Magnetically tuneable nonlinear electronic properties of ZnO/La(Sr)MnO3 ceramic composites

Polycrystalline, multiphase ceramic composites exhibiting voltage-limiting nonlinear current-voltage (I-V) relations with the nonlinearity coefficient ( a) of 220 - 360 have been realized by co-sintering ZnO and La0.6Sr0.4MnO3. The I - V characteristics are symmetrical on voltage reversal to the negative quadrant. By selecting the sintering temperature ( 1573 - 1673 K) and duration ( 1 - 4 h), the interdiffusion of Mn into ZnO and of Zn into La0.6Sr0.4MnO3 could be controlled so that La and Sr segregation leads to the formation of p-type La0.6Sr0.4Mn1-yZnyO3 (LSMZO) permeating along the intergrain regions of n- type ZnO1-gamma : Mn, thereby preserving interior n-p-n heterojunction characteristics at the grain boundaries. Possible mechanisms proposed for large nonlinearity in the I - V curves are (i) avalanche multiplication of minority carrier through impact ionization of majority carriers or (ii) Zener tunnelling of charge carriers across the depleted Schottky potential barriers prevailing at n- ZnO1-gamma : Mn/p-LSMZO ( and p-LSMZO/ n-ZnO1-gamma : Mn) interfaces on the left- and right- hand side of p-LSMZO intergrain phase, wherein the type of conduction mechanism is predominantly dependant on the transport properties and thickness of intergranular LSMZO phase. The I-V characteristics are modified under an applied magnetic field by way of decreasing the turn-on voltage leading to higher currents. Such magnetically tuneable electrical nonlinearity arises from the prevalence of spin-polarized conduction in the ferromagnetic insulating LSMZO which prevails at the intergrain regions of n- ZnO1-gamma : Mn. Magnetization studies of bulk LSMZO show the conversion from ferromagnetic metallic (y > 0.03) to ferromagnetic insulating ( y = 0.03 - 0.08) and paramagnetic insulating ( y > 0.08) phases on increasing the zinc content. However, the n-ZnO1-gamma : Mn grains remain paramagnetic insulator and the magnetically tuneable I - V curves are not dependent on the ferromagnetic ordering therein.