Electronic Structure, Magnetic Behavior, and Transport Property of Fe-based Multicomponent Alloys : 철 기반 합금의 전자 구조, 자성 특성 및 이동 물성

다국어 초록 (Multilingual Abstract)

Electronic structure, magnetic behavior, and transport property of Fe-rich multicomponent alloys have been theoretically investigated by using the first principles full-potential linearized augmented plane-wave (FLAPW) method. The included alloying species have ranged from sp-elements (SP) through transition metals (TM) to rare earths (RE). The parametric analysis with average valency shows that the Slater-Pauling theory of 3d-TM is extendable to Fe-rich multicomponent alloys with late 4d-5d TM and late 3sp-6sp elements. Based on the calculated magnetic moments of Fe-X and Fe-Si-X (X=TM, SP) alloys, we have shown that the magnetization variation of Fe-Si-X alloys can be interpreted as the Si-absent binary alloy pattern repetition at the shifted reference moment. Further we have compactified the blending effects of new solute species beyond ternary alloys. In essence, the pattern repetition has been found to be minimalistic. We have proposed Fe-Si-X-Y (X=Co, Pd, Pt and Y=Al, Sb, Bi) as a viable candidate for the magnetically optimized products of electrical steels.
We have applied the orbital polarization (OP) formalism to the calculations of magnetic properties of Fe-X [X=Lanthanides (Ln)] alloys. The orbital moment enhancements are substantial in the quarter filled or three quarter filled Lanthanides and the calculated hyperfine fields are in good agreement with experimental observations. We have computationally proved the interaction mechanism of Campbell-Brooks model where 4f-5d hybridization directly renders ferromagnetic coupling between itinerant electron spins in Ln 5d bands and itinerant-like electron spins in Fe 3d minority-bands. Non-metallic analogue of this interaction is the superexchange mechanism in which the spin orientation is conserved in the dominant hopping process. We have calculated the magnetocrystalline anisotropy energy (MAE) by choosing the two magnetization directions of [001] and [110]. The magnetic hardness of Fe-Ln alloys is found to be in Er-Ho-Dy-Tm-Sm order. By employing the BoltzTraP code, we have calculated the electrical resistivity and the magnetic susceptibility of Fe-Ln alloys. The overall trend of susceptibility is incremental whereas the overall behavior of resistivity is decremental as the alloyed Ln becomes heavier. The susceptibility peaks are found in Fe-Sm, Fe-Dy, and Fe-Tm. The largest resistivity is found in Fe-Sm and the smallest in Fe-Gd. We have suggested Nd as a promising alloying option for the Fe-based soft magnet because of its moment enhancing character, high resistivity, and relatively small magnetic anisotropy when alloyed with Fe.
New formalism has been developed for the unified treatment of formation energies and applied to α-Fe with B impurities. This formalism is generally applicable to systems with arbitrary interfaces such as surface and grain-boundary. For the Fe-B system, both formation and surface energies are found to decrease as B impurities become close to the free surface. In bulk α-Fe, the solution type of B impurity is determined by the geometrical factors rather than the chemical bonding factors. In slab α-Fe, the geometrical pressure induces B impurities to segregate toward the surface. It has been found that the boundary energy determines the stability of total system.
As a consecutive work of B distribution in α-Fe, we have investigated the most plausible diffusion paths for both interstitial and substitutional B impurities, and provided the quantitative information of how the impurity’s mobility is enhanced in the presence of monovacancies near impurities. The migration barrier of interstitial B has been estimated to be 0.64 eV. We have classified the substitutional B diffusions into three cases: (i) Step-by-step diffusions of B and Fe, (ii) Dissociation of a substitutional B into an interstitial B and a vacancy, and subsequently their independent diffusions, and (iii) Simultaneous rotations of substitutional B and its nearby Fe atom. Compared with the interstitial case, the inspected rotational diffusion has a much larger migration barrier of 5.48 eV and other two diffusions are thought to be energetically improbable due to the absence of stable or metastable positions along the diffusion paths. In contrast, the interactive impurity-vacancy-host complexes describe the energetically feasible diffusion paths, in which B impurity can travel freely in any directions via zigzag-motions inside the bcc matrix. For the vertical and horizontal motions of substitutional B, the maximum migration barriers have been evaluated to be 1.02 and 0.70 eV, respectively. The substitutional B traveling accordingly becomes energetically competitive with the interstitial B motion. The discovered piecewise diffusion paths and their combinatory motions are system-independent and generally applicable to any bcc matrices with substitutional impurities.

목차 (Table of Contents)

• 1. Introduction 1
• 1-1. Ferromagnetism & Fe-based Alloys 1
• 1-2. Overview 2
• 2. Computational Theory 3
• 2-1. Density Functional Theory 3
• 2-2. Local Density Approximation 5
• 3. Fe-X and Fe-Si-X (X=TM, SP) Alloys 7
• 3-1. Introduction 7
• 3-2. Computational Detail 9
• 3-3. RKKY-type Theory 12
• 3-4. Fe-X (X= 3d-5d Transition Metals) Alloys 15
• 3-5. Fe-X (X= 3sp-6sp Elements) Alloys 21
• 3-6. Fe-Si-X (X=TM, SP) Alloys 25
• 3-7. Conclusion 28
• 4. Fe-X (X=Lanthanides) Alloys 29
• 4-1. Introduction 29
• 4-2. Computational Detail 30
• 4-3. Boltzmann Theory: Conduction in Metals 31
• 4-4. Orbital Polarization Potential 34
• 4-5. Spin and Orbital Moment 36
• 4-6. Hyperfine Field 42
• 4-7. Magnetocrystalline Anisotropy Energy (MAE) 47
• 4-8. Resistivity and Susceptibility 49
• 4-9. Conclusion 51
• 5. Fe-(B, C, N) Alloys: B Distribution in Fe-B Alloys 53
• 5-1. Introduction 53
• 5-2. Computational Detail 55
• 5-3. Formalism 56
• 5-4. Bulk Result 58
• 5-5. Surface Result 68
• 5-6. Conclusion 75
• 6. Special Topic: B Diffusion in Fe-B Alloys 77
• 6-1. Introduction 77
• 6-2. Computational Detail 78
• 6-3. Diffusion Mechanism inside Bulk 78
• 6-4. Vacancy Mediated Diffusion 83
• 6-4-1. Case I and II 83
• 6-4-2. Case III and IV 86
• 6-5. Combined Motions of B impurity 89
• 6-6. Conclusion 94
• 7. Conclusion 95
• Bibliography 98
• Acknowledgements 107