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Doped HfO2 and HfO2-ZrO2 compounds are gaining significant interest thanks to their ferroelectric properties in ultrathin films. Here, we show that ZrO2 could be a playground for doping and strain engineering to increase the thickness in epitaxial thin films. Based on surface energy considerations supported by ab initio calculations, we show that a pure ZrO2 epitaxial thin film exhibits a ferroelectric rhombohedral phase (r-phase, with R3m space group) more stable than for the Hf0.5Zr0.5O2 (HZO) and pure HfO2 cases. In particular, for a thickness up to 37 nm we experimentally evidence a single (111)-oriented r-phase in ZrO2 films deposited on La2/3Sr1/3MnO3-buffered DyScO3(110) substrate, while a tetragonal phase is observed alongside the rhombohedral one on SrTiO3(001). The formation of this r-phase is discussed and compared between HfO2, ZrO2 and HZO, highlighting the role of surface energy., 21 pages, 14 figures, 2 tables, including supplemental information

Silicon photonics has been largely developed as a platform to address the future challenges in several applications including datacom, sensing or optical communications, among others. However, the properties of silicon itself is not enough to overcome all limitations in terms of speed, power consumption and scalability. New strategies have then been encouraged based on the hybrid integration of new materials in the silicon photonics platform. In this paper, we will introduce the recent advances in the hybrid integration of doped crystalline-oxides on silicon and silicon nitride waveguides. Especially, Yttria-stabilized zirconia (YSZ) with a lattice parameter compatible with the silicon lattice has been considered because it exhibits promising linear and nonlinear optical properties: low propagation loss, no two photon absorption (TPA) due to its large bandgap energy, a large transparency window from the ultraviolet to the mid-infrared and a good Kerr effect. Furthermore, YSZ can be doped with many dopants to develop active photonic devices with strong second- and third-order nonlinearities and light emission. We have recently demonstrated propagation loss in YSZ waveguides as low as 2dB/cm at a wavelength of 1380 nm, a nonlinear refractive index (Kerr effect) comparable with the SiN coefficient and light amplification in Er3+ doped YSZ on SiN waveguides. The recent results are very promising to pave the way for the development of low cost and low power consumption devices.

Magnetoelectric material-based cantilever resonators have been considered as a promising solution for magnetic sensing applications. However, most applications focus on bulk piezoelectric (e.g., PZT) laminated composites, which put a critical limit for miniaturizing into micrometer-sized devices. This work aims at demonstrating the potential of a micro-resonator approach with lower power consumption and smaller size. It reports on the fabrication and characterization of a resonant cantilever based on a freestanding multi-ferroic PZT/Tb–Fe–Co thin film multilayer, where the magnetic signal is sensed by measuring the shift of the device resonant frequency. The Tb–Fe–Co layer acts as a magnetic field sensing layer, while the PZT thin film integrated in the capacitor geometry acts as a micro-transducer to obtain an electrical signal. For a magnetic field less than 0.2 T, a sensitivity as high as 487 Hz/T is measured for the sensor under a vacuum environment. While the sensor design has to be further optimized to improve the performance, it is promising as a micro-magnetoelectric sensor for magnetic field sensing.

Hard X-ray nanodiffraction provides a unique nondestructive technique to quantify local strain and structural inhomogeneities at nanometer length scales. However, sample mosaicity and phase separation can result in a complex diffraction pattern that can make it challenging to quantify nanoscale structural distortions. In this work, a k-means clustering algorithm was utilized to identify local maxima of intensity by partitioning diffraction data in a three-dimensional feature space of detector coordinates and intensity. This technique has been applied to X-ray nanodiffraction measurements of a patterned ferroelectric PbZr0.2Ti0.8O3 sample. The analysis reveals the presence of two phases in the sample with different lattice parameters. A highly heterogeneous distribution of lattice parameters with a variation of 0.02 Å was also observed within one ferroelectric domain. This approach provides a nanoscale survey of subtle structural distortions as well as phase separation in ferroelectric domains in a patterned sample.

