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

TL;DR

This study investigates how thallium doping in BaFe2As2 influences magnetic and structural transitions, revealing a non-monotonic behavior driven by magneto-elastic coupling and charge doping effects.

Contribution

It provides new insights into the effects of thallium substitution on magneto-elastic coupling and transition temperatures in BaFe2As2, combining experimental and theoretical approaches.

Findings

Neel and structural transition temperatures increase at low Tl doping

Transition temperatures decrease at higher Tl doping due to charge doping

Shorter Fe-Fe bonds enhance magneto-elastic coupling

Abstract

Within the BaFe2As2 crystal lattice, we partially substitute thallium for barium and report the effects of interlayer coupling in Ba1-xTlxFe2As2 crystals. We demonstrate the unusual effects of magneto-elastic coupling and charge doping in this iron-arsenide material, whereby Neel temperature rises with small x, and then falls with additional x. Specifically, we find that Neel and structural transitions in BaFe2As2 (TN =Ts= 133 K) increase for x=0.05 (TN = 138 K, Ts = 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 (TN = Ts = 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.