Search

International audience; Phase Change Materials are materials that exhibit a large optical change in their complex index of refraction upon switching between their various electronic or structural states. As such, they offer a promising way to implement tunability and reconfigurability in the next generation of nano-photonic devices. In this contribution, we will review our recent studies on spin-crossover molecular compounds that can be evaporated and micro-structured. We will also report their use to make resonant and broadband thermo-optic modulators operating in the visible wavelength range respectively based on metal-mirror Fabry-Perot interferometers and frustrated total internal reflection systems.

International audience; The spin-state switching stability of the benchmark [Fe(HB(tz)3)2] (tz = 1,2,4-triazol-1-yl) complex is examined in single crystals, thin films and ITO/[Fe(HB(tz)3)2]/Al metal–insulator–metal junctions. In each case, the switching behavior remains virtually unaltered for more than 10 000 endurance cycles in ambient air, even after more than one-year storage time. The structural/morphological integrity of the samples is not affected to any appreciable extent. On the other hand, the junctions exhibit a slow decrease of their conductance. Based on a comparison with analogous ITO/[Zn(HB(tz)3)2]/Al junctions, we attribute this phenomenon to the degradation of the device itself, rather than to the deterioration of the SCO properties. These results highlight the intrinsic stability of certain spin crossover compounds, providing impetus for future work, aiming at both the fundamental understanding of this switching stability and its exploitation in technological applications.

Multilayer crossbar junctions composed of ITO/[Fe(HB(1,2,4-triazol-1-yl)3)2]/M (with M  =  Al or Ca) were fabricated and investigated for their resistance switching properties. Current–voltage–temperature maps revealed ON/OFF resistance ratios as high as 400, with the ON and OFF states defined, respectively, as the low-resistance, low spin state and the high-resistance, high spin state of the spin crossover layer. Similar results were obtained with Al and Ca cathodes indicating that the charge transport in the insulating spin crossover film is at the origin of the resistance switching instead of electron injection at the electrodes. The reproducibility and stability of the device properties were also studied.

International audience; We have investigated the effects of low-energy ion beam irradiations using argon clusters on the chemical and electronic properties of LaAlO3/SrTiO3 (LAO/STO) heterointerfaces by combining x-ray photoelectron spectroscopy (XPS) and electrical transport measurements. Due to its unique features, we demonstrate that a short-time cluster ion irradiation of the LAO surface induces significant modifications in the chemical properties of the buried STO substrate with (1) a lowering of Ti atoms oxidation states (from Ti4+ to Ti3+ and Ti2+) correlated to the formation of oxygen vacancies at the LAO surface and (2) the creation of new surface states for Sr atoms. Contrary to what is generally observed by using higher energy ion beam techniques, this leads to an increase of the electrical conductivity at the LAO/STO interface. Our XPS data clearly reveal the existence of dynamical processes on the titanium and strontium atoms, which compete with the effect of the cluster ion beam irradiation. These relaxation effects are in part attributed to the diffusion of the ion-induced oxygen vacancies in the entire heterostructure since an increase of the interfacial metallicity is also evidenced far from the irradiated area. This paper highlights the possibility of tuning the electrical properties of LAO/STO interfaces by surface engineering, confirming experimentally the intimate connection between LAO chemistry and electronic properties of LAO/STO interfaces.

International audience; The spatiotemporal dynamics of the spin transition have been thoroughly investigated in single crystals of the mononuclear spin-crossover (SCO) complex [Fe(HB(tz)3)2] (tz=1,2,4-triazol-1-yl) by optical microscopy. This compound exhibits an abrupt spin transition centered at 334 K with a narrow thermal hysteresis loop of ∼1 K (first-order transition). Most single crystals of this compound reveal exceptional resilience upon repeated switching (several hundred cycles), which allowed repeatable and quantitative measurements of the spatiotemporal dynamics of the nucleation and growth processes to be carried out. These experiments revealed remarkable properties of the thermally induced spin transition: high stability of the thermal hysteresis loop, unprecedented large velocities of the macroscopic low-spin/high-spin phase boundaries up to 500 µm/s, and no visible dependency on the temperature scan rate. We have also studied the dynamics of the low-spin → high-spin transition induced by a local photothermal excitation generated by a spatially localized (Ø=2μm) continuous laser beam. Interesting phenomena have been evidenced both in quasistatic and dynamic conditions (e.g., threshold effects and long incubation periods, thermal activation of the phase boundary propagation, stabilization of the crystal in a stationary biphasic state, and thermal cutoff frequency). These measurements demonstrated the importance of thermal effects in the transition dynamics, and they enabled an accurate determination of the thermal properties of the SCO compound in the framework of a simple theoretical model.

