Role of annealing atmosphere on structure, dielectric and magnetic properties of La<SUB>2</SUB>CoMnO<SUB>6</SUB> and La<SUB>2</SUB>MgMnO<SUB>6</SUB>

Polycrystalline samples of La<SUB>2</SUB>MMnO<SUB>6</SUB> (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<SUB>O2</SUB>) and characterized by powder XRD, SQUID magnetometry, ac impedance spectroscopy, and electron paramagnetic resonance techniques. Monoclinic (P2<SUB>1</SUB>/n) and rhombohedral (Requation image) lattices were observed for La<SUB>2</SUB>CoMnO<SUB>6</SUB> and La<SUB>2</SUB>MgMnO<SUB>6</SUB>, respectively. On annealing in inert atmosphere, La<SUB>2</SUB>MgMnO<SUB>6</SUB> partially converted to monoclinic La<SUB>2</SUB>CoMnO<SUB>6</SUB> type structure, whereas no structural change was observed in La<SUB>2</SUB>CoMnO<SUB>6</SUB>. Dielectric studies of La2CoMnO6 indicated relaxor like behavior with polaronic conduction, which systematically decreased with the increase in p<SUB>2</SUB> of the annealing atmosphere. Magnetic studies indicated multiple ferromagnetic phase transitions in La<SUB>2</SUB>CoMnO<SUB>6</SUB> and a spin-glass like phase transition in La<SUB>2</SUB>MgMnO<SUB>6</SUB>. The fraction of ferromagnetic phases of La<SUB>2</SUB>CoMnO<SUB>6</SUB> 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