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.
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Nano Lett., (2022)
Disentangling Structural and Electronic Properties in V2O3 Thin Films: A Genuine Nonsymmetry Breaking Mott Transition
F. Mazzola, S.K. Chaluvadi, V. Polewczyk, D. Mondal, J. Fujii, P. Rajak, M. Islam, R. Ciancio, L. Barba, M. Fabrizio, G. Rossi, P. Orgiani, and I. Vobornik
Phase transitions are key in determining and controlling the quantum properties of correlated materials. Here, by using the combination of material synthesis and photoelectron spectroscopy, we demonstrate a genuine Mott transition undressed of any symmetry breaking side effects in the thin films of V2O3. In particular and in contrast with the bulk V2O3, we unveil the purely electronic dynamics approaching the metal–insulator transition, disentangled from the structural transformation that is prevented by the residual substrate-induced strain. On approaching the transition, the spectral signal evolves slowly over a wide temperature range, the Fermi wave-vector does not change, and the critical temperature is lower than the one reported for the bulk. Our findings are fundamental in demonstrating the universal benchmarks of a genuine nonsymmetry breaking Mott transition, extendable to a large array of correlated quantum systems, and hold promise of exploiting the metal–insulator transition by implementing V2O3 thin films in devices.
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Nanomaterials, 2(10), 1709, (2022)
Normal-State Transport Properties of Infinite-Layer Sr1−xLaxCuO2 Electron-Doped Cuprates in Optimal- and Over-Doped Regimes
Transport properties of electron-doped cuprate Sr1−xLaxCuO2 thin films have been investigated as a function of doping. In particular, optimal- and over-doped samples were obtained by tuning the Sr:La stoichiometric ratio. Optimal-doped samples show a non-Fermi liquid behavior characterized by linear dependence of the resistivity from room temperature down to intermediate temperature (about 150–170 K). However, by approaching temperatures in the superconducting transition, a Fermi-liquid behavior-characterized by a T2-scaling law-was observed. Once established, the transition from a linear-T to a quadratic-T2 behavior was successfully traced back in over-doped samples, even occurring at lower temperatures. In addition, the over-doped samples show a crossover to a linear-T to a logarithmic dependence at high temperatures compatible with anti-ferromagnetic spin fluctuations dominating the normal state properties of electron-doped cuprates.
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ACS Omega, (2022)
Electronic Properties of Fully Strained La1–xSrxMnO3 Thin Films Grown by Molecular Beam Epitaxy (0.15 ≤ x ≤ 0.45)
S.K. Chaluvadi, V. Polewczyk, A.Y. Petrov, G. Vinai, L. Braglia, J.M. Diez, V. Pierron, P. Perna, L. Mechin, P. Torelli, and P. Orgiani
The structural, electronic, and magnetic properties of Sr-hole-doped epitaxial La1–xSrxMnO3 (0.15 ≤ x ≤ 0.45) thin films deposited using the molecular beam epitaxy technique on 4° vicinal STO (001) substrates are probed by the combination of X-ray diffraction and various synchrotron-based spectroscopy techniques. The structural characterizations evidence a significant shift in the LSMO (002) peak to the higher diffraction angles owing to the increase in Sr doping concentrations in thin films. The nature of the LSMO Mn mixed-valence state was estimated from X-ray photoemission spectroscopy together with the relative changes in the Mn L2,3 edges observed in X-ray absorption spectroscopy (XAS), both strongly affected by doping. CTM4XAS simulations at the XAS Mn L2,3 edges reveal the combination of epitaxial strain, and different MnO6 crystal field splitting give rise to a peak at ∼641 eV. The observed changes in the occupancy of the eg and the t2g orbitals as well as their binding energy positions toward the Fermi level with hole doping are discussed. The room-temperature magnetic properties were probed at the end by circular dichroism.
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Adv. Sci., 2105114, (2022)
Evidence of a 2D Electron Gas in a Single-Unit-Cell of Anatase TiO2 (001)
A. Troglia, C. Bigi, I. Vobornik, J. Fujii, D. Knez, R. Ciancio, G. Dražić, M. Fuchs, D. Di Sante, G. Sangiovanni, G. Rossi, P. Orgiani, G. Panaccione
The formation and the evolution of electronic metallic states localized at the surface, commonly termed 2D electron gas (2DEG), represents a peculiar phenomenon occurring at the surface and interface of many transition metal oxides (TMO). Among TMO, titanium dioxide (TiO2), particularly in its anatase polymorph, stands as a prototypical system for the development of novel applications related to renewable energy, devices and sensors, where understanding the carrier dynamics is of utmost importance. In this study, angle-resolved photo-electron spectroscopy (ARPES) and X-ray absorption spectroscopy (XAS) are used, supported by density functional theory (DFT), to follow the formation and the evolution of the 2DEG in TiO2 thin films. Unlike other TMO systems, it is revealed that, once the anatase fingerprint is present, the 2DEG in TiO2 is robust and stable down to a single-unit-cell, and that the electron filling of the 2DEG increases with thickness and eventually saturates. These results prove that no critical thickness triggers the occurrence of the 2DEG in anatase TiO2 and give insight in formation mechanism of electronic states at the surface of TMO.
