Anisotropic magnetoelastic coupling in iron arsenide superconductors: an x-ray diffraction study in high magnetic field

We report high-resolution single crystal x-ray diffraction measurements of underdoped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ in pulsed magnetic fields as high as 28 Tesla. Our direct measurements confirm earlier reports of strong and highly anisotropic magnetoelastic coupling in iron arsenides. We observe magnetic field induced de-twinning of orthorhombic samples, and characterize the magnitude of the effect as a function of temperature and field. We identify a range of field and temperature where samples can be 100\% de-twinned by magnetic fields less than 30 Tesla. The effect shows a notable insensitivity to SDW ordering, but varies rapidly in the vicinity of the superconducting transition.