Lapo Bogani

Molecular spintronics using single-molecule magnets

A revolution in electronics is in view, with the contemporary evolution of the two novel disciplines of spintronics and molecular electronics. A fundamental link between these two fields can be established using molecular magnetic materials and, in particular, single-molecule magnets. Here, we review the first progress in the resulting field, molecular spintronics, which will enable the manipulation of spin and charges in electronic devices containing one or more molecules. We discuss the advantages over more conventional materials, and the potential applications in information storage and processing. We also outline current challenges in the field, and propose convenient schemes to overcome them.

Single chain magnets: where to from here?

Single chain magnets (SCMs) are an interesting class of molecular polymeric materials displaying slow relaxation of the magnetization. They provide, at low temperatures, a magnetic hysteretic behaviour for a single polymeric chain. Although their behaviour evokes better-known magnetic nanoparticles and single-molecule magnets (SMMs), the similarity is mainly apparent. The fundamental differences in the physical origin of the magnetic behaviour offer perspectives that are still largely unexplored. Here we review the progress made in the synthesis, characterization and theoretical understanding of SCMs, highlighting differences and similarities with SMMs. For each of the points we then present a perspective of the advantages offered by this class of materials and we point out the main aspects that remain to be developed in the field.

Magnetic Anisotropy of Dysprosium(III) in a Low-Symmetry Environment: A Theoretical and Experimental Investigation

A mixed theoretical and experimental approach was used to determine the local magnetic anisotropy of the dysprosium(III) ion in a low-symmetry environment. The susceptibility tensor of the monomeric species having the formula [Dy(hfac)(3)(NIT-C(6)H(4)-OEt)(2)], which contains nitronyl nitroxide (NIT-R) radicals, was determined at various temperatures through angle-resolved magnetometry. These results are in agreement with ab initio calculations performed using the complete active space self-consistent field (CASSCF) method, validating the predictive power of this theoretical approach for complex systems containing rare-earth ions, even in low-symmetry environments. Susceptibility measurements performed with the applied field along the easy axis eventually permitted a detailed analysis of the temperature and field dependence of the magnetization, providing evidence that the Dy ion transmits an antiferromagnetic interaction between radicals but that the Dy-radical interaction is ferromagnetic.

Anchoring of rare-earth-based single-molecule magnets on single-walled carbon nanotubes.

A new heteroleptic bis(phthalocyaninato) terbium(III) complex 1, bearing a pyrenyl group, exhibits temperature and frequency dependence of ac magnetic susceptibility, typical of single-molecule magnets. The complex was successfully attached to single-walled carbon nanotubes (SWNTs) using pi-pi interactions, yielding a 1-SWNT conjugate. The supramolecular grafting of 1 to SWNTs was proven qualitatively and quantitatively by high-resolution transmission electron microscopy, emission spectroscopy, and atomic force spectroscopy. Giving a clear magnetic fingerprint, the anisotropy energy barrier and the magnetic relaxation time of the 1-SWNT conjugate are both increased in comparison with the pure crystalline compound 1, likely due to the suppression of intermolecular interactions. The obtained results propose the 1-SWNT conjugate as a promising constituent unit in magnetic single-molecule measurements using molecular spintronics devices.

Finite-size effects in single chain magnets: an experimental and theoretical study.

The problem of finite-size effects in s=1/2 Ising systems showing slow dynamics of the magnetization is investigated introducing diamagnetic impurities in a Co2+-radical chain. The static magnetic properties have been measured and analyzed considering the peculiarities induced by the ferrimagnetic character of the compound. The dynamic susceptibility shows that an Arrhenius law is observed with the same energy barrier for the pure and the doped compounds while the prefactor decreases, as theoretically predicted. Multiple spin reversal has also been investigated.

Single-molecule-magnet carbon-nanotube hybrids.

