Charge trapping in optimally doped epitaxial manganite thin films

Abstract

We have studied the thickness dependence of the magnetotransport properties of ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_3$ thin films epitaxially grown on ${\mathrm{SrTiO}}_3,$ ${\mathrm{LaAlO}}_3,$ and ${\mathrm{NdGaO}}_3$ single-crystalline substrates. When thickness decreases, a global disruption of the magnetoelectronic properties occurs, namely the resistivity and the low-temperature magnetoresistance increase while the metal-to-insulator transition temperature ${(T}_{\mathrm{P}})$ is lowered. We state that the electronic properties of these films, especially close to the film/substrate interface, differ from those of the bulk material. This is confirmed by nuclear-magnetic-resonance measurements which provide evidence that these films have an inhomogeneous magnetoelectronic nanostructure with distinguishable regions containing localized charges. These regions are scattered within the films, with a higher density close to interfaces in the case of ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_3$ films on ${\mathrm{SrTiO}}_3$ but more homogeneously distributed for films grown on ${\mathrm{NdGaO}}_3.$ Since our manganite films have a virtually unrelaxed crystal structure, the thickness dependence of $T_{\mathrm{P}}$ can neither be related to the strain states nor to dimensional effects. Alternatively, we show that the coexistence of different electronic phases leads to a modification of the carrier density in the metallic regions and, presumably, to an enhancement of the disorder in the Mn-O bond length and Mn-O-Mn angles. We will argue that the conjunction of both factors promotes a decrease of the double exchange transfer integral and, consequently, accounts for the reduction of the Curie temperature for the thinnest films. The possible mechanisms responsible for this phase separation are discussed in terms of the microstructure of the interfaces between the manganite and the insulating perovskite.