Rare-earth pyrocoboltates [formula omitted]Co2O7 (R = Ce, Yb): Effective spin-[formula omitted] antiferromagnetic insulators with strong geometrical frustration

Abstract

The two new rare-earth pyrocoboltates R2Co2O7 (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 χdc(T), AC magnetic susceptibility χac(T) and isothermal magnetization M(B) measurements. The observed effective magnetic moment of Ce2Co2O7 indicates a contribution from the Co sublattice while that of Yb2Co2O7 is mainly from the localized 4f13 electronic configuration of Yb3+ ion. A magnetic ordering of the antiferromagnetic-type is observed in Ce2Co2O7 at TN≃ 34 K. This ordering is accompanied by the opening of a magnetic spectral gap, Δ=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 TN,χac(T) measurements reveal a dome-like feature centered at ≃ 20 K in high frequency which indicates the presence of possible short-range correlation. In Yb2Co2O7, a magnetic phase transition is observed below TN=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(B) of Yb2Co2O7 shows a metamagnetic-like feature at a critical field of HC≈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.