Magnetic ordering and structural distortion in a PrFeAsO single crystal studied by neutron and x-ray scattering

We report the magnetic ordering and structural distortion in PrFeAsO crystals, the basis compound for one of the oxypnictide superconductors, using high-resolution x-ray diffraction, neutron diffraction, and x-ray resonant magnetic scattering (XRMS). We find the structural tetragonal-to-orthorhombic phase transition at $T_{\mathrm{S}}=147\mathrm{K}$, the AFM phase transition of the Fe moments at $T_{\mathrm{Fe}}=72\mathrm{K}$, and the Pr AFM phase transition at $T_{\mathrm{Pr}}=21\mathrm{K}$. Combined high-resolution neutron diffraction and XRMS show unambiguously that the Pr moments point parallel to the longer orthorhombic $\mathit{a}$ axis and order antiferromagnetically along the $\mathit{a}$ axis but ferromagnetically along the $\mathit{b}$ and $\mathit{c}$ directions in the stripelike AFM order. The temperature-dependent magnetic order parameter of the Pr moments shows no evidence for a reorientation of moments.

Figure 1

(a) High-resolution x-ray diffraction scans along the $[H,0,0]$ direction through $(2,0,7)$ charge Bragg peak for selected temperatures. The lines present the fitted curves using Lorentzian-squared line shapes. (b) Orthorhombic distortion, determined from fits to the $(2,0,7)$ and ${(0,2,7)}^{\prime }$ charge Bragg peaks, as a function of temperature upon cooling and warming. The blue bar indicates the tetragonal-to-orthorhombic structural transition temperature ($T_{\mathrm{S}}$). Error bars are smaller than symbols and represent one standard deviation.

Figure 2

Neutron diffraction rocking scans through (a) the $(1,0,1)$ magnetic Bragg peak and (c) the $(1,0,0)$ magnetic Bragg peak measured at selected temperatures. Temperature evolution of magnetic Bragg peak intensity measured (b) at $(1,0,1)$ magnetic Bragg peak and (d) at $(1,0,0)$ magnetic Bragg peak. Fe ordering temperature ($T_{\mathrm{Fe}}$) and Pr ordering temperature ($T_{\mathrm{Pr}}$) are marked with a green bar and orange bars, respectively.

Figure 3

High-resolution neutron diffraction scans along $[H,0,0]$ through (a), (b) the $(2,0,6)$ nuclear Bragg peak measured with the second harmonic of the incident neutrons ($\frac{\lambda }{2}$) and (c), (d) the $(1,0,3)$ and ${(0,1,3)}^{\prime }$ magnetic Bragg peaks measured with the primary neutrons ($\lambda$). Open symbols are data measured at $T=155\mathrm{K}$. Filled circles are data taken at $T=30\mathrm{K}$ in (a), (b) and $T=2.5\mathrm{K}$ in (c), (d). The magnetic peaks are fitted with one or two Gaussian line shapes. Resulting fits are shown with solid lines. Dashed lines present two-Gaussian fits.

Figure 4

(a) Energy scans through the $(1,0,5)$ magnetic peak (filled circles) in the $\sigma \text{−}\pi$ scattering geometry at $T=3\mathrm{K}$ and the measured fluorescence (open squares) under the same experimental conditions without a polarization analyzer. (b) Distribution of structural Bragg peaks at $T=3\mathrm{K}$ measured along $[H,0,0]$ after the alignment is optimized for $\mathbf{Q}=(2,0,5)$ (filled squares) and ${(0,2,5)}^{\prime }$ Bragg peaks (open squares). (c) Magnetic Bragg peaks at $T=3\mathrm{K}$ measured along $[H,0,0]$ at positions corresponding to the $(2,0,5)$ and ${(0,2,5)}^{\prime }$ Bragg peaks. A magnetic Bragg peak appears only at $\mathbf{Q}=(1,0,5)$ (filled circles) but not at $\mathbf{Q}={(0,1,5)}^{\prime }$ (open circles).

Figure 5

Azimuthal-angle ($0^{\circ }\le \mathrm{\Psi }\le {40}^{\circ }$) dependence of the normalized magnetic Bragg peak intensity measured at (a) $\mathit{Q}=(1,0,5)$ and (b) ${(0,1,5)}^{\prime }$ at $T=3\mathrm{K}$. Inset shows the schematic diagram of our experimental setup for the azimuthal-angle scans. Lines present calculations using the sine-squared function. (c) Temperature-dependent XRMS signals at $\mathit{Q}=(1,0,5)$ (filled symbols) and ${(0,1,5)}^{\prime }$ (open symbols) with $\mathrm{\Psi }={20}^{\circ }$. The lines present guides to the eye. (d) Proposed magnetic structure at $T=3\mathrm{K}$ with Pr moments pointing along the longer orthorhombic $\mathit{a}$ axis.