Manganese valence and magnetotransport in ultrathin films of La$_{0.67}$Ca$_{0.33}$MnO$_3$

TL;DR

This study investigates how strain and substrate choice affect the microstructure, Mn valence, and magnetotransport properties of ultrathin La$_{0.67}$Ca$_{0.33}$MnO$_3$ films, revealing charge compensation layers and variations in magnetic and transport behaviors.

Contribution

It provides a comparative analysis of epitaxial La$_{0.67}$Ca$_{0.33}$MnO$_3$ films on different substrates, highlighting the effects of strain and miscut on microstructure, valence state, and magnetotransport properties.

Findings

Charge compensation layer with enhanced Mn valence near interface.

Reduced saturation magnetization in strained films.

Variation in metal-insulator transition temperature depending on substrate.

Abstract

We report a comparative study of the properties of very thin films of La$_{0.67}$Ca$_{0.33}$MnO$_3$ grown epitaxially under strain on flat SrTiO$_3$ (STO), lattice matched on NdGaO$_3$ (NGO), and strained on $1^{\circ}$-miscut STO. We use transmission electron microscopy and electron energy loss spectroscopy to study the microstructure, composition, and Mn valence state. Near the interface we find no significant segregation, but a charge compensation layer where the valence is enhanced over the nominal value of 3.3+, and a relaxation to this value over several nanometer. The transport properties show well-known behavior for the films on flat STO and NGO, namely values of the metal-insulator transition temperature which are strongly (STO) or only little (NGO) reduced with respect to the bulk value. The reduction in films on miscut STO however is less strong than in films on flat STO, even though they appear similar as to strain state and interface layer. Magnetically, we find reduced values of the saturation magnetization for the strained films with respect to the bulk value, which cannot only be ascribed to the interface layer.