Nature of the Magnetic Order in ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$

We present a detailed powder and single-crystal neutron diffraction study of the spin chain compound ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$. Below 25 K, the system orders magnetically with a modulated partially disordered antiferromagnetic structure. We give a description of the magnetic interactions in the system which is consistent with this magnetic structure. Our study also reveals that the long-range magnetic order coexists with a shorter-range order with a correlation length scale of $\sim 180\AA$ in the $ab$ plane. Remarkably, on cooling, the volume of material exhibiting short-range order increases at the expense of the long-range order.

Figure 1

Single-crystal neutron diffraction data for ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$ taken on the time-of-flight SXD instrument at ISIS [26]. Neutron intensity patterns were recorded in the ($hk0$) scattering plane at three different temperatures (a) $T=30\mathrm{K}$, (b) 20 K, and 2.2 K (not shown), above and below the magnetic ordering temperature, $T_N=25\mathrm{K}$. A refinement of the data [27] (${\chi }^2=9.18$, $R_{F^2}=18.84$, $R_{w{F}^2}=24.53$) revealed that no structural transitions occur on cooling from 30 down to 2 K; there are no significant variations in the lattice reflections or the atomic parameters, which are in good agreement with those in the literature [12, 17].

Figure 2

A selection of the results of the powder measurements on the time-of-flight GEM diffractometer at ISIS. (a) Neutron intensity of the (110) nuclear Bragg peak in ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$ at different temperatures; no variations in intensity are observed except for an increase of the background at $T=30\mathrm{K}$ due to a paramagnetic contribution. (b) Neutron diffraction pattern at 18 K (●) and refinement shown for two values of the propagation vector (red dotted line, $k_z=1.00$) and (blue dashed line, $k_z=1.01$) [20].

Figure 3

(a) Schematic of the proposed incommensurate magnetic structure for 30 unit cells along the $c$ axis with $k_z$ increased to 1.05 to emphasize the modulation of the moments. The magnitude and direction at sites in three adjacent chains are shown. (b) Magnetic exchange interactions in ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$ $J_1$ (black lines) FM; $J_2$ (red dashed lines) AFM; $J_3$ (blue dotted lines) AFM. Co II sites at (0, 0, $\frac{1}{4}$) and (0, 0, $\frac{3}{4}$) are labeled 1 and 2.

Figure 4

Transverse scans in the $ab$ plane through the (100) magnetic Bragg peak at (a) $T=5\mathrm{K}$ and (b) $T=21\mathrm{K}$. The data were collected on the 4-circle TriCS diffractometer at PSI. Each peak is fitted (full line) by the sum of a broad and a narrow Gaussian function, together with a flat background. (c) FWHM and (d) integrated intensity of the broad (△) and narrow (■) Gaussian components of the (100) peak as a function of $T$. The total integrated intensity (●) is also shown.