P. Ohresser

Robust spin crossover and memristance across a single molecule

A nanoscale molecular switch can be used to store information in a single molecule. Although the switching process can be detected electrically in the form of a change in the molecules conductance, adding spin functionality to molecular switches is a key concept for realizing molecular spintronic devices. Here we show that iron-based spin-crossover molecules can be individually and reproducibly switched between a combined high-spin, high-conduction state and a low-spin, low-conduction state, provided the individual molecule is decoupled from a metallic substrate by a thin insulating layer. These results represent a step to achieving combined spin and conduction switching functionality on the level of individual molecules.

Direct observation of a highly spin-polarized organic spinterface at room temperature

Organic semiconductors constitute promising candidates toward large-scale electronic circuits that are entirely spintronics-driven. Toward this goal, tunneling magnetoresistance values above 300% at low temperature suggested the presence of highly spin-polarized device interfaces. However, such spinterfaces have not been observed directly, let alone at room temperature. Thanks to experiments and theory on the model spinterface between phthalocyanine molecules and a Co single crystal surface, we clearly evidence a highly efficient spinterface. Spin-polarised direct and inverse photoemission experiments reveal a high degree of spin polarisation at room temperature at this interface. We measured a magnetic moment on the molecules nitrogen π orbitals, which substantiates an ab-initio theoretical description of highly spin-polarised charge conduction across the interface due to differing spinterface formation mechanisms in each spin channel. We propose, through this example, a recipe to engineer simple organic-inorganic interfaces with remarkable spintronic properties that can endure well above room temperature.

Magnetic behaviour of TbPc2 single-molecule magnets chemically grafted on silicon surface

Single-molecule magnets (SMMs) are among the most promising molecular systems for the development of novel molecular electronics based on the spin transport. Going beyond the investigations focused on physisorbed SMMs, in this work the robust grafting of Terbium(III) bis(phthalocyaninato) complexes to silicon surface from a diluted solution is achieved by rational chemical design yielding the formation of a partially oriented monolayer on the conducting substrate. Here, by exploiting the surface sensitivity of X-ray circular magnetic dichroism we evidence an enhancement of the magnetic bistability of this single-molecule magnet, in contrast to the dramatic reduction of the magnetic hysteresis that characterises monolayer deposits evaporated on noble and ferromagnetic metals. Photoelectron spectroscopy investigations and density functional theory analysis suggest a non-innocent role played by the silicon substrate, evidencing the potentiality of this approach for robust integration of bistable magnetic molecules in electronic devices.

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.

DEIMOS: A beamline dedicated to dichroism measurements in the 350–2500 eV energy range

The DEIMOS (Dichroism Experimental Installation for Magneto-Optical Spectroscopy) beamline was part of the second phase of the beamline development at French Synchrotron SOLEIL (Source Optimisée de Lumière à Energie Intermédiaire du LURE) and opened to users in March 2011. It delivers polarized soft x-rays to perform x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and x-ray linear dichroism in the energy range 350-2500 eV. The beamline has been optimized for stability and reproducibility in terms of photon flux and photon energy. The main end-station consists in a cryo-magnet with 2 split coils providing a 7 T magnetic field along the beam or 2 T perpendicular to the beam with a controllable temperature on the sample from 370 K down to 1.5 K.

Antiferromagnetic coupling of TbPc2 molecules to ultrathin Ni and Co films

Summary The magnetic and electronic properties of single-molecule magnets are studied by X-ray absorption spectroscopy and X-ray magnetic circular dichroism. We study the magnetic coupling of ultrathin Co and Ni films that are epitaxially grown onto a Cu(100) substrate, to an in situ deposited submonolayer of TbPc2 molecules. Because of the element specificity of the X-ray absorption spectroscopy we are able to individually determine the field dependence of the magnetization of the Tb ions and the Ni or Co film. On both substrates the TbPc2 molecules couple antiferromagnetically to the ferromagnetic films, which is possibly due to a superexchange interaction via the phthalocyanine ligand that contacts the magnetic surface.

LEED analysis of ultra-thin cobalt layers grown on Cr(100)

Abstract Cobalt overlayers grown at room temperature on a chromium (100) surface have been studied with low energy electron diffraction (LEED) up to a thickness of four atomic layers. A quantitative analysis via dynamical LEED calculations is given for zero and two cobalt layers grown on Cr(100). A metastable bcc phase of cobalt is evidenced. The influence of nitrogen contamination of the films is discussed.

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.

Efficient Spin-Flip Excitation of a Nickelocene Molecule

Inelastic electron tunneling spectroscopy (IETS) within the junction of a scanning tunneling microscope (STM) uses current-driven spin-flip excitations for an all-electrical characterization of the spin state of a single object. Usually decoupling layers between the single object, atom or molecule, and the supporting surface are needed to observe these excitations. Here we study the surface magnetism of a sandwich nickelocene molecule (Nc) adsorbed directly on Cu(100) by means of X-ray magnetic circular dichroism (XMCD) and density functional theory (DFT) calculations and show with IETS that it exhibits an exceptionally efficient spin-flip excitation. The molecule preserves its magnetic moment and magnetic anisotropy not only on Cu(100), but also in different metallic environments including the tip apex. By taking advantage of this robusteness, we are able to functionalize the microscope tip with a Nc, which can be employed as a portable source of inelastic excitations as exemplified by a double spin-flip excitation process.