arXiv subjects

Emile Fourneau

Publications and source records attributed to Emile Fourneau.

5 recordsLinked to original sources

arXiv2026

Electrical manipulation of oxygen stoichiometry in multiterminal YBa$_2$Cu$_3$O$_{7-\delta}$ junctions

Local manipulation of oxygen stoichiometry offers a route to control the electronic properties of complex oxides, yet the selective modification of individual current-carrying branches through oxygen redistribution remains unexplored in multiterminal high-temperature superconducting junctions. In a YBa$_2$Cu$_3$O$_{7-\delta}$ Y-shaped three-terminal device, we demonstrate the possibility to electrically control oxygen vacancy migration on a hand-picked terminal while largely preserving the other two. Oxygen-depleted propagating fronts are directly visualized by the resulting change in optical reflectivity and they are linked to the evolution of the electrical response. The process is highly directional and determined by the polarity of the applied current, allowing for the creation of either a converging or a diverging propagating front from the central node of the Y-shaped device. The associated changes in resistance exhibit relaxation on a timescale of minutes, driven by the vacancy concentration gradient. Effects of oxygen migration are also mapped by Kelvin Probe Force Microscopy and Scanning Laser Microscopy, which probe work-function changes and spatially resolved variations in the superconducting transition, respectively. Notably, the Tc contrast revealed by the latter provides a quantitative handle on the underlying oxygen content, enabling direct visualization of oxygen redistribution. Finite-element modeling and nanoprobe X-ray diffraction qualitatively reproduce the observed vacancy redistribution. These results establish a post-fabrication route to locally tune properties of superconducting multiterminal devices such as nanocryotrons, yTron, and tunable weak links.

cond-mat.supr-con↗

arXiv2026

Engineering Magnetic Anisotropy in Permalloy Films via Atomic Force Nanolithography

Atomic force nanolithography provides a precise method for sculpting magnetic thin films, enabling controlled engineering of magnetic anisotropy in soft ferromagnets at the microscale. We demonstrate that nanoscale groove arrays patterned into permalloy (Ni80Fe20) films induce a robust in-plane uniaxial anisotropy, with the easy axis aligned along the groove direction. The anisotropy field is shown to increase with decreasing groove period and increasing engraving depth, offering continuous tunability of magnetic hardness within a single fabrication step. Artificially engraved microstructures further allow domain configurations and domain-wall trajectories to be directed along predefined pathways, exemplified by the creation of a chessboard-like magnetic landscape. Owing to its adaptability to diverse ferromagnetic materials and arbitrary corrugation geometries, this approach provides a versatile platform for tailoring in-plane magnetic anisotropy. Concrete applications are demonstrated in the design of magnonic elements and anisotropic magnetoresistance sensors.

cond-mat.mtrl-sci↗

arXiv2025

Anisotropy by design in superconducting Nb thin films via ultrashort pulse laser irradiation

The ability to fabricate anisotropic superconducting layers a la carte is desired in technologies such as fluxon screening or removal in field-resilient devices, flux lensing in ultra-sensitive sensors, or in templates for imprinting magnetic structures in hybrid magnetic/superconducting multilayers. In this work, we demonstrate tailored superconductivity in polycrystalline niobium thin films exposed to femtosecond ultraviolet laser pulses. The samples exhibit significant changes in their superconducting properties, directly connected with the observed topography, crystallite geometry, and lattice parameter modifications. On the mesoscopic scale, quasi-parallel periodic ripple structures (about 260 nm of spatial period) gradually form on the film surface by progressively increasing the laser energy per pulse, Ep. This gives way to a stepwise increase of the critical current anisotropy and magnetic flux channeling effects along the ripples. As demonstrated in our resistive and inductive measurements, these superstructures determine the electromagnetic response of the sample within the regime dominated by flux-pinning. Time-dependent Ginzburg-Landau simulations corroborate the topographical origin of the customized anisotropy. Concurrently, intrinsic superconducting parameters (critical field and temperature) are moderately and isotropically depressed upon increasing Ep, as is the lattice parameter of Nb. These findings promote pulsed laser processing as a flexible, one-step, and scalable lithography-free technique for versatile surface functionalization in microelectronic superconducting technology.

cond-mat.supr-con↗

arXiv2020

A roadmap for the design of four-terminal spin valves and the extraction of spin diffusion length

Graphene is a promising substrate for future spintronics devices owing to its remarkable electronic mobility and low spin-orbit coupling. Hanle precession in spin valve devices is commonly used to evaluate the spin diffusion and spin lifetime properties. In this work, we demonstrate that this method is no longer accurate when the distance between inner and outer electrodes is smaller than six times the spin diffusion length, leading to errors as large as 50% for the calculations of the spin figures of merit of graphene. We suggest simple but efficient approaches to circumvent this limitation by addressing a revised version of the Hanle fit function. Complementarily, we provide clear guidelines for the design of four-terminal spin valves able to yield flawless estimations of the spin lifetime and the spin diffusion coefficient.

cond-mat.mes-hall↗

arXiv2020

On the origin of the giant spin detection efficiency in tunnel barrier based electrical spin detector

Efficient conversion of a spin signal into an electric voltage in mainstream semiconductors is one of the grand challenges of spintronics. This process is commonly achieved via a ferromagnetic tunnel barrier where non-linear electric transport occurs. In this work, we demonstrate that non-linearity may lead to a spin-to-charge conversion efficiency larger than 10 times the spin polarization of the tunnel barrier when the latter is under bias of a few mV. We identify the underlying mechanisms responsible for this remarkably efficient spin detection as the tunnel barrier deformation and the conduction band shift resulting from a change of applied voltage. In addition, we derive an approximate analytical expression for the detector spin sensitivity $P_{\textrm{det}}(V)$. Calculations performed for different barrier shapes show that this enhancement is present in oxide barriers as well as in Schottky tunnel barriers even if the dominant mechanisms differs with the barrier type. Moreover, although the spin signal is reduced at high temperatures, it remains superior to the value predicted by the linear model. Our findings shed light into the interpretation and understanding of electrical spin detection experiments and open new paths to optimize the performance of spin transport devices.

physics.app-ph↗