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

Research on double perovskites R 2 NiMnO 6 with small R 3+ 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-x Y x NiMnO 6 samples were synthesized via a hydrothermal method. Compared to other synthesis methods, the hydrothermal method promotes the stabilization of Ni 2+ and Mn 4+ ions, and stabilizes structural distortion for R 2 NiMnO 6 with small R 3+ cations. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images revealed a rock-salt arrangement of Ni 2+ and Mn 4+ ions. By the variation in the R 3+ ionic radius, the magnetic interactions were continuously tuned. Compared to R MnO 3 and R 2 CoMnO 6 , the less pronounced deviation from the expected trend in the magnetic ordering temperature with the R 3+ ionic radius highlighted the significant role of orbital ordering on enhancing the relative strength of NNN AFM with respect to NN FM interactions.