Formation and Etching of the Insulating Sr-Rich V5+ Phase at the Metallic SrVO3 Surface Revealed by Operando XAS Spectroscopy Characterizations
V. Polewczyk, M. Mezhoud, M. Rath, O. El-Khaloufi, F. Bassato, A. Fouchet, W. Prellier, M. Frégnaux, D. Aureau, L. Braglia, G. Vinai, P. Torelli, U. Lüders
In the search of low cost and more efficient electronic devices, here the properties of SrVO3 transparent conductor oxide (TCO) thin film are investigated, both visible-range optically transparent and highly conductive, it stands as a promising candidate to substitute the standard indium-tin-oxide (ITO) in applications. Its surface stability under water (both liquid and vapor) and other gaseous atmospheres is especially addressed. Through the use of spectroscopy characterizations, X-ray photoemission and operando X-ray absorption measurements, the formation of a thin Sr-rich V5+ layer located at the surface of the polycrystalline SrVO3 film with aging is observed, and for the first time how it can be removed from the surface by solvating in water atmosphere. The surface recovery is associated to an etching process, here spectroscopically characterized in operando conditions, allowing to follow the stoichiometric modification under reaction. Once exposed in oxygen atmosphere, the Sr-rich V5+ layer forms again. The findings improve the understanding of aging effects in perovskite oxides, allowing for the development of functionalized films in which it is possible to control or to avoid an insulating surface layer. This constitutes an important step towards the large-scale use of V-based TCOs, with possible implementations in oxide-based electronics.
Our Research
Appl. Surf. Sci., 155918, (2023)
Electronically ordered ultrathin Cr2O3 on Pt(111) in presence of a multidomain Graphene intralayer
S. Fiori, D. Dagur, M. Capra, A. Picone, A. Brambilla, P. Torelli, G. Panaccione, G. Vinai
In the last decade, reducing the dimensionality of materials to few atomic layers thickness has allowed exploring new physical properties and functionalities otherwise absent out of the two dimensional limit. In this regime, interfaces and interlayers play a crucial role. Here, we investigate their influence on the electronic properties and structural quality of ultrathin Cr2O3 on Pt(111), in presence of a multidomain graphene intralayer. Specifically, by combining Low-Energy Electron Diffraction, X-ray Photoelectron Spectroscopy and X-ray Absorption Spectroscopy, we confirm the growth of high-quality ultrathin Cr2O3 on bare Pt, with sharp surface reconstructions, proper stoichiometry and good electronic quality. Once a multidomain graphene intralayer is included at the metal/oxide interface, the Cr2O3 maintained its correct stoichiometry and a comparable electronic quality, even at the very first monolayers, despite the partially lost of the morphological long-range order. These results show how ultrathin Cr2O3 films are slightly affected by the interfacial epitaxial quality from the electronic point of view, making them potential candidates for graphene-integrated heterostructures.
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Adv. Mater. Interfaces, 2201337, (2022)
Visible Light Effects on Photostrictive/Magnetostrictive PMN-PT/Ni Heterostructure
D. Dagur, V. Polewczyk, A.Y. Petrov, P. Carrara, M. Brioschi, S. Fiori, R. Cucini, G. Rossi, G. Panaccione, P. Torelli, G. Vinai
The possibility of modifying the ferromagnetic response of a multiferroic heterostructure via fully optical means exploiting the photovoltaic/photostrictive properties of the ferroelectric component is an effective method for tuning the interfacial properties. In this study, the effects of 405 nm visible-light illumination on the ferroelectric and ferromagnetic responses of (001) Pb(Mg1/3Nb2/3)O3-0.4PbTiO3 (PMN-PT)/Ni heterostructures are presented. By combining electrical, structural, magnetic, and spectroscopic measurements, how light illumination above the ferroelectric bandgap energy induces a photovoltaic current and the photostrictive effect reduces the coercive field of the interfacial magnetostrictive Ni layer are shown. Firstly, a light-induced variation in the Ni orbital moment as a result of sum-rule analysis of x-ray magnetic circular dichroic measurements is reported. The reduction of orbital moment reveals a photogenerated strain field. The observed effect is strongly reduced when polarizing out-of-plane the PMN-PT substrate, showing a highly anisotropic photostrictive contribution from the in-plane ferroelectric domains. These results shed light on the delicate energy balance that leads to sizeable light-induced effects in multiferroic heterostructures, while confirming the need of spectroscopy for identifying the physical origin of interface behavior.
