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.
The double perovskite La 2 FeMnO 6 is ideally expected to be a ferrimagnet with a low saturation moment of 1 µ B / f.u. Inhomogeneity in the Fe/Mn sites, along with other lattice disorders, can modify the exchange interactions in the material and result in a net saturation moment of more than 1 µ B / f.u. Here, the origin of complicated magnetic behavior is examined of a pure phase La 2 FeMnO 6 sample prepared by the sol-gel method. XRD analysis established that the material crystallizes in the orthorhombic Pbnm symmetry. The comprehensive analysis of x-ray photoelectron spectroscopy, x-ray near edge structure, and magnetic measurements acknowledge an antiferromagnetic coupling between Fe 3+ and Mn 3+ cations, thereby, resulting in the ferrimagnetic ground state for La 2 FeMnO 6 . The magnetic hysteresis loop obtained at 5 K shows a large coercivity of ∼820 Oe and a saturation moment of 1.6 µ B / f.u., hinting at partial B-site ordering in La 2 FeMnO 6 . The ac susceptibility and dc magnetization measurements indicate the existence of magnetic glassy states (of cluster-glass type) with two distinct dynamical freezing points at ∼27 and 92 K, along with a Griffiths-like phase in the material. The experimental results are discussed taking several possible exchange interactions among Fe 3+ and Mn 3+ ions in the system into consideration. The magnetic complexity of this system makes it attractive for fundamental research and technological applications.
Among multifunctional double perovskite oxides, La2NiMnO6 has recently drawn significant attention due to its importance both in terms of understanding of fundamental physics and potential for device applications. The relative alteration in Ni:Mn ratio strongly influences the structural and magnetic properties of La2NiMnO6. The cation ratio and degree of cation order significantly affect the magnetic coupling of the two B‐site cations in these compounds. In the present study, La2Ni1−xMn1+xO6 (x = −0.25, 0, 0.25) samples with different Ni:Mn ratio have been prepared using sol–gel method and modifications of the above physical properties from that of a stoichiometric sample of La2NiMnO6 are discussed. The crystalline structures of the samples varied with different ionic ratios. While all samples exhibited ferromagnetic behavior, long‐range Ni/Mn magnetic ordering was detected in selected samples only. The experimental values of saturation magnetization were smaller than the theoretical spin‐only moments, which suggests a less ordered state for all samples. Due to an increased antiferromagnetic interaction caused by antisite disorders, the saturation magnetization decreases while the coercive field increases with decreasing Mn content.
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