Doping strengthens magnetism in an iron-based crystal

Thallium-doping (5%) of BaFe₂As₂ crystal causes a surprising rise of the antiferromagnetic transition temperature (T_N), related to magneto-elastic coupling. This is the first example of the increase of T_N with chemical doping in BaFe₂As₂. Understanding the role of chemical doping for changing magnetism or causing superconductivity can lead to design principles for magnets and superconductors that operate at desired temperatures. 

BaFe₂As₂ crystals were chemically doped with Tl, for the possibility of creating a similar high-temperature superconductor (T_c = 38 K) found in hole-doped Ba_1-xK_xFe₂As₂. However, and unexpectedly, the crystals exhibited highly unusual behavior for which T_N first increases in 5% chemical substitution, before the expected drop with 9% in Ba_1-xTl_xFe₂As₂. Ba_1-xTl_xFe₂As₂ behaves differently from the traditionally explored Ba_1-xK_xFe₂As₂, due to the differences in structural details upon doping.

This study used a combination of crystal synthesis, temperature-dependent single-crystal X-ray diffraction, ORNL’s neutron diffraction user facility at the High Flux Isotope Reactor, and first principles calculations, as well as characterization of the electrical resistance, magnetic susceptibility, and heat capacity. 

A. S. Sefat, L. Li, H. B. Cao, M. A. McGuire, B. Sales, R. Custelcean, and D. S. Parker, “Anomalous magneto-elastic and charge doping effects in thallium-doped BaFe₂As₂,” Scientific Reports (2016).  http://dx.doi.org/10.1038/srep21660