Neutron-Diffraction Measurements of Magnetic Order and a Structural Transition in the Parent ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ Compound of FeAs-Based High-Temperature Superconductors

The recent discovery of superconductivity in $(\mathrm{Ba},\mathrm{K}){\mathrm{Fe}}_2{\mathrm{As}}_2$, which crystallizes in the ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$ (122) structure as compared with the LnFeAsO (Ln is lanthanide) systems that possess the ZrCuSiAs (1111) structure, demonstrates the exciting potential of the FeAs-based materials for high-$T_C$ superconductivity. Here we report neutron diffraction studies that show a tetragonal-to-orthorhombic distortion associated with the onset of $\mathbf{q}=(101)$ antiferromagnetic order in ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, with a saturation moment $0.87(3){\mu }_B$ per Fe that is orientated along the longer $a$ axis of the $ab$ planes. The simultaneous first-order structural and magnetic transition is in contrast with the separated transitions previously reported in the 1111-type materials. The orientational relation between magnetic alignment and lattice distortion supports a multiorbital nature for the magnetic order.

Figure 1

The resistivity of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ as a function of temperature, measured while cooling then warming, shows a sharp drop at $T_S\sim 142\mathrm{K}$. The resistivity anomaly is associated with a first-order structural transition, as indicated by the hysteresis in neutron diffraction intensity at diffraction angle $2\theta =95.8°$. Inset: Neutron diffraction spectra measured above and below the structural transition, showing the splitting of the peak below $T_S$.

Figure 2

Neutron powder diffraction spectra at (a) 175 and (b) 5 K. The high-temperature spectrum is refined with the tetragonal $I4/mmm$ space group; the low temperature one with the orthorhombic $Fmmm$ space group together with the magnetic structure shown in Fig. 3. The (220) Bragg peak in (a) is split into the (400) and (040) in (b) by the orthorhombic structural distortion. Magnetic peaks at 5 K are highlighted in the inset of (b).

Figure 3

Magnetic and crystal structures of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$. The magnetic Bragg profiles of two magnetic models are shown in two rows, respectively. The moment orientation in the two Fe layers of the orthorhombic $Fmmm$ unit cell is highlighted on the side. The measured magnetic Bragg peak is well represented by the (121) index (bottom), not the (211) (top). Therefore, the magnetic wave vector is (101); namely, the Fe magnetic moments are aligned antiferromagnetically along the $a$ and $c$ axes, and ferromagnetically along the $b$ axis (bottom).

Figure 4

Magnetic Bragg peak (101) as a function of temperature. The solid line represents the least-squares fit to mean-field theory for the squared order parameter.