Vincent Polewczyk's postdoctoral activities were essentially focused on multiferroic (ferroelectric/ferromagnetic) materials. From the MBE growths to characterization measurements, passing through micro or nano-patterning steps.
V2O3 presents a complex interrelationship between the metal–insulator transition and the structural rhombohedral-monoclinic one in temperature, as a function of sample thickness. Whilst in bulk V2O3 the two transitions coincide on the temperature scale, at 15 nm thickness a fully independent Mott-like transition occurs at lower temperature, with no corresponding structural changes perhaps related to epitaxial strain. It is therefore of relevance to investigate the thin and ultrathin film growth to pinpoint the chemical, electronic and structural phase phenomenology and the role of the interface with the substrate. Here we present results on the thickness dependent properties of V2O3 from 1 nm up to 40 nm thick as grown on c-plane Al2O3 substrates by exploiting variable sampling depth probes. The surface morphology of stoichiometric ultra-thin V2O3 layers evolves from islands-like to continuous flat film with thickness, with implications on the overall properties.
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.
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.
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.
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NFFA is a Progetto Internazionale financed by MIUR through CNR
(Istituto Officina dei Materiali, Trieste) and Elettra-Sincrotrone Trieste
and managed by the Commissione NFFA chaired by Giorgio Rossi
(Università di Milano and IOM-CNR).