Carbon nanotubes (CNTs) hold great promise for sensing and nanoelectronics, as core components of chemical and biological ultra-sensitive probes and of field-effect transistors (FETs). CNT–SQUID devices in particular could constitute magnetic detectors with single-molecule sensitivity, thus offering a viable route to the long-sought readout of magnetic information stored in individual single-molecule magnets (SMMs). SMMs are metal-ion clusters with a large easy-axis magnetic anisotropy, exhibiting a magnetic hysteresis loop at low temperature and suggested as components for quantum computing and molecular spintronics. To date, the chemistry needed to bridge the domains of CNTs and SMMs has remained unexplored. CNT hybrids with gold or magnetic nanoparticles, proteins, enzymes, or luminescent molecules are currently under intense investigation. 8] The resulting materials usually entail a large number of nanoparticles or molecules per CNT, whereas CNT–SMM detectors and spintronic devices require the sequential addition of a small but very controlled number of nanomagnets. Grafting through covalent bonds might introduce electron scattering centers that may limit the performance of CNT devices. By contrast, noncovalent p-stacking interactions with pristine CNTs should largely preserve the CNT conductance, while guaranteeing SMM–CNT interaction. Herein we report the assembly of CNT–SMM hybrids using a tailor-made tetrairon(III) SMM, [Fe4(L)2(dpm)6] (1; Hdpm= dipivaloylmethane), designed to graft onto the walls of CNTs. The ligand L (H3L= 2-hydroxymethyl-2-(4(pyren-1-yl)butoxy)methylpropane-1,3-diol), features an alkyl chain with a terminal pyrenyl group and was synthesized as in Figure 1a. Reduction of 4-pyren-1-yl-butyric acid gives 4-(1-pyrenil)butanol, which is then coupled with 4-bromomethyl-1-methyl-2,6,7-trioxa-bicyclo[2.2.2]octane. A twosteps deprotection of the trimethylol function affords H3L, which is finally treated with the preformed complex [Fe4(OMe)6(dpm)6] (2) to give 1 in excellent yield (95%). The molecular structure of 1 (Figure 1b,c), determined by single-crystal X-ray diffraction, shows a tetrairon(III) propeller-like core with idealized D3 symmetry held together by two triply deprotonated H3L ligands lying at opposite sides of the molecular plane (see Supporting Information). The molecular size of 1 is 1.6–2.3 nm (av.: 1.9 nm). Low-temperature high-frequency (HF)-EPR spectra at 190 and 230 GHz (Figure 2a) and variable-temperature magnetic-susceptibility measurements show the presence of an S= 5 high-spin ground state with an easy-axis magnetic anisotropy (D= 0.409 cm ; Supporting Information). Indeed, single-crystal magnetic measurements reveal a hysteresis loop below 1 K with characteristic quantum-tunneling steps (Figure 2b), confirming the SMM behavior. CNT–FETs were obtained by electron-beam lithography on degenerately n-doped silicon wafers covered with a 300 nm thick SiO2 layer. Single CNTs were located by atomic force microscopy (AFM) and connected by palladium leads separated by 300 nm gaps. The hybrids were then produced by immersion of the CNT–FETs in a 3.1 10 m solution of 1 in 1,2-dichloroethane (DCE) for 30 min, followed by extensive washing with pure DCE. H NMR, ESI-MS, and fluorescence techniques demonstrate that the complex is completely stable in solution in the conditions used for the deposition (Supporting Information). The grafting was reiterated to follow the progressive addition of SMMs. After each treatment a few SMMs were found to stick onto the CNT (Figure 3a), while some others were also located on the surrounding surface. The isostructural complex containing H3L’= 2-hydroxymethyl-2-phenylpropane-1,3-diol did not graft onto CNTs in the same experimental conditions. This result is a strong indication that 1 has been grafted as a result of the pyrenyl functionalities. [*] Dr. L. Bogani, Dr. N. Bendiab, Dr. W. Wernsdorfer Institut N el, CNRS 25 Av. des Martyrs, 38042 Grenoble, Cedex 9 (France) Fax: (+33)4-7688-1191 E-mail: lbogani@hotmail.com

Evidence of intermolecular π-stacking enhancement of second-harmonic generation in a family of single chain magnets

The second-order nonlinear optical properties of a family of rare-earth-based single chain magnets are presented (using a fundamental field at wavelength λ = 766 nm), together with an analysis of the origin of the second-harmonic generation (SHG) process. By studying the fundamental crystal symmetries and the geometrical arrangement of the constituting elements of the system in the unit cell we show that the main contribution to the nonlinear process arises from intermolecular π-stacking interactions.

