Welcome to our website !

The Oxitronics team is part of the CNRS/Thales laboratory located within Thales Research and Technology in Palaiseau, France. Our main scientific interests are in oxide electronics, spintronics, multiferroics, ferroelectrics and oxide interfaces. This website contains information on our past and current research, our group members and our publications. See also our events page for information on workshops, schools, etc.

Tuning up or down the critical thickness in LAO/STO with metal overlayers

TOC LAOSTO.pngThe quasi two- dimensional electron system (q2DES) that forms at the interface between LaAlO3 (LAO) and SrTiO3 (STO) has attracted much attention from the oxide electronics community. One of its hallmark features is the existence of a critical LAO thickness of 4 unit-cells (uc) for interfacial conductivity to emerge; another is the extreme sensitivity of its transport properties to electrostatic boundary conditions. This surface-interface coupling was previously exploited to modulate both carrier densities and mobilities of the q2DES through the controlled adsorption of polar solvents and by capping with different materials. In our recent work just published in Advanced Materials, we investigate in detail the chemical, electronic and transport properties of several LAO(1-2 uc)/STO samples capped with different metals (Ti, Ta, Co, Ni80Fe20 – NiFe -, Nb, Pt, Pd and Au) grown in a ultra-high vacuum (UHV) system combining pulsed laser deposition (to grow the LAO), sputtering (to grow the metal) and in situ X-ray photoemission spectroscopy (XPS). The results confirm that for several metals a q2DES forms at 1-2 uc of LAO. Additionally, XPS shows that the appearance of interfacial conductivity is accompanied by a partial oxidation of the metal, a phenomenon that is strongly linked with the q2DES properties and with the formation of defects in this system. In contrast, for noble metals, the q2DES does not form at low LAO thicknesses and instead the critical thickness is increased above 4 unit cells. We discuss the results in terms of a hybrid mechanism that incorporates both electrostatic and chemical effects.

Tuning up or down the critical thickness in LaAlO3/SrTiO3 through in situ deposition of metal overlayers
D.C. Vaz et al ; Adv. Mater. 10.1002/adma.201700486 (2017)

Multi-stimuli manipulation of antiferromagnetic domains assessed by second-harmonic imaging

More than 80% of known magnetic substances have dominant antiferromagnetic interactions. As it generates no stray field, the antiferromagnetic order is very discreet, which makes the processes of nucleation or growth of domains, as well as their responses to external stimuli at the microscopic scale, a virtually uncharted territory. The scarcity of real‑space imaging techniques devoted to this class of magnetic materials is a major bottleneck to understanding the fundamental basis of their manipulation.BFO-SHG.png

In this paper just published in Nature Materials, we use second harmonic generation with unprecedented sub-micron resolution to image antiferromagnetic order in BiFeO3 thin films. We provide a direct visualization of the antiferromagnetic domains in a single ferroelectric one. these antiferromagnetic domains can be manipulated thanks to magnetoelectric coupling in this archetypal multiferroic. More unexpectedly, we are also able  to manipulate the antiferromagnetic domains independently of the ferroelectric polarization, using electric fields significantly lower than the ferroelectric coercivity or  using optical stimuli such as THz pulses generated by a femtosecond laser. This opens horizons to manipulate the antiferromagnetic order in multiferroics and brings new insights into the emerging field of  antiferromagnetic spintronics.

This work was performed in collaboration with CEA-Saclay.

