = S, Se, Te) alloys in type II arrangement have also been discussed.

Rare earth-based quaternary Heusler compounds MCoVZ (M = Lu, Y; Z = Si, Ge) with tunable band characteristics for potential spintronic applications

Under uniform strain, there are natural physical transitions from spin-filter magnetic semiconductor (MS) to spin-gapless semiconductor (SGS) to half-metal (HM) for rare earth-based equiatomic quaternary Heusler (EQH) compounds with the formula LuCoVZ, and from HM → SGS → MS → SGS → HM for EQH compounds with the formula YCoVZ.

Structural, electronic, magnetic, half-metallic, mechanical, and thermodynamic properties of the quaternary Heusler compound FeCrRuSi: A first-principles study

In this paper, we have investigated the structural, electronic, magnetic, half-metallic, mechanical, and thermodynamic properties of the equiatomic quaternary Heusler (EQH) compound FeCrRuSi using the density functional theory (DFT) and the quasi-harmonic Debye model. Our results reveal that FeCrRuSi is a half-metallic material (HMM) with a total magnetic moment of 2.0 μB in agreement with the well-known Slater-Pauling rule Mt = Zt − 24. Furthermore, the origin of the half-metallic band gap in FeCrRuSi is well studied through a schematic diagram of the possible d-d hybridization between Fe, Cr and Ru elements. The half-metallic behavior of FeCrRuSi can be maintained in a relatively wide range of variations of the lattice constant (5.5–5.8 Å) under uniform strain and the c/a ratio (0.96–1.05) under tetragonal distortion. The calculated phonon dispersion, cohesive and formation energies, and mechanical properties reveal that FeCrRuSi is stable with an EQH structure. Importantly, the compound of interest has been prepared and is found to exist in an EQH type structure with the presence of some B2 disorder. Moreover, the thermodynamic properties, such as the thermal expansion coefficient α, the heat capacity CV, the Grüneisen constant γ, and the Debye temperature ΘD are calculated.

L21 and XA Ordering Competition in Hafnium-Based Full-Heusler Alloys Hf2VZ (Z = Al, Ga, In, Tl, Si, Ge, Sn, Pb)

For theoretical designing of full-Heusler based spintroinc materials, people have long believed in the so-called Site Preference Rule (SPR). Very recently, according to the SPR, there are several studies on XA-type Hafnium-based Heusler alloys X2YZ, i.e., Hf2VAl, Hf2CoZ (Z = Ga, In) and Hf2CrZ (Z = Al, Ga, In). In this work, a series of Hf2-based Heusler alloys, Hf2VZ (Z = Al, Ga, In, Tl, Si, Ge, Sn, Pb), were selected as targets to study the site preferences of their atoms by first-principle calculations. It has been found that all of them are likely to exhibit the L21-type structure instead of the XA one. Furthermore, we reveal that the high values of spin-polarization of XA-type Hf2VZ (Z = Al, Ga, In, Tl, Si, Ge, Sn, Pb) alloys have dropped dramatically when they form the L21-type structure. Also, we prove that the electronic, magnetic, and physics nature of these alloys are quite different, depending on the L21-type or XA-type structures.

Electronic, Magnetic, Half-Metallic, and Mechanical Properties of a New Equiatomic Quaternary Heusler Compound YRhTiGe: A First-Principles Study

We apply First-principles theory to study the electronic structure as well as the magnetic and mechanical characteristics of YRhTiGe, a newly-designed Y-based quaternary equiatomic Heusler compound. This compound is half-metallic in nature with a ferromagnetic ground state. The total magnetic moment of YRhTiGe is 2 μB and it obeys the Slater-Pauling rule, Mt = Zt − 18, where Mt and Zt are the total magnetic moment and total number of valence electrons, respectively. The magnetic and half-metallic behaviors at its equilibrium and strained lattice constants have been discussed in detail. In addition, for FM-type YRhTiGe, its polycrystalline mechanical features such as Poisson’s ratio, Lame constants, Kleinman parameter and hardness, are also computed according to the well-known Voigt-Reuss-Hill approximation. We investigate the mechanical anisotropy of YRhTiGe using the directional dependences of the Young’s modulus and the shear modulus. Finally, we prove this compound is structurally and mechanically stable. This theoretical investigation provides further insight into the application of Y-based compounds as spintronic materials.

Anti-site-induced diverse diluted magnetism in LiMgPdSb-type CoMnTiSi alloy

The effect of three kinds of anti-site disorder to electronic structure and magnetic properties of the LiMgPdSb-type CoMnTiSi alloy are investigated. It was found the Mn-Ti anti-site disorder can induce the diluted magnetism in CoMnTiSi matrix. The magnetic structure has an oscillation between the ferromagnetic and antiferromagnetic states with the different degree of Mn-Ti anti-site disorder. Two novel characteristics: the diluted antiferromagnetic half-metallicity and the diluted zero-gap half-metallity are found in the different degree range of the Mn-Ti anti-site disorder. The Co-Mn and Co-Ti anti-site disorder have little effect on the magnetic properties. The width of energy gap and the intensity of DOS at the Fermi level can be adjusted by the degree of Co-Mn or Co-Ti anti-site disorder. The independent control to the carrier concentration and magnetization can be realized by introducing the different anti-site disorder.