First-Principles Predictions of Magnetic Properties of Heusler Compounds: Half-metallicity, Permanent Magnetism, and Anomalous Hall Conductivity

Abstract

Magnetic materials encompass a broad range of materials, which are applied in a diversity of fields. For example, magnetic materials are key component of motors, power generators, and spintronic devices. Even though there is a several type of magnetic materials, due to some issues at practical level, the demand of finding new magnetic material is still in need. To identify new magnetic materials, first-principles calculations are indeed faster, safer, and resource-saving compared to experiments. Among several classes of materials, Heusler compounds stand out as potential candidates in terms of material designs with an extensive tunability which can be tailored by varying chemical substitutions and structural motifs. In this work, using density functional theory (DFT), the magnetic and electronic properties of hypothetical Heusler compounds are predicted and attention is paid to half-metallicity, permanent magnetism, and anomalous Hall effect. This dissertation is organized as follow. In the chapter 1, we present a brief introduction of Heusler compounds. We shed light on overview of Heusler compounds such as definitions, chemical formulars, and types of Heusler compounds. Finally, we discuss the concept of half-metallicity, permanent magnetism, and anomalous Hall conductivity. In the chapter 2, we present an overview of density functional theory (DFT) by describing the theoretical foundations, advantages, and drawbacks due to approximation in exchange-correlation functional. The chapter 3 aims to perform a systematic density functional study on alkali-metal-based half-Heusler compounds, namely ACrZ (A = Li, Na, and K; Z = As, Sb, and P), to identify the optimal half-metal (HM) for practical applications. Unlike most HMs proposed so far, the majority of ACrZ compounds in our study exhibit a wide band gap (1.60-2.38 eV) and retain robust half-metallicity even at the surface. Furthermore, the half-metallicity is robust under severe strain, up to 10 % , their stability, robust half-metallicity at the surface and under strain, and good lattice mismatch with zinc-blende semiconductors, we propose LiCrZ and NaCrZ (Z = As and Sb) as promising compounds for practical applications to spintronics. In the chapter 4, due to the high price and limited source of rare earth (RE) elements and heavy metals (HM), demand for finding new permanent magnets (PM) without including 4d and 5d HM or RE elements is growing fast. A large saturation magnetization μ_0 M_S and uniaxial magnetocrystalline anisotropy Ku (Ku > 0) are demanded as necessary conditions to be a PM. We theoretically investigate structural stability and intrinsic magnetic properties of Fe2MnSn Heusler compound, adopting cubic, tetragonal, and hexagonal, to identify a new potential PM. The most stable phase is hexagonal, followed by inverse tetragonal, inverse cubic, and regular cubic. The large enough magnetization μ_0 M_S of 1.28 T-1.59 T is predicted, for all structure phases. The inverse tetragonal phase with an uniaxial Ku of 2.36 MJ·m3 is more desirable to be PM compared to hexagonal one with a biaxial Ku of -0.72 MJ·m3. Furthermore, inverse tetragonal phase can be stabilized by H and N interstitial doping, indicated by negative formation energies of -8.55 and -35.19 kJ·mol-1. Particularly, on the one hand, H has better intrinsic magnetic properties [μ_0 M_S = 1.66 T, K_u = 1.66 MJ·m3, (BH)max = 56 MGOe, and κ = 0.93] than N [μ_0 M_S = 1.36 T, K_u = 1.07 MJ·m3, (BH)max = 46 MGOe, and κ = 0.73]. On the other, N shows a better thermal stability since even having higher formation energy than hexagonal phase by 5.03 kJ·mol-1. Recently, it was shown that anomalous Hall conductivity (AHC) can be tailored via tuning Berry curvature curvature regardless of magnetization, which may pave a new way to achieve large AHC without net magnetic moment. In the chapter 5, using the PAW and FLAPW methods implemented in VASP and Fleur codes, AHC of a ferrimagnet quaternary Heusler compounds TiZrMnAl is investigated. Among three possible structural phases, ⍺-phase is energetically most stable, by energy differences of 0.34 eV/fu and 0.03 eV/fu compared to β and γ-phase. The local magnetic moment of Mn is antialigned with those of Ti and Zr. As a result, total magnetic moment is fully compensated (mtot = 0.0 μ_B) in ⍺- and β-phase, and nearly compensated (mtot = 0.1 μ_B) in γ-phase. Interestingly, ⍺-phase possesses a large AHC of 1470 Ω-1cm-1, while β- and γ-phase show moderate AHC of 200 and 100 Ω-1cm-1, consistently in both the PAW and FLAPW methods. The large AHC in ⍺-phase is discussed based on the global Berry curvature over whole Brillouin zone.