Role of Antisite Disorder, Rare-Earth Size, and Superexchange Angle on Band Gap, Curie Temperature, and Magnetization of R2NiMnO6 Double Perovskites

Homogeneous solid solutions of sol gel-prepared R2NiMnO6 (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 R3+ ionic radii (r(R)(3+)). The magnetic ordering temperature (T-C) decreases from 270 K for La2NiMnO6 to 80 K for Ho2NiMnO6 as r(R)(3+) decreases from 1.16 angstrom (La3+) to 1.02 angstrom (Ho3+). An additional magnetic anomaly is observed in Nd2NiMnO6, Sm2NiMnO6, Tb2NiMnO6, and Dy2NiMnO6 at lower temperatures, which originates from the 3d-4f coupling between Mn-Ni and Nd3+/Sm3+/Tb3+/Dy3+ 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 R3+, the local structure shows an expansion of NiO6 octahedra and almost unchanged of MnO6 octahedra. X-ray-absorption near-edge spectroscopy reveals a majority of Ni2+ and Mn4+ 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.