Speaker
Description
Grain boundaries (GBs) play an important role in polycrystalline magnetic materials because they act as obstacles to magnetic domain wall motion. Therefore, magnetic coercivity, remanence, saturation magnetization, and the Curie temperature are known to depend strongly on grain size. An atomistic description of magnetic interactions requires construction of an effective Heisenberg Hamiltonian containing exchange interaction parameters $J_{ij}$ between pairs of atomic sites. The values of $J_{ij}$ can be obtained from ab initio calculations. Knowledge of these parameters further enables simulations of larger systems and/or excited states at nonzero temperatures using Monte Carlo techniques. Information about $J_{ij}$ is available in the literature for many bulk magnetic materials. However, so far there has been no information about $J_{ij}$ in the vicinity of GBs, where the symmetry of the crystal lattice is reduced and atoms have a different chemical environment.
In the present work, we provide a detailed analysis of $J_{ij}$ obtained from ab initio calculations in the vicinity of a clean Σ5(310) GB in bcc Fe, as well as of a GB with segregated P impurities. Negative $J_{ij}$ values were found for interactions across the clean GB, indicating a preferred antiferromagnetic alignment of magnetic moments on these atoms. This demonstrates the strong influence of the GB on exchange interactions in Fe. Monte Carlo results show that, despite pronounced local perturbations, realistic GB densities cause only a small reduction in the Curie temperature because bulk-like regions dominate the global magnetic transition. A substantial decrease in the Curie temperature appears only when the GB volume fraction is artificially increased.