Negative Piezoelectric Coefficient in Ferromagnetic1H-LaBr2 Monolayer

Magnetic materials are of great importance for energy harvesting and conversion and information technologies, as exemplified by permanent magnets and spintronic applications, respectively. There is a strong impetus to develop new high performance magnets to satisfy the increasing requirements of such technologies. On the other hand, high-throughput (HTP) screening based on density functional theory calculations provides an efficiency way to predict novel materials with target properties. Therefore, it is meaningful to accelerate the development of functional magnetic materials by using the high-throughput screening method. In this thesis, we endeavored to perform HTP engineering of both 3D and 2D magnetic materials, focusing on spintronic and permanent magnet applications. For the spintronic applications, based on HTP density functional theory calculations, we performed a systematic screening by examining peculiar electronic structure. For instance, we have done HTP screening for spin-gapless semiconductors (SGSs) in quaternary Heusler alloys XX'YZ. Following an empirical rule, we focused on compounds with 21, 26, or 28 valence electrons, resulting in 12000 possible chemical compositions. After systematically evaluating the thermodynamic, mechanical, and dynamical stabilities, we have identified 70 so far unreported SGSs, confirmed by explicit electronic structure calculations with proper magnetic ground states, of which 17 candidates have a distance to the convex hull smaller than 0.10 eV/atom. It is demonstrated that all four types of SGSs can be realized, defined based on the spin characters of the touching bands around the Fermi energy. Particularly, it is found that the type-II SGSs exhibit promising transport properties for spintronic applications, such as large anisotropic magnetoresistance and anomalous Nernst effects driven by spin-orbit coupling. On the other hand, as to 2D materials, we carried out a systematic HTP screening for in-plane ordered MXene (i-MXene), which can be obtained by etching the main group element (A) away from the recently synthesized in-plane ordered MAX (i-MAX) phase. Such 2D i-MXenes provide us a new playground for 2D magnetic materials. It is observed that the spin configurations, hence the magnetic anisotropy, can be tuned by strain. We found that five i-MXenes have a significantly large out-of-plane magnetic anisotropy energy (>0.5 meV/f.u.), and the four ferromagnetic candidates have high Curie temperature based on the 2D Ising model. Additionally, we found i-MXene can realize large Seebeck effect, antiferromagnetic topological insulator, and spin-gapless semiconductors, making them interesting for future studies. In terms of designing permanent magnets, we carried out a systematic HTP screening for rare-earth free permanent magnets by incorporating light interstitials (H, B, C, N) into magnetic full Heusler alloys. We successfully identified 32 candidates with an out-of-plane magnetic anisotropy larger than 0.4 MJ/m$^3$ as well as 10 cases with large in-plane anisotropy. Detailed analysis reveals that the interstitials are very efficient in inducing global tetragonal distortions, whereas the local chemical bonding and changes in the crystalline environment result in significant enhancement of MAE. We strongly believe this provides an efficient way to tailor MAE, and such newly predicted permanent magnet candidates are promising gap magnets between rare earth based Sm-Co and Nd-Fe-B and the transition metal based AlNiCo and ferrite. To summarise, in addition to screening over various types of functional magnetic materials, my work provides valuable solutions to the pending challenges of HTP design of magnetic materials. We have established automated workflows, which can be easily applied on the other magnetic materials. This paves the way for further design of advanced magnetic materials with optimal performance

The recent emergence of two-dimensional (2D) Janus materials has opened a new avenue for spintronic and optoelectronic applications. However, 2D magnetic Janus materials and Janus monolayer-based magnetic heterostructures are yet to be fully studied. Herein, the stability and electronic structure of 2D Janus V2I3Br3 and V2I3Cl3 monolayers, and the electronic and magnetic properties of 2D graphyne/Janus V2I3Br3 (γ-GY/V2I3Br3) heterostructures are calculated based on the density functional theory. Janus V2I3Br3 and V2I3Cl3 monolayers are ferromagnetic semiconductors with good stability and direct band gap. By combing the graphyne layer, the Janus V2I3Br3 monolayer shows half-metallic characteristics. The electrical conductivity of the Janus V2I3Br3 monolayer in γ-GY/V2I3Br3 heterostructures is further improved, which is very favorable for the applications of the γ-GY/V2I3Br3 heterostructure in battery anodes. Moreover, the Janus V2I3Br3 monolayer possesses a smaller perpendicular magnetic anisotropy (PMA), and the PMA can be effectively enhanced by combing γ-GY. Herein, the enhanced PMA was discovered to depend on the stacking patterns of γ-GY and V2I3Br3 monolayers. Biaxial strains can further affect the PMA of the γ-GY/V2I3Br3 heterostructure. Meanwhile, at a compressive strain, the Janus V2I3Br3 monolayer in the γ-GY/V2I3Br3 heterostructure realizes the transition from the magnetic half-metallic to the magnetic metal state. These results can enrich the applications and designs of γ-GY/V2I3Br3 magnetic heterostructures in spintronic devices and energy fields.

