Control of co-existing phases and charge transport in a nanostructured manganite film by field effects with an electric double layer as the gate dielectric

Abstract

We report bipolar control of co-existing phases in a nanostructured film of manganite, La_0.85Ca_0.15MnO₃, using an applied gate bias in a field effect (FE) device configuration. Low hole-doped manganite La_0.85Ca_0.15MnO₃ shows the co-existence of different electronic phases and a ferromagnetic insulating (FMI) state at low temperatures. The FE device with manganite as the channel uses an electric double layer (EDL) as a gate dielectric which can induce a large specific charge density (≥10¹³ cm⁻²) in the channel when a moderate gate bias is applied. The nanostructured nature of the manganite film enhances the field effect by controlling transport within the nano-grain as well as by controlling the depletion layer and the potential barrier at the grain boundaries. We observed a large modulation in the resistance of the film, ∼±40%, at room temperature for a moderate gate bias (V_G) of ±4 V, which increased to ∼±100% at 100 K. The field-effect-induced charges alter the relative fraction of the co-existing phases as well as the characteristic temperatures, such as the orthorhombic–orthorhombic (O–O′) transition temperature, the ferromagnetic transition temperature, and the onset temperature of the low temperature FMI state. The change in the relative fraction of the phases was found to have an exponential dependence on the gate bias. By using the shift in the temperature of the O–O′ transition with gate bias as a reference, we could establish that the change in hole concentration, brought about by the field effect, closely mimics (quantitatively) the changes brought about via chemical substitution, including the change in the activation energy of transport in the paramagnetic insulating region.