Structural transformation, Griffiths phase and metal-insulator transition in polycrystalline Nd_2-x Sr _x NiMnO₆...

Polycrystalline double perovskite Nd_2-x Sr _x NiMnO₆ (x  =  0, 0.2, 0.4, 0.5 and 1) samples were synthesized using the solid state reaction method. There occurs a structural transformation from monoclinic (P2₁/n, for x  =  0 to x  =  0.5) to cubic (Fm [Formula: see text] m, for x  =  1) with increasing Sr doping. Raman spectroscopy reveals the increase in static disorder with doping. The Curie temperature (T _C) shows a small increase from x  =  0 to 0.5 (T _C ~ 200 K), but for x  =  1, T _C increases drastically upto ~264 K. The deviation of 1/χ(T) from Curie-Weiss behaviour for doped samples with exponent less than one, indicates a development of the Griffiths phase with doping. The systematic reduction in magnetic moment at 5 K suggests an increase in anti-site disorders with doping. Mn 3s x-ray photoemission spectra show an increase in exchange splitting, indicating a decrease in the valency of Mn. The x-ray absorption spectra at Ni and Mn 2p edges show that the formal valence remains 2+  (Ni) and 4+  (Mn) for all the samples, with changes in spectral weights. Ni 2p x-ray photoemission spectra show characteristic feature similar to Ni³⁺ systems, only for x  =  1 sample. Our GGA-based calculations for the ordered supercell, predict half metallic character for doping (x  >  0) samples due to delocalization of Ni e_g orbitals. The calculations with anti-site disorders yield drastic reduction in Ni moments, with the disordered anti-ferromagnetic phase having lowest energy at maximum doping. Temperature dependent resistivity measurements exhibit a clear metallic region for x  =  0.2 sample, while for higher dopings (x  >  0.2), the metallicity gets suppressed due to increase in anti-site disorders in these samples.