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Register a new userEpitaxial growth and electronic properties of mixed valence YbAl3 thin films
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We report the growth of thin films of the mixed valence compound YbAl$_{3}$ on MgO using molecular-beam epitaxy. Employing an aluminum buffer layer, epitaxial (001) films can be grown with sub-nm surface roughness. Using x-ray diffraction, in situ low-energy electron diffraction and aberration-corrected scanning transmission electron microscopy we establish that the films are ordered in the bulk as well as at the surface. Our films show a coherence temperature of 37 K, comparable to that reported for bulk single crystals. Photoelectron spectroscopy reveals contributions from both $textit{f}^{13}$ and $textit{f}^{12}$ final states establishing that YbAl$_{3}$ is a mixed valence compound and shows the presence of a Kondo Resonance peak near the Fermi-level.
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The double perovskite Sr2CrReO6 is an interesting material for spintronics, showing ferrimagnetism up to 635 K with a predicted high spin polarization of about 86%. We fabricated Sr2CrReO6 epitaxial films by pulsed laser deposition on (001)-oriented SrTiO3 substrates. Phase-pure films with optimum crystallographic and magnetic properties were obtained by growing at a substrate temperature of 700 degree C in pure O2 of 6.6x10-4 mbar. The films are c-axis oriented, coherently strained, and show less than 20% anti-site defects. The magnetization curves reveal high saturation magnetization of 0.8 muB per formula unit and high coercivity of 1.1 T, as well as a strong magnetic anisotropy.
The electronic and magnetic properties of transition metal dichalcogenides are known to be extremely sensitive to their structure. In this paper we study the effect of structure on the electronic and magnetic properties of mono- and bilayer $VSe_2$ films grown using molecular beam epitaxy. $VSe_2$ has recently attracted much attention due to reports of emergent ferromagnetism in the 2D limit. To understand this important compound, high quality 1T and distorted 1T films were grown at temperatures of 200 $^text{o}$C and 450 $^text{o}$C respectively and studied using 4K Scanning Tunneling Microscopy/Spectroscopy. The measured density of states and the charge density wave (CDW) patterns were compared to band structure and phonon dispersion calculations. Films in the 1T phase reveal different CDW patterns in the first layer compared to the second. Interestingly, we find the second layer of the 1T-film shows a CDW pattern with 4a $times$ 4a periodicity which is the 2D version of the bulk CDW observed in this compound. Our phonon dispersion calculations confirm the presence of a soft phonon at the correct wavevector that leads to this CDW. In contrast, the first layer of distorted 1T phase films shows a strong stripe feature with varying periodicities, while the second layer displays no observable CDW pattern. Finally, we find that the monolayer 1T $VSe_2$ film is weakly ferromagnetic, with ~ $3.5 {mu}_B$ per unit similar to previous reports.
La0.7Sr0.3MnO3 (LSMO) films with extraordinarily wide atomic terraces are epitaxially grown on SrTiO3 (100) substrates by pulsed laser deposition. Atomic force microscopy measurements on the LSMO films show that the atomic step is ~ 4 {AA} and the atomic terrace width is more than 2 micrometers. For a 20 monolayers (MLs) LSMO film, the magnetization is determined to be 255 +- 15 emu/cm3 at room temperature, corresponding to 1.70 + - 0.11 Bohr magneton per Mn atom. As the thickness of LSMO increases from 8 MLs to 20 MLs, the critical thickness for the temperature dependent insulator-to-metal behavior transition is shown to be 9 MLs. Furthermore, post-annealing in oxygen environment improves the electron transport and magnetic properties of the LSMO films.
The growth and characterization of epitaxial Co3O4(111) films grown by oxygen plasma-assisted molecular beam epitaxy on single crystalline a-Al2O3(0001) is reported. The Co3O4(111) grows single crystalline with the epitaxial relation Co3O4(111)[-12-1]||a-Al2O3(0001)[10-10], as determined from in situ electron diffraction. Film stoichiometry is confirmed by x-ray photoelectron spectroscopy, while ex situ x-ray diffraction measurements show that the Co3O4 films are fully relaxed. Post-growth annealing induces significant modifications in the film morphology, including a sharper Co3O4/a-Al2O3 interface and improved surface crystallinity, as shown by x-ray reflectometry, atomic force microscopy and electron diffraction measurements. Despite being polar, the surface of both as-grown and annealed Co3O4(111) films are (1 * 1), which can be explained in terms of inversion in the surface spinel structure.
The metastable orthorhombic phase of hafnia is generally obtained in polycrystalline films, whereas in epitaxial films, its formation has been much less investigated. We have grown Hf0.5Zr0.5O2 films by pulsed laser deposition, and the growth window (temperature and oxygen pressure during deposition and film thickness) for epitaxial stabilization of the ferroelectric phase is mapped. The remnant ferroelectric polarization, up to around 24 uC/cm2, depends on the amount of orthorhombic phase and interplanar spacing and increases with temperature and pressure for a fixed film thickness. The leakage current decreases with an increase in thickness or temperature, or when decreasing oxygen pressure. The coercive electric field (EC) depends on thickness (t) according to the coercive electric field (Ec) - thickness (t)-2/3 scaling, which is observed for the first time in ferroelectric hafnia, and the scaling extends to thicknesses down to around 5 nm. The proven ability to tailor the functional properties of high-quality epitaxial ferroelectric Hf0.5Zr0.5O2 films paves the way toward understanding their ferroelectric properties and prototyping devices.
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