A comparative study of structural, magnetic, dielectric behaviors and impedance spectroscopy for bulk and nanometric double perovskite Sm2CoMnO6

Abstract

A comparative study regarding the structural, magnetic, dielectric and conductivity behavior and impedance spectroscopy of double perovskite Sm2CoMnO6 (SCMO) with different grain sizes of nearly 35 nm (SCMO_N) and 1 μm (SCMO_B) synthesized by a chemical sol-gel route has been performed in detail. The high-resolution recorded XRD pattern confirms the crystalline phase with monoclinic symmetry (space group P21/n) in SCMO_B. The XRD pattern of SCMO_N discloses the coexistence of both monoclinic and orthorhombic (Pnma) phases in the system. The magnetic measurements reveal that, in SCMO_N, the increase of the disordered phase (orthorhombic phase) results in an increase in the Curie temperature and a decrease in the magnetic moment. SCMO_B shows ferromagnetic behavior with two magnetic transitions, whereas the absence of a high-temperature magnetic transition reveals the disorder nature of the SCMO_N sample. The value of effective paramagnetic moment peff (5.75 μB) of SCMO_B is comparable to the calculated value of the spin-only moment for intermediate-spin Co3+ and high-spin Mn3+, as well as high-spin Mn2+ and Co4+, whereas the observed reduced peff (5.5 μB) value is attributed to the spin-only moment due to the increased number of high-spin Mn3+ and low-spin Co3+ in SCMO_N. SCMO_B exhibits a high dielectric constant with prominent high-frequency dielectric dispersion as compared to SCMO_N. The impedance spectroscopy studies disclose different conduction processes at the grain and grain boundary where the grain boundary obeys Arrhenius behavior and the grains follow the variable range hopping (VRH) model. SCMO_N shows high grain and grain boundary resistance as compared to SCMO_B. The grain and grain boundary capacitance show an anomaly near the dielectric relaxation. The nature of the temperature-dependant exponent s in the ac conductivity predicts that the overlapping large polaron tunneling mechanism (QLPT) is the dominant conduction mechanism for both the SCMO_B and SCMO_N samples.