LaCoO3 Films Research Articles

Perovskite LaCoO3 thin films on single crystal substrates: MOCVD growth and characterization

Metal-Organic Chemical Vapor Deposition (MOCVD) has been applied to the fabrication of lanthanum cobaltite (LaCoO3) films on MgO (001) and LaAlO3 (001) substrates. The films have been deposited using a multi-metal source, consisting of the La (hfa)3•diglyme and Co (tta)2•tmeda (H-hfa=1,1,1,5,5,5-hexafluoro-2,4-pentanedione; diglyme=CH3O-(CH2CH2O) 2CH3, Htta=α-thenoyltrifluoroacetone; tmeda=N,N,N′,N′-tetramethyl-ethylendiammine) precursor mixture. Structural and morphological characterization of films, carried out using X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM), proved the present MOCVD approach a suitable effective route for the growth of pure LaCoO3 films. Composition and purity has been assessed through energy dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) analyses.

Simple polymer assisted deposition and strain-induced ferromagnetism of LaCoO3 epitaxial thin films

LaCoO3 epitaxial films with different thicknesses (~20, 50 and 80nm) were grown on (001) SrTiO3 substrates by a simple polymer assisted deposition method. X-ray diffraction analyses including θ/2θ symmetric scan, ω-scan and in-plane φ-scan indicate that single-phase (001) oriented LaCoO3 films with a pseudotetragonal structure were grown on (001) SrTiO3 substrates successfully, with a biaxial tensile strain from +2.42% to +2.60% and tetragonal distortion from 1.47% to 1.63%. Due to the lattice relaxation effect in epitaxial thin film, the biaxial tensile strain is slightly relaxed when the thickness of the LaCoO3 film increases, resulting in an increase of the c-axis constant in contrast to a decrease of the in-plane constants of the film. It is different from LaCoO3 bulk with a nonmagnetic ground state that all the epitaxial films exhibit a ferromagnetic transition at TC~85K. Combining with the structural and magnetic analyses, it is shown that the strain-induced ferromagnetism in LaCoO3 epitaxial films, corresponding to the higher spin states, origins from the decrease of the energy difference between eg and t2g levels, which is caused by an increase of the unit-cell volume and suppression of the CoO6 octahedral rotations. In addition, the change of FC curve with the thickness of the film reveals that the ferromagnetism is enhanced by the thickness decrease of LaCoO3 film due to the increase of the biaxial tensile strain.

Strain-Induced Spin States in Atomically Ordered Cobaltites

Epitaxial strain imposed in complex oxide thin films by heteroepitaxy is recognized as a powerful tool for identifying new properties and exploring the vast potential of materials performance. A particular example is LaCoO(3), a zero spin, nonmagnetic material in the bulk, whose strong ferromagnetism in a thin film remains enigmatic despite a decade of intense research. Here, we use scanning transmission electron microscopy complemented by X-ray and optical spectroscopy to study LaCoO(3) epitaxial thin films under different strain states. We observed an unconventional strain relaxation behavior resulting in stripe-like, lattice modulated patterns, which did not involve uncontrolled misfit dislocations or other defects. The modulation entails the formation of ferromagnetically ordered sheets comprising intermediate or high spin Co(3+), thus offering an unambiguous description for the exotic magnetism found in epitaxially strained LaCoO(3) films. This observation provides a novel route to tailoring the electronic and magnetic properties of functional oxide heterostructures.

Microstructural Investigations of Ca and La Doped CoO Thin Films Prepared by Pulsed Laser Deposition Technique

Two types of different oxides multifunctional thin films were deposited by PLD technique on the surface of Si and MgO substrates. First of them was CoO doped with Ca content characterized by fcc (halite) structure and second one was perovskite-type LaCoO3 compound. Their microstructure, chemical/phase composition and morphology were examined by means of diverse techniques (SEM, TEM, EDS). For the surface topography observations of thin films the Atomic Force Microscopy (AFM) was applied. Nanohardness and scratch tests (adhesion measurements) were also performed for estimation of (Co,Ca)O and LaCoO3 films mechanical properties and quality. Obtained results confirm that using PLD technique it is possible to carry stoichiometric composition of different compounds from the target to single crystal substrate however the microstructure and properties of obtained thin films are highly influenced by the substrate’s material and laser ablation parameters (laser wavelength, energy density, time and target-substrate distance).

