Anomalous magneto-elastic and charge doping effects in thallium-doped BaFe₂As₂

Within the BaFe₂As₂ crystal lattice, we partially substitute thallium for barium and report the effects of interlayer coupling in Ba_1-xTl_xFe₂As₂ crystals. We demonstrate the unusual effects of magneto-elastic coupling and charge doping in this iron-arsenide material, whereby Néel temperature rises with small x, and then falls with additional x. Specifically, we find that Néel and structural transitions in BaFe₂As₂ (T_N = T_s = 133 K) increase for x = 0.05 (T_N = 138 K, T_s  = 140 K) from magnetization, heat capacity, resistivity, and neutron diffraction measurements. Evidence from single crystal X-ray diffraction and first principles calculations attributes the stronger magnetism in x = 0.05 to magneto-elastic coupling related to the shorter intraplanar Fe-Fe bond distance. With further thallium substitution, the transition temperatures decrease for x = 0.09 (T_N = T_s = 131 K), and this is due to charge doping. We illustrate that small changes related to 3d transition-metal state can have profound effects on magnetism.