Rare-earth pyrocoboltates R 2 Co2O7 (R = Ce, Yb): Effective spin- 1 2 antiferromagnetic insulators with strong geometrical frustration

The two new rare-earth pyrocoboltates $R_2$Co₂O₇ (R = Ce, Yb) adopt the layered monoclinic Ca₂Nb₂O₇-type structure with the space group P2₁ (No. 4). Both compounds have been investigated by DC magnetic susceptibility ${\chi }_{\mathrm{dc}}\left(T\right)$, AC magnetic susceptibility ${\chi }_{\mathrm{ac}}\left(T\right)$ and isothermal magnetization $M\left(B\right)$ measurements. The observed effective magnetic moment of Ce₂Co₂O₇ indicates a contribution from the Co sublattice while that of Yb₂Co₂O₇ is mainly from the localized $4f^{13}$ electronic configuration of Yb³⁺ ion. A magnetic ordering of the antiferromagnetic-type is observed in Ce₂Co₂O₇ at $T_N\simeq$ 34 K. This ordering is accompanied by the opening of a magnetic spectral gap, $\Delta =57.7$ K ($~$ 5.0 meV) in the spin-wave spectrum and it survives in an external magnetic field of 7 T. In addition to the transition at $T_N,{\chi }_{\mathrm{ac}}\left(T\right)$ measurements reveal a dome-like feature centered at $\simeq$ 20 K in high frequency which indicates the presence of possible short-range correlation. In Yb₂Co₂O₇, a magnetic phase transition is observed below $T_N=2.94$ K, which is suppressed below 1.9 K in an external magnetic field of 3 T. In the low-T region where the crystal electric field effects can be neglected, an effective spin-1/2 state is realized in Yb₂Co₂O₇, with a Landé-g factor of 3.76 which is comparable to values observed from electron spin resonance (ESR) measurements on related systems. In addition, $M\left(B\right)$ of Yb₂Co₂O₇ shows a metamagnetic-like feature at a critical field of $H_C\approx 2.5$ T. Signatures from both compounds reveal the presence of strong magnetic frustrations with frustration index f of 9.17 and 15.12 for Ce₂Co₂O₇ and Yb₂Co₂O₇, respectively.