The Foundation for a Ground-Up & Robust Approach to Computational Magnetic Materials Discovery
Guy Moore, University of California, Berkeley
Bridging the atomistic picture of magnetism to larger length scales is an important challenge for the design and discovery of technologically relevant magnetic materials. We present a multiscale computational approach for obtaining magnetic exchange constants and their derived continuum properties from density functional theory, DFT+U+J. The Heisenberg exchange constants are highly sensitive to two important prerequisites: the magnetic ground-state, as well as Hubbard U and Hund J values. The on-site U/J corrections are calculated using a custom linear response workflow that we’ve built within the atomate code framework. Additionally, we have implemented and benchmarked a “source-free” exchange-correlation magnetic field in VASP, which is implemented using a fully parallelized fast Poisson solver.
This source-free functional, paired with a custom particle swarm optimization strategy, SpinPSO, has resulted in improved agreement with a variety of magnetic ground-states that were measured using neutron diffraction. The custom U and J values and optimized spin moment ground-state are used as inputs to the calculation of exchange constants using VASP+Wannier90 and TB2J via the single-particle Green’s function approach. Equipped with calculated exchange constants, we study the finite temperature behavior using a custom Monte Carlo code, which is MPI and OpenMP parallelized and written in Cython. Additionally, we present a general approach for obtaining the continuum Ginzburg Landau (GL) free energy functional from the microscopic Hamiltonian. Starting from this functional, we probe how microstructure influences hysteresis using a custom Python-based micromagnetic code, which is also parallelized using MPI. This ground-up computational approach will allow for the discovery of magnetic materials with technological applications ranging from spintronics to cost-effective magnetocaloric materials for magnetic refrigeration.
Abstract Author(s): Guy C. Moore