Manipulating the ferromagnetism in narrow-bandwidth Pr_1-xCa_xMnO₃ (0 ≤ x ≤ 0.6) by means of the Mn-Ru t_2g ferromagnetic super-exchanges

The concurrent ferromagnetic and metal-insulator transitions via the double-exchange route and electronic phase separation scenario represent the core ingredients of the physics of manganites. In this work, a Ca²⁺ and Ru⁴⁺ co-substitution of Pr³⁺ and Mn³⁺ in narrow-bandwidth and insulating PrMnO₃, namely, Pr_1-xCa_xMn_1-xRu_xO₃ (PCMRO, x ≤ 0.6), is carried out in order to investigate an alternative approach to effectively manipulate the ferromagnetism of PrMnO₃-based manganites. It is revealed that PCMRO over the whole substitution range is homogeneous solid solution with increased lattice distortion. The preference of Ru⁴⁺ valence state and the absence of Mn⁴⁺ valence state disable the Mn³⁺-Mn⁴⁺ e_g-orbital double-exchange, and the random occupation of Ru⁴⁺ in the lattice excludes the charge ordering and electronic phase separation. While all these consequences should favor antiferromagnetic insulating states, nevertheless, a high-temperature ferromagnetic transition is triggered by the co-substitution and the magnetization can reach up to ∼1.0 μ_B/f.u. at x ∼ 0.2–0.3, much bigger than the moment (<0.1 μ_B/f.u.) of Pr_1−xCa_xMnO₃ in the weak ferromagnetic insulator state. It is suggested that this strong ferromagnetism is substantially ascribed to the Mn³⁺-Ru⁴⁺ t_2g-orbital ferromagnetic super-exchange, and a simple geometric network illustration of the magnetism and electrical transport is presented