One of the major issues in the field of molecular magnetism is to better understand and predict the correlations between the structural properties of molecule-based compounds and their magnetic properties, all of which may be tunable using “bottom-up” synthesis methods. In particular, the understanding and control of the magnetic anisotropy at the atomic scale is essential, especially with the aim to design Single-Molecule Magnets (SMM) with higher blocking temperatures. In this context, this thesis work is focused on two mains subjects. The first part deals with the determination and the characterization of the local magnetic anisotropy in low-nuclearity molecular complexes based on transition ions. Polarised neutron diffraction (PND) allows us, for the first time, to directly access the local susceptibility tensor in a Low-Spin Fe(3+) mononuclear complex as well as in two, mononuclear and dinuclear, High-Spin Co(2+) complexes. This innovative approach leads to the establishment of unique and direct magneto-structural correlations, by relating the local magnetic principal directions with the coordination environment of the metallic ions and, in particular, with the local distortion axes. We have also carried out an original investigation by inelastic neutron scattering (INS) of a Mn(3+) thermo-induced spin-transition compound in both High-Spin (HS) and Low-Spin (LS) states. On the basis of this study, we were able to propose an anisotropic spin-Hamiltonian model in both HS and LS phases, and their relationships with the structure of the molecule are discussed. In a second more exploratory part of the thesis, we have performed by small-angle neutron scattering (SANS) a complete study of the structural and magnetic properties of Prussian blue analogues (PBA) ferromagnetic nanoparticles CsNiCr. The effects of size, organization and concentration on their superparamagnetic properties have been clearly highlighted. In particular, a strong magnetic contribution has been observed for the smallest particles (5 nm diameter) which results from the manifestation of a collective process, while the biggest (28 nm diameter) appear to be in a multi-domain state., Un des enjeux majeurs dans le domaine du magnétisme moléculaire est de mieux comprendre et prévoir, dans les composés à base moléculaire, les corrélations qui existent entre les propriétés structurales (modulables à partir de méthodes de synthèse de type « bottom-up ») et les propriétés magnétiques. En particulier, la compréhension et la maîtrise de l’anisotropie magnétique à l’échelle locale est primordiale, notamment en vue de concevoir des molécules-aimants avec de plus hautes températures de blocage. Dans ce contexte, ce travail de thèse s’organise autour de deux grands axes. La première partie se concentre sur la détermination et la caractérisation de l’anisotropie magnétique locale dans des complexes moléculaires d’ions de transition de faible nucléarité. La diffraction de neutrons polarisés (PND) nous a permis, pour la première fois, de mettre clairement en évidence le tenseur de susceptibilité magnétique locale dans un complexe moléculaire mononucléaire de Fe(3+) Bas-Spin ainsi que dans deux complexes, mononucléaire et dinucléaire, de Co(2+) Haut-Spin. Cette approche novatrice mène à l’établissement de relations magnéto-structurales claires et directes, en reliant les directions magnétiques locales propres à l’environnement de coordination des ions métalliques et en particulier aux axes locaux de distorsion. Nous avons également mené l’étude originale d’un complexe à transition de spin thermo-induite de Mn(3+) par diffusion inélastique de neutrons (INS) dans les deux phases Haut-Spin (HS) et Bas-Spin (BS). Cette étude nous a conduits à la proposition d’un modèle d’hamiltonien de spin anisotrope dans les deux états HS et BS, en relation avec la structure du complexe. Dans une seconde partie plus exploratoire de la thèse, nous avons mené une étude complète des propriétés structurales et magnétiques de nanoparticules ferromagnétiques d’analogue du bleu de Prusse CsNiCr, par diffusion de neutrons aux petits angles (SANS). Les effets de taille, d’organisation et de concentration sur leurs propriétés superparamagnétiques ont ainsi été clairement mis en évidence. En particulier, nous avons mis en exergue, pour les particules de plus petite taille (5 nm de diamètre), une contribution magnétique qui résulte de la manifestation d’un phénomène collectif, tandis que celles de plus grande taille (28 nm de diamètre) apparaissent être dans un état complètement multidomaine.