Comparative study of conductive delafossites with and without frustrated spins on a triangular lattice, PdM O2 (M = Cr; Co)

We report a comparative study of the conductive oxides PdMO2 (M = Cr, Co) consisting of a layered structure with a triangular lattice of M3+ ions: PdCrO2 (S = 3 = 2) is magnetically active showing an antiferromagnetic (AF) transition at TN ' 37.5 K, whereas PdCoO2 (S = 0) is not magnetically active. Both compounds exhibit metallic conductivity down to low temperatures, but the temperature dependence of the resistivity is quite different reflecting the difference in their magnetic properties. Especially, the resistivity of PdCrO2 exhibits strong sub-linear temperature dependence in a wide temperature range above TN. Such dependence is markedly different from that of ordinary magnetic metals with non-frustrated localized spin moments. The estimated entropy at TN is substantially smaller than that expected for a system with S = 3/2, suggesting that the spin correlations still remain much above TN. These facts indicate that it is the short-range correlations of frustrated spins that leads to the unusual sub-linear temperature dependence of the resistivity.