Tetragonal-orthorhombic phase coexistence under magnetic fields in BaFe$_2$As$_2$ and Sr(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$: evidence of magnetically driven structural transition

TL;DR

This study uses synchrotron x-ray diffraction, resistivity, and heat capacity measurements to show that magnetic fields influence the structural phase coexistence in BaFe$_2$As$_2$ and Sr(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$, supporting magnetism as the driver of structural transitions.

Contribution

It provides direct experimental evidence that magnetic fields affect the structural phase coexistence, highlighting magnetism's role in driving structural and nematic transitions in 122 Fe pnictides.

Findings

Magnetic field favors tetragonal domains near $T_{AF}$.

Structural transition is sensitive to magnetic fields in the studied compounds.

Results support magnetism as the primary driver of structural and nematic transitions.

Abstract

Synchrotron x-ray diffraction experiments were performed on BaFe$_2$As$_2$ and Sr(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ single crystals as a function of temperature and applied magnetic field along the tetragonal $[1 \bar{1} 0]$ direction, complemented by electrical resistivity and specific heat experiments. For a BaFe$_2$As$_2$ crystal with spin-density-wave antiferromagnetic ordering temperature $T_{AF}=132.5$ K and onset of the orthorhombic phase at $T_{o}=137$ K, the magnetic field favors the growth of tetragonal domains that compete with orthorhombic ones for $T \gtrsim T_{AF}$. For a Sr(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ crystal with more separated transitions ($T_{AF} = 132$ K and $T_{o} = 152$ K), the crystal structure also shows significant field-dependence in a narrow temperature interval close to $T_{AF}$. These results favor magnetism as the driver of the structural and nematic transitions in 122 Fe pnictides.