Enabling the light control of complex systems on ultrashort timescales gives rise to rich physics with promising applications. Although crucial, the quantitative determination of both the longitudinal and the shear photoinduced strains still remains challenging. Here, by scrutinizing asymmetric Bragg peaks pairs $(\ifmmode\pm\else\textpm\fi{}h01)$ in ${\mathrm{BiFeO}}_{3}$ using picosecond time-resolved x-ray diffraction experiments, we simultaneously determine the longitudinal and shear strains. Importantly, we reveal a difference in the dynamical response of the longitudinal strain with respect to the shear one due to an interplay of quasilongitudinal and quasitransverse acoustic modes, well reproduced by our model. Finally, we show that the relative amplitude of those strains can be explained only if both thermal and nonthermal processes contribute to the acoustic phonon photogeneration process.

A ferroelectric BaTiO3 thin film grown on a NdScO3 substrate was studied using x-ray photon correlation spectroscopy (XPCS) to characterize thermal fluctuations near the a/b to a/c domain structure transformation present in this low-strain material, which is absent in the bulk. XPCS studies provide a direct comparison of the role of domain fluctuations in first- and second-order phase transformations. The a/b to a/c domain transformation is accompanied by a decrease in fluctuation timescales, and an increase in intensity and correlation length. Surprisingly, domain fluctuations are observed up to 25 degrees C above the transformation, concomitant with the growth of a/c domains and coexistence of both domain types. After a small window of stability, as the Curie temperature is approached, a/c domain fluctuations are observed, albeit slower, potentially due to the structural transformation associated with the ferroelectric to paraelectric transformation. The observed time evolution and reconfiguration of domain patterns highlight the role played by phase coexistence and elastic boundary conditions in altering fluctuation timescales in ferroelectric thin films.

A role for vacancies Hafnia-based materials are of interest because of their potential use in microelectronic components. Hafnia-oxide is a ferroelectric material, but whether the polarization switching comes from the polar crystal phases or the migration of oxygen vacancies has remained an open question. Nukala et al. attempted to resolve this controversy by conducting electron microscopy during the operation of a hafnium zirconium oxide capacitor. The authors found that vacancy migration is intertwined with the ferroelectric switching, which has implications for the use of these materials in a range of microelectronic applications. Science , this issue p. 630

Hard X-ray nanodiffraction provides a unique nondestructive technique to quantify local strain and structural inhomogeneities at nanometer length scales. However, sample mosaicity and phase separation can result in a complex diffraction pattern that can make it challenging to quantify nanoscale structural distortions. In this work, a k-means clustering algorithm was utilized to identify local maxima of intensity by partitioning diffraction data in a three-dimensional feature space of detector coordinates and intensity. This technique has been applied to X-ray nanodiffraction measurements of a patterned ferroelectric PbZr

Ferroelectric thin films are investigated for their potential in photovoltaic (PV) applications, owing to their high open-circuit voltage and switchable photovoltaic effect. The direction of the ferroelectric polarization can control the sign of the photocurrent through the ferroelectric layer, theoretically allowing for 100% switchability of the photocurrent with the polarization, which is particularly interesting for photo-ferroelectric memories. However, the quantitative relationship between photocurrent and polarization remains little studied. In this work, a careful investigation of the polarization-dependent photocurrent of epitaxial Pb(Zr, Ti)O3 thin films has been carried out, and has provided a quantitative determination of the unswitchable part of ferroelectric polarization. These results represent a systematic approach to study and optimize the switchability of photocurrent, and more broadly to get important insights on the ferroelectric behavior in all types of ferroelectric layers in which pinned polarization is difficult to investigate.

Nonlinear all-optical technology is an ultimate route for the next-generation ultrafast signal processing of optical communication systems. New nonlinear functionalities need to be implemented in photonics, and complex oxides are considered as promising candidates due to their wide panel of attributes. In this context, yttria-stabilized zirconia (YSZ) stands out, thanks to its ability to be epitaxially grown on silicon, adapting the lattice for the crystalline oxide family of materials. We report, for the first time to the best of our knowledge, a detailed theoretical and experimental study about the third-order nonlinear susceptibility in crystalline YSZ. Via self-phase modulation-induced broadening and considering the in-plane orientation of YSZ, we experimentally obtained an effective Kerr coefficient of n ^ 2 YSZ = 4.0 ± 2 × 10 − 19 m 2 · W − 1 in an 8% (mole fraction) YSZ waveguide. In agreement with the theoretically predicted n ^ 2 YSZ = 1.3 × 10 − 19 m 2 · W − 1 , the third-order nonlinear coefficient of YSZ is comparable with the one of silicon nitride, which is already being used in nonlinear optics. These promising results are a new step toward the implementation of functional oxides for nonlinear optical applications.

