Effects of Bi Substitution on Magnetic and Transport Properties of La0.7-x Bi x Ag0.3MnO3 Ceramics

Colossal magnetoresistance, CMR ceramics with starting composition of La0.7-x Bi (x) Ag0.3MnO3 (x=0-0.2) were synthesized using the conventional solid-state synthesis method to investigate the effects of Bi and Ag on their magnetic and electrical transport properties as well as their magnetoresistance behavior. Magnetic susceptibility measurements showed that the La0.7-x Bi (x) Ag0.3MnO3 samples with x=0, 0.10 and 0.15 exhibit single paramagnetic to ferromagnetic transition at Curie temperature, T (C) , which was observed to decrease from 289.5 K (x=0) to 186.5 K (x=0.15) while the x=0.2 sample showed two magnetic transitions at T (C1) (160.5 K) and T (C2) (214.0 K). Electrical resistivity measurements showed metal-insulator transition behavior for all samples. Bi substitution caused resistivity to increase while metal-insulator transition temperature, T (MI) shifted to lower temperature from 252.7 K (x=0) to 136.3 K (x=0.20). The metallic region of the rho(0,T) curve below T (MI) for all samples was well fitted to the equation rho=rho (o) +rho (2) T (2)+rho (4.5) T (4.5) indicating a combination of grain or domain boundary, electron-electron and electron-magnon scattering mechanism while the insulator region was governed by the Variable Range Hopping (VRH) model at T (MI)< T <theta (D) /2 and adiabatic small polaronic model (SPH) at T >theta (D) /2. The increase of hopping activation energy, E (a) for the latter is suggested to be due to possible hybridization between Bi 6s(2) lone pair and O orbital. Bi3+ substitution was also observed to enhance intrinsic MR at the vicinity of T (MI) due to increase in DE interaction when external magnetic field was applied. On the other hand, the substitution also caused reduction of extrinsic MR effect at low temperatures, which is suggested to be due to reduction of Mn spin disorder at grain boundaries as a result of the presence of small amount of Ag secondary phase.