Study of the linear and nonlinear response properties of the zinc-blende AlP, GaP and their AlxGa1-xP ternary alloys using first principle calculations

The conventional magnetic ordering models, exchange interaction, super-exchange (SE) interaction and double exchange (DE) interaction models relating to the valence electron structure in the materials, were proposed about in or before the 1950's, the time when there was little experimental evidence. Since the 1970's, more and more experimental results for the valence electron states have been reported. These experimental results suggested that the conventional magnetic ordering models need improving. i) Many experimental results, including the electron energy-loss spectra (EELS), X-ray absorption spectra (XAS), and X-ray photoelectron spectra (XPS), indicate that there are O<sup>–</sup> anions in addition to O<sup>2–</sup> anions in oxides, and that the percentage of O<sup>–</sup> anions may reach 30% or more. This suggests that the SE model and DE model both need to improving, in which all oxygen anions are assumed to be O<sup>2–</sup> anions. ii) Several experimental results, including gamma radiation diffraction, XAS and magnetic circular dichroism spectra (XMCD), suggest that part of 4s electrons enter into 3d orbits and transit into the 3d electrons in the process of forming metals from free atoms. The effect of the orbital magnetic moment on the magnetic moment of a bulk metal is far smaller than the spin magnetic moments. These provide the evidence of exploring the relation between magnetic moment and electrical resistivity of the magnetic metal. iii) Using density function theory (DFT) to fit physical properties yields plenty of results for many materials, but there exist serious difficulties for magnetic materials. This is due to magnetic ordering energy is included in the exchange correlation energy, which has been find no phenomenological expression so far, and has to be fitted using various models in DFT calculation. These investigations provide an opportunity to improve magnetic ordering models. Therefore, our group proposed three models of magnetic ordering in typical magnetic materials, they including an O 2p itinerant electron model for magnetic oxides (IEO model), a new itinerant electron model for magnetic metal (IEM model), and a Weiss electron pair (WEP) model for the origin of magnetic ordering energy. Replacing the SE model and DE model with the IEO model, the magnetic structures of Co, Ni, Cu doped spinel ferrites as well as Cr and Ti doped spinel ferrites can be explained. The dependence of the magnetic moment on the Sr content in perovskite manganites La<sub>1–<i>x</i></sub>Sr<sub><i>x</i></sub>MnO<sub>3</sub> can also be explained, for which there have been many ongoing disputes about the cation distributions. With the IEM model, we can explain qualitatively the relation of the magnetic moment with the resitivity for each of Fe, Co, Ni, Cu metals, and fit the curves of the resistivity of NiCu alloy versus test temperature and the Cu doped level. With the WEP model, we can explain why Fe, Co, Ni metal, NiCu alloys, Fe<sub>3</sub>O<sub>4</sub> and La<sub>0.7</sub>Sr<sub>0.3</sub>MnO<sub>3</sub> oxides have different Curie temperature values. The new itinerant electron model is different from the classical model in the following three elementary characteristics. First, the s electrons in free 3d transition metal atoms are divided into two parts when they form a metal or alloy. One part of these s electrons enter into the d orbits and change into the d electrons. and the other part of those electrons are the free electrons which are no longer called the s electrons. Second, only the d electrons occupying the outer orbit of an ion core in a metal or alloy may form itinerant electrons with a certain probability, while the remaining d electrons are local electrons. Third, whether in a magnetic metal or in a magnetic oxide, the transition of the itinerant electrons is the spin-dependent transition below the Curie temperature, and the transition probability decreases with test temperature increasing. The transition of the itinerant electrons turns into the spin-independent transition when the temperature is above the Curie temperature. In this paper, first, we introduce several typical experimental results of the valence electron states. Then, we present the new magnetic ordering models proposed by our group and analyze the elementary differences between the new models and the conventional models. Finally, we point out the challenge to the future work.

Electrochemical properties of Li2NiPO4F were studied using density functional theory. The obtained voltage, electronic band gap, capacity (∼ for 2 Li+ extraction) and energy density are achieved as 5.33 V, 4.0 eV, 287.3 mAh g−1 and 1531.31 Wh kg−1, respectively. Although, the electrochemical properties of Li2NiPO4F are promising, large electronic band gap would certainly pose a limitation for its commercial application. Nb is a transition metal and its electronegativity is 1.6 which is less than the electronegativity of 2.19 for P. This implies, less operating voltage would be obtained if we replace P in Li2NiPO4F by Nb to form Li2NiNbO4F. However, electronic configuration of Nb is [Kr] 4d45 s1 and the valance state of Nb in Li2NiNbO4F is +5, which in turn specify that, localized Nb d states will reside in conduction band of Li2NiNbO4F and hence the electronic band-gap would be less owing to this localized Nb-d states. Our speculation gets verified by the calculated properties of Li2NiNbO4F obtained through DFT as follows; Voltage, electronic band gap, capacity (∼ for 2 Li+ extraction) and energy density achieved, respectively, are 5.01 V, 3.64 eV (less than LiFePO4), 215.71 mAh g−1, 1080.71 Wh kg−1. Lower electronic band gap of Li2NiNbO4F makes it an alternative to Li2NiPO4F.