Unconventional ferroelectricity, robust at reduced nanoscale sizes, exhibited by hafnia-based thin-films presents tremendous opportunities in nanoelectronics. However, the exact nature of polarization switching remains controversial. Here, we investigate epitaxial Hf0.5Zr0.5O2 (HZO) capacitors, interfaced with oxygen conducting metals (La0.67Sr0.33MnO3, LSMO) as electrodes, using atomic resolution electron microscopy while in situ electrical biasing. By direct oxygen imaging, we observe reversible oxygen vacancy migration from the bottom to the top electrode through HZO and reveal associated reversible structural phase transitions in the epitaxial LSMO and HZO layers. We follow the phase transition pathways at the atomic scale and identify that these mechanisms are at play both in tunnel junctions and ferroelectric capacitors switched with sub-millisecond pulses. Our results unmistakably demonstrate that oxygen voltammetry and polarization switching are intertwined in these materials., Comment: 29 pages (including supplementary), 4 main figures, 6 supplementary figures

On-chip optical amplifiers operating at telecom wavelengths are crucial elements for signal recovering and routing in photonic integrated circuits. In this work, we present the optical and structural properties of Er-doped yttria-stabilized zirconia (YSZ) thin films for the implementation of on-chip optical amplifiers in hybrid multifunctional photonic platforms. Photoluminescence excitation measurements have revealed strong luminescence at 1530 nm under 960 nm wavelength pumping, with lifetime values around 2 ms and a strong Er3++–Er3+ interaction for Er3+ doping concentrations beyond 1.5 atomic percentage (at%). This work contributes to establish the solid foundations for a class of Er-doped on-chip amplifiers using the robust and stable YSZ host matrix.

Ferroelectric materials exhibit the largest dielectric permittivities and piezoelectric responses in nature, making them invaluable in applications from supercapacitors or sensors to actuators or electromechanical transducers. The origin of this behavior is their proximity to phase transitions. However, the largest possible responses are most often not utilized due to the impracticality of using temperature as a control parameter and to operate at phase transitions. This has motivated the design of solid solutions with morphotropic phase boundaries between different polar phases that are tuned by composition and that are weakly dependent on temperature. Thus far, the best piezoelectrics have been achieved in materials with intermediate (bridging or adaptive) phases. But so far, complex chemistry or an intricate microstructure has been required to achieve temperature-independent phase-transition boundaries. Here, we report such a temperature-independent bridging state in thin films of chemically simple BaTiO3. A coexistence among tetragonal, orthorhombic, and their bridging low-symmetry phases are shown to induce continuous vertical polarization rotation, which recreates a smear in-transition state and leads to a giant temperature-independent dielectric response. The current material contains a ferroelectric state that is distinct from those at morphotropic phase boundaries and cannot be considered as ferroelectric crystals. We believe that other materials can be engineered in a similar way to contain a ferroelectric state with gradual change of structure, forming a class of transitional ferroelectrics. Similar mechanisms could be utilized in other materials to design low-power ferroelectrics, piezoelectrics, dielectrics, or shape-memory alloys, as well as efficient electro- and magnetocalorics.

Multiferroic tunnel junctions (MFTJs) with Hf0.5Zr0.5O2 barriers are reported to show both tunneling magnetoresistance effect (TMR) and tunneling electroresistance effect (TER), displaying four resistance states by magnetic and electric field switching. Here we show that, under electric field cycling of large enough magnitude, the TER can reach values as large as 106%. Moreover, concomitant with this TER enhancement, the devices develop electrical control of spin polarization, with sign reversal of the TMR effect. Currently, this intermediate state exists for a limited number of cycles and understanding the origin of these phenomena is key to improve its stability. The experiments presented here point to the magneto-ionic effect as the origin of the large TER and strong magneto-electric coupling, showing that ferroelectric polarization switching of the tunnel barrier is not the main contribution.