Multi-stimuli manipulation of antiferromagnetic domains assessed by second-harmonic imaging
J.-Y. Chauleau et al ; Nature Mater. 10.1038/nmat4899 (2017)

Phase diagram of rare-earth nickelates mapped from theory

Rare-earthfig-nickelates-web.jpg nickelates are intringuing perovskite oxides showing metal-insulator transition tuneable by the rare-earth size, and complex antiferromagnetic order at low temperature. Yet, a complete theoretical description of their rich phase diagram was  missing. In this work just out in NPJ Quantum Materials, we have used first-principles simulations to describe their electronic and magnetic experimental ground state. We show that the insulating phase is characterized by a split of the electronic states of the two Ni sites (i.e. resembling low-spin 4+ and high-spin 2+) with a concomitant shift of the oxygen-2p orbitals toward the depleted Ni cations. Therefore, from the point of view of the charge, the two Ni sites appear nearly identical whereas they re in fact distinct. Performing such calculations for several nickelates, we have built a theoretical phase diagram that reproduces all their key features, namely a systematic dependence of the MIT with the rare-earth size and the crossover between a second to first order transition for R=Pr and Nd. Our results hint at strategies to control the electronic and magnetic phases of perovskite oxides by fine tuning of the level of covalence.

This work was performed thanks to collaboration with LIST.

Complete phase diagram of rare-earth nickelates from first-principles
J. Varignon et al ; NPJ Quant. Mater. 2, 21 (2017)

Learning with solid-state ferroelectric synapses

CP_IA-Biomem_CNRS_image.pngArtificial neural networks show enhanced performance for key applications such as data mining or pattern recognition, but need to be implemented in hardware to make these applications accessible to everyone. Memristors are the electronic equivalent of synapses, whose variable connecting strength is at the heart of the learning process.

Accurate modelling of memristor dynamics is essential for the development of autonomous learning in artificial neural networks. In this paper, we demonstrate that spike-timing-dependent plasticity can be harnessed from inhomogeneous polarization switching in ferroelectric memristors. Combining time-dependent transport measurements, ferroelectric domain imaging, and effective-Hamiltonian-based atomistic molecular dynamics simulations, we show that the ferroelectric switching underlying resistance changes in these devices can be described by a nucleation-limited model. Using this physical model, we can reliably predict the conductance evolution of ferroelectric synapses with varying neural inputs. These results pave the way toward low-power hardware implementations of billions of reliable and predictable artificial synapses in future brain-inspired computers.

This work was performed thanks to collaboration between CNRS-Thales, Univ. of Bordeaux, Univ. of Arkansas, Centre de Nanosciences et de Nanotechnologies, and Thales Research and Technology.

Learning through ferroelectric domain dynamics in solid-state synapses
S. Boyn et al ; Nature Comm. 8, 14736 (2017)

See also the Press Release by CNRS (in French), the news piece by Phys.org and the articles in Le Monde and Le Temps.

Tunable spin texture in multiferroic BiFeO3

Agbelele-press.png

Non collinear spin textures such as skyrmions are currently under focus owing to their fundamental interest (associated with the influence of the  topology of the spin configuration on electronic properties) and to their potential for applications in data storage.

Multiferroic BiFeO3 naturally possesses a non collinear spin order which is also controlable by an electric field.  In BiFeO3, spins order along a 62-nm-period cycloid. In our study just out in Advanced Materials, we have shown that when BiFeO3 is grown in thin film form, both epitaxial strain and the application of an external magnetic field lengthen this period and may eventually destroy the cycloidal state. By combining several stimuli (strain, magnetic field, electric field), it thus appears possible to stabilize various non-collinear spin states, which bears a strong potential for spintonics and magnonics.

This work was done in collaboration with the Laboratoire MPQ (Université Paris-Diderot), the GPM (Université de Rouen), the SPMS lab (Ecole Centrale Paris), the ESRF, the ISIS facility and the Moscow Institute of Science and Technology.

Strain and Magnetic Field Induced Spin-Structure Transitions in Multiferroic BiFeO3
A. Agbelele et al, Adv. Mater. 1602327 (2017) ; DOI: 10.1002/adma.201602327

See also the News piece by Labex NanoSaclay (in French).

Mathieu Grisolia defends his PhD thesis

On Monday, September 26, our group member Mathieu Grisolia successfully defended his PhD “Novel interfacial electronic states between correlated insulators“. Exploring novel concepts in correlated oxides was an exciting but arduous task, but Mathieu was certainly up to the challenge. Congratulations !