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We introduce the concept of geometric frustration in plasmonic arrays of nanoelements. In particular, we present the case of a hexagonal lattice of Au nanoasterisks arranged so that the gaps between neighboring elements are small and lead to a strong near-field dipolar coupling. Besides, far-field interactions yield higher-order collective modes around the visible region that follow the translational symmetry of the lattice. However, dipolar excitations of the gaps in the hexagonal array are geometrically frustrated for interactions beyond nearest neighbors, yielding the destabilization of the low energy modes in the near infrared. This in turn results in a slow dynamics of the optical response and a complex interplay between localized and collective modes, a behavior that shares features with geometrically frustrated magnetic systems.

Treballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Curs: 2022-2023, Tutora: Arantxa Fraile Rodríguez, This work has consisted on processing and analysing synchrotron-based magnetic Transmission X-ray Microscopy images to obtain the internal magnetic spin texture in individual nanoflowers (NF) of about 200 nm in average size. By extracting 2D maps of the internal configuration of the magnetic moments of the cores within each NF for several tens of NF, it has been possible to visualise these different internal spin textures, all of them having in common a total magnetization close to zero. Those have been compared with previous studies, and are in agreement with prior OOMMF simulations. Due to the great variety of these configurations, a categorisation of NF subfamilies has been developed. An analysis of how the individual configurations are affected by the distance between NF has also been studied.

Treballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Curs: 2022-2023, Tutora: Arantxa Fraile Rodriguez, NiO nanoparticles (NP) exhibit relevant physical features when compared to the bulk materials, such as charge-transfer effects and interesting antiferromagnetic (AF) spin structure. Chemical and magnetic studies are often limited to ensembles of NiO NP which are limited to averaging their individual electronic, and magnetic properties with respect to their individual characteristics. In this work, we have analyzed with single-particle sensitivity the chemical, and magnetic properties of three samples of NiO nanoparticles in a size range of 40-60 nm, synthesized by wet chemistry routes, by means of synchrotron-based X-ray Photoemission Electron Microscopy (X-PEEM) combined with X-ray Magnetic Linear Dichroism (XMLD) around the Ni L3,2 edges. The analysis of the local Xray absorption spectra indicates a strong dependence of the oxidation state of the individual NP with the oxidation protocol of each sample. Furthermore, first evaluation of the XMLD signal with respect to the linear polarization angle, a signature of AF order, indicates different AF spin easy axis orientations between individual NiO NP

Treballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Curs: 2020, Tutors: Albert Queraltó, Teresa Puig, Arantxa Fraile Rodríguez, The future fabrication of high-temperature-superconductor-based coated conductors depends on the current research of thin films’ growth processes, especially in developing low cost, scalable and industrially processing methods. A new transient liquid assisted growth method is explored through the growth of 𝑌𝐵𝑎$𝐶𝑢'𝑂) − 𝐵𝑎𝑍𝑟𝑂' and 𝑌𝐵𝑎$𝐶𝑢'𝑂)thin films. The characterization of spin-coating deposed thin films verified c-axis epitaxy, although the presence of secondary phases within the film were also confirmed. 𝐵𝑎𝑍𝑟𝑂' nanoparticles proved to be useful to enhance density superconducting properties at high fields, furthermore a 1.77 MA cm-2 current density was achieved for 𝑌𝐵𝑎$𝐶𝑢'𝑂) − 𝐵𝑎𝑍𝑟𝑂' films at 5 K, whereas for lesser fields 𝑌𝐵𝑎$𝐶𝑢'𝑂) current density was up to 19.94 MA cm-2 also at 5 K. Critical temperatures of (89.2 ± 0.6)K and (90.6 ± 0.4) K were obtained for the films, respectively. First steps towards a new combinational chemistry approach for the inkjet printed samples were also performed

Treballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Curs: 2020, Tutora: Arantxa Fraile Rodríguez, The effect of topography in the Raman signal from solutions of 4-mercaptopyridine molecules is determined when deposited with different concentrations on two different Au substrates with plasmonic properties: continuous thin films and ordered lattice of nanoelements. The topography is characterized through Atomic Force Microscopy imaging. The results show how the nanostructured substrate affects the topography presenting similar configuration to more diluted solutions

Treballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Curs: 2018, Tutora: Arantxa Fraile Rodríguez, The system formed by a 10 nm thin film of Ni and a 10 μm-thick V2O3 layer shows a reversible enhancement of the coercivity (∼300%) due to the structural phase transition (metal/insulator) of the V2O3layer. Performing micromagnetic simulations and comparing the results with experimental data we have studied the coupling mechanism on the Ni/V2O3 interface. We have proven that this coupling is responsible for the coercivity enhancement and that the magnetisation domains in the Ni do not follow exactly the metal/insulator domains on the V2O3. Finally, we have determined that the results of the simulations are in agreement with a phase coexistence in V2O3.