Ferroelectric tunnel barriers in between two ferromagnetic electrodes (multiferroic tunnel junctions, or MFTJs) hold great promise for future microelectronic devices. Here, we utilize Hf0.5Zr0.5O2 (HZO) tunnel barriers with an ultralow thickness of only 2 nm, epitaxially grown on La0.7Sr0.3MnO3 ferromagnetic bottom electrodes and with cobalt top electrodes. Both tunneling electroresistance and tunneling magnetoresistance effects are observed, demonstrating four nonvolatile resistance states in HZO-based junctions. The large band gap and excellent homogeneity of the HZO tunnel barriers enable a high yield of working devices, as well as devices with sizes of tens of micrometers. This allows working with fixed electrodes, as opposed to the use of scanning probes, bringing MFTJs closer to applications.

International audience

New optical materials for hybrid photonic integration on silicon platform have become a hot research topic aiming at providing additional functionalities. In this regard, functional oxides are a very interesting class of materials due to their singular properties. Material engineering is commonly employed to tune and manipulate such properties at will, thus being functional oxides often used to build active reconfigurable elements in complex systems. Transparent oxides with moderate refractive indexes are targeted for hybrid integration due to the rewarding benefits envisioned. Yttria-Stabilized Zirconia (YSZ) is a chemically stable oxide1 with a transparency range that spans from the visible to the mid-IR2, with a refractive index around 2.1, which makes this functional oxide interesting for the development of low-loss waveguides when grown over a low contrast substrate. While these optical properties are very interesting for various applications, including on-chip optical communications and sensing, YSZ has remained almost unexplored in photonics up to now. Nevertheless, this complex functional oxide shows interesting optical properties such as low-moderate propagation losses of 2 dB/cm at telecom wavelengths3. In our work, we explore the deposition of erbium doped YSZ by pulsed layer deposition (PLD) on a multilayer approach providing outstanding luminescence in correspondence with C-band of telecommunication window (λ=1530 nm) and in the visible range by means of up-conversion processes. The optical properties of active layers of Er-doped YSZ grown on waveguides in different platforms and under resonant pumping will be discussed in this paper, as well as their propagation losses. Based on the preliminary study of these active hybrid systems, we envision light amplification functionalities based on rare-earth doped functional oxides.

International audience; After decades of searching for robust nanoscale ferroelectricity that could enable integration into the next generation memory and logic devices, hafnia-based thin films have appeared as the ultimate candidate because their ferroelectric (FE) polarization becomes more robust as the size is reduced. This exposes a new kind of ferroelectricity, whose mechanism still needs to be understood. Towards this end, thin films with increased crystal quality are needed. We report the epitaxial growth of Hf0.5Zr0.5O2 (HZO) thin films on (001)-oriented La0.7Sr0.3MnO3/SrTiO3 (STO) substrates. The films, which are under epitaxial compressive strain and are predominantly (111)-oriented, display large FE polarization values up to 34 μC/cm 2 and do not need wake-up cycling. Structural characterization reveals a rhombohedral phase, different from the commonly reported polar orthorhombic phase. This unexpected finding allows us to propose a compelling model for the formation of the FE phase. In addition, these results point towards nanoparticles of simple oxides as a vastly unexplored class of nanoscale ferroelectrics.

Multiferroics are gathering solid-state matter in which several types of orders are simultaneously allowed, as ferroelectricity, ferromagnetism (or antiferromagnetism), ferroelasticity, or ferrotoroidicity. Among all, the ferroelectric/ferromagnetic couple is the most intensively studied because of potential applications in novel low-power magnetoelectric devices. Switching of one order thanks to the other necessarily proceeds via the nucleation and growth of coupled domains. This review is an introduction to the basics of ferroelectric/ferromagnetic domain formation and to the recent microscopy techniques devoted to domains imaging, providing new insights into the archetypal multiferroic domain morphologies. Some relevant examples are also given to illustrate some of the unexpected properties of domain walls, as well as the way these domain walls can be manipulated altogether thanks to various types of magnetoelectric coupling.

