Electronic and magnetic properties of the kagome systems ${\text{YBaCo}}_4{\text{O}}_7$ and ${\text{YBaCo}}_{3}M{\text{O}}_7$ $\left(M=\text{Al},\text{Fe}\right)$

We present a combined experimental and theoretical x-ray absorption spectroscopy study of the class of cobaltates ${\text{YBaCo}}_4{\text{O}}_7$ and ${\text{YBaCo}}_{3}M{\text{O}}_7$ $\left(M=\text{Al},\text{Fe}\right)$. The focus is on the local electronic and magnetic properties of the transition metal ions in these geometrically frustrated kagomé compounds. For the mixed valence cobaltate ${\text{YBaCo}}_4{\text{O}}_7$, both the ${\text{Co}}^{2+}$ and ${\text{Co}}^{3+}$ are found to be in the high-spin state. The stability of these high-spin states in tetrahedral coordination is compared with those in the more studied case of octahedral coordination. For the compound ${\text{YBaCo}}_3{\text{FeO}}_7$, we find exclusively ${\text{Co}}^{2+}$ and ${\text{Fe}}^{3+}$ as charge states.

Figure 1

(Color online) Experimental $\text{Co}{L}_{2,3}$ XAS spectra of ${\text{YBaCo}}_3{\text{AlO}}_7$, ${\text{YBaCo}}_3{\text{FeO}}_7$, and ${\text{YBaCo}}_4{\text{O}}_7$. The ${\text{YBaZn}}_3{\text{AlO}}_7$ spectrum shown at the bottom serves as a reference for the $\text{Ba}{M}_{4,5}$ edges. The inset depicts the oxygen $K$ XAS spectrum of ${\text{YBaCo}}_4{\text{O}}_7$ collected by the total electron yield (TEY) and fluorescence yield (FY) methods.

Figure 2

(Color online) Experimental $\text{Co}{L}_{2,3}$ XAS spectra of ${\text{YBaCo}}_3{\text{FeO}}_7$, ${\text{YBaCo}}_3{\text{AlO}}_7$, and ${\text{YBaCo}}_4{\text{O}}_7$ after subtraction of the $\text{Ba}{M}_{4,5}$ white lines. The theoretical calculation for ${\text{Co}}^{2+}$ in tetrahedral symmetry is to be compared with the experimental spectra of ${\text{YBaCo}}_3{\text{FeO}}_7$ and ${\text{YBaCo}}_3{\text{AlO}}_7$. The bottom curve depicts the weighted sum of a calculation for ${\text{Co}}^{2+}$ and ${\text{Co}}^{3+}$ as a simulation for ${\text{YBaCo}}_4{\text{O}}_7$.

Figure 3

(Color online) XAS spectrum on the $\text{Fe}{L}_{2,3}$ edges of ${\text{YBaCo}}_3{\text{FeO}}_7$. Reference spectra of ${\text{Fe}}^{2+}$ in $\text{FeO}\left(O_h\right)$ and ${\text{Fe}}^{3+}$ in ${\text{Fe}}_2{\text{O}}_3\left(O_h\right)$ are also given. The spectrum at the bottom is a simulation for ${\text{Fe}}^{3+}$ in $T_d$ symmetry.

Figure 4

(Color online) Experimental $\text{Co}{L}_{2,3}$ XAS spectrum of the ${\text{Co}}^{3+}$ ion in ${\text{YBaCo}}_4{\text{O}}_7$ as extracted from the subtraction of the ${\text{YBaCo}}_4{\text{O}}_7$ spectrum with the ${\text{YBaCo}}_3{\text{AlO}}_7$ one in a 4:3 weight ratio. Theoretical simulations for the ${\text{Co}}^{3+}$ ion in $T_d$ symmetry are also included for the high-spin $\left(S=2\right)$ and intermediate-spin $\left(S=1\right)$ state cases.

Figure 5

(Color online) Energy level diagrams for ${\text{Co}}^{2+}$ in a tetrahedral crystal field (panel (a), $T_d$, negative $10Dq$) and octahedral crystal field [panel (b), $O_h$, positive $10Dq$]. The full-multiplet calculations include covalency due to $\text{O}2p\text{-Co}3d$ hybridization.

Figure 6

(Color online) Energy level diagrams for ${\text{Co}}^{3+}$ in a tetrahedral crystal field [panel (a), $T_d$, negative $10Dq$] and octahedral crystal field [panel (b), $O_h$, positive $10Dq$]. The full-multiplet calculations include covalency due to $\text{O}2p\text{-Co}3d$ hybridization.