S. N. Khan, Aftab Alam, D. D. Johnson
In Fe-based superconductors, moments measured at low temperatures determine the "ordered" moments (along the global axis of quantization) in the antiferromagnetic (AFM) state. Assessed "ordered" Fe moments (0.8-1.04 $\mu_{B}$) are half of 1.6 \mu_{B} from density-functional theory (DFT), a discrepancy not yet understood. Also, evolution of the magnetism and its dynamics over microscopic length scales and experimental time scales is still lacking. We study via DFT the stability of magnetic planar defects -- antiphase and domain boundaries, twins, and $nano$twins -- and their effect on moments. Along with DFT high moments, we find two low-moment states confined near defect boundaries, making a single local-moment picture inappropriate for long-range magnetic order that arises only in a fraction of the sample. The magnetic correlation length coincide with distances between twin boundaries, while the other nano-sized defects are extremely low in energy. These competing defects are either weakly mobile (phasons) or have low-energy fluctuations that lower moments from spatial and/or time averaging probed in experiment. If so, controlling the energy window in a neutron scattering experiment can help alter the time averaging to see such an effect.
View original:
http://arxiv.org/abs/1303.4024
No comments:
Post a Comment