No abstract

Homogeneous solid solutions of sol−gel-prepared R 2 NiMnO 6 (R = La, Pr, Nd, Sm, Gd, Tb, Dy, Y, and Ho) double perovskites crystallize in a B-site-ordered monoclinic structure (P2 1 /n space group). Monoclinic distortion enhances with decreasing R 3+ ionic radii (r R 3+ ). The magnetic ordering temperature (T C ) decreases from 270 K for La 2 NiMnO 6 to 80 K for Ho 2 NiMnO 6 as r R 3+ decreases from 1.16 Å (La 3+ ) to 1.02 Å (Ho 3+ ). An additional magnetic anomaly is observed in Nd 2 NiMnO 6 , Sm 2 NiMnO 6 , Tb 2 NiMnO 6 , and Dy 2 NiMnO 6 at lower temperatures, which originates from the 3d−4f coupling between Mn−Ni and Nd 3+ /Sm 3+ /Tb 3+ /Dy 3+ magnetic moments. Further, high saturation magnetization is achieved for all samples, indicating that they are atomically ordered and have less antisite disorders. Upon a decrease in the size of R 3+ , the local structure shows an expansion of NiO 6 octahedra and almost unchanged of MnO 6 octahedra. X-ray-absorption near-edge spectroscopy reveals a majority of Ni 2+ and Mn 4+ ions in all samples. Softening of phonon modes results in the elongation of the Ni/Mn−O bond length. Finally, a correlation among lattice parameters, structural distortion, octahedral tilting, superexchange angle, electronic band gap, Curie temperature, and the rare-earth ionic radius is established.

No abstract

No abstract