arXiv subjects

Nathalie Viart

Publications and source records attributed to Nathalie Viart.

13 recordsLinked to original sources

arXiv2026

Decoding Superconductivity in La$_3$Ni$_2$O$_{7-δ}$ Thin Films via Ozone-Driven Structure and Oxidation Tuning

The discovery of superconductivity in bulk Ruddlesden-Popper La$_3$Ni$_2$O$_7$(LNO327) under high hydrostatic pressure has redefined the recent experimental consensus that nickelate superconductivity is restricted to systems with a $3d^9$ electronic configuration and square-planar coordination. However, the structural and electronic prerequisites for stabilizing superconductivity, whether under pressure or at ambient conditions in the case of thin films, remain poorly understood, largely due to the metastable nature of the LNO327 phase. Here, we present a detailed structural study of epitaxial La$_3$Ni$_2$O$_{7-δ}$ thin films by using scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS). Grown via pulsed laser deposition onto SrLaAlO$_4$ substrates, those films exhibit distinct superconducting properties as a function of the different post-annealing conditions used. By correlating the rich landscape of stacking polymorphs with transport behavior, this work establishes a framework for understanding the metastable superconducting phase in bilayer nickelate thin films. Our findings underscore the critical role of homogeneity in oxygen stoichiometry, epitaxial strain and structural motif in stabilizing superconductivity, offering a clear pathway for designing ambient-pressure superconducting nickelates.

cond-mat.supr-con↗

arXiv2026

Strain-driven magnetic anisotropy and spin reorientation in epitaxial Co V 2 O 4 spinel oxide thin films

CoV___O___ (CVO) stands out among spinel vanadates for its ultra-short V-V distances, placing it at the brink of itinerant electron behaviour-an ideal playground for strain engineering. In this work, we exploit this sensitivity by growing high-quality epitaxial CVO thin films on SrTiO___ (001) and MgO (001), inducing compressive and tensile strain, respectively. Using pulsed laser deposition under ultra-low oxygen pressure, we achieve high crystalline quality and straincontrolled tetragonal distortions: c > a under compression (STO) and c < a under tension (MgO). Resonant elastic X-ray scattering confirms a normal spinel structure, with cobalt occupying tetrahedral sites and vanadium octahedral ones. Both strain types reduce charge transport, driving the system into a highly resistive state. Magnetic measurements reveal strain-driven anisotropy switching: STO films transition from out-of-plane to in-plane easy axis below 90 K, while MgO films flip from in-plane to out-of-plane below 45 K. These results highlight CVO's exceptional responsiveness to lattice strain, unlocking a path to finely tunable electronic and magnetic properties. With its strong spin-lattice coupling and potential in spin Hall magnetoresistance, strained CVO emerges as a compelling platform for next-generation lowpower spintronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Tailoring the breathing-mode distortions in nickelate-ferroelectric heterostructures

In transition metal oxides electron-electron interaction and lattice degree of freedom are basic ingredients of emergent phenomena, such as metal-to-insulator transition (MIT) and superconductivity. Perovskite rare-earth nickelates are largely studied for their temperature-driven MIT which is accompanied by a breathing mode distortion, and associated to a bond-disproportionation of the expanded (3d8L0) and compressed (3d8L2) NiO6 octahedra. Steric effects control the onset temperature of the MIT, the latter being concomitant or not with a complex antiferromagnetic spin arrangement depending upon the choice of the rare earth ion (TMIT>TNeel). Interface engineering of oxygen octahedra tilting, as imposed by the symmetry and orientation of the substrate, has resulted in an efficient pathway to modify both TMIT and TNeel, hence, suggesting a key role of the electron-phonon coupling for both transport and magnetic properties in nickelate thin films. Here, via a combination of resonant elastic X-ray scattering and transport experiments, we show a control over both TMIT and TNeel in heteroepitaxial PZT(d)/NNO(7 nm)//STO heterostructures, which are characterized by different strains and polarization states of the PZT layer grown at different thicknesses d. We found the expected NNO bulk behaviour, for a fully relaxed PZT layer showing a monodomain polarization state. On the other side, an almost 30 K difference, is found for a fully strained PZT characterized by a multidomain texture of the polarization state. We discuss our results in terms of an altered breathing distortion pattern of the underlying nickelate layer as supported by X-ray absorption spectroscopy measurements. We infer that locally different polar distortions controlled by a combination of polarization direction and strength of the strain state play the main role in the observed TMIT and TNeel variations.

