Relevance of thermal disorder in the electronic and spin ultrafast dynamics of iron in the low-perturbation regime
G.M. Pierantozzi, A. De Vita, R. Cucini, A.M. Finardi, T. Pincelli, F. Sirotti, J. Fujii, C. Dri, G. Brajnik, R. Sergo, G. Cautero, G. Panaccione, and G. Rossi
Understanding the ultrafast demagnetization of transition metals requires pump-probe experiments sensitive to the time evolution of the electronic, spin, and lattice thermodynamic baths. By means of time-resolved photoelectron energy and spin-polarization measurements in the low-pump-fluence regime on iron, we disentangle the different dynamics of hot electrons and demagnetization in the subpicosecond and picosecond time range. We observe a broadening of the Fermi-Dirac distribution, following the excitation of nonthermal electrons at specific region of the iron valence band. The corresponding reduction of the spin polarization is remarkably delayed with respect to the dynamics of electronic temperature. The experimental results are corroborated with a microscopic 3-temperature model highlighting the role of thermal disorder in the quenching of the average spin magnetic moment, and indicating Elliot-Yafet type spin-flip scattering as the main mediation mechanism, with a spin-flip probability of 0.1 and a rate of energy exchange between electrons and lattice of 2.5Kfs−1.
Our Research
Adv. Funct. Mat., 2301056, (2023)
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
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. Applied, 18, 044009, (2022)
All-Optical Generation and Time-Resolved Polarimetry of Magnetoacoustic Resonances via Transient Grating Spectroscopy
P. Carrara, M. Brioschi, E. Longo, D. Dagur, V. Polewczyk, G. Vinai, R. Mantovan, M. Fanciulli, G. Rossi, G. Panaccione, and R. Cucini
The generation and control of surface acoustic waves (SAWs) in a magnetic material are objects of an intense research effort focused on magnetoelastic properties, with fruitful ramifications in spin-wave-based quantum logic and magnonics. We implement a transient grating setup to optically generate SAWs also seeding coherent spin waves via magnetoelastic coupling in ferromagnetic media. In this work we report on SAW-driven ferromagnetic resonance (FMR) experiments performed on polycrystalline Ni thin films in combination with time-resolved Faraday polarimetry, which allows extraction of the value of the effective magnetization and of the Gilbert damping. The results are in full agreement with measurements on the very same samples from standard FMR. Higher-order effects due to parametric modulation of the magnetization dynamics, such as down-conversion, up-conversion, and frequency mixing, are observed, testifying the high sensitivity of this technique.
Our Research
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.
Our Research
Appl. Sci., 12(3), 1489, (2022)
HAADF STEM and Ab Initio Calculations Investigation of Anatase TiO2/LaAlO3 Heterointerface
M. Islam, P. Rajak, D. Knez, S.K. Chaluvadi, P. Orgiani, G. Rossi, G. Dražić, R. Ciancio
The understanding of the origin of a two-dimensional electron gas (2DEG) at the surface of anatase TiO2 remains a challenging issue. In particular, in TiO2 ultra-thin films, it is extremely difficult to distinguish intrinsic effects, due to the physics of the TiO2, from extrinsic effects, such as those arising from structural defects, dislocations, and the presence of competing phases at the film/substrate interface. It is, therefore, mandatory to unambiguously ascertain the structure of the TiO2/substrate interface. In this work, by combining high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), density functional theory calculations, and multislice image simulations, we have investigated the nature of strainless anatase TiO2 thin films grown on LaAlO3 substrate. In particular, the presence of oxygen vacancies in anatase TiO2 has been proved to stabilize the formation of an extra alloy layer, Ti2AlO4, by means of interface rearrangement. Our results, therefore, elucidate why the growth of anatase TiO2 directly on LaAlO3 substrate has required the deposition of a TiOx extra-layer to have a 2DEG established, thus confirming the absence of a critical thickness for the TiO2 to stabilize a 2DEG at its surface. These findings provide fundamental insights on the underlying formation mechanism of the 2DEG in TiO2/LAO hetero-interfaces to engineer the 2DEG formation in anatase TiO2 for tailored applications.
NFFA Thesis
University of Milan Master's Degree Thesis, (2021)
Transient grating spectroscopy with a magnetic probe: a study on phonon-driven magnons in a ferromagnetic thin film
M. Brioschi (Supervisors: G. Rossi, G. Panaccione, R. Cucini)
Materials and heterostructures that exhibit coupling between elastic and magnetic degrees of freedom are of both fundamental and technological interest. In particular, they have great potential for novel energy-efficient spintronic devices because acoustic waves can generate coherent and long-living spin waves through inverse magnetostriction, which consists in variations in the magnetization due to lattice deformations. As optical methods are versatile, non-invasive and contactless, an all-optical approach has been implemented and applied to study magnetoelastic coupling in a ferromagnetic film on a glass substrate. The present thesis work was performed at the NFFA-SPRINT facility of IOM-CNR in the Fermi@Elettra hall at Trieste, where I actively contributed to the realization and characterization of an all new experimental setup which is able to combine transient grating spectroscopy with a time-resolved Faraday polarimetry.
NFFA Thesis
University of Milan Bachelor's Degree Thesis, (2019)
All-resolved photoemission spectroscopy of Fe(100) and passivated Fe-p(1x1)O surfaces
A.M. Finardi (Supervisors: G. Rossi, I. Vobornik, J. Fujii)
Le proprietà ottiche, elettroniche e magnetiche dei solidi e delle loro superfici dipendono dalla struttura degli stati elettronici entro alcuni eV dal livello di Fermi. I calcoli della struttura elettronica a bande sono efficaci solo nel caso di materiali a bassa interazione elettrone-elettrone (correlazione). L'esperimento e la guida necessaria per lo studio delle proprietà elettroniche dei solidi e delle loro superfici, ed in particolare la spettroscopia di fotoemissione (photoemission spectroscopy - PES) che si basa sulla misura dello spettro energetico degli elettroni emessi da un solido eccitato da un fascio di fotoni monocromatici di energia eccedente la funzione lavoro. La risoluzione dell'angolo di emissione (Angle-resolved photemission spectroscopy - ARPES) permette di avere informazioni sulla legge di dispersione En(k) dello stato elettronico iniziale, mentre la misura del grado di polarizzazione in spin del fascio di elettroni completa il set di numeri quantici, fornendo un dato molto importante per lo studio delle correlazioni elettroniche.
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