Character of the structural and magnetic phase transitions in the parent and electron doped BaFe2As2 compounds

TL;DR

This study investigates the structural and magnetic phase transitions in BaFe2As2 and doped variants, revealing a two-step transition process and the evolution of transition characteristics with doping, supported by experimental and theoretical analysis.

Contribution

It provides detailed experimental evidence and a mean-field theoretical model explaining the coupled structural and magnetic transitions in BaFe2As2 and doped compounds.

Findings

Structural transition is second-order at T_S=134.5 K

Magnetic transition is first-order at T_N=133.75 K

Transitions evolve towards split second-order with doping

Abstract

We present a combined high-resolution x-ray diffraction and x-ray resonant magnetic scattering (XRMS) study of as-grown BaFe2As2. The structural/magnetic transitions must be described as a two-step process. At T_S = 134.5 K we observe the onset of a second-order structural transition from the high-temperature paramagnetic tetragonal structure to a paramagnetic orthorhombic phase, followed by a discontinuous step in the structural order parameter that is coincident with a first-order antiferromagnetic (AFM) transition at T_N = 133.75 K. These data, together with detailed high-resolution x-ray studies of the structural transition in lightly doped Ba(Fe{1-x}Co{x})2As2 and Ba(Fe{1-x}Rh{x})2As2 compounds, show that the structural and AFM transitions do, in fact, occur at slightly different temperatures in the parent BaFe2As2 compound, and evolve towards split secondorder transitions as the doping concentration is increased. We estimate the composition for the tricritical point for Co-doping and employ a mean-field approach to show that our measurements can be explained by the inclusion of an anharmonic term in the elastic free energy and magneto-elastic coupling in the form of an emergent Ising-nematic degree of freedom.