We investigated the structural, magnetic, transport, and thermoelectric properties of polycrystalline delafossite oxides, $\mathrm{Cu}{\mathrm{Cr}}_{1-x}{\mathrm{Mg}}_x{\mathrm{O}}_2$ $(0⩽x⩽0.04)$. We found that, with the substitution of ${\mathrm{Mg}}^{2+}$ ions at the Cr site, the resistivity and Seebeck coefficient drastically decrease without a change of Néel temperature $(T_N=26\mathrm{K})$. In contrast to the $x$-independent $T_N$, the deduced Curie-Weiss temperature linearly varies from $-170\text{to}-100\mathrm{K}$ with the increase of $x$. A crossover occurs in the conductivity from a thermal activation behavior to variable range-hopping one at $T_{\mathit{\text{cross}}}$, around which the temperature dependence of magnetization starts to deviate from the Curie-Weiss form. Furthermore, the negative magnetoresistance effect for $\mathrm{Cu}{\mathrm{Cr}}_{0.96}{\mathrm{Mg}}_{0.04}{\mathrm{O}}_2$ compound is observed only in the variable range-hopping regime and is enhanced around $T_N$. These results clearly indicate the coupling between the doped hole and the local spin at the Cr site.

• Received 9 March 2005

DOI:https://doi.org/10.1103/PhysRevB.72.144403