In this dissertation, the manipulation of the magnetic properties of ultra-thin ferromagnetic (FM) films by varying the interfacial coupling on both sides of the film, e.g. with the capping layer and with the substrate, has been studied by means of X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). Firstly, XAS and XMCD measurements were carried out at the Fe L2;3 absorption edges of an ultra-thin iron wedge grown on a ferroelectric [Pb(Mg1/3Nb2/3)O3]0.68- [PbTiO3]0.32 (PMN-PT) (001) substrate using the total electron yield mode. Upon switching the ferroelectric polarization of the substrate from Pup to Pdown, we observe a strong change of approximately 20% of the Fe total magnetic moment for a film thickness of 1:5 nm. However, when the dimensionality of the system increases, i.e. for a film thickness of 3 nm, the same FE polarization only weakly affect the Fe magnetic moments. Taking the sampling depth of the measurement method into account, this difference is compatible with a magnetic anisotropy change taking place only in the first interfacial layer in contact with the ferroelectric substrate. We attribute this interfacial coupling to a charge accumulation or depletion at the interface. Similar measurements were carried out for different systems such as Pd/Fe/PMN-PT system in which the 3 nm Cr capping was replaced by the 3 nm Pd capping. In this heterostructure system, we did not observe a strong anisotropy for Pup as the remanent moments are observed along both the in-plane and out-of-plane directions. Upon switching the FE polarization of the PMN-PT to down in Pd/Fe/PMN-PT system, the remanence magnetic moments increase along the in-plane direction while they decrease along the out-of-plane direction. Further, we report an increase in the saturation moments for the Pd/Fe/PMN-PT system compared to the bcc Fe. In order to gain further insight into the magnetic properties at the interface between oxides and metals, we investigated ultra-thin films of 1:2 nm thickness Co40Fe40B20 (CoFeB), using the XMCD technique, by probing at the Fe and Co L2;3 absorption edges. Systems with different oxide interfaces such as Ta/CoFeB/MgO, Ta/CoFeB/BaTiO3 (BTO) and Ta/CoFeB/Ba0.1Sr0.9TiO3 (BSTO) samples have been investigated. For the CoFeB/MgO system, the XMCD measurement revealed that magnetization along in-plane and out-of-plane direction are similar, i.e., the system is very isotropic. However, when the same thickness of CoFeB is grown on BSTO and BTO substrates, the easy axis of the magnetization aligns along the in-plane direction. The magnetic moments for Fe and Co shown by all the three systems are less than the bulk bcc Fe and bulk hcp Co, which is attributed to a dead layer at the CoFeB/Ta interface. We also report an enhancement in the orbital moments for Fe and Co atoms in comparison to bulk bcc Fe and bulk hcp Co due to the interface with different oxides. Lastly, the effect of capping layers of Tantalum (Ta), Platinum (Pt) and Ruthenium (Ru) on the CoFeB/MgO system has been studied. The Ta/CoFeB/MgO system shows an isotropic behavior, while the Pt/Ru capped/CoFeB/MgO systems show an in-plane magnetization. The Fe and Co magnetic moments were calculated using the XMCD sum rules, in which we clearly observed the change in moments among the three systems. Pt/CoFeB/MgO system shows an increase in the magnetic moments of Fe and Co atoms as compared to the Ta/CoFeB/MgO system. On the other hand, Ru/CoFeB/MgO system shows a decrease in the Fe moments as compared to Ta/CoFeB/MgO. The Pt/CoFeB/MgO system shows Co moments similar to the bulk hcp Co. XAS spectra at the Fe and Co L2;3 edges, show a shift in the energy of the XAS peak when capped with Pt and Ru in comparison to Ta capped films. In addition the Ru/CoFeB/MgO systems shows a shoulder at the Fe L3-edge due to the interaction of Ru and Fe orbitals. Whereas, the Pt/CoFeB/MgO system shows a shift of the Co L3-edge to lower energy in comparison to the Ta/CoFeB/MgO system which can be the result of interaction between Pt and Co orbitals.

Treballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Curs: 2016, Tutor: Arantxa Fraile Rodríguez, Magnetic properties of thin films can be modified through proximity effects with multilayer structures. Materials that undergo both a structural phase transition (SPT) and a metal to insulator transition (MIT) have been used for this purpose. This crystallographic change induces a stress to the adjacent layer that changes the magnetic properties of the film through magnetoelastic anisotropy. This work is devoted to the study of magnetic properties of Ni/V2O3 affected by the SPT of the V2O3. In order to achieve this goal, images obtained with synchrotron-based photoemission electron microscopy have been analyzed to spatially resolve the magnetic domains orientations and observe its evolution with an applied magnetic field. For this purpose-built algorithm has been developed and used in addition to other existing image and data analysis tools. Continuous distributions of coercitivites and magnetizations have been obtained in addition to a rather abrupt magnetic reversal mechanism