This study is focused on the link between the structural and electric properties of BaTiO3 thin films grown on SrRuO3-buffered (001) SrTiO3 substrates, SrRuO3 acting as bottom electrode. The growth regime and film structure are here tuned through the growth pressure for pulsed laser deposition in the 1–200 mTorr range. The dielectric, ferroelectric and leakage current properties are systematically measured for the different strain states of the BaTiO3 thin films on SrRuO3. The results are discussed with the help of ab initio calculations on the effects of Ba- and Ti-vacancies on BaTiO3 lattice parameters. A sharp increase of the dielectric constant is evidenced in the high pressure region, where the tetragonality of the BaTiO3 is decreasing rapidly with growth pressure. We interpret this divergence of the dielectric function as the signature of the vicinity of the phase boundary between the out-of-plane and in-plane orientations of the tetragonal BTO films.

The spinel $\mathrm{CoC}{\mathrm{r}}_{2}{\mathrm{O}}_{4}$ (CCO) is one of the few bulk multiferroics with net magnetic moment. However, studies on the properties of CCO thin films are scarce. Here, we investigate the interplay between microstructure and magnetism of a series of CCO epitaxial thin films by means of x-ray diffraction, magnetometry, and aberration-corrected scanning transmission electron microscopy (STEM). Optimum pristine CCO films can be grown on a substrate with spinel structure $(\mathrm{MgA}{\mathrm{l}}_{2}{\mathrm{O}}_{4})$, despite the large lattice mismatch of $\ensuremath{\sim}3%$. On the contrary, a substrate with lower lattice mismatch $(\ensuremath{\sim}1%)$ but with the rock salt structure (MgO) favors the degradation of the CCO crystal quality by forming antiphase boundaries (APBs), drastically weakening the magnetization, in agreement with reports for other spinel films. Nevertheless, our results also show that the type and number of APBs can be tuned by changing the growth temperature to favor the ferromagnetic alignment between antiphase domains, giving rise to a partial recovery of the magnetization.

The purpose of this work is to explore an alternative approach for high speed and low power consumption optical modulation based on the use of the Pockels effect in silicon. Unfortunately, silicon is a centro-symmetric crystal leading to a vanishing of the second order nonlinear coefficient, i.e. no Pockels effect. To overcome this limitation, on possibility would be to break the crystal symmetry by straining the silicon lattice with the epitaxial growth of crystalline functional oxides. Indeed, the induced strain due to lattice parameter mismatch and the difference in the thermal expansion coefficients between oxides and silicon are strong and may induce strong strain into silicon. Furthermore, functional oxides can exhibit very interesting multiferroicity and piezoelectricity properties that pave the way to a new route to implement silicon photonic circuits with unprecedented functionalities.

Epitaxial strain in ferroelectric films offers the possibility to enhance the piezoelectric performance utilizing low crystal symmetries and high density of domain walls. Ferroelectric BaTiO3 has been predicted to order in a variety of phases and domain configurations when grown under low strain on low mismatched substrates, but little experimental evidence of that region of the phase diagram exist. Here, epitaxial BaTiO3 thin films are grown on NdScO3 substrates under ≈0.1% strain. A monoclinic ca1/ca2 phase, with 90° periodic in-plane domain configuration and small additional out-of-plane component of polarization, is stabilized at room temperature and investigated using piezoelectric force microscopy and X-ray diffraction. Above 50 °C, this phase is transformed into an a/c phase with alternating in-plane and out-of-plane polarizations and forming zigzag domain walls between up-polarized and down-polarized superdomains. Both types of domain patterns are highly anisotropic, giving rise to very long domain walls. Above 130 °C, the paraelectric phase is observed. The occurrence of a phase transition close to room temperature, a low symmetry ca1/ca2 phase, and the formation of periodic domains make of this material a promising candidate for high piezoelectric response., Parts of this research were carried out at the light source Petra III at DESY, a member of the Helmholtz Association (HGF). Work at ICN2 was supported by the Severo Ochoa Excellence programme.

We show that the magnetic anisotropy in spinel-structure CoCr$_2$O$_4$ thin films exhibits a strain dependence in which compressive strain induces an out-of-plane magnetic easy axis and tensile strain an in-plane easy axis, exactly opposite to the behavior reported for the related compound CoFe$_2$O$_4$. We use density functional theory calculations within the LSDA+U approximation to reproduce and explain the observed behavior. Using second-order perturbation theory, we analyse the anisotropy tensor of the Co$^{2+}$ ions in both octahedral and tetrahedral coordination, allowing us to extend our results to spinels with general arrangements of Co$^{2+}$ ions., 8 pages, 7 figures

International audience; 2D electron systems (2DESs) in functional oxides are promising for applications, but their fabrication and use, essentially limited to SrTiO3-based heterostructures, are hampered by the need for growing complex oxide overlayers thicker than 2 nm using evolved techniques. It is demonstrated that thermal deposition of a monolayer of an elementary reducing agent suffices to create 2DESs in numerous oxides.