The classical and quantum dynamics of molecular spins on graphene

Controlling the dynamics of spins on surfaces is pivotal to the design of spintronic1 and quantum computing2 devices. Proposed schemes involve the interaction of spins with graphene to enable surface-state spintronics3,4, and electrical spin-manipulation4-11. However, the influence of the graphene environment on the spin systems has yet to be unraveled12. Here we explore the spin-graphene interaction by studying the classical and quantum dynamics of molecular magnets13 on graphene. While the static spin response remains unaltered, the quantum spin dynamics and associated selection rules are profoundly modulated. The couplings to graphene phonons, to other spins, and to Dirac fermions are quantified using a newly-developed model. Coupling to Dirac electrons introduces a dominant quantum-relaxation channel that, by driving the spins over Villain’s threshold, gives rise to fully-coherent, resonant spin tunneling. Our findings provide fundamental insight into the interaction between spins and graphene, establishing the basis for electrical spin-manipulation in graphene nanodevices.

Straightforward Synthesis of Substituted p‐Quinones: Isolation of a Key Intermediate and Use as a Bridging Ligand in a Diruthenium Complex

Quinones are naturally occurring redox active molecules that function in vital electron-transport processes, often in conjugation with a transition-metal center. p-Quinones, such as vitamin K derivatives, ubiquinones or plastoquinones, play important roles in photosynthesis, respiration, and information-transfer processes. Substituted p-quinones have been extensively used as ligands in coordination chemistry in recent years. The remarkable properties that such ligands impart to their metal complexes make such compounds useful in a variety of fields, such as homogenous catalysis, supramolecular chemistry, coordination polymers, and as bridging ligands in combination with redox active metal centers such as ruthenium. The last field has gained tremendous attention in recent years because of ambiguities arising in oxidation-state formulations also with “organometallic-type” non-innocent ligands. In addition, the creation of quinone-based molecular magnets is a lively field of research that has produced many interesting results. 2,5-Diamino-1,4-benzoquinone 1 and its substituted derivatives have been known for decades. The synthesis of 1 has previously been reported and such syntheses are rarely straightforward one-pot reactions. Some years back Braunstein et al. reported a straightforward and “green” synthesis of a new class of molecules that occurs through transamination. Such molecules, which are isomers of 1 and its derivatives, are best described as zwitterions, 6. Inspired by this process we looked for an elegant synthesis for the parent compound 1 and for possible intermediates formed during the transamination process. Herein we report a simple one-pot synthesis of 1 and its mono(2 and 3) and dialkyl (4 and 5) derivatives (Scheme 1). To elucidate

Coupling between magnetic and optical properties of stable Au–Fe solid solution nanoparticles

Au-Fe nanoparticles constitute one of the simplest prototypes of a multifunctional nanomaterial that can exhibit both magnetic and optical (plasmonic) properties. This solid solution, not feasible in the bulk phase diagram in thermal equilibrium, can be formed as a nanostructure by out-of-equilibrium processes. Here, the novel magnetic, optical and magneto-optical properties of ion-implanted Au-Fe solid solution nanoparticles dispersed in a SiO(2) matrix are investigated and correlated. The surface plasmon resonance of the Au-Fe nanoparticles with almost equicomposition is strongly damped when compared to pure Au and to Au-rich Au-Fe nanoparticles. In all cases, the Au atoms are magnetically polarized, as measured by x-ray magnetic circular dichroism, and ferromagnetically coupled with Fe atoms. Although the chemical stability of Au-Fe nanoparticles is larger than that of Fe nanoparticles, both the magnetic moment per Fe atom and the order temperature are smaller. These results suggest that electronic and magnetic properties are more influenced by the hybridization of the electronic bands in the Au-Fe solid solution than by size effects. On the other hand, the magneto-optical transitions allowed in the vis-nIR spectral regions are very similar. In addition, we also observe, after studying the properties of thermally treated samples, that the Au-Fe alloy is stabilized, not by surface effects, but by the combination of the out-of-equilibrium nature of the ion implantation technique and by changes in the properties due to size effects.