Metal-insulator transition, giant negative magnetoresistance, and ferromagnetism inLaCo1−yNiyO3

Abstract

We have investigated the transport and magnetic properties of the perovskite ${\mathrm{LaCo}}_{1\ensuremath{-}y}{\mathrm{Ni}}_{y}{\mathrm{O}}_{3},$ an alloy of ${\mathrm{LaCoO}}_{3}$ (a semiconductor that exhibits spin-state transitions) and ${\mathrm{LaNiO}}_{3}$ (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 ${(T}_{0})$ 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 ${T}_{0}$ 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 ${\mathrm{LaCoO}}_{3}$ with Ni leads to a rapid destruction of the low spin-state for ${\mathrm{Co}}^{3+}$ ions, followed by the onset of distinct ferromagnetic interactions at higher Ni content. Similar to ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3},$ 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 ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{3}$ 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 ${\mathrm{LaCo}}_{1\ensuremath{-}y}{\mathrm{Ni}}_{y}{\mathrm{O}}_{3}$ 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.