With shrinking device sizes, controlling domain formation in nanoferroelectrics becomes crucial. Periodic nanodomains that self-organize into so-called 'superdomains' have been recently observed, mainly at crystal edges or in laterally confined nanoobjects. Here we show that in extended, strain-engineered thin films, superdomains with purely in-plane polarization form to mimic the single-domain ground state, a new insight that allows a priori design of these hierarchical domain architectures. Importantly, superdomains behave like strain-neutral entities whose resultant polarization can be reversibly switched by 90 degrees, offering promising perspectives for novel device geometries.

International audience; NiFe2O4(111) ultrathin films (3-5 nm) have been grown by oxygen-assisted molecular beam epitaxy and integrated as effective spin-filter barriers. Structural and magnetic characterizations have been performed in order to investigate the presence of defects that could limit the spin filtering efficiency. These analyses have revealed the full strain relaxation of the layers with a cationic order in agreement with the inverse spinel structure but also the presence of antiphase boundaries. A spin-polarization up to þ25% has been directly measured by the Meservey-Tedrow technique in Pt(111)/NiFe2O4(111)/c-Al2O3(111)/Al tunnel junctions. The unexpected positive sign and relatively small value of the spin-polarization are discussed, in comparison with predictions and previous indirect tunnelling magnetoresistance measurements

We report on the thickness dependence of the ferroelastic domains of PbTiO3 films grown on (110)-DyScO3 with low thicknesses (up to 240 nm), which fall outside the validity range of the square root law proposed by Roytburd. For slow-grown films, the data reveal the linear thickness dependence predicted by Pertsev and Zembilgotov, using a complete elastic description, while a 2/3 scaling exponent is found for fast-grown films. Extremely long domains running all through the samples have been observed in the latter case, compared to the short domains observed in slow-grown films. These differences are ascribed to the in-plane anisotropy for domain wall nucleation, which is likely caused by the anisotropic elastic modulus of the substrate. © 2013 AIP Publishing LLC., This work was supported by NanoNextNL, a micro and nanotechnology consortium of the Government of the Netherlands and 130 partners.

International audience; We report on the correlations between magnetic anisotropy and strain state in CoFe$_2$O$_4$ ultrathin films grown on MgAl$_2$O$_4$(100) and MgAl$_2$O$_4$(111) substrates. By local strain analysis using the geometric phase method, a significant in-plane compressive strain is observed for the (001) orientation while a full relaxation is detected for the (111) orientation. The relaxation process in CoFe$_2$O$_4$(111) layers induces interface dislocations and a large amount of antiphase boundaries while a pseudomorphic growth is observed for the (001) direction, decreasing significantly the density of antiphase boundaries. By comparing the magnetoelastic energy terms, the correlation between strain state and resultant magnetization is discussed.

A spin filtering effect has been evidenced in epitaxial MnFe${}_{2}$O${}_{4}$ tunnel barriers directly by Meservey-Tedrow experiments. The asymmetry of the Zeeman-split tunneling conductance curves of the superconducting Al spin analyzer in Pt(111)/MnFe${}_{2}$O${}_{4}$(111)/\ensuremath{\gamma}-Al${}_{2}$O${}_{3}$(111)/Al tunnel junctions revealed a positive spin-polarization (up to +9$%$), proving the potential of manganese ferrite for generation of a spin-polarized current. A negatively polarized spin filtering being expected theoretically, different mechanisms are discussed to explain both sign and amplitude of the measured spin-polarization.

