Influence of the deposition configuration on the composition, structure and morphology of La0.6Y0.07Ca0.33MnO3−δ thin films obtained by pulsed laser deposition

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Abstract

We present a structural and morphological study on La0.6Y0.07Ca0.33MnO3−δ (LYCMO) thin films deposited by laser ablation onto MgO-buffered silicon wafers using two deposition configurations named on-axis and off-axis. All thin films have been deposited at high temperatures in an oxygen atmosphere. A comparative study between the on-axis and off-axis samples is performed by transmission electron microscopy, electron diffraction and energy dispersive X-ray spectroscopy. The films are polycrystalline and show a columnar morphology. The off-axis sample shows a better crystalline texture, although slight compositional variations have been found.

Introduction

During the last decade, the interest of the international scientific community has been attracted by the ‘colossal’ magnetoresistive (CMR) perovskite-like manganites with the general formula LnAMnO3, where Ln represents a metal from the lanthanide class (La, Pr, Sm) and A a bivalent alkaline metal (Ca, Sr, Ba). Their importance is justified by the amplitude of the magnetoresistive effect, reaching up to 106% in special temperature and magnetic field conditions [1]. Their particular magnetic and transport properties are qualitatively explained by the ‘double exchange’ mechanism proposed by Zener in 1951 [2]. The quantitative aspect is determined by the growth morphology [3], lattice strain [4], chemical composition [5], etc. Throughout this work we briefly report on morphological, compositional and structural properties of LYCMO films obtained by pulsed laser deposition (PLD) onto MgO-buffered Si wafers using two deposition configurations.

Section snippets

Experimental

Thin films have been deposited by laser ablation using a KrF excimer laser (λ=248 nm and τFWHM≈20 ns). The experiments were performed in a 10 Pa flowing oxygen atmosphere in order to provide a fresh oxygen source all along the deposition process. The Si 〈100〉 substrate was maintained at 750°C during deposition. We used two kinds of target–substrate configurations: an ordinary one, with the substrate parallel and facing the target — we will call it on-axis geometry, and a second, modified

EDS results

The chemical composition of the LYCMO films was determined by EDS using a Kevex system installed on a Jeol JSM-840 scanning electron microscope operated at 10 kV. On each sample, EDS spectra were acquired from five areas of 30×40 μm2 denoted by 1–5 from the center to the border of the deposition. The oxygen content was estimated by considering the sample as made up of the corresponding metal oxides. The atomic concentrations were calculated and a local stoichiometry was determined for each

Conclusions

Both deposition configurations allow the growth of well-crystallised LYCMO thin films. For the same temperature and oxygen pressure conditions, the on-axis film is more homogeneous from the chemical point of view, while the off-axis sample shows an enhanced texture. The characteristic inclined growth morphology in the off-axis case is due to the large tangential velocity component of the particles reaching the substrate surface.

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