Role of annealing atmosphere on structure, dielectric and magnetic properties of La₂CoMnO₆ and La₂MgMnO₆

Abstract

Polycrystalline samples of La₂MMnO₆ (M = Co and Mg) were prepared by a combined gel-combustion and high temperature reaction method. The samples were annealed in different oxygen partial pressure (p_O2) and characterized by powder XRD, SQUID magnetometry, ac impedance spectroscopy, and electron paramagnetic resonance techniques. Monoclinic (P2₁/n) and rhombohedral (Requation image) lattices were observed for La₂CoMnO₆ and La₂MgMnO₆, respectively. On annealing in inert atmosphere, La₂MgMnO₆ partially converted to monoclinic La₂CoMnO₆ type structure, whereas no structural change was observed in La₂CoMnO₆. Dielectric studies of La2CoMnO6 indicated relaxor like behavior with polaronic conduction, which systematically decreased with the increase in p₂ of the annealing atmosphere. Magnetic studies indicated multiple ferromagnetic phase transitions in La₂CoMnO₆ and a spin-glass like phase transition in La₂MgMnO₆. The fraction of ferromagnetic phases of La₂CoMnO₆ was significantly dependent on the annealing environments. The variations of magnetic and dielectric properties of samples were related to the fluctuation of oxidation state of transition metal ions and oxygen vacancies in the samples.