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

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.