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Electronic memory and computing devices currently rules our digital lives, creating and consuming more than 10^21 bytes of data per year. This amount is expected to grow exponentially, questing for a so-called “hardware revolution”. Bit size-reduction governed the development of solid-state-memory and semiconductor technologies in the last decades. Yet, the innovative concept of an “universal memory” emerged for transcending the size-node. Indeed, this novel system combines high-speed computation and high-density storage skills. Memory devices based on Phase-Change Materials (PCMs) may serve to the scope, defying to scale reduction and both having a competitive erase/read speed with respect to the primary memory systems -as Static/Dynamic Random Access Memory (SRAM, DRAM)- and the large capacity of non-volatile secondary/tertiary storage systems -as Solid State Disk (SSD), Hard Disk Drive (HDD), Digital Video Disc (DVD). The appeal of Phase-Change Materials arises from their ability to rapidly and reversibly switch between the amorphous and crystalline states, via optical or electrical pulses. Interestingly, the two structural phases own significantly different physical properties, in particular in terms of reflectivity and conductivity. Many binary or ternary compounds display phase-change features, still Ge-Sb-Te (GST) alloys are the prominent members of this class of materials and are largely employed in industry. However, the main drawback is the PCMs relatively high operation power consumption, in the form of energy required for the phase transformation and of energy dissipation through -primarily- thermal diffusion. This thesis follows a GST dimensional strategy for power optimization -while maintaining advanced performances- for future integration in novel memory devices. The followed approach includes the case of 2-D highly-textured superlattice structures -of alternately deposited GeTe bilayers and Sb2Te3 quintuple layers- and of 0-D Ge2Sb2Te5 nanoparticles. Chapter 1 reviews the state-of-the-art of Phase-Change Materials, that stimulated the research questions addressed in the present work. Extended X-Ray Absorption Spectroscopy (EXAFS) -briefly presented in the first part of Chapter 2- is a powerful experimental tool for investigating a material’s atomic structure. As described in Chapter 3, one of the two allotrope crystalline phases of (GeTe)-(Sb2Te3) superlattices is revealed in details via EXAFS measurements performed at the Ge and Sb K-edges. The emerged structural picture is commented in light of the proposed models in literature, advising a power-saving yet over-simplified switching process occurring in the superlattice structure. Chapter 4 tackles the problem of power consumption by experimentally demonstrating the energy boost on the optical phase-change process occurring in 0-D Ge2Sb2Te5 nanoparticles. Here, a stable but reversible transition from the crystalline to the amorphous state of nanoparticles is induced with a single low-fluence femtosecond laser pulse. Thermodynamic, optical and structural considerations corroborate the experimental evidence. The laser source together with the setup used for the optical measurements are described in the second part of Chapter 2. The optical arrangement -conceived for time-resolved measurements- led to follow also the relaxation pathways of photo-excited nanoparticles below the threshold fluence for permanent amorphization. The results of this study are unveiled in Chapter 5. The ultrafast dynamics are compared to theoretical simulation and modelled with a phenomenological rate equation. Remarks on the resulting time-scales and the underlying interaction mechanisms -questioning the nature of the resonant bonding- close the Chapter.

Scientific reports 6, 22353 (2016). doi:10.1038/srep22353, Published by Nature Publishing Group, London

We investigate the evolution of both the occupied and unoccupied electronic structure in representative compounds of the infinitely adaptive superlattice series (Sb2)m-Sb2Te3 (m = 0–3) by means of angle-resolved photoemission spectroscopy and time-delayed two-photon photoemission, combined with first-principles band-structure calculations. We discover that the topological nature of the surface states and their spin texture are robust, with dispersions evolving from linear (Dirac-like) to parabolic (Rashba-like) along the series, as the materials evolve from semiconductors to semimetals. Our findings provide a promising strategy for engineering the topological states with the desired flexibility needed for realizing different quantum phenomena and spintronics applications.

Here we demonstrate that the 0-dimensional confinement of Ge2Sb2Te5 results in a drastic reduction of the minimum critical fluence required for optical-induced amorphization when compared to the thin-film cases. We show that by using single-shot laser pulses, the investigated nanoparticles display a crystalline-to-amorphous transition, satisfying a mandatory requirement of a bit-memory element. These unprecedented results open a viable route to boost energy efficient phase-change processes.

The study of ultrafast magnetodynamics has entered a new era thanks to the groundbreaking technological advances in free-electron laser (FEL) light sources. The advent of these light sources has made possible unprecedented experimental schemes for time-resolved x-ray magneto-optic spectroscopies, which are now paving the road for exploring the ultimate limits of out-of-equilibrium magnetic phenomena. In particular, these studies will provide insights into elementary mechanisms governing spin and orbital dynamics, therefore contributing to the development of ultrafast devices for relevant magnetic technologies. This topical review focuses on recent advancement in the study of non-equilibrium magnetic phenomena from the perspective of time-resolved extreme ultra violet (EUV) and soft x-ray spectroscopies at FELs with highlights of some important experimental results.

