Magnetic and Transport Properties of La₀.₇Sr₀.₃Co₁₋yMnyO₃

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While both La₀.₇Sr₀.₃MnO₃ and La₀.₇Sr₀.₃CoO₃ are ferromagnets with metallic conductivity below Tc (360K and 220K, respectively), partial substitutions of Co by Mn in La₀.₇Sr₀.₃Co₁₋yMnyO₃ (y < 0.1) drastically suppress the ferromagnetic long-range order pre-established by Co-O-Co double exchange and tune the conductivity towards insulating behavior. Since Mn-O-Mn double-exchange interactions are avoidable at low Mn-substitution levels, the deterioration of ferromagnetism and conductivity thus provides evidence for no Mn-O-Co double exchange (but antiferromagnetic superexchange) in the present system. At y = 0.1, the ferromagnetism is no longer observed; the system becomes an insulating spin-glass with Tg≈ 62K as estimated from the ac-susceptibility data using the conventional critical slowing-down scaling law. With further substitution (y≥ 0.3), the ferromagnetism is recovered (Tc = 165K for y = 0.3 and 200K for y = 0.5) while the resistivity continues increasing and exhibits insulating behavior. These results indicate that the substitution is not simply a mixture of La₀.₇Sr₀.₃MnO₃ and La₀.₇Sr₀.₃CoO₃, but produces a more complicated scenario of spin states, interactions and disorder.