Nonlinear Relation Between Current and Voltage in Nickel-Doped Lanthanum-Strontium Polycrystalline Manganites
Manganites of the La<sup>3+</sup><sub>0.81+x</sub>Sr<sup>2+</sup><sub>0.19-x</sub>Mn<sup>3+</sup><sub>0.81-x</sub>Mn<sup>4+</sup><sub>0.19</sub>Ni<sup>2+</sup><sub>x</sub>O<sup>2-</sup><sub>3</sub> system (x=0.100; 0.125) were synthesized using traditional ceramic processing. The obtained samples have a rhombohedral structure, are ferromagnetic with Curie points of 216 and 181~K, respectively, and exhibit semiconductive type of conduction in the temperature range of 110-200 K. The sample with x=0.125 has submicron crystallite sizes and higher resistance. In manganite with nickel content of 0.100~f.e. at T=112 K, N-shaped current-voltage (I-V) characteristic is observed. Manganite La<sub>0.935</sub>Sr<sub>0.065</sub>Mn<sub>0.875</sub>Ni<sub>0.125</sub>O<sub>3</sub> has S-shaped I-V characteristic in the temperature range of 132-172 K. With a decrease in temperature, the differential conductivity on the upper branch of I-V characteristic decreases and becomes negative at 122 K. As a result, for the first time, current-voltage characteristic was detected with two sections of negative differential resistance of various types, on which dU<0 at dI>0 and dI<0 at dU>0. Experimental data are explained by competing effects of local self-heating of the samples and changes in the parameters of zone structure with increasing voltage, taking into account the role of microinhomogeneities, the possibility of current channels occurrence and tunneling of charge carriers between regions with different densities of states. The study of regularities and mechanisms of I-V characteristic formation with coexisting sections of negative differential resistance of various types is important for understanding the nature of nonlinear phenomena in complex-substituted manganites as strongly correlated systems and for obtaining the required properties of these materials.