First-principles study of magnetic ground and excited-state properties of metallic antiferromagnets

Kang, Kisung

Permalink

https://hdl.handle.net/2142/115808 Copy

Description

Title

First-principles study of magnetic ground and excited-state properties of metallic antiferromagnets

Author(s)

Kang, Kisung

Issue Date

2022-04-18

Director of Research (if dissertation) or Advisor (if thesis)

Cahill, David G.

Doctoral Committee Chair(s)

Schleife, André

Committee Member(s)

• Hoffmann, Axel
• Mason, Nadya

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Antiferromagnets
• Density functional theory
• Computational materials science

Abstract

Metallic antiferromagnets have recently been investigated as a new candidate for future magnetic memory devices because of their potential for faster spin dynamics, the possibility of compact data storage, and robustness against external stimulations. Especially, its metallicity enables the electrical methods for reading and writing utilized in magnetic memory devices. However, the characterization of metallic antiferromagnets is not easily implemented by the conventional methods for ferromagnets due to zero net magnetization caused by the compensatory magnetic configuration of metallic antiferromagnets. This thesis work implements a comprehensive computational study about ground and excited-state properties of metallic antiferromagnets to find the physical origins of magnetic phenomena through fundamental magnetic interactions and quasiparticle energy dispersion relations. First-principles methods can overcome the overarching problems in experiments, visualizing the atomistic magnetic structure and quantifying the magnetic interactions of metallic antiferromagnets. This work integrates individually developed computational methods, including classical means like atomistic spin dynamics and linear spin-wave theory and quantum mechanical approaches such as density functional theory and the Korringa-Kohn-Rostoker method. Magnetic interactions are important parameters to determine the ground state magnetic structure. Exchange interactions between magnetic moments define the antiferromagnetic configuration, while magnetocrystalline anisotropy indicates which direction the entire magnetic structure orients. For magnetocrystalline anisotropy, the significance of the contribution from the magnetic dipole-dipole interactions to out-of-plane anisotropy of metallic antiferromagnets is confirmed in this work. Calculated exchange interactions and magnetocrystalline anisotropy of metallic antiferromagnets are closely connected to excited state properties through magnon dispersion relations. Excited properties of metallic antiferromagnets originate from quasiparticle contributions of electrons, phonons, and magnons, and their relations can be explained by quasiparticle energy dispersion of metallic antiferromagnets. Electronic band structure can confirm the metallicity of target antiferromagnets, while magnon dispersion enables the estimation of the switching speed through the lowest spin-wave frequency at the long-wavelength limit. Characterization of metallic antiferromagnets accompanies external stimulations to generate unique excited state properties. External magnetic field effect to metallic antiferromagnet can be quantified as magnetic susceptibility calculated by the newly proposed method with moment-tilted states. The thermal effect can be introduced by atomistic spin dynamics simulation, estimating N\'{e}el temperature of metallic antiferromagnets indicated by a peak of magnetic heat capacity. (Magneto-)Optical properties of metallic antiferromagnets are also affected by these external stimulations and give additional characteristic information of materials. The forbidden polar magneto-optical Kerr effect can be generated by magnetic symmetry breaking when an external magnetic field is applied, and its signal strength is determined by the splitting from spin-orbit coupling effect, exchange splitting, and magnetic susceptibility. Calculated temperature-dependent optical spectra of ferromagnetic BCC Fe delineate that intraband transitions can be captured by supercells with perturbed atomic position and magnetic structure, and thermal demagnetization can be observed as a peak redshifting of imaginary optical conductivity caused by the reduction of exchange splitting. In conclusion, this thesis work implements the comprehensive first-principles investigation about ground and excited-state properties of metallic antiferromagnets. Based on the atomistic analysis, investigation for metallic antiferromagnets as future candidates for magnetic memory devices can make further progress with first-principles methods.

Graduation Semester

2022-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Kisung Kang

Owning Collections