Incommensurate magnetic ground state revealed by resonant x-ray scattering in the frustrated spin system ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$

We have performed a resonant x-ray scattering study at the Co pre-$K$ edge on a single crystal of ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$. The measurements reveal an abrupt transition to a magnetically ordered state immediately below $T_N=25\mathrm{K}$, with a magnetic correlation length in excess of $5500\mathrm{\AA }$ along the $c$ axis chains. There is no evidence for modifications to the ${\mathrm{Co}}^{3+}$ spin state. A temperature dependent modulation in the magnetic order along the $c$ axis and an unusual decrease in the magnetic correlation lengths on cooling are observed. The results are compatible with the onset of a partially disordered antiferromagnetic structure in ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$.

Figure 1

(Color online) Azimuth scan on the (3 2 0) reflection collected in the $\sigma \text{−}\pi$ channel at $E=7.707\mathrm{keV}$ and $T=4.2\mathrm{K}$. Azimuth reference (0 0 1). The dotted line shows a purely $E1\text{−}E1$ fit and the dashed line is a fit including $E1\text{−}E1$ and $E2\text{−}E2$ contributions. The inset shows the geometry of the scattering and the fixed reference system $u_1$, $u_2$, and $u_3$ used.

Figure 2

Evolution of the (3 2 0) magnetic reflection (black symbols) versus $T$ taken at $\psi =0$ and $E=7.707\mathrm{keV}$. Panels contain (a) the integrated intensity, (b) the FWHM along $\vec{c}$ ($L$ scan), (c) the FWHM in the $ab$ plane ($HK$ scan), and (d) the evolution of the center position in the $L$ direction. The $T$ dependence of the corresponding parameters for the (3 3 0) charge peak (gray symbols) is shown for comparison.

Figure 3

Photon-energy dependence around the Co K-absorption edge of the intensity of the (3 2 0) magnetic reflection. Data were collected in the $\sigma \text{−}\pi$ channel at $T=4.5$ (black circles) and $20\mathrm{K}$ (gray diamonds). Corrections for self-absorption have been applied. The absorption coefficient given by the fluorescence yield is shown by the dotted line.