van der Waals materials provide a versatile toolbox for the emergence of new quantum phenomena and fabrication of functional heterostructures. Among them, the trihalide VI3 stands out for its unique magnetic and structural landscape. Here we investigate the spin and orbital magnetic degrees of freedom in the layered ferromagnet VI3 by means of temperature-dependent X-ray absorption spectroscopy and X-ray magnetic circular and linear dichroism. We detect localized electronic states and reduced magnetic dimensionality, due to electronic correlations. We furthermore provide experimental evidence of (a) an unquenched orbital magnetic moment (up to 0.66(7) μB/V atom) in the ferromagnetic state and (b) an instability of the orbital moment in the proximity of the spin reorientation transition. Our results support a coherent picture where electronic correlations give rise to a strong magnetic anisotropy and a large orbital moment and establish VI3 as a prime candidate for the study of orbital quantum effects.

This thesis completes my work as doctoral student of the Scuola di Dottorato in Fisica, Astrofisica e Fisica Applicata at the Universit`a degli Studi di Milano, that has been carried out since November 2019 at the Istituto Officina dei Materiali of the Consiglio Nazionale delle Ricerche (IOM-CNR) in the premises of the Elettra - Sincrotrone Trieste and FERMI@Elettra infrastructures and in the framework of the NFFA facility.
My experimental activity employed complementary spectroscopy and polarimetry techniques oriented to address the characterisation of electronic and spin properties of systems with decreasing dimensionality. This programme has been conducted by exploiting state-of-the-art infrastructures to generate visible, UV and EUV ultrashort pulses (tabletop lasers and HHG at NFFA-SPRINT laboratory) and soft X-ray synchrotron light (at Elettra, Diamond and ESRF synchrotron light sources).
I used photoemission as the main tool in my investigation, supplementing my results with absorption spectroscopy. I focused on three materials, Fe(001)-p(1x1)O/MgO, EuSn2P2 and VI3, of high interest in modern and next-generation magnetic devices.
In the three systems I studied the electronic band structure to identify key features hinting at the bound electrons behaviour. I investigated the properties of the magnetically ordered phases and found evidence of the reduced dimensionality in the emergence of atypical spin ordering and the increasingly manifest electron correlation phenomena.
The information retained by band electrons is critical to access the spin polarisation of the bands and to give insight into the effects of spatial confinement on the spin degree of freedom.

Two-dimensional van der Waals magnetic semiconductors display emergent chemical and physical properties and hold promise for novel optical, electronic and magnetic “few-layers” functionalities. Transition-metal iodides such as CrI3 and VI3 are relevant for future electronic and spintronic applications; however, detailed experimental information on their ground state electronic properties is lacking often due to their challenging chemical environment. By combining X-ray electron spectroscopies and first-principles calculations, we report a complete determination of CrI3 and VI3 electronic ground states. We show that the transition metal-induced orbital filling drives the stabilization of distinct electronic phases: a wide bandgap in CrI3 and a Mott insulating state in VI3. Comparison of surface-sensitive (angular-resolved photoemission spectroscopy) and bulk-sensitive (X-ray absorption spectroscopy) measurements in VI3 reveals a surface-only V2+ oxidation state, suggesting that ground state electronic properties are strongly influenced by dimensionality effects. Our results have direct implications in band engineering and layer-dependent properties of two-dimensional systems.

La misura della polarizzazione in spin di un fascio di elettroni fotoemessi da una superficie ferromagnetica permette di studiare in modo diretto la struttura elettronica determinata dall’interazione di scambio e quindi il momento magnetico di spin del sistema, caratterizzandone il comportamento magnetico. Da una parte lo sviluppo del campo della spintronica, dall’altra la richiesta sempre crescente di strumenti e dispositivi di immagazzinamento e trattamento dati ad alte prestazioni, marcano la necessità di esplorare le configurazioni degli stati elettronici e le loro eccitazioni.