Scientific Highlights 2018



26 October 2018

Relationship between crystal structure and multiferroic orders in orthorhombic perovskite manganites

Abstract:
We use resonant and nonresonant x-ray diffraction measurements in combination with first-principles electronic structure calculations and Monte Carlo simulations to study the relationship between crystal structure and multiferroic orders in the orthorhombic perovskite manganites, o−RMnO3 (R is a rare-earth cation or Y). In particular, we focus on how the internal lattice parameters (Mn-O bond lengths and Mn-O-Mn bond angles) evolve under chemical pressure and epitaxial strain, and the effect of these structural variations on the microscopic exchange interactions and long-range magnetic order. We show that chemical pressure and epitaxial strain are accommodated differently by the crystal lattice of o-RMnO3, which is key for understanding the difference in magnetic properties between bulk samples and strained films. Finally, we discuss the effects of these differences in the magnetism on the electric polarization in o−RMnO3.
Keywords: Multiferroics; All-electron density functional calculations; Frustrated magnetism; X-ray diffraction;

Facility: Thin Films and Interfaces, LMX, SYN, ETHZ

Reference: N.S. Fedorova et al., Phys. Rev. Materials, 2, 104414 (2018)

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25 October 2018

Rolling dopant and strain in Y-doped BiFeO3 epitaxial thin films for photoelectrochemical water splitting

Abstract:
We report significant photoelectrochemical activity of Y-doped BiFeO3 (Y-BFO) epitaxial thin films deposited on Nb:SrTiO3 substrates. The Y-BFO photoanodes exhibit a strong dependence of the photocurrent values on the thickness of the films, and implicitly on the induced epitaxial strain. The peculiar crystalline structure of the Y-BFO thin films and the structural changes after the PEC experiments have been revealed by high resolution X-ray diffraction and transmission electron microscopy investigations. The crystalline coherence breaking due to the small ionic radius Y-addition was analyzed using Willliamson-Hall approach on the 2θ-ω scans of the symmetric (00 l) reflections and confirmed by high resolution TEM (HR-TEM) analysis. In the thinnest sample the lateral coherence length (L∥) is preserved on larger nanoregions/nanodomains. For higher thickness values L∥ is decreasing while domains tilt angles (αtilt) is increasing. The photocurrent value obtained for the thinnest sample was as high as Jph = 0.72 mA/cm2, at 1.4 V(vs. RHE). The potentiostatic scans of the Y-BFO photoanodes show the stability of photoresponse, irrespective of the film’s thickness. There is no clear cathodic photocurrent observation for the Y-BFO thin films confirming the n-type semiconductor behavior of the Y-BFO photoelectrodes.
Keywords: Ferroelectrics; Multiferroics; Photocatalytic watersplitting; Surfaces; Interfaces; thin films

Facility: Thin Films and Interfaces, LMX, National Institute for Laser, Plasma and Radiation Physics, Romania; National Institute of Material Physics, Romania

Reference: F. Haydous et al., Sci. Rep. 8, 15826 (2018)

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18 September 2018

Influence of Plume Properties on Thin Film Composition in Pulsed Laser Deposition

Abstract:
Despite the apparent simplicity of pulsed laser deposition, consistent deposition of thin films with the desired thickness, composition, crystallinity, and quality still remains challenging. This article explores the influence of process parameters with respect to film thickness and composition, two key aspects for thin films which have a very strong effect on film properties, possible applications, and characterization. Using five perovskite materials, a systematic analysis of different process parameters, e.g., target material, deposition pressure, fluence, substrate temperature or target to substrate distance, is performed. The results are classified under target ablation, plasma expansion, and substrates effects, which provide vital guidance to reduce the degree of trial and error when producing thin films. Moreover, they enable the understanding of what should be considered, and avoided for the deposition of thin films.
Keywords: Pulsed Laser Ablation; Laserinduced plasma; Plasma spectroscopy; Thin films; Thin film properties;

Facility: Thin Films and Interfaces, LMX, ETHZ

Reference: A. Ojeda, M. Döbeli, and T. Lippert, Adv. Mat. Interfaces 5, 1701062 (2018)

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