Christophe Cartier dit Moulin

XAS and XMCD Investigation of Mn12 Monolayers on Gold

The deposition of Mn(12) single molecule magnets on gold surfaces was studied for the first time using combined X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) methods at low temperature. The ability of the proposed approach to probe the electronic structure and magnetism of Mn(12) complexes without significant sample damage was successfully checked on bulk samples. Detailed information on the oxidation state and magnetic polarization of manganese ions in the adsorbates was obtained from XAS and XMCD spectra, respectively. Partial reduction of metal ions to Mn(II) was clearly observed upon deposition on Au(111) of two different Mn(12) derivatives bearing 16-acetylthio-hexadecanoate and 4-(methylthio)benzoate ligands. The average oxidation state, as well as the relative proportions of Mn(II), Mn(III) and Mn(IV) species, are strongly influenced by the deposition protocol. Furthermore, the local magnetic polarizations are significantly decreased as compared with bulk Mn(12) samples. The results highlight an utmost redox instability of Mn(12) complexes at gold surfaces, presumably accompanied by structural rearrangements, which cannot be easily revealed by standard surface analysis based on X-ray photoelectron spectroscopy and scanning tunnelling microscopy.

Photomagnetic CoFe Prussian blue analogues: role of the cyanide ions as active electron transfer bridges modulated by cyanide-alkali metal ion interactions.

X-ray absorption spectra at the Co L(2,3)-edges were analyzed by means of ligand field multiplet calculations in different states of three photomagnetic CoFe Prussian blue analogues of chemical formula Cs(2)Co(4)[Fe(CN)(6)](3.3) x 11 H(2)O, Rb(2)Co(4)[Fe(CN)(6)](3.3) x 11 H(2)O and Na(2)Co(4)[Fe(CN)(6)](3.3) x 11 H(2)O. These simulations of the experimental spectra allowed the quantification of the crystal field parameter (10Dq). This determination led us (i) to evidence different behaviors of the Co(III)(LS) and Co(II)(HS) ions in the three-dimensional structure related to their electronic configurations, (ii) to propose an approach based on the electronic density distribution along the Co-NC-Fe linkage to account for the energy position of the states implied in the switching properties of the compounds, and (iii) to explain the different photomagnetic properties observed as a function of the size of the inserted alkali cation by competing interactions between the cyanide ion and the transition metal ions within the CoFe cyanide bimetallic network on the one hand and the cyanide ion and the alkali metal ions on the other hand.

Investigation of the photoinduced magnetization of copper octacyanomolybdates nanoparticles by X-ray magnetic circular dichroism

Through an extensive set of SQUID magnetic measurements, X-ray absorption spectroscopy, and X-ray magnetic circular dichroism, we have determined the nature of the metastable photomagnetic phase in the cyano-bridged 3D network Cs(2)Cu(7)[Mo(CN)(8)](4). The photomagnetic effect is induced by the photoconversion of Mo(IV) ions in low spin (LS) configuration (S = 0) into Mo(IV) ions in high spin (HS) configuration (S = 1). The magnetic and spectroscopic measurements fully support the LS to HS conversion, whereas the previously invoked charge transfer mechanism Mo(IV) + Cu(II) ⇒ Mo(V) + Cu(I) can be completely ruled out.

Remnant magnetization of Fe8 high-spin molecules: X-ray magnetic circular dichroism at 300 mK

Fe8 high-spin molecules exhibit quantum spin tunneling at very low temperatures. Eight Fe3+ ions are sixfold coordinated and magnetically coupled through oxygen bridges. The net magnetization (MS=20 μB per molecule) results from competing antiferromagnetic interactions between the various Fe3+ ions (S=5/2). Because of the structural anisotropy of these molecules, the magnetization curve presents a hysteresis loop with staircases below 2 K. The staircases of the hysteresis loop are due to the quantum spin tunneling, which is temperature dependent for 400 mK<T<2 K and temperature independent below 400 mK. In order to reach temperatures low enough to observe a remnant magnetization, we developed a H3e−H4e dilution refrigerator. This device, that takes advantage of the thermodynamic particularities of the H3e−H4e mix, let us obtain x-ray magnetic circular dichroism (XMCD) spectra at 300 mK for this molecule. It has been possible to register an XMCD remnant signal, without magnetic field applied, at the iron L...

Photo-Induced Electron Transfer and Magnetic Switching in CoFe Cyanides: Study of the Metastable State

Abstract Photo-induced magnetisation has been recently evidenced by Hashimoto et al. in CoFe cyanide[1]. We synthesized a diamagnetic RbFeCo cyanide able to present the same photomagnetic effect. The magnetic properties of the excited state, the electronic structure and the local structure of the ground and the excited states have been investigated. The conditions required to observe the phenomenon and the mechanism of the electron transfer are discussed.

