Susceptibility Anisotropy in an Iron Arsenide Superconductor Revealed by X-Ray Diffraction in Pulsed Magnetic Fields

In addition to unconventional high-$T_c$ superconductivity, the iron arsenides exhibit strong magnetoelastic coupling and a notable electronic anisotropy within the $a-b$ plane. We relate these properties by studying underdoped $\mathrm{Ba}({\mathrm{Fe}}_{\mathbf{1}\mathrm{\text{−}}\mathbf{x}}{\mathrm{Co}}_{\mathbf{x}}{)}_{\mathbf{2}}{\mathrm{As}}_{\mathbf{2}}$ by x-ray diffraction in pulsed magnetic fields up to 27.5 T. We exploit magnetic detwinning effects to demonstrate anisotropy in the in-plane susceptibility, which develops at the structural phase transition despite the absence of magnetic order. The degree of detwinning increases smoothly with decreasing temperature, and a single-domain condition is realized over a range of field and temperature. At low temperatures we observe an activated behavior, with a large hysteretic remnant effect. Detwinning was not observed within the superconducting phase for accessible magnetic fields.

Figure 1

(a) High-resolution x-ray diffraction shows the (2, 2, 0) Bragg peak splitting at the structural phase transition ($T_S\sim 70\mathrm{K}$). (b) Our $x=0.045$ single crystal, mounted on a sapphire post. We perform diffraction in the horizontal [HHL] plane, in transmission through the c-face of our crystal. We apply magnetic fields along the vertical $[1\overline{1}0]$ direction, coaxial with the sapphire post. (c) Orthorhombic strain, defined as $2(a-b)/(a+b)$, as a function of temperature showing the relative difference in the lattice constants below $T_S$. The spin density wave transition $T_m$ is also indicated for reference. (d) Schematic of twin domains within the $a-b$ plane. Orthorhombic strain and the density of twinning are exaggerated for effect. The highly anisotropic magnetic ordering pattern of Fe spins is also illustrated.

Figure 2

Evolution of twin reflections at $T=30\mathrm{K}$, while magnetic field is pulsed to 27.5 T. Top: The applied magnetic field as a function of time. The square pulses denote the 20 kHz frame rate collection of our one-dimensional strip detector, which generates a series of $30\mathrm{\text{−}}\mu \mathrm{s}$ exposures of the Bragg scattering along arcs in reciprocal space. Bottom: Arcs collected at both twin peak positions and projected onto the tetragonal ($H$, 2, 0) line in reciprocal space. The peak at higher momentum transfer is completely suppressed at high field, with the intensity transferred to the other peak. This indicates a complete elimination of the twin domains with magnetic field applied along the longer $a$ axis, in favor of the domains with magnetic field along the shorter $b$ axis.

Figure 3

Suppression of the $B\parallel a$ twin domains, as a function of temperature and applied magnetic field. Here, 100% suppression signifies a completely detwinned volume, while 0% suppression corresponds to a twin population balance that is completely unchanged by applied magnetic fields. Left: Data collected on the [($2+\Delta$), 2, 0] Bragg feature, after cooling in zero field from 80 K and pulsing magnetic fields to 27.5 T. Data for rising and falling fields are shown, with time during the pulse increasing along the $x$ axis. Right: Cuts through the same data, highlighting the effect at the highest measured peak fields, and also the residual remnant detwinning after the pulse is over. The temperatures of the structural ($T_s$), magnetic ($T_m$), and superconducting ($T_c$) phase transitions are indicated. Detwinning in the paramagnetic phase is clear.