Metal-oxide tunnel junctions made by optical lithography were studied by X-ray diffraction (imaging approach with diffraction contrast) to access local distortions of the crystalline planes of the constituting layers. The effect of the size (10–150 µm) and shape (disc, square and rectangle) of the junction was investigated., X-ray diffraction techniques are used in imaging mode in order to characterize micrometre-sized objects. The samples used as models are metal–oxide tunnel junctions made by optical lithography, with lateral sizes ranging from 150 µm down to 10 µm and various shapes: discs, squares and rectangles. Two approaches are described and compared, both using diffraction contrast: full-field imaging (topography) and raster imaging (scanning probe) using a micrometre-sized focused X-ray beam. It is shown that the full-field image gives access to macroscopic distortions (e.g. sample bending), while the local distortions, at the micrometre scale (e.g. tilts of the crystalline planes in the vicinity of the junction edges), can be accurately characterized only using focused X-ray beams. These local defects are dependent on the junction shape and larger by one order of magnitude than the macroscopic curvature of the sample.

In spintronics, spin filtering is a physical phenomenon which has the potential to produce highly spin-polarized electron currents by the spin-selective transport of electrons across a ferromagnetic tunnel barrier. While spin filtering at room temperature is highly attractive for applications, the selection of materials having the necessary magnetic and electronic properties is quite limited. The insulating ferrites XFe2O4 (X= Co, Ni, Mn) whose Curie temperatures are well above 300 K are promising candidates for spin filtering at room temperature. In this thesis, I present an in-depth study of epitaxial MnFe2O4, NiFe2O4 and CoFe2O4 ultrathin films for room temperature spin filtering, by optimizing their crystalline growth by molecular beam epitaxy. The structural, chemical, magnetic and electrical properties were studied by a number of in situ and ex situ characterization techniques, demonstrating the potential of these oxides to act as magnetic tunnel barriers at room temperature. The spin filters were then integrated into tunnel junctions in order to perform spin-polarized tunneling transport experiments, involving either the Meservey-Tedrow technique or tunneling magnetoresistance (TMR) measurements. These experiments revealed for the first time the spin filtering capability of MnFe2O4(111) tunnel barriers and TMR effects were also observed at room temperature in nanometric tunnel junctions based on CoFe2O4(111) spin filter, showing the highest spin polarization value for a tunnel current at room temperature by spin filtering., Dans le domaine de l'électronique de spin, le filtrage de spin est un phénomène physique qui permet de générer des courants d'électrons polarisés en spin grâce au transport dépendant du spin à travers une barrière tunnel ferromagnétique. Alors que le filtrage de spin à température ambiante est très attractif pour les applications, il existe peu de matériaux ayant les proprieties électriques et magnétiques requises. Les ferrites isolants XFe2O4 (X= Co, Ni, Mn), qui présentent des températures de Curie nettement supérieures à 300 K, sont de bons candidats pour jouer le rôle de filtre à spin à température ambiante. Dans cette thèse, je présente une étude approfondie des ferrites MnFe2O4, NiFe2O4 et CoFe2O4 en films ultraminces épitaxiés pour le filtrage de spin à température ambiante, en me penchant sur leur croissance par épitaxie par jets moléculaires. Les proprieties structurales, chimiques, magnétiques et électriques ont été étudiées par plusieurs méthodes de caractérisation in situ et ex situ, qui ont permis de démontrer le fort potentiel de ces oxydes pour jouer le rôle de barrière tunnel magnétique à temperature ambiante. Les filtres à spin ont ensuite été intégrés dans des jonctions tunnel afin de faire des mesures de transport tunnel polarisé en spin, soit par la méthode de Meservey-Tedrow, soit par des mesures de magnétorésistance tunnel (TMR). Ces mesures ont révélé pour la première fois un effet de filtrage de spin à travers MnFe2O4(111) et des effets de TMR ont été obtenus dans des nano-jonctions tunnel à base de CoFe2O4(111), permettant d'obtenir la plus forte polarisation en spin actuelle à température ambiante par effet de filtrage de spin.

${\text{CoFe}}_{2}{\text{O}}_{4}/{\text{Fe}}_{3}{\text{O}}_{4}$ bilayers represent an unusual exchange-coupled system whose properties are due to the nature of the oxide-oxide superexchange interactions at the interface. In this work, we analyze the magnetization reversal behavior and magnetic interface structure using a variety of advanced techniques. Low-temperature magnetization, magnetotransport, and polarized neutron reflectivity measurements suggest that the magnetization configuration at the bilayer interface mimics that of an antiphase boundary found in spinel ferrite thin films, thus explaining the unique nature of the magnetic behavior.