cond-mat.mtrl-sci↗

arXiv2024

Superconductivity in PrNiO2 infinite-layer nickelates

Several reports about infinite-layer nickelate thin films suggest that the superconducting critical temperature versus chemical doping phase diagram has a dome-like shape, similar to cuprates. Here, we demonstrate a highly reproducible superconducting state in undoped PrNiO$_2$ thin films grown onto SrTiO$_3$. Scanning transmission electron microscopy measurements demonstrate coherent and defect-free infinite-layer phase, a high structural quality with no unintentional chemical doping and a total absence of interstitial oxygen. X-ray absorption measurements show very sharp features at the Ni L$_{3,2}$-edges with a large linear dichroism, indicating the preferential hole-occupation of Ni$^{1+}$-3d$_{x^2-y^2}$ orbitals in a square planar geometry. Resonant inelastic X-ray scattering measurements reveal sharp magnon excitations of 200\,meV energy at magnetic Brillouin zone boundary, highly resonant at the Ni$^{1+}$ absorption peak. The results indicate that, when properly stabilized, infinite-layer nickelate thin films are superconducting without chemical doping.

cond-mat.supr-con↗

arXiv2024

Freestanding perovskite and infinite-layer nickelate membranes

Following the discovery of superconductivity in hole-doped NdNiO$_2$ infinite-layer thin films, extensive research has been conducted particularly to compare these materials with cuprates. Superconductivity has also been observed in nickelate thin films with other rare-earth elements like Pr and La, but not in their bulk forms, suggesting a critical role for substrate-induced strain/interface or dimensionality effects. In this study, we use water-soluble (Ca,Sr)$_3$Al$_2$O$_6$ sacrificial layers to fabricate freestanding perovskite nickelate membranes and explore topotactic reduction without the SrTiO$_3$ substrate as a template. NdNiO$_3$-based heterostructure membranes transferred from a LaAlO$_3$ substrate exhibit better metallic behavior and higher hysteresis than those transferred from SrTiO$_3$, owing to the different strain states that the NdNiO$_3$ layer experience when grown on these two substrates. Despite the expected X-ray diffraction shifts, membranes reduced with CaH$_2$ display insulating characteristics, similar to bulk infinite-layer nickelates. Our findings strongly indicate that a template is necessary to stabilize a coherent and robust infinite-layer phase with optimal transport properties.

cond-mat.mtrl-sci↗

arXiv2023

Room-temperature magnetism and controlled cation distribution in vanadium ferrite thin films

Spinel oxides demonstrate significant technological promise due to the vast array of interrelated physical properties that their unique structure supports. Specifically, the Fe1+xV2-xO4 spinel system garners extensive interest due to the presence of orbitally ordered states and multiferroism. This study focuses on the elaboration of high-quality Fe2VO4 (x = 1) thin films on MgO substrates via pulsed laser deposition. Structural analyses confirm the epitaxial growth of the films, their high crystallinity and fully strained nature. The cationic distribution and stoichiometry were investigated using Resonant Elastic X-ray Scattering experiments, in conjunction with comprehensive characterization of the films' physical and electrical properties. The films exhibit room-temperature magnetism, with a magnetization consistent with the (Fe3+)Td[Fe2+V3+2]OhO4 inverse spinel structure unveiled by anomalous diffraction. This work represents the inaugural successful deposition of Fe2VO4 thin films, thereby expanding the family of spinel vanadium oxide thin films with a new member that demonstrates room-temperature magnetic properties.