Our Research
Phys. Rev. Materials, 4, 114418, (2020)
Interplay between morphology and magnetoelectric coupling in Fe/PMN-PT multiferroic heterostructures studied by microscopy techniques
F. Motti, G. Vinai, V. Bonanni, V. Polewczyk, P. Mantegazza, T. Forrest, F. Maccherozzi, S. Benedetti, C. Rinaldi, M. Cantoni, D. Cassese, S. Prato, S.S. Dhesi, G. Rossi, G. Panaccione, and P. Torelli
A ferromagnetic (FM) thin film deposited on a substrate of Pb(Mg1/3Nb2/3)O3−PbTiO3 (PMN-PT) is an appealing heterostructure for the electrical control of magnetism, which would enable nonvolatile memories with ultralow-power consumption. Reversible and electrically controlled morphological changes at the surface of PMN-PT suggest that the magnetoelectric effects are more complex than the commonly used “strain-mediated” description. Here we show that changes in substrate morphology intervene in magnetoelectric coupling as a key parameter interplaying with strain. Magnetic-sensitive microscopy techniques are used to study magnetoelectric coupling in Fe/PMN-PT at different length scales, and compare different substrate cuts. The observed rotation of the magnetic anisotropy is connected to the changes in morphology, and mapped in the crack pattern at the mesoscopic scale. Ferroelectric polarization switching induces a magnetic field-free rotation of the magnetic domains at micrometer scale, with a wide distribution of rotation angles. Our results show that the relationship between the rotation of the magnetic easy axis and the rotation of the in-plane component of the electric polarization is not straightforward, as well as the relationship between ferroelectric domains and crack pattern. The understanding and control of this phenomenon is crucial to develop functional devices based on FM/PMN-PT heterostructures.
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Rev. Sci. Instrum., 91, 085109, (2020)
An integrated ultra-high vacuum apparatus for growth and in situ characterization of complex materials
G. Vinai, F. Motti, A.Yu. Petrov, V. Polewczyk, V. Bonanni, R. Edla, B. Gobaut, J. Fujii, F. Suran, D. Benedetti, F. Salvador, A. Fondacaro, G. Rossi, G. Panaccione, B.A. Davidson and P. Torelli
Here, we present an integrated ultra-high vacuum apparatus—named MBE-Cluster —dedicated to the growth and in situ structural, spectroscopic, and magnetic characterization of complex materials. Molecular Beam Epitaxy (MBE) growth of metal oxides, e.g., manganites, and deposition of the patterned metallic layers can be fabricated and in situ characterized by reflection high-energy electron diffraction, low-energy electron diffraction, Auger electron spectroscopy, x-ray photoemission spectroscopy, and azimuthal longitudinal magneto-optic Kerr effect. The temperature can be controlled in the range from 5 K to 580 K, with the possibility of application of magnetic fields H up to ±7 kOe and electric fields E for voltages up to ±500 V. The MBE-Cluster operates for in-house research as well as user facility in combination with the APE beamlines at Sincrotrone-Trieste and the high harmonic generator facility for time-resolved spectroscopy.
From our users
J. Elec. Spectr. Rel. Phenom., 146902, (2019)
Operando photoelectron emission spectroscopy and microscopy at Elettra soft X-ray beamlines: From model to real functional systems
M. Amati, V. Bonanni, L. Braglia, F. Genuzio, L. Gregoratti, M. Kiskinova, A. Kolmakov, A. Locatelli, E. Magnano, A.A. Matruglio, T.O. Menteş, S. Nappini, P. Torelli, P. Zeller
Implementation of in-situ and operando experimental set-ups for bridging the pressure gap in characterization techniques based on monitoring of photoelectron emission has made significant achievements at several beamlines at Elettra synchrotron facility. These set-ups are now operational and have been successfully used to address unsolved issues exploring events occurring at solid–gas, solid–liquid and solid-solid interfaces of functional materials. The sections in the article communicate the research opportunities offered by the current set-ups at APE, BACH, ESCAmicroscopy and Nanospectroscopy beamlines and outline the next steps to overcome the present limits.
Our Research
J. Phys. Chem. C., 123, 24511-24519, (2019)
Study of Gaseous Interactions on Co3O4 Thin Film Coatings by Ambient Pressure Soft X-ray Absorption Spectroscopy
R. Edla, L. Braglia, V. Bonanni, A. Miotello, G. Rossi, P. Torelli
The redox process of pretreated Co3O4 thin film coatings has been studied by ambient pressure soft X-ray absorption spectroscopy. The Co3O4 coatings were composed of nanoparticles of about 10 nm in size as prepared by pulsed laser deposition. The thin film coatings were pretreated in He or in H2 up to 150 °C prior to exposure to the reactive gases. The reactivity toward carbon monoxide and oxygen was monitored by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy during gas exposures. The results indicate that the samples pretreated in He show reactivity only at high temperature, while the samples pretreated in H2 are reactive also at room temperature. X-ray photoemission spectroscopy measurements in ultra-high vacuum and NEXAFS simulations with the CTM4XAS code further specify the results.
