Double perovskites R2NiMnO6 with small R3+ cations: Magnetic interactions tuned by R3+ ionic radius and the role of orbital ordering

Research on double perovskites R₂NiMnO₆ with small R³⁺ cations encounters challenges in following aspects: Firstly, stabilizing significant structural distortions is relatively difficult, and minimizing defects such as mixed oxidation states and antisite disorder also poses challenges owing to thermodynamic considerations. Secondly, the theoretically predicted ↑↑↓↓ spin alignment, arising from competition between nearest-neighboring ferromagnetic (NN FM) and next-nearest-neighboring antiferromagnetic (NNN AFM) interactions, remains controversial. In this study, double perovskite Lu_2-xY_xNiMnO₆ samples were synthesized via a hydrothermal method. Compared to other synthesis methods, the hydrothermal method promotes the stabilization of Ni²⁺ and Mn⁴⁺ ions, and stabilizes structural distortion for R₂NiMnO₆ with small R³⁺ cations. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images revealed a rock-salt arrangement of Ni²⁺ and Mn⁴⁺ ions. By the variation in the R³⁺ ionic radius, the magnetic interactions were continuously tuned. Compared to RMnO₃ and R₂CoMnO₆, the less pronounced deviation from the expected trend in the magnetic ordering temperature with the R³⁺ ionic radius highlighted the significant role of orbital ordering on enhancing the relative strength of NNN AFM with respect to NN FM interactions.