Magnetic Anisotropy of Cyanide‐Bridged Core and Core–Shell Coordination Nanoparticles Probed by X‐ray Magnetic Circular Dichroism

The local symmetry and local magnetic properties of 6 nm-sized, bimetallic, cyanide-bridged CsNiCr(CN)6 coordination nanoparticles 1 and 8 nm-sized, trimetallic, CsNiCr(CN)6@CsCoCr(CN)6 core-shell nanoparticles 2 were studied by X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). The measurements were performed at the Ni(II), Co(II), and Cr(III) L2,3 edges. This study revealed the presence of distorted Ni(II) sites located on the particle surface of 1 that account for the uniaxial magnetic anisotropy observed by SQUID measurements. For the core-shell particles, a combination of the exchange anisotropy between the core and the shell and the pronounced anisotropy of the Co(II) ions is the origin of the large increase in coercive field from 120 to 890 Oe on going from 1 to 2. In addition, XMCD allows the relative orientation of the magnetic moments throughout the core-shell particles to be determined. While for the bimetallic particles of 1, alignment of the magnetic moments of Cr(III) ions with those of Ni(II) ions leads to uniform magnetization, in the core-shell particles 2 the magnetic moments of the isotropic Cr(III) follow those of Co(II) ions in the shell and those of Ni(II) ions in the core, and this leads to nonuniform magnetization in the whole nanoobject, mainly due to the large difference in local anisotropy between the Co(II) ions belonging to the surface and the Ni(II) ions in the core.

Large Orbital Magnetic Moment Measured in the [TpFeIII(CN)3]− Precursor of Photomagnetic Molecular Prussian Blue Analogues

Photomagnetism in three-dimensional Co/Fe Prussian blue analogues is a complex phenomenon, whose detailed mechanism is not yet fully understood. Recently, researchers have been able to prepare molecular fragments of these networks using a building block synthetic approach from mononuclear precursors. The main objective in this strategy is to isolate the smallest units that show an intramolecular electron transfer to have a better understanding of the electronic processes. A prior requirement to the development of this kind of system is to understand to what extent electronic and magnetic properties are inherited from the corresponding precursors. In this work, we investigate the electronic and magnetic properties of the FeTp precursor (N(C4H9)4)[TpFe(III)(CN)3], (Tp being tris-pyrazolyl borate) of a recently reported binuclear cyanido-bridged Fe/Co complex. X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements at the Fe L2,3 edges (2p → 3d) supported by ligand field multiplet calculations have allowed to determine the spin and orbit magnetic moments. Inaccuracy of the spin sum rule in the case of low-spin Fe(III) ion was demonstrated. An exceptionally large value of the orbital magnetic moment is found (0.9 μB at T = 2 K and B = 6.5 T) that is likely to play an important role in the magnetic and photomagnetic properties of molecular Fe/Co Prussian blue analogues.

A chemical model of intermediate states implied in the switching properties of CoFe Prussian blue analogues: how a cell parameter lengthening can cause a crystal field parameter increase

A series of CoFe Prussian blue analogues of chemical formula Rb2Co4−xZnx[Fe(CN)6]3.3·11H2O (x = 0, 1, 1.95 and 2.7) has been synthesized along which the MII/CoIII ions ratio at the Co site has been tuned. The long range order and the electronic structure of the Co ions have been investigated by combined powder X-ray diffraction and X-ray absorption spectroscopy (XAS) measurements. The cell parameter of the face-centered cubic structure lengthens as the MII/CoIII ions ratio increases without phase demixing. The study of the electronic structure of the Co ions by XAS shows that the coordination polyhedra of the CoII(HS) ions play an important role in the flexibility of the cubic structure. The variation of the cell parameter in the series of compounds is accompanied by the variation of the CoII–NC bond angle which allows the expansion or contraction of the cubic structure accompanying the electronic switch without phase demixing. Due to this structural re-arrangement, a lengthening of the cell parameter unusually produces an increase of the Co ion crystal field. Such a re-arrangement occurs in the course of the photo-induced electron transfer.

Molecular Magnetism: Design of High-TC Prussian Blue Molecule Based Magnets

Molecular magnetism is a field of rational synthesis of new systems, using the exchange interaction models between magnetic centers and the flexibility of coordination chemistry. We define first some theoretical approaches commonly used in this field and we show how they can help the building of molecule-based magnets. Orbital model combined with the Neel molecular field approach is used to increase the Curie temperature up to room temperature for Prussian blue family compounds.