Our Research
Adv. Electron. Mater., 7, 1900150, (2019)
Reversible Modification of Ferromagnetism through Electrically Controlled Morphology
G. Vinai, F. Motti, V. Bonanni, A. Petrov, S. Benedetti, C. Rinaldi, M. Stella, D. Cassese, S. Prato, M. Cantoni, G. Rossi, G. Panaccione, P. Torelli
Converse magnetoelectric coupling in artificial multiferroics is generally modeled through three possible mechanisms: charge transfer, strain mediated effects or ion migration. Here the role played by electrically controlled morphological modifications on the ferromagnetic response of a multiferroic heterostructure, specifically FexMn1−x ferromagnetic films on piezoferroelectric PMN‐PT [001] substrates, is discussed. The substrates present, in correspondence to electrical switching, fully reversible morphological changes at the surface, to which correspond reproducible modifications of the ferromagnetic response of the FexMn1−x films. Topographic analysis by atomic force microscopy shows the formation of surface cracks (up to 100 nm in height) upon application of a sufficiently high positive electric field (up to 6 kV cm−1). The cracks disappear after application of negative electric field of the same magnitude. Correspondingly, in operando X‐ray magnetic circular dichroic spectroscopy at Fe edge in FexMn1−x layers and micro‐MOKE measurements show local variations in the intensity of the dichroic signal and in the magnetic anisotropy as a function of the electrically driven morphological state. This morphologic parameter, rarely explored in literature, directly affects the ferromagnetic response of the system. Its proof of electrically reversible modification of the magnetic response adds a new possibility in the design of electrically controlled magnetic devices.
Our Research
Rev. Sci. Instrum., 89, 054101, (2018)
A reaction cell for ambient pressure soft x-ray absorption spectroscopy
C. Castán-Guerrero, D. Krizmancic, V. Bonanni, R. Edla, A. Deluisa, F. Salvador, G. Rossi, G. Panaccione, and P. Torelli
We present a new experimental setup for performing X-ray Absorption Spectroscopy (XAS) in the soft X-ray range at ambient pressure. The ambient pressure XAS setup is fully compatible with the ultra high vacuum environment of a synchrotron radiation spectroscopy beamline end station by means of ultrathin Si3N4 membranes acting as windows for the X-ray beam and seal of the atmospheric sample environment. The XAS detection is performed in total electron yield (TEY) mode by probing the drain current from the sample with a picoammeter. The high signal/noise ratio achievable in the TEY mode, combined with a continuous scanning of the X-ray energies, makes it possible recording XAS spectra in a few seconds. The first results show the performance of this setup to record fast XAS spectra from sample surfaces exposed at atmospheric pressure, even in the case of highly insulating samples. The use of a permanent magnet inside the reaction cell enables the measurement of X-ray magnetic circular dichroism at ambient pressure.
Our Research
Appl. Phys. Lett., 112, 022404, (2018)
X-ray magnetic circular dichroism discloses surface spins correlation in maghemite hollow nanoparticles
V. Bonanni, M. Basini, D. Peddis, A. Lascialfari, G. Rossi, and P. Torelli
The spin-spin correlations in hollow (H) and full (F) maghemite nanoparticles (NPs) have been studied by X-ray magnetic circular dichroism (XMCD). An unexpected XMCD signal was detected and analyzed under the application of a small field (μ0H = 160 Oe) and at remanence for both F and H NPs. Clear differences in the magnitude and in the lineshape of the XMCD spectra between F and H NPs emerged. By comparing XMCD measurements performed with a variable degree of surface sensitivity, we were able to address the specific role played by the surface spins in the magnetism of the NPs.
Our Research
J. Phys.: Conf. Ser., 903, 012061, (2017)
Influence of Mn diffusion on IrMn thickness threshold for the onset of exchange bias in IrMn/Co bilayers
G. Vinai, L. Frangou, C. Castan-Guerrero, V. Bonanni, B. Gobaut, S. Auffret, I.L. Prejbeanu, B. Dieny, V. Baltz and P. Torelli
In this work, we studied the influence of the buffer layer composition on the IrMn thickness threshold for the onset of exchange bias in IrMn/Co bilayers. By means of magnetometry, x-ray absorption and x-ray photoelectron spectroscopy, we investigated the magnetic and chemical properties of the stacks. We demonstrated a higher diffusion of Mn through the Co layer in the case of a Cu buffer layer. This is consistent with the observation of larger IrMn thickness threshold for the onset of exchange bias.
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