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Adv. Electron. Mater., 2101338, (2022)
Oxygen-Driven Metal–Insulator Transition in SrNbO3 Thin Films Probed by Infrared Spectroscopy
P. Di Pietro, C. Bigi, S.K. Chaluvadi, D. Knez, P. Rajak, R. Ciancio, J. Fujii, F. Mercuri, S. Lupi, G. Rossi, F. Borgatti, A. Perucchi, P. Orgiani
The occurrence of oxygen-driven metal–insulator-transition (MIT) in SrNbO3 (SNO) thin films epitaxially grown on (110)-oriented DyScO3 has been reported. SNO films are fabricated by the pulsed laser deposition technique at different partial O2 pressure to vary the oxygen content and their structural, optical, and transport properties are probed. SNO unit cell has been found to shrink vertically as the oxygen content increases but keeping the epitaxial matching with the substrate. The results of Fourier-transform infra-red spectroscopy show that highly oxygenated SNO samples (i.e., grown at high oxygen pressure) show distinct optical conductivity behavior with respect to oxygen deficient films, hence demonstrating the insulating character of the formers with respect to those fabricated with lower pressure conditions. Tailoring the optical absorption and conductivity of strontium niobate epitaxial films across the MIT will favor novel applications of this material.
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Sensors, 21(18), 6114, (2021)
ZnO Thin Films Growth Optimization for Piezoelectric Application
V. Polewczyk, R. Magrin Maffei, G. Vinai, M. Lo Cicero, S. Prato, P. Capaldo, S. Dal Zilio, A. di Bona, G. Paolicelli, A. Mescola, S. D’Addato, P. Torelli and S. Benedetti
The piezoelectric response of ZnO thin films in heterostructure-based devices is strictly related to their structure and morphology. We optimize the fabrication of piezoelectric ZnO to reduce its surface roughness, improving the crystalline quality, taking into consideration the role of the metal electrode underneath. The role of thermal treatments, as well as sputtering gas composition, is investigated by means of atomic force microscopy and x-ray diffraction. The results show an optimal reduction in surface roughness and at the same time a good crystalline quality when 75% O2 is introduced in the sputtering gas and deposition is performed between room temperature and 573 K. Subsequent annealing at 773 K further improves the film quality. The introduction of Ti or Pt as bottom electrode maintains a good surface and crystalline quality. By means of piezoelectric force microscope, we prove a piezoelectric response of the film in accordance with the literature, in spite of the low ZnO thickness and the reduced grain size, with a unipolar orientation and homogenous displacement when deposited on Ti electrode.
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Phys. Rev. Materials, 5, 034413, (2021)
Tuning the magnetic properties of V2O3/CoFeB heterostructures across the V2O3 structural transition
V. Polewczyk, S.K. Chaluvadi, P. Orgiani, G. Panaccione, G. Vinai, G. Rossi, and P. Torelli
In this work, we investigate the effects of the V2O3 structural phase transition on the magnetic properties of an amorphous magnetic thin film of CoFeB in contact with it. V2O3 thin films are deposited epitaxially on sapphire substrates, reaching bulklike properties after few nm of growth. By means of temperature dependent Kerr effect characterizations, we prove that crossing the V2O3 structural phase transition induces reproducible and reversible changes to CoFeB magnetic properties, especially to its coercive field. By decreasing the oxide layer thickness, its effects on the magnetic layer decreases, while reducing the magnetic layer thickness maximizes it, with a maximum of 330% coercive field variation found between the two V2O3 structural phases. By simply tuning the temperature, this systematic study shows that the engineering of V2O3 structural transition induces large interfacial strain and thus strong magnetic property variations to an amorphous thin film, opening wide possibilities in implementing strain-driven control of the magnetic behavior without strict requirements on epitaxial coherence at the interface.
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J. Magn. Magn. Mater., 515, 167257, (2020)
Thermal assisted tailoring of magnetic coercivity in Iron thin films on unstable Lithium Niobate substrate
V. Polewczyk, G. Vinai, F. Motti, S. Santhosh, S. Benedetti, G. Rossi, P. Torelli
In this work, we present an investigation on the effects of thermal annealing on the magnetic response of Lithium Niobate/Fe samples. Fe thin films have been deposited on Lithium Niobate Z-cut ferroelectric substrates by vapor phase epitaxy. A series of annealing treatments were performed on the samples, monitoring the evolution of their magnetic properties, both at the surface and on the volume. The combination of structural, magnetic, chemical and morphological characterizations shows that the modification of the chemical properties, i.e. the phase decomposition, of the substrate upon annealing affects drastically the magnetic behavior of the interfacial Fe layer. By tuning the annealing temperature, the magnetic coercive field value can be increased by an order of magnitude compared to the as-grown value, keeping the same in-plane isotropic behavior. Since no evident differences were recorded in the Fe layer from the chemical point of view, we attribute the origin of this effect to an intermixing process between a fragment of the substrate and the Fe thin film upon critical temperature annealing, process that is also is responsible for the observed changes in roughness and morphology of the magnetic thin film.