Ferromagnetism enhanced by structural relaxation of biaxially compressed LaCoO3 films

Epitaxial LaCoO3 films were synthesized on LaAlO3 substrates to explore the role of epitaxial strain and structure on the ferromagnetism observed in these biaxially compressed films. Coherent strain and tetragonal structure were only achieved in thin film samples grown using higher energy densities. The strain relaxed with increasing thickness and was accompanied by increasing mosaic spread. Higher magnetization values were consistently seen in fully relaxed films grown using lower laser energy density. These results suggest that epitaxial strain is not the only factor determining the ferromagnetism and that the microstructure and defects may play a significant role.

Lattice structure and magnetization of LaCoO3 thin films

We investigate the structure and magnetic properties of thin films of the LaCoO3 compound. Thin films are deposited by pulsed laser deposition on various substrates in order to tune the strain from compressive to tensile. Single-phase (001) oriented LaCoO3 layers were grown on all substrates despite large misfits. The tetragonal distortion of the films covers a wide range from -2% to 2.8%. Our LaCoO3 films are ferromagnetic with Curie temperature around 85 K, contrary to the bulk. The total magnetic moment is below 1μ B /Co3+, a value relatively small for an exited spin-state of the Co3+ ions, but comparable to values reported in literature. A correlation of strain states and magnetic moment of Co3+ ions in LaCoO3 thin films is observed.

Direct measurement of ferromagnetic ordering in biaxially strained LaCoO3 thin films

LaCoO 3 undergoes a transition from a nonmagnetic to a paramagnetic semiconductor at 80 K, associated with a spin-state transition of the Co3+ ions. It was proposed that the temperature of the spin-state transition depends strongly on the LaCoO3 lattice parameter, suggesting that strain can stabilize different spin states at different temperatures. By combining atomic-resolution Z-contrast imaging, electron diffraction, and angular-resolved electron energy-loss spectroscopy (EELS) with in situ cooling experiments, we show that epitaxially strained LaCoO3 (001) thin films grown on LaAlO3 (001) do not undergo a low temperature spin-state transition. Our EELS study explores the origins of the ferromagnetic ordering in strained LaCoO3 films.

Unusual two-dimensional electrical charge transport at the surface of polycrystalline perovskite ultrathin films

The discontinuity of the lattice at the surface leads to an inherent presence of dangling bonds and atomic coordinations that differ from that of the bulk. As a consequence of this effect, we report on a two-dimensional confinement of the charge transport at the surface of polycrystalline perovskite oxide. Studying the surface transport separately from the bulk effect was approached by the investigation of ultrathin and stress-free LaCoO3 films that are grown on amorphous and nonconducting substrates using pulsed-spray evaporation chemical vapor deposition. The electrical characterization demonstrates an intriguing surface localization of the charge carriers. This surface trapping, which is observed above room temperature, dominates the electrical transport up to a temperature that depends on the film thickness, e.g., up to 440 K for 8-nm-thick films. This high-temperature effect, which is attributed to the surface adsorption of oxygen, points at a largely ignored surface effect in the study of thin and ultrathin films of transition metal oxides. Desorbing surface oxygen was experimentally shown to disrupt the two-dimensional confinement of the charge transport.

Pulsed Spray Evaporation CVD of Functional Complex Oxides: Interplay between the Structure, Electrical Properties and Surface Reactivity

The growth of pure phase of a number of perovskites and spinels is reported using pulsed spray evaporation (PSE) CVD. Beside the near bulk properties of 250 nm-thick perovskites an unusual confinement of the charge transport was observed in ultrathin, < 10 nm, polycrystalline LaCoO3 films resulting in an apparent electrical conductivity that is higher than that of single crystal. In the second part of this report, a doping strategy was used as tool to tune the properties of the spinel structure of cobalt oxide, enabling the establishment of direct correlations between its properties and catalytic activity. Iron doping of Co3O4 has enabled the control of the reducibility of the spinel with direct consequences on the catalytic oxidation of CO, and the control of the abundance of basic adsorption sites which affected the catalytic oxidation of ethanol.

A versatile single-source precursor for the synthesis of LaCoO3 films

The present communication is focused on the synthesis of the versatile heterobimetallic LaCo(ODiEt)5 (ODiEt = OC(CH2CH3)2CH2OCH3) alkoxide compound and on its innovative and unprecedented use as single-source precursor in the preparation of LaCoO3 films by both sol–gel and Chemical Vapour Deposition techniques. Nanostructured LaCoO3 films characterized by thermal stability of the perovskite structure up to 900 °C have been obtained thus evidencing the versatility of the adopted source compound. Morphological analysis revealed that high-quality smooth, well-adherent and crack-free layers have been prepared.

Grain growth of LaCoO3 film dispersed with a second phase prepared by a sol-gel technique

Grain growth of LaCoO3 film dispersed with a second phase prepared by a sol-gel technique