Metal-insulator transition, giant negative magnetoresistance, and ferromagnetism in LaCo_1-yNi_yO₃

We have investigated the transport and magnetic properties of the perovskite LaCo_1-yNi_yO₃, an alloy of LaCoO ₃ (a semiconductor that exhibits spin-state transitions) and LaNiO₃ (a paramagnetic metal). The metal-insulator transition (MIT) was found to occur at y = 0.40. On the insulating side of the transition the conductivity obeys Mott variable range hopping with a characteristic temperature (T₀) that varies with y in a manner consistent with the predictions of the scaling theory of electron localization. On the metallic side the low temperature conductivity (down to 0.35 K) varies as T ^1/2 due to the effects of electron-electron interaction in the presence of disorder. The composition dependence of the low-temperature conductivity in the critical region fits the scaling theory of electron localization with a conductivity critical exponent close to unity, consistent with the scaling of T₀ in the insulating phase. A large negative magnetoresistance is observed (up to 70% in 17 T) which increases monotonically with decreasing temperature and is smoothly decreased through the MIT. The magnetic properties show that doping LaCoO₃ with Ni leads to a rapid destruction of the low spin-state for Co³⁺ ions, followed by the onset of distinct ferromagnetic interactions at higher Ni content. Similar to La_1-xSr_xCoO₃, the system shows a smooth evolution from spin-glass to ferromagnetic ground states, which is interpreted in terms of the formation of ferromagnetic clusters. In contrast to La _1-xSr_xCoO₃ further doping does not lead to a bulk ferromagnetlike state with a large T_C, despite the clear existence of ferromagnetic interactions. We suggest that this is due to a limitation of the strength of the ferromagnetic interactions, which could be related to the fact that Ni rich clusters are not thermodynamically stable. The ferromagnetic clusters in LaCo_1-yNi_yO₃ do not percolate with increasing y explaining the lack of a high-T_C ferromagnetic state and the fact that the MIT is a simple Mott-Anderson transition rather than a percolation transition. Finally, in contrast to previous works (which focused on a single composition) we find no clear correlation between freezing temperature and the onset of magnetoresistance.