Effect of Uniaxial Strain on the Structural and Magnetic Phase Transitions in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$

We report neutron scattering experiments probing the influence of uniaxial strain on both the magnetic and structural order parameters in the parent iron pnictide compound, ${\mathrm{BaFe}}_2{\mathrm{As}}_2$. Our data show that modest strain fields along the in-plane orthorhombic $b$ axis can affect significant changes in phase behavior simultaneous to the removal of structural twinning effects. As a result, we demonstrate in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ samples detwinned via uniaxial strain that the in-plane $C_4$ symmetry is broken by both the structural lattice distortion and long-range spin ordering at temperatures far above the nominal (strain-free) phase transition temperatures. Surprising changes in the magnetic order parameter of this system under relatively small strain fields also suggest the inherent presence of magnetic domains fluctuating above the strain-free ordering temperature in this material.

Figure 1

(a) Picture of the detwinning mount used for neutron scattering experiments. (b) Reference data (from Ref. 8) refit with a single Gaussian fit quantifying the structural phase transition in strain-free Ba-122. (c) Single Gaussian fits to radial scans through the (2, 0, 0) reflection showing a broadening below $T_S$ using data from Ref. 8.

Figure 2

Peak widths and lattice parameters determined from Gaussian fits to the $Q=(2,0,0)$ nuclear reflection plotted in (a),(b), respectively. Fit parameters show the evolution of the phase behavior as pressure along the $b$ axis is increased incrementally in the same crystal. The insets show the global picture of the phase transition. Radial scans through $Q=(2,0,0)$ both above and below $T_S$, with (c) 0 MPa and (d) 0.7 MPa of uniaxial pressure applied. Horizontal bars show the FWHM of the Gaussian fits.

Figure 3

(a) Intensity ($I\propto M^2$) at the $Q=(1,0,3)$ AF peak position normalized by the (0, 0, 4) nuclear peak plotted as a function of temperature for different pressures applied along the $b$ axis. The inset shows the magnetic phase transition over the entire temperature range. Dashed lines are power-law fits, as described in the text. (b) Pressure dependence of $T_{\mathrm{AF}}$, $T_S$, and $T_{\mathrm{AF},\mathrm{onset}}$. Open symbols denote measurements on a separate crystal at an intermediate pressure. (c) Scan $Q=(1,0,3)$ at $T=142\mathrm{K}$ with nonmagnetic background subtracted. Horizontal bar indicates the FWHM of the (1, 0, 3) reflection measured at 3 K. (d) HB-1A data measuring the magnetic peak intensity (circles) at $Q=(1,0,3)$ under an unknown $P>1\mathrm{MPa}$. Squares show the $a$-axis lattice parameter measured under the same strain at the $Q=(2,0,0)$ nuclear position. Triangles show the same $a$-axis measurement with the strain released. Solid lines are mean-field fits.