Fadi Choueikani

Exchange bias and room-temperature magnetic order in molecular layers

Molecular semiconductors may exhibit antiferromagnetic correlations well below room temperature. Although inorganic antiferromagnetic layers may exchange bias single-molecule magnets, the reciprocal effect of an antiferromagnetic molecular layer magnetically pinning an inorganic ferromagnetic layer through exchange bias has so far not been observed. We report on the magnetic interplay, extending beyond the interface, between a cobalt ferromagnetic layer and a paramagnetic organic manganese phthalocyanine (MnPc) layer. These ferromagnetic/organic interfaces are called spinterfaces because spin polarization arises on them. The robust magnetism of the Co/MnPc spinterface stabilizes antiferromagnetic ordering at room temperature within subsequent MnPc monolayers away from the interface. The inferred magnetic coupling strength is much larger than that found in similar bulk, thin or ultrathin systems. In addition, at lower temperature, the antiferromagnetic MnPc layer induces an exchange bias on the Co film, which is magnetically pinned. These findings create new routes towards designing organic spintronic devices.

Magnetic Cooling at a Single Molecule Level: a Spectroscopic Investigation of Isolated Molecules on a Surface

A sub-monolayer distribution of isolated molecular Fe14 (bta)6 nanomagnets is deposited intact on a Au(111) surface and investigated by X-ray magnetic circular dichroism spectroscopy. The entropy variation with respect to the applied magnetic field is extracted from the magnetization curves and evidences high magnetocaloric values at the single molecule level.

Spin-Dependent Hybridization between Molecule and Metal at Room Temperature through Interlayer Exchange Coupling

We experimentally and theoretically show that the magnetic coupling at room temperature between paramagnetic Mn within manganese phthalocyanine molecules and a Co layer persists when separated by a Cu spacer. The molecules magnetization amplitude and direction can be tuned by varying the Cu-spacer thickness and evolves according to an interlayer exchange coupling mechanism. Ab initio calculations predict a highly spin-polarized density of states at the Fermi level of this metal-molecule interface, thereby strengthening prospective spintronics applications.

Enhancing the magnetic anisotropy of maghemite nanoparticles via the surface coordination of molecular complexes.

Superparamagnetic nanoparticles are promising objects for data storage or medical applications. In the smallest—and more attractive—systems, the properties are governed by the magnetic anisotropy. Here we report a molecule-based synthetic strategy to enhance this anisotropy in sub-10-nm nanoparticles. It consists of the fabrication of composite materials where anisotropic molecular complexes are coordinated to the surface of the nanoparticles. Reacting 5 nm γ-Fe2O3 nanoparticles with the [CoII(TPMA)Cl2] complex (TPMA: tris(2-pyridylmethyl)amine) leads to the desired composite materials and the characterization of the functionalized nanoparticles evidences the successful coordination—without nanoparticle aggregation and without complex dissociation—of the molecular complexes to the nanoparticles surface. Magnetic measurements indicate the significant enhancement of the anisotropy in the final objects. Indeed, the functionalized nanoparticles show a threefold increase of the blocking temperature and a coercive field increased by one order of magnitude.

First glimpse of the soft x-ray induced excited spin-state trapping effect dynamics on spin cross-over molecules

The dynamics of the soft x-ray induced excited spin state trapping (SOXIESST) effect of Fe(phen)2(NCS)2 (Fe-phen) powder have been investigated by x-ray absorption spectroscopy (XAS) using the total electron yield method, in a wide temperature range. The low-spin (LS) state is excited into the metastable high-spin (HS) state at a rate that depends on the intensity of the x-ray illumination it receives, and both the temperature and the intensity of the x-ray illumination will affect the maximum HS proportion that is reached. We find that the SOXIESST HS spin state transforms back to the LS state at a rate that is similar to that found for the light induced excited spin state trapping (LIESST) effect. We show that it is possible to use the SOXIESST effect in combination with the LIESST effect to investigate the influence of cooperative behavior on the dynamics of both effects. To investigate the impact of molecular cooperativity, we compare our results on Fe-phen with those obtained for Fe{[Me2Pyrz]3BH}2 (Fe-pyrz) powder, which exhibits a similar thermal transition temperature but with a hysteresis. We find that, while the time constant of the dynamic is identical for both molecules, the SOXIESST effect is less efficient at exciting the HS state in Fe-pyrz than in Fe-phen.

