Structure and properties of the kagome compound ${\text{YBaCo}}_3{\text{AlO}}_7$

Large single crystals of ${\text{YBaCo}}_3{\text{AlO}}_7$ were synthesized in a floating-zone furnace. Powder and single-crystal x-ray data confirm that the structure belongs to the swedenborgite compound family ($P{6}_{3}mc$, $a=6.28098\left(3\right)\text{Å}$, and $c=10.21200\left(5\right)\text{Å}$). ${\text{YBaCo}}_3{\text{AlO}}_7$ can reversibly absorb/desorb oxygen up to about ${\text{O}}_{7.5}$ according to thermo gravimetric measurements. The magnetic substructure of corner-sharing $\left(\text{Co}/\text{Al}\right){\text{O}}_4$ tetrahedra forms kagome-like layers, resulting in a geometric frustration. The magnetization data indicate a highly degenerate spin state with a freezing temperature of $T_f=16.8\text{K}$. Relaxation of magnetic remanence, including aging effects, frequency dependence of ac susceptibility near the transition temperature, and lack of entropy change at $T_f$ allow the conclusion that ${\text{YBaCo}}_3{\text{AlO}}_7$ is a spin glass or a “cluster glass” with randomly frozen domains of antiferromagnetically interacting spins at low temperatures. The origin of this glassy spin state is a combination of Co and Al disorder at the tetrahedral sites and the high frustration between the sites caused by kagome geometry.

Figure 1

(Color online) A perspective view of the atomic structure of ${\text{YBaCo}}_3{\text{AlO}}_7$. The smaller balls (red) are oxygen atoms, and the three different sites are marked with O1, O2, and O3. The oxygen coordination of Y and Ba are indicated with bonding lines between the metals and oxygen atoms. The kagome net is highlighted with thick lines between the Co2 (light blue) sites. The Co1 (blue) sites connect the layers. The hexagonal unit cell is marked with a thin green line, the coordination system is shown, and dashed green lines indicate the different (Co/Al)-O-(Co/Al) angles discussed in the text.

Figure 2

Thermogravimetric data of a powdered ${\text{YBaCo}}_3{\text{AlO}}_7$ single crystal. (a) Heat treatment in oxygen atmosphere (50 ml/min). The oxygen content has been calculated with the assumption that the starting compound had the ideal stoichiometry. (b) Weight changes at isothermal conditions $\left(305°\text{C}\right)$. The flowing gas (${\text{N}}_2$ or ${\text{O}}_2$) has been changed at the vertical lines. The oxygen content has been calculated as in Fig. 2a.

Figure 3

(Color online) (a) The magnetic-susceptibility $\left(\chi \right)$ is plotted as function of temperature for ${\text{YBaCo}}_3{\text{AlO}}_7$. The two data sets represent the measurement with the field $\left(H\right)$ parallel and perpendicular to the unique axis $\left(c\right)$. The inset upper right is a magnification of the low-temperature region. (b) The same data as in Fig. 3a is again plotted but $\chi$ is displayed on a reciprocal scale. The dashed line represents a Curie-Weiss fit to extract the paramagnetic moment and the Weiss constant $\left({\theta }_{CW}\right)$.

Figure 4

(Color online) (a) Magnetic relaxation with different waiting times before shutting of the outer field. A field cooling was performed down to 9 K, 7.8 K below the magnetic freezing temperature with a subsequent waiting time ($t_{\text{wait}}$–upper right) to let the disordered state solidify before the outer field was shut off, and the residual magnetic signal defined as magnetization at time $t\left(M_t\right)$ divided by the magnetization with field $\left(M_0\right)$ was recorded as function of time. The curves represent different $t_{\text{wait}}$ as indicated at upper right in the figure. The field was applied perpendicular to the unique axis $\left(c\right)$. (b) The anisotropy of the relaxation is shown in for $H$ parallel and perpendicular to $c$ using the same $t_{\text{wait}}\left(=1000\text{s}\right)$. The scale for the $H\parallel c$ data is displayed left and for the $H\perp c$ on the right side. In both graphs, only every ${10}^{\text{th}}$ data point is shown.

Figure 5

(Color online) Frequency-dependent ac susceptibility with the field parallel to the 001 axis of ${\text{YBaCo}}_3{\text{AlO}}_7$. The lower inset shows the imaginary part of the susceptibility and the upper-right inset is a power plot, where the frequency is plotted as function of the relative transition temperature. The dashed line represents a linear fit to extract the dynamic exponent $\left(zv\right)$ and the upper frequency limit $\left({\omega }_0\right)$.

Figure 6

Low-temperature specific-heat data for ${\text{YBaCo}}_3{\text{AlO}}_7$ measured in zero field. Lines are drawn to guide the eye, and $T_f$ from the magnetic data is indicated. The upper inset shows the data up to room temperature.