Thermal Stability and Intrinsic Magnetism of MnBiGe alloys : A Potential Gap Permanent Magnet = MnBi-Ge 구조의 고유 자기 특성 연구

다국어 초록 (Multilingual Abstract)

The low-temperature phase of MnBi has long attracted attention as a rare-earth–free permanent magnet due to its unusual increase in magnetic anisotropy and coercivity with temperature. Using systematic density functional theory, density functional perturbation theory, and Monte Carlo simulations, we investigate the structural stability and intrinsic magnetic properties of MnBi under metal and metalloid substitution. Our results show that the low in-plane magnetocrystalline anisotropy of MnBi can reorient into a strong uniaxial anisotropy (Ku) through Mn relocation at interstitial sites and lattice expansion at elevated temperatures. Among 11 candidate substitutions, only Ge at the Bi site preserves phase stability while enhancing intrinsic magnetic properties. In addition, in-plane anisotropy transforms into strong out-of-plane anisotropy upon 3–12 at.% Mn-site substitution with Cr, Co, Cu, or Zn. Specifically, Mn1-xCrxBi (x<0.2) is predicted to exhibit a large Ku of 2.5 MJ/m3 and Curie temperature Tc up to 726 K, in contrast to the values of -0.3 MJ/m3 and 750 K for pristine MnBi. The origin of the enhanced magnetization reversal and uniaxial anisotropy is traced to energy-level shifts in the strongly spin-orbit-coupled Bi 6p states. These findings demonstrate the potential for improving the intrinsic magnetic performance of MnBi through substitutional doping with nonmagnetic metalloid elements. Keywords: Permanent Magnet, Magnetic Anisotropy, Saturation Magnetization, Density Functional Theory, Substitution Doping

목차 (Table of Contents)

• Abstract i
• Table of Contents ii
• List of Tables iii
• List of Figures iv
• Chapter 1. Introduction 1
• 1.1. Permanent magnetism 1
• 1.1.1.Magnetic Properties 2
• 1.1.1.1. The Magnetic Hysteresis Loop 2
• 1.1.1.2. Intrinsic Magnetic Properties 5
• 1.1.1.3. Temperature Dependent Ferromagnetism 7
• 1.1.2. Structural and Dynamic Properties 8
• 1.2. The research Objective : MnBi as a “Gap” Permanent Magnet 10
• Chapter 2. Methodology 13
• 2.1. Density Functional Theory 13
• 2.2. Density Functional Perturbation Theory 17
• 2.3. Computational Details 19
• Chapter 3. Research results 21
• 3.1. Ga/Ge Substitutional Doping of MnBi 21
• 3.2. Transition Metal Doping of MnBi 34
• 3.2. Conclusion 40
• References 41
• 국문초록 47
• iii