Current switching in semiconducting LaMn1−xMgxO3+δ

The semiconducting LaMn1−xMgxO3+δ (LMMO) exhibits highly nonlinear, voltage-limiting, conduction (current switching) in the current–voltage (J–E) relationship. The nonlinearity is observed at field strengths less than 50 V/cm and below the magnetic transition temperature (Tc). The current density in the conducting region is very high, reaching tens of A/cm2. The threshold electric field (ETH) shifts to lower values in presence of an external magnetic field. No current switching is observed in ferromagnetic metallic, La0.67Ca0.33MnO3+δ (LCMO) specimens. The nonlinearity observed in LMMO cannot be explained on the basis of antiferromagnetic/paramagnetic insulating phases co-existing with ferromagnetic metallic phases below Tc, because such giant clusters are observed in LCMO. Substitution of Mg increases the Mn4+ concentration, however, does not induce a transition to metallic state below Tc even at high hole concentration; instead, displays the behaviour of localisation. Localisation of carriers is due to the increased hole hopping distance (Mn3+–O–Mg2+–O–Mn4+) and by the spin canting. Below Tc, in the canted ferromagnetic state, the carriers can be delocalised by applying an electric field. The electric field enhances the mobility of the carriers and increases the hopping probability. Above ETH, large number of carriers are delocalised and set in for conduction, yielding high current density.