Engineering the magnetic coupling and anisotropy at the molecule–magnetic surface interface in molecular spintronic devices

A challenge in molecular spintronics is to control the magnetic coupling between magnetic molecules and magnetic electrodes to build efficient devices. Here we show that the nature of the magnetic ion of anchored metal complexes highly impacts the exchange coupling of the molecules with magnetic substrates. Surface anchoring alters the magnetic anisotropy of the cobalt(II)-containing complex (Co(Pyipa)2), and results in blocking of its magnetization due to the presence of a magnetic hysteresis loop. In contrast, no hysteresis loop is observed in the isostructural nickel(II)-containing complex (Ni(Pyipa)2). Through XMCD experiments and theoretical calculations we find that Co(Pyipa)2 is strongly ferromagnetically coupled to the surface, while Ni(Pyipa)2 is either not coupled or weakly antiferromagnetically coupled to the substrate. These results highlight the importance of the synergistic effect that the electronic structure of a metal ion and the organic ligands has on the exchange interaction and anisotropy occurring at the molecule–electrode interface.

High-efficiency B 4 C/Mo 2 C alternate multilayer grating for monochromators in the photon energy range from 0.7 to 3.4 keV

An alternate multilayer (AML) grating has been prepared by coating an ion etched lamellar grating with a B4C/Mo2C multilayer (ML) having a layer thickness close to the groove depth. Such a structure behaves as a 2D synthetic crystal and can reach very high efficiencies when the Bragg condition is satisfied. This AML coated grating has been characterized at the SOLEIL Metrology and Tests Beamline between 0.7 and 1.7 keV and at the four-crystal monochromator beamline of Physikalisch-Technische Bundesanstalt (PTB) at BESSY II between 1.75 and 3.4 keV. A peak diffraction efficiency of nearly 27% was measured at 2.2 keV. The measured efficiencies are well reproduced by numerical simulations made with the electromagnetic propagation code CARPEM. Such AML gratings, paired with a matched ML mirror, constitute efficient monochromators for intermediate energy photons. They will extend the accessible energy for many applications as x-ray absorption spectroscopy or x-ray magnetic circular dichroism experiments.

Fast continuous energy scan with dynamic coupling of the monochromator and undulator at the DEIMOS beamline

In order to improve the efficiency of X-ray absorption data recording, a fast scan method, the Turboscan, has been developed on the DEIMOS beamline at Synchrotron SOLEIL, consisting of a software-synchronized continuous motion of the monochromator and undulator motors. This process suppresses the time loss when waiting for the motors to reach their target positions, as well as software dead-time, while preserving excellent beam characteristics.

Versatile variable temperature insert at the DEIMOS beamline for in situ electrical transport measurements

The design and the first experiments are described of a versatile cryogenic insert used for its electrical transport capabilities. The insert is designed for the cryomagnet installed on the DEIMOS beamline at the SOLEIL synchrotron dedicated to magnetic characterizations through X-ray absorption spectroscopy (XAS) measurements. This development was spurred by the multifunctional properties of novel materials such as multiferroics, in which, for example, the magnetic and electrical orders are intertwined and may be probed using XAS. The insert thus enables XAS to in situ probe this interplay. The implementation of redundant wiring and careful shielding also enables studies on operating electronic devices. Measurements on magnetic tunnel junctions illustrate the potential of the equipment toward XAS studies of in operando electronic devices.

Preparation for B4C/Mo2C multilayer deposition of alternate multilayer gratings with high efficiency in the 0.5-2.5 keV energy range

This paper presents a study of B4C/Mo2C multilayers mirrors with the aim of using it in the achievement of Alternate MultiLayer (AML) grating. Such component allows a high efficiency in the 500-2500 eV energy range for the DEIMOS beamline. Multilayers were deposited on silicon substrate. They are characterized by reflectometry under grazing incidence. Numerical adjustments were performed with a model of two layers in the period without any interfacial. A prototype of AML grating was fabricated and characterized. The efficiency of the first order of diffraction was worth 15% at 1700 eV.