$\left(\mathrm{RE}\right){\mathrm{Ni}}_{0.3}{\mathrm{Co}}_{0.7}{\mathrm{O}}_3$ $(\mathrm{RE}=\mathrm{L}\mathrm{a},\mathrm{N}\mathrm{d},\mathrm{S}\mathrm{m})$ oxides with the perovskite structure have been studied. All of these compounds show a spin-glass behavior at low temperatures. Isostructural compounds in which Ni or Co are replaced by a nonmagnetic element do not show spin-glass behavior. Therefore, simultaneous presence of Ni and Co is needed to develop competitive ferro- and antiferromagnetic interactions in the $\left(\mathrm{RE}\right){\mathrm{Ni}}_{0.3}{\mathrm{Co}}_{0.7}{\mathrm{O}}_3$ compounds. Moreover, large negative magnetoresistance below freezing temperature is observed in these samples. We conclude the existence of an electronic transfer between ${\mathrm{Ni}}^{3+}$ and ${\mathrm{Co}}^{3+}$ ions in these compounds which gives rise to ferromagnetic interactions.

• Received 28 October 1997

DOI:https://doi.org/10.1103/PhysRevLett.80.2401