The magnetism of ferrite ultrathin films has been probed at nanoscale by spin-polarized tunneling transport in CoFe2O4-based room-temperature spin filters. By significantly reducing the Pt/CoFe2O4/Al2O3/Co junctions’ size (cross sections ∼5 nm) in comparison with usual microjunctions, transport through a ferrite single magnetic domain has been achieved. Our tunnel magnetoresistance measurements at the nanoscale have revealed very well defined resistance states and the generation of the highest spin filtering efficiency (−8%) at room temperature. A uniaxial magnetic anisotropy has been evidenced in CoFe2O4, showing that these magnetotransport measurements offer a unique probe of a single magnetic domain in ferrite ultrathin films.

We report on the significantly enhanced in-plane magnetic properties of CoFe2O4 (001) ultrathin layers (5 nm) grown on MgAl2O4 (001) in comparison to films deposited on MgO (001). The predicted inverse spinel structure is confirmed by x-ray magnetic circular dichroism measurements and transmission electronic microscopy studies reveal a significant in-plane compressive strain, responsible for the strong film anisotropy. These results show that strain engineering can be used to tailor the magnetic properties of oxide ultrathin films. A large compressive strain restores bulk magnetic properties for CoFe2O4 films at tunnel barrier thicknesses.

We report on the morphology, transport, and magnetic properties of ultrathin films of 4,4′-bis(diphenyl-2,6-thiapyrannylidene) (DITPY-Ph4), a planar quinoid organic compound with potential applications in electronics. Atomic force microscopy (AFM) shows that highly homogeneous and continuous DITPY-Ph4 ultrathin films with thicknesses as low as 2 nm can be grown on epitaxial Fe3O4(111). Current-sensing AFM performed on ultrathin layers evidences an insulating behavior with a nonresonant tunneling conduction regime. Vibrating sample magnetometry achieved on Fe3O4/DITPY-Ph4/Co organic-inorganic heterojunctions reveals the decoupling between the two magnetic electrodes and demonstrates that DITPY-Ph4 can be used to build magnetic tunnel junctions.

Recently, annealing of ion irradiated high Tc Josephson iunctions (JJs) has been studied experimentally in the perspective of improving their reproducibility. Here we present numerical simulations based on random walk and Monte Carlo calculations of the evolution of JJ characteristics such as the transition temperature Tc′ and its spread ΔTc′, and compare them with experimental results on junctions irradiated with 100 and 150 keV oxygen ions, and annealed at low temperatures (below 80 °C). We have successfully used a vacancy-interstitial annihilation mechanism to describe the evolution of the Tc′ and the homogeneity of a JJ array, analyzing the evolution of the defects density mean value and its distribution width. The annealing first increases the spread in Tc′ for short annealing times due to the stochastic nature of the process, but then tends to reduce it for longer times, which is interesting for technological applications.

We report the growth and magnetic properties of AFe2O4/Alq3/Co heterostructures (AFe, Co) containing a continuous Alq3 thin layer with a low roughness, down to a few nanometers, and we demonstrate the magnetic decoupling between AFe2O4 and Co electrodes showing the interest of these systems for spintronics devices.

We have studied the annealing effects on the transport properties of High Tc Josephson Junctions (JJ) made by ion irradiation. Several JJ were measured for different annealing times and the experimental data were compared to numerical simulations. We have successfully used a vacancy-interstitial annihilation mechanism to describe the evolution of the JJ coupling temperature (TJ) and the homogeneity of a JJ array, related to the evolution of the defects density mean value and its distribution width. For sufficient long annealing times (t > 600 min), ΔTJ was significatively reduced. This result appears to be very encouraging for future applications where the spread in JJ characteristics has to be as low as possible.

The authors have studied the annealing effect in the transport properties of high Tc Josephson junctions (JJs) made by ion irradiation. Low temperature annealing (80°C) increases the JJ coupling temperature (TJ) and the IcRn product, where Ic is the critical current and Rn the normal resistance. They have found that the spread in JJ characteristics can be reduced by sufficient long annealing times, increasing the reproducibility of ion irradiated Josephson junctions. The characteristic annealing time and the evolution of the spread in the JJ characteristics can be explained by a vacancy-interstitial annihilation process rather than by an oxygen diffusion one.