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

Abstract

Polycrystalline, multiphase ceramic composites exhibiting voltage-limiting nonlinear current–voltage (I–V) relations with the nonlinearity coefficient (α) 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−γ : 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−γ : Mn/p-LSMZO (and p-LSMZO/n-ZnO1−γ : 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−γ : 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−γ : Mn grains remain paramagnetic insulator and the magnetically tuneable I–V curves are not dependent on the ferromagnetic ordering therein.