cond-mat.mtrl-sci↗

arXiv2023

Spin transport properties of spinel vanadate-based heterostructures

Spin-orbit coupling and breaking of inversion symmetry are necessary ingredients to enable a pure spin current-based manipulation of the magnetization via the spin-orbit torque effect. Currently, magnetic insulator oxides with non-dissipative characteristics are being explored. When combined with non-magnetic heavy metals, known for their large spin-orbit coupling, they offer promising potential for energy-efficient spin-orbitronics applications. The intrinsic electronic correlations characterizing those strongly correlated oxides hold the promises to add extra control-knobs to the desired efficient spin-wave propagation and abrupt magnetization switching phenomena. Spinel vanadate FeV2O4 (FVO) exhibits several structural phase transitions which are accompanied by an intricate interplay of magnetic, charge and orbital orderings. When grown as a thin film onto SrTiO3, the compressive strain state induces a perpendicular magnetic anisotropy, making FVO-based heterostructures desirable for spin-orbitronics applications. In this study, we have optimised the deposition of stoichiometric and epitaxial Pt/FVO heterostructures by Pulsed Laser Deposition and examined their spin-related phenomena. From angle-dependent magnetotransport measurements, we observed both Anisotropic Magnetoresistance (AMR) and Spin Hall Magnetoresistance (SMR) effects. Our findings show the SMR component as the primary contributor to the overall magnetoresistance, whose high value of 0.12% is only comparable to properly optimized oxide-based systems.

cond-mat.mtrl-sci↗

arXiv2023

Charge distribution across capped and uncapped infinite-layer neodymium nickelate thin films

Charge ordering (CO) phenomena have been widely debated in strongly-correlated electron systems mainly regarding their role in high-temperature superconductivity. Here, we elucidate the structural and charge distribution in NdNiO$_{2}$ thin films prepared with and without capping layers, and characterized by the absence and presence of CO. Our microstructural and spectroscopic analysis was done by scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) and hard x-ray photoemission spectroscopy (HAXPES). Capped samples show Ni$^{1+}$, with an out-of-plane (o-o-p) lattice parameter of around 3.30 angstroms indicating good stabilization of the infinite-layer structure. Bulk-sensitive HAXPES on Ni-2p shows weak satellite feature indicating large charge-transfer energy. The uncapped samples evidence an increase of the o-o-p parameter up to 3.65 angstroms on the thin-film top, and spectroscopies show signatures of higher valence in this region (towards Ni$^{2+}$). Here, 4D-STEM demonstrates (3,0,3) oriented stripes which emerge from partially occupied apical oxygen. Those stripes form quasi-2D coherent domains viewed as rods in the reciprocal space with $Δ\text{q}_{z} \approx 0.24$ r.l.u. extension located at Q = ($\pm \frac{1}{3},0,\pm \frac{1}{3}$) r.l.u. and Q = ($\pm \frac{2}{3},0,\pm \frac{2}{3}$) r.l.u. The stripes associated with oxygen re-intercalation concomitant with hole doping suggests a possible link to the previously reported CO in infinite-layer nickelate thin films.

cond-mat.mtrl-sci↗

arXiv2022

Synthesis of infinite-layer nickelates and influence of the capping-layer on magnetotransport

The recent discovery of a zero-resistance state in nickel-based compounds has generated a re-excitement about the long-standing problem in condensed matter of high-critical-temperature superconductivity, in light of the analogies between infinite-layer nickelates and cuprates. However, despite some formal valence and crystal symmetry analogies, the electronic properties of infinite-layer nickelates are remarkably original accounting, among other properties, of a unique Nd5d-Ni3d hybridization. This designates infinite-layer nickelates as a new class of oxide superconductors which should be considered on their own. Here we report about Nd1-xSrxNiO2 (x = 0, 0.05 and 0.2) thin films synthesized with and without a SrTiO3 capping-layer, showing very smooth and step-terraced surface morphologies. Angle-dependent anisotropic magnetoresistance measurements performed with a magnetic field rotating in-plane or out-of-plane with respect to the sample surface, rendered important information about the magnetic properties of undoped SrTiO3-capped and uncapped samples. The results point at a key role of the capping-layer in controlling the magnitude and the anisotropy of the anisotropic magnetoresistance properties. We discuss this control in terms of a combined effect between the Nd-Ni hybridization and an intra-atomic exchange coupling between the Nd-4f and Nd-5d states, the latter essentially contributing to the (magneto)transport. Further studies foresee the influence of the capping layer on infinite-layer nickelates with no magnetic rare-earth.

cond-mat.mtrl-sci↗

arXiv2022

Unveiling Unconventional Ferroelectric Switching in Multiferroic Ga0.6 Fe1.4O3 Thin Films Through Multiscale Electron Microscopy Investigations

