Thallium-doped BaFe$_{\mathrm{2}}$As$_{\mathrm{2}}$ crystals: The unusual competition between magneto-elastic coupling and charge doping

We partially substitute thallium for barium and report the effects of interlayer coupling in Ba$_{\mathrm{1-x}}$Tl$_{\mathrm{x}}$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ crystals. We demonstrate the unusual competition between magneto-elastic coupling and charge doping in an iron-arsenide material, whereby $T_{N}$ temperature rises in BaFe$_{\mathrm{2}}$As$_{\mathrm{2}}$, and then falls with additional Tl-doping. Evidence from temperature-dependent bulk thermodynamic and transport properties, and neutron diffraction results will be presented. Using LDA, we illustrate that small changes related to 3$d$ transition-metal state can have profound effects on magnetism.