Nonlinear current-voltage relations in polycrystalline perovskite manganites ceramics

Highly nonlinear current-voltage (J proportional toE(alpha)) relations with voltage-limiting characteristics are observed for Mg-doped lanthanum calci um manganite polycrystalline ceramics with nonlinearity coefficient, alpha =2-9 at low-electric-field strengths of 2-5 V/mm, below magnetic transition temperatures. The current density increases with external magnetic field, so that magnetically tunable low-voltage varistors are realized. The alpha increases on annealing at 1375 K in atmospheres of lower p(O2), and becomes more pronounced with decreasing grain size, indicating that nonlinear beha vior is related to the outdiffusion of oxygen through the grain-boundary-la yer regions. The increasing deficiency in Mn3+/Mn4+ pairs reduces the hole- hopping probability leading to insulating barriers in the grain-boundary-la yer regions. The barrier height is lowered by the external electric field, facilitating the charge-carrier movement between the grains. Since the cond uction is due to spin-polarized tunneling, the external magnetic field incr eases the current. (C) 2001 American Institute of Physics.