Direct detection experiments for light dark matter are making enormous leaps in reaching previously unexplored model space. Several recent proposals rely on collective excitations, where the experimental sensitivity is highly dependent on detailed properties of the target material, well beyond just nucleus mass numbers as in conventional searches. It is thus important to optimize the target choice when considering which experiment to build. We carry out a comparative study of target materials across several detection channels, focusing on electron transitions and single (acoustic or optical) phonon excitations in crystals, as well as the traditional nuclear recoils. We compare materials currently in use in nuclear recoil experiments (Si, Ge, NaI, CsI, CaWO$_4$), a few which have been proposed for light dark matter experiments (GaAs, Al$_2$O$_3$, diamond), as well as 16 other promising polar crystals across all detection channels. We find that target- and dark matter model-dependent reach is largely determined by a small number of material parameters: speed of sound, electronic band gap, mass number, Born effective charge, high frequency dielectric constant, and optical phonon energies. We showcase, for each of the two benchmark models, an exemplary material which has a better reach than in any currently proposed experiment.

The electronic structures, Born effective charges ( BEC ), and interatomic force constants of BaMO 3 (M = Ti, Zr, Hf, Sn) in the cubic perovskite structure are computed using first‐principles density functional theory. Comparison with the M aximally‐localized W annier function description of the electronic structures for BaTiO 3 shows that the change of a single constituent (Ti to Zr, Hf, and Sn) has pronounce effects on the orbital hybridizations with the oxygen, with significant implications for the nature of the BEC anomalous of the compounds. Examination of the real‐space interatomic force constants shows that the key interatomic interactions are strongly dependent on the constituent's dynamical effective charge and the very delicate compensation between the short‐range interactions and long‐range dipole–dipole interactions gives rise to the ferroelectric structural instabilities.

Ab initio electronic, dynamic, and thermodynamic properties of isotopic lithium hydrides ( 6LiH, 6LiD, 6LiT, 7LiH, 7LiD, and 7LiT)

Using a first-principles approach based on density-functional theory, the electronic, dielectric, and dynamical properties of the two phases of lithium niobate are studied. In particular, the spontaneous polarization, the optical dielectric tensors, the Born effective charges, and the zone-center phonons are computed. The Born effective charges are found to be significantly larger than the nominal ionic charges of the ions, a feature similar to what is observed in related ${\mathrm{ABO}}_{3}$ compounds and attributed to the hybridization between the O $2p$ and Nb $4d$ states. The analysis of the zone-center phonons in the paraelectric phase reveals an unstable ${A}_{2u}$ mode to be responsible for the phase transition. The origin of the structural instability is attributed to destabilizing long-range dipolar interactions, which are not fully compensated by stabilizing short-range forces. Finally, the identification of the E modes in the ferroelectric phase, which is still the cause for debates in spite of the numerous experimental and theoretical studies, is discussed and an assignation based on the analysis of the mode-oscillator strengths and the angular dispersion relation of extraordinary phonons is proposed.

Two-dimensional (2D) magnetic materials exhibit a wide array of fascinating magnetic properties, making them highly attractive for spintronic applications such as high-density nonvolatile memories and multifunctional nano-devices. Recently, chromium tellurides (CrTex) have attracted significant attention due to their metallic band structure, strong magnetic anisotropy, and tunable exchange couplings. The unique tunability of magnetic properties in a metallic ground state makes CrTexa promising platform for generating, controlling, and manipulating spin currents. This review summarizes recent advances in large-scale 2D magnetic CrTexepitaxial thin films, emphasizing synthesis techniques that produce high-quality, large-area films. It explores the role of self-intercalation and heterostructure engineering in tailoring the magnetic and structural properties of these materials. We also review the band structure, magnetic characteristics, and spin dynamics of CrTex, with a particular emphasis on thickness-dependent band dispersion, magnetic anisotropy, and the emergence of skyrmions. Moreover, this review highlights the applications of CrTexin spintronics, covering the anomalous Hall effect, topological Hall effect, spin valves, and spin-orbit torque devices. The goal of this review is to furnish readers with a comprehensive overview of the intriguing properties of CrTexcompounds and to inspire further innovative studies on the vast potential of 2D magnetic materials for next-generation spintronic and quantum devices.

Ultra-thin ferromagnets with large magnetic anisotropy by assembling MnCl3 superatoms on SbAs monolayer

A theoretical study of dielectric tensor, Born effective charges, LO-TO splitting and infrared response of Al4SiC4 and Al4Si2C5

Lattice dynamical and thermodynamic properties study of ceria using density functional theory and Hubbard correction

Magnetic topological materials in two-dimensional: theory, material realization and application prospects

Novel bipolar magnetic semiconductor materials formed by adsorption of methyl or halomethyles on the graphene: A spin-polarized density functional theory study