A multiscale computational approach to magnetic materials discovery

Bridging the atomistic picture of magnetism to larger length scales is an important challenge for the design and discovery of technologically relevant magnetic materials. For example, the well known multiferroic bismuth ferrite exhibits unique magnetoelectric coupling, in which the spin-orbit interaction induces a magnetic cycloid ground-state with a wavelength that exceeds fifty nanometers. In order to capture the varying lengthscales of these phenomena, we present a multiscale computational approach for obtaining magnetic exchange constants and their derived continuum properties from microscopic lattice Hamiltonians fit to density functional theory (DFT) calculations. The exchange constants are obtained using VASP+Wannier90 and TB2J via the single-particle Green's function approach. Equipped with the lattice Hamiltonian, we study the finite temperature behavior using a custom Monte Carlo code. 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 evolve the system in time to probe how microstructure influences hysteresis using a custom MPI-parallelized Python-based micromagnetic code. This framework allows for a rigorous treatment of dynamics across a magnetic phase transition, and specifically the role of Langevin fluctuations at the critical point. This ground-upwards multiscale computational approach will allow for the discovery of magnetic materials, with technological applications ranging from spintronics to quantifying coercivity in permanent magnets, as well as identifying new cost-effective magnetocaloric materials for magnetic refrigeration.