Our Research
Nano Lett., 20, 6444–6451, (2020)
Unveiling Oxygen Vacancy Superstructures in Reduced Anatase Thin Films
D. Knez, G. Dražić, S.K. Chaluvadi, P. Orgiani, S. Fabris, G. Panaccione, G. Rossi, and R. Ciancio
Oxygen vacancies are known to play a crucial role in tuning the physical properties and technological applications of titanium dioxide TiO2. Over the last decades, defects in substoichiometric TiO2 have been commonly associated with the formation of TinO2n–x Magnéli phases, which are extended planar defects originating from crystallographic shear planes. By combining advanced transmission electron microscopy techniques, electron energy-loss spectroscopy and atomistic simulations, we reach new understanding of the oxygen vacancy induced structural modulations in anatase, ruling out the earlier shear-plane model. Structural modulations are instead shown to be due to the formation of oxygen vacancy superstructures that extend periodically inside the films, preserving the crystalline order of anatase. Elucidating the structure of oxygen defects in anatase is a crucial step for improving the functionalities of such material system and to engineer devices with targeted properties.
Our Research
Appl. Phys. Lett., 113, 052403, (2018)
Room temperature biaxial magnetic anisotropy in La0.67Sr0.33MnO3 thin films on SrTiO3 buffered MgO (001) substrates for spintronic applications
S.K. Chaluvadi, F. Ajejas, P. Orgiani, O. Rousseau, G. Vinai, A. Petrov, P. Torelli, A. Pautrat, J. Camarero, P. Perna, and L. Mechin
Spintronics exploits the magnetoresistance effects to store or sense the magnetic information. Since the magnetoresistance strictly depends on the magnetic anisotropy of a system, it is fundamental to set a defined anisotropy to the system. Here, we investigate half-metallic La0.67Sr0.33MnO3 thin films by means of vectorial Magneto-Optical Kerr Magnetometry and found that they exhibit pure biaxial magnetic anisotropy at room temperature if grown onto a MgO (001) substrate with a thin SrTiO3 buffer. In this way, we can avoid unwanted uniaxial magnetic anisotropy contributions that may be detrimental for specific applications. The detailed study of the angular evolution of the magnetization reversal pathways and critical fields (coercivity and switching) discloses the origin of the magnetic anisotropy, which is magnetocrystalline in nature and shows fourfold symmetry at any temperature.
Our Research
Phys. Rev. B, 97, 094423, (2018)
Strain-induced magnetization control in an oxide multiferroic heterostructure
F. Motti, G. Vinai, A. Petrov, Bruce A. Davidson, B. Gobaut, A. Filippetti, G. Rossi, G. Panaccione, and P. Torelli
Controlling magnetism by using electric fields is a goal of research towards novel spintronic devices and future nanoelectronics. For this reason, multiferroic heterostructures attract much interest. Here we provide experimental evidence, and supporting density functional theory analysis, of a transition in La0.65Sr0.35MnO3 thin film to a stable ferromagnetic phase, that is induced by the structural and strain properties of the ferroelectric BaTiO3 (BTO) substrate, which can be modified by applying external electric fields. X-ray magnetic circular dichroism measurements on Mn L edges with a synchrotron radiation show, in fact, two magnetic transitions as a function of temperature that correspond to structural changes of the BTO substrate. We also show that ferromagnetism, absent in the pristine condition at room temperature, can be established by electrically switching the BTO ferroelectric domains in the out-of-plane direction. The present results confirm that electrically induced strain can be exploited to control magnetism in multiferroic oxide heterostructures.
Our Research
APL Mater., 3, 116107, (2015)
Unraveling the magnetic properties of BiFe0.5Cr0.5O3 thin films
G. Vinai, A. Khare, D. S. Rana, E. Di Gennaro, B. Gobaut, R. Moroni, A. Petrov, U. Scotti di Uccio, G. Rossi, F. Miletto Granozio, G. Panaccione, and P. Torelli
We investigate the structural, chemical, and magnetic properties on BiFe0.5Cr0.5O3 (BFCO) thin films grown on (001) (110) and (111) oriented SrTiO3 (STO) substrates by x-ray magnetic circular dichroism and x-ray diffraction. We show how highly pure BFCO films, differently from the theoretically expected ferrimagnetic behavior, present a very weak dichroic signal at Cr and Fe edges, with both moments aligned with the external field. Chemically sensitive hysteresis loops show no hysteretic behavior and no saturation up to 6.8 T. The linear responses are induced by the tilting of the Cr and Fe moments along the applied magnetic field.
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