Valence, spin, and orbital state of Co ions in one-dimensional ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$: An x-ray absorption and magnetic circular dichroism study

We have investigated the valence, spin, and orbital state of the Co ions in the one-dimensional cobaltate ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$ using x-ray absorption and x-ray magnetic circular dichroism at the $\mathrm{Co}\text{−}{L}_{2,3}$ edges. The Co ions at both the octahedral ${\mathrm{Co}}_{\mathrm{oct}}$ and trigonal ${\mathrm{Co}}_{\mathrm{trig}}$ sites are found to be in a $3+$ state. From the analysis of the dichroism we established a low-spin state for the ${\mathrm{Co}}_{\mathrm{oct}}$ and a high-spin state with an anomalously large orbital moment of $1.7{\mu }_B$ at the ${\mathrm{Co}}_{\mathrm{trig}}^{3+}$ ions. This large orbital moment along the $c$-axis chain and the unusually large magnetocrystalline anisotropy can be traced back to the double occupancy of the $d_2$ orbital in trigonal crystal field.

Figure 1

$\mathrm{Co}\text{−}{L}_{2,3}$ XAS spectra of ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$, $\mathrm{CoO}$, and ${\mathrm{EuCoO}}_3$.

Figure 2

(Color online) (a) Measured soft-x-ray absorption spectra with parallel (${\mu }^{+}$, red solid line) and antiparallel (${\mu }^{-}$, black dashed line) alignment between photon spin and magnetic field, together with the difference spectrum (${\mu }^{+}-{\mu }^{-}$, blue dashed-dotted line). (b) Simulated sum spectra assuming a doubly occupied $d_2$ orbital for the ${\mathrm{Co}}_{\mathrm{trig}}$ and low-spin (LS) ${\mathrm{Co}}_{\mathrm{oct}}$ ions; (c) and (d) Contribution of the ${\mathrm{Co}}_{\mathrm{trig}}$ and ${\mathrm{Co}}_{\mathrm{oct}}$ ions to the simulated sum spectra.

Figure 3

(Color online) (a) Measured soft-x-ray magnetic circular dichroism spectrum (XMCD, ${\mu }^{+}-{\mu }^{-}$); (b) simulated XMCD spectrum assuming a doubly occupied $d_0$ orbital for the ${\mathrm{Co}}_{\mathrm{trig}}$ and low-spin (LS) ${\mathrm{Co}}_{\mathrm{oct}}$ ions.

Figure 4

(Color online) Total-energy level diagram for the ${\mathrm{Co}}_{\mathrm{trig}}$ ion as a function of the exchange field $H_{ex}$ along the $z$ direction ($c$ axis, solid black lines) and along the $x$ direction (perpendicular to the $c$ axis, dashed blue lines), with (left-hand panel) the $d_2$ and (right-hand panel) the $d_0$ orbital doubly occupied.