title: High-throughput screening of multifunctional magnetic materials
Creators: name: Gao, Qiang
Publisher: name: UNSPECIFIED
Year: 2020

description: 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 (&gt;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