Anisotropic magnetoelastic coupling in single-crystalline CeFeAsO as seen via high-resolution x-ray diffraction

Single-crystal synchrotron x-ray diffraction studies of CeFeAsO reveal strong anisotropy in the charge-correlation lengths along or perpendicular to the in-plane antiferromagnetic (AFM) wave vector at low temperatures, indicating an anisotropic two-dimensional magnetoelastic coupling. The high-resolution setup allows to distinctly monitor each of the twin domains by virtue of a finite misfit angle between them that follows the order parameter. In addition, we find that the in-plane correlations, above the orthorhombic (O)-to-tetragonal (T) transition, are shorter than those in each of the domains in the AFM phase, indicating a distribution of the in-plane lattice constants. This strongly suggests that the phase above the structural O-to-T transition is virtually T with strong O-T fluctuations that are probably induced by spin fluctuations.

Figure 1

Schematic illustration of the formation of twin domains when the T structure transfers into the O phase in CeFeAsO. The O distortion in pnictides proceeds by shearing the T planar square into two rhombuses with angles $\frac{\pi }{2}\pm \delta$ (preserving the length of the square), creating twin domains. $\delta$ is the shearing angle. Here we just show one rhombus for clarity. $J_1$ and $J_2$ represent the NN and NNN AFM exchange interactions, respectively. $a_{\mathrm{T}}$ (T symmetry), and $a_{\mathrm{O}}$ and $b_{\mathrm{O}}$ (O symmetry) are the in-plane lattice constants. The arrows on the Fe ions represent the spins of iron ions.

Figure 2

(a) Illustration of the x-ray diffraction setup at the 6-ID-B beamline at APS at Argonne National Laboratory. The $\chi$ angle indicates the directions for tilting the CeFeAsO single crystal with respect to Q. (b) Temperature evolution of the $\chi$ angle of the $h$ and $k$ domains. The vertical dashed line implies the structural O-T transition temperature $T_{\mathrm{S}}$. The horizontal dashed lines indicate the $\chi$-angle difference between the $h$ and $k$ domains at 80 K. The solid lines are guides to the eye.

Figure 3

Temperature evolution of (a) the $h$ domain and (b) the $k$ domain. The solid line in (a) and the lower solid line in (b) indicate the Lorentzian-fit center of the longitudinal $h$ scans at $\mathbf{Q}={\left(h05\right)}_{\mathrm{O}}$. The upper solid line in (b) indicates the Lorentzian-fit center of the longitudinal $k$ scans at $\mathbf{Q}={\left(0k5\right)}_{\mathrm{O}}$. (c) Temperature evolutions of the integrated intensities of the longitudinal $h$ and $k$ scans at $\mathbf{Q}={\left(h05\right)}_{\mathrm{O}}$ and ${\left(0k5\right)}_{\mathrm{O}}$, respectively, and (d) the O distortion (squares) (right) in the crystalline $ab$ plane, namely, $S\equiv (a_{\mathrm{O}}-b_{\mathrm{O}})/(a_{\mathrm{O}}+b_{\mathrm{O}})$, and the domain-angle misfit (stars) (left), namely, $\Delta \chi ={\chi }_k-{\chi }_h$. (a${}^{\prime }$) The $\sim$2 K temperature hysteresis effect of the Lorentzian-fit centers indicative of a first-order O-T structural transition. Points A and B in (c) indicate the kink positions probably associated with the AFM transition, as described in the test. The vertical dashed lines in (a)–(d) indicate the structural O-T transition temperature $T_{\mathrm{S}}$. The solid lines in (a)–(c) and (a${}^{\prime }$) are guides to the eye.

Figure 4

Temperature evolutions of (a) the FWHM $\kappa$ of the longitudinal $h$ and $k$ scans at $\mathbf{Q}={\left(h05\right)}_{\mathrm{O}}$ and ${\left(0k5\right)}_{\mathrm{O}}$, respectively, and (b) the resistivity measurement (left) and its first derivative (right) indicate the temperatures of the AFM transition of iron ions ($T_{\mathrm{N}}^{\mathrm{Fe}}$) as well as the O-T structural transition ($T_{\mathrm{S}}$). (a${}^{\prime }$) shows the longitudinal $h$ scans at $\mathbf{Q}={\left(h05\right)}_{\mathrm{O}}$ and at temperatures of 80, 145, and 161 K. To estimate the instrumental resolution effect, we also showed the corresponding $l$ scan of the ${\left(004\right)}_{\mathrm{O}}$ reflection at 80 K. The Q values and observed intensities were shifted and normalized, respectively, for comparison. The vertical dashed lines in (a) and (b) indicate the AFM and the O-T structural transition temperatures $T_{\mathrm{N}}$ and $T_{\mathrm{S}}$, respectively. The solid lines in (a) are guides to the eye.