Here we report a giant, completely reversible magneto-electric coupling of 100 nm polycrystalline Co layer in contact with ZnO nanorods. When the sample is under an applied bias of ±2 V, the Co magnetic coercivity is reduced by a factor 5 from the un-poled case, with additionally a reduction of total magnetic moment in Co. Taking into account the chemical properties of ZnO nanorods measured by X-rays absorption near edge spectroscopy under bias, we conclude that these macroscopic effects on the magnetic response of the Co layer are due to the microstructure and the strong strain-driven magneto-electric coupling induced by the ZnO nanorods, whose nanostructuration maximizes the piezoelectric response under bias.

Physical review / B 96(18), 184420 (2017). doi:10.1103/PhysRevB.96.184420, We report a high-resolution resonant inelastic extreme ultraviolet (EUV) scattering study of the quantum Heisenberg antiferromagnet KCoF$_3$. By tuning the EUV photon energy to the cobalt M$_{23}$ edge, a complete set of low-energy 3d spin-orbital excitations is revealed. These low-lying electronic excitations are modeled using an extended multiplet-based mean-field calculation to identify the roles of lattice and magnetic degrees of freedom in modifying the resonant inelastic x-ray scattering (RIXS) spectral line shape. We have demonstrated that the temperature dependence of RIXS features upon the antiferromagnetic ordering transition enables us to probe the energetics of short-range spin correlations in this material., Published by APS, Woodbury, NY

Scientific reports 6, 38796 (2016). doi:10.1038/srep38796, In the past few years, we have been witnessing an increased interest for studying materials properties under non-equilibrium conditions. Several well established spectroscopies for experiments in the energy domain have been successfully adapted to the time domain with sub-picosecond time resolution. Here we show the realization of high resolution resonant inelastic X-ray scattering (RIXS) with a stable ultrashort X-ray source such as an externally seeded free electron laser (FEL). We have designed and constructed a RIXS experimental endstation that allowed us to successfully measure the d-d excitations in KCoF3 single crystals at the cobalt M$_{2,3}$-edge at FERMI FEL (Elettra-Sincrotrone Trieste, Italy). The FEL-RIXS spectra show an excellent agreement with the ones obtained from the same samples at the MERIXS endstation of the MERLIN beamline at the Advanced Light Source storage ring (Berkeley, USA). We established experimental protocols for performing time resolved RIXS experiments at a FEL source to avoid X ray-induced sample damage, while retaining comparable acquisition time to the synchrotron based measurements. Finally, we measured and modelled the influence of the FEL mixed electromagnetic modes, also present in externally seeded FELs, and the beam transport with ~120 meV experimental resolution achieved in the presented RIXS setup., Published by Nature Publishing Group, London

A long-standing question for avant-garde data storage technology concerns the nature of the ultrafast photoinduced phase transformations in the wide class of chalcogenide phase-change materials (PCMs). Overall, a comprehensive understanding of the microstructural evolution and the relevant kinetics mechanisms accompanying the out-of-equilibrium phases is still missing. Here, after overheating a phase-change chalcogenide superlattice by an ultrafast laser pulse, we indirectly track the lattice relaxation by time resolved x-ray absorption spectroscopy (tr-XAS) with a sub-ns time resolution. The approach to the tr-XAS experimental results reported in this work provides an atomistic insight of the mechanism that takes place during the cooling process; meanwhile a first-principles model mimicking the microscopic distortions accounts for a straightforward representation of the observed dynamics. Finally, we envisage that our approach can be applied in future studies addressing the role of dynamical structural strain in PCMs.

A new-concept approach to room temperature magnetic gas sensing has been developed, based on newly designed Co/ZnO hybrid nanostructures. The sensor prototype has been demonstrated to be sensitive, reversible, fast and scalable. In this work, the role of the Co/ZnO surface and interface in the gas-sensing mechanism has been clarified. In order to attain a comprehensive understanding of the physics governing the remarkable properties of the Co/ZnO stack, an extensive electronic and structural investigation has been carried out. The reaction at room temperature with target gases involves the surface and the lateral faces of the ZnO nanorods, with the formation of structural defects and vacancies. In particular, it has been discovered that the stress enhancement as well as the change in the polarizability of the ZnO nanorods are transduced by Co in a change of its magnetization. The interplay between these phenomena may provide versatile approaches to tune the intrinsic electronic, magnetic and optical properties of the hybrid nanostructure.

We here discuss the growth process and the properties of single-layer graphene on a vicinal Rh(533) surface. The structural anisotropy of the substrate leads to a moire cell with nonequivalent lattice vectors in the directions parallel and orthogonal to the steps. Our results indicate that the high structural quality of the carbon layer, combined with the weaker interaction with the substrate and the higher thermal stability with respect to graphene on Rh(111), is strongly influenced by the presence of the surface steps, which play a fundamental role in the defect-healing mechanism first predicted by earlier theoretical calculations.