Guodong Liu

New quarternary half metallic material CoFeMnSi

A good field to develop new materials with half metallicity is the quaternary Heusler alloys. The preferred route is to combine the compounds that have been already grown in Heusler structure. As a typical example, the quaternary LiMgPdSb-type CoFeMnSi have been investigated in detail. For the quaternary LiMgPdSb-type compounds, three nonequivalent structures exist. From the calculated density of state (DOS) results, it can be seen that one superstructure shows half metallicity. The second superstructure has a pseudogap at the Fermi level. The third superstructure shows metallic behavior. Based on the analysis of the measured XRD pattern and magnetization curve, we can deduce that the CoFeMnSi compound is crystallized in the structure where half metallicity occurs.

A new spin gapless semiconductors family: Quaternary Heusler compounds

By using first-principles calculations, we investigate the band structures of a series of quaternary LiMgPdSn-type Heusler compounds. Our calculation results show that five compounds, CoFeMnSi, CoFeCrAl, CoMnCrSi, CoFeVSi and FeMnCrSb, possess unique electronic structures characterized by a half-metallic gap in one spin direction while they have a zero-width gap in the other spin direction showing a spin gapless semiconducting behavior. We further analyse the electronic and magnetic properties of all quaternary Heusler alloys involved, and reveal a semi-empirical general rule (the total valence electrons number should be 26 or 28) for indentifying spin gapless semiconductors in Heusler compounds. The influences of lattice distortion and main-group element change have also been discussed.

Search for new half-metallic ferromagnets in semi- Heusler alloys NiCrM (M = P, As, Sb, S, Se and Te)

The first-principles calculation within density-functional theory is used to search for new candidates of half-metallic ferromagnets in semi-Heusler alloys NiCrM (M = P, As, Sb, S, Se and Te). Our calculations predict that NiCrP, NiCrSe and NiCrTe are half-metallic ferromagnets (HMFs) with magnetic moments of nearly 3 or 4 μB/fu and HM gaps of 0.263, 0.047 and 0.102 eV, respectively. Other compounds are so-called nearly HMFs. Substitution of the sp atoms cannot be responsible for the formation of the band gap, but results in a shift in the Fermi level and the loss of half-metallicity. The half-metallicity of NiCrP and NiCrTe can be retained when the lattice parameter is changed by about 2%–3%.

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.

Unprecedentedly Wide Curie-Temperature Windows as Phase-Transition Design Platform for Tunable Magneto-Multifunctional Materials

A series of unprecedentedly wide Curie-temperature windows (CTWs) between 40 and 450 K are realized by employing the isostructural alloying principle for the strongly coupled magnetostructural phase transitions in a single host system. The CTWs provide a design platform for magneto-multifunctional multiferroic alloys that can be manipulated in a quite large temperature space in various scales and patterns, as well as by multiple physical fields.

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.

Realization of multifunctional shape-memory ferromagnets in all-d-metal Heusler phases

Heusler ferromagnetic shape-memory alloys (FSMAs) normally consist of transition-group d-metals and main-group p-elements. Here, we report the realization of FSMAs in Heusler phases that completely consist of d metals. By introducing the d-metal Ti into NiMn alloys, cubic B2-type Heusler phase is obtained and the martensitic transformation temperature is decreased efficiently. Strong ferromagnetism is established by further doping Co atoms into the B2-type antiferromagnetic Ni-Mn-Ti austenite. Based on the magnetic-field-induced martensitic transformations, collective multifunctional properties are observed in Ni(Co)-Mn-Ti alloys. The d metals not only facilitate the formation of B2-type Heusler phases but also establish strong ferromagnetic coupling and offer the possibility to tune the martensitic transformation.

Half-metallic ferromagnetism in zinc-blende CrBi and the stability of the half-metallicity of zinc-blende CrM (M = P, As, Sb, Bi)

First-principles calculations within density-functional theory are used to investigate zinc-blende-structured CrBi. Our calculation predicts ZB-type CrBi to be a true half-metallic ferromagnet with a magnetic moment of 3 μB per formula. Its half-metallicity can be maintained over a relatively wide range of lattice constant (from −8 to 8%). The trends with varying lattice constant for the ZB-type half-metallic system CrM (M = P, As, Sb, Bi) are also studied.

First-principles study on quaternary Heusler compounds ZrFeVZ (Z = Al, Ga, In) with large spin-flip gap

First-principles calculations were used to systematically investigate the structural, electronic and half-metallic properties of newly designed quaternary Heusler compounds ZrFeVZ (Z = Al, Ga, In). The calculated results show that these three compounds have an excellent half-metallicity in their ferrimagnetic ground state. ZrFeVZ (Z = Al, Ga, In) compounds exhibit the large spin-flip gaps of 0.348 eV, 0.428 eV and 0.323 eV at their equilibrium lattice constants, respectively. The total spin magnetic moment is 2 μB for all the three compounds, which is in agreement with the Mt = Zt − 18 rule. The half-metallic properties of these three compounds are quite robust to the hydrostatic and tetragonal strain, and can also be kept when the electron correlation (U) is considered. For the ZrFeVIn compound, the spin-flip band gap achieves the maximum value (0.57 eV) under a small strain. The calculated Curie temperatures based on the mean field approximation (MFA) method are 818.04 K, 826.66 K, and 751.70 K for the ZrFeVAl, ZrFeVGa, and ZrFeVIn compounds, respectively. We hope that our current work may trigger Heusler compounds containing 4d transition metal elements and with quite large spin-flip band gaps for application in future spintronics devices.