Understanding the polarization switching mechanisms at play in ferroelectric materials is crucial for their exploitation in electronic devices. The conventional centrosymmetric reference structure-based mechanism which accounts for ferroelectricity in most of the usual displacive ferroelectric materials is too energy-demanding for some newly diagnosed ferroelectric materials such as the Ga2-xFexO3 (0.8 < x < 1.4) compounds. Some alternative theoretical propositions have been made and need experimental confirmation. A dual-scale electron microscopy study is performed on thin films of the Ga0.6Fe1.4O3 multiferroic compound. A wide scale precession-assisted electron diffraction tomography study first allows the determination of the structure the compound adopts in thin films, and even permits the refinement of the atomic positions within this structure. Cationic mobility is suggested for two of the atomic positions through the existence of extra electronic density. A local in situ high resolution scanning transmission electron microscopy study then allows confirming these mobilities by directly spotting the cationic displacements on successively acquired images. The whole study confirms an unconventional switching mechanism via local domain wall motion in this compound.

cond-mat.mtrl-sci↗

arXiv2022

Thermal spin current generation in the multifunctional ferrimagnet Ga$_{0.6}$Fe$_{1.4}$O$_{3}$

In recent years, multifunctional materials have attracted increasing interest for magnetic memories and energy harvesting applications. Magnetic insulating materials are of special interest for this purpose, since they allow the design of more efficient devices due to the lower Joule heat losses. In this context, Ga$_{0.6}$Fe$_{1.4}$O$_3$ (GFO) is a good candidate for spintronics applications, since it can exhibit multiferroicity and presents a spin Hall magnetoresistance similar to the one observed in a yttrium iron garnet (YIG)/Pt bilayer. Here, we explore GFO utilizing thermo-spin measurements in an on-chip approach. By carefully considering the geometry of our thermo-spin devices we are able to quantify the spin Seebeck effect and the spin current generation in a GFO/Pt bilayer, obtaining a value comparable to that of YIG/Pt. This further confirms the promises of an efficient spin current generation with the possibility of an electric-field manipulation of the magnetic properties of the system in an insulating ferrimagnetic material.

cond-mat.mtrl-sci↗

arXiv2021

Spin current transport in hybrid Pt / multifunctional magnetoelectric Ga0.6Fe1.4O3 bilayers

The low power manipulation of magnetization is currently a highly sought-after objective in spintronics. Non ferromagnetic large spin-orbit coupling heavy metal (NM) / ferromagnet (FM) heterostructures offer interesting elements of response to this issue, by granting the manipulation of the FM magnetization by the NM spin Hall effect (SHE) generated spin current. Additional functionalities, such as the electric field control of the spin current generation, can be offered using multifunctional ferromagnets. We have studied the spin current transfer processes between Pt and the multifunctional magnetoelectric Ga0.6Fe1.4O3 (GFO). In particular, via angular dependent magnetotransport measurements, we were able to differentiate between magnetic proximity effect (MPE)-induced anisotropic magnetoresistance (AMR) and spin Hall magnetoresistance (SMR). Our analysis shows that SMR is the dominant phenomenon at all temperatures and is the only one to be considered near room temperature, with a magnitude comparable to those observed in Pd/YIG or Pt/YIG heterostructures. These results indicate that magnetoelectric GFO thin films show promises for achieving an electric-field control of the spin current generation in NM/FM oxide-based heterostructures.

cond-mat.mtrl-sci↗

arXiv2017

Domain wall architecture in tetragonal ferroelectric thin films

Domain walls in ferroelectrics exhibit a plethora of phases and functionalities not found in the bulk. The interplay of electrostatic, chemical, topological, and distortive inhomogeneities at the walls can be so complex, however, that this obstructs their technological performance. In tetragonal ferroelectrics like PbZrxTi1-xO3, for example, the desired functional 180° domain walls within out-of-plane-polarized c-domains are interspersed by in-plane-polarized a-domains and the associated network of domain walls remains challenging to analyze. Here we use a combination of STEM and optical second harmonic generation (SHG) to determine the relation between strain, film thickness, local electric fields and the resulting domain and domain-wall structures across the entire thickness of a set of PZT films. We quantify the distribution of a-domains in the c-domain matrix of the films. Using locally applied electric fields we control the a/c distribution and induce the technologically preferable 180° domain walls. We find that these voltage induced walls are tilted and exhibit a mixed Ising-Néel type transverse rotation of polarization across the wall with a specific nonlinear optical response.

cond-mat.mes-hall↗