2 Methods and codes developments

Ab initio linear response methods provide a powerful tool to investigate magnetic interactions and excitations in materials of interest. We calculate bare and full dynamic spin susceptibilities to describe magnetic interactions and spin excitations in various materials. This method is useful for investigating the magnetic interactions and excitations in itinerant systems since the Stoner excitations and spin-wave excitations are treated on equal footing.

Tight-binding code is developed to investigate the detailed band structures and magnetic properties such as exchange coupling and magnetocrystalline anisotropy. We start from the realistic TB model constructed using wannier90 or non-orthogonal basis and construct Hamiltonian and Green’s functions. Some of the functions are listed below.

• Bandstructure, spectra functions, Fermi surface
• Green’s functions (GF)
• Spin-orbit coupling (SOC)
• Perturbation theory treatment of SOC using GF
• Pairwise exchange coupling and magnetocrystalline anisotropy using linear response
• Orthogonal and non-orthogonal basis

Spinwave theory, linear or nonlinear, is used in our research to compare with inelastic neutron scattering measurements or estimate Curie temperatures of 2D materials. One of the later examples is our applications of renormalized spin-wave theory on magnetic 2D van der Waals materials.

• Nonlinear renormalized spinwave theory
• Disorder effects on spin excitations

Micromagnetics is used to investigate extrinsic magnetic properties in permanent magnets or combined with the string method to investigate magnetic skyrmions’ growth mechanism.

• Combination of micromagnetics and the string method