Chiara Danieli

Magnetic memory of a single-molecule quantum magnet wired to a gold surface

In the field of molecular spintronics, the use of magnetic molecules for information technology is a main target and the observation of magnetic hysteresis on individual molecules organized on surfaces is a necessary step to develop molecular memory arrays. Although simple paramagnetic molecules can show surface-induced magnetic ordering and hysteresis when deposited on ferromagnetic surfaces, information storage at the molecular level requires molecules exhibiting an intrinsic remnant magnetization, like the so-called single-molecule magnets (SMMs). These have been intensively investigated for their rich quantum behaviour but no magnetic hysteresis has been so far reported for monolayers of SMMs on various non-magnetic substrates, most probably owing to the chemical instability of clusters on surfaces. Using X-ray absorption spectroscopy and X-ray magnetic circular dichroism synchrotron-based techniques, pushed to the limits in sensitivity and operated at sub-kelvin temperatures, we have now found that robust, tailor-made Fe(4) complexes retain magnetic hysteresis at gold surfaces. Our results demonstrate that isolated SMMs can be used for storing information. The road is now open to address individual molecules wired to a conducting surface in their blocked magnetization state, thereby enabling investigation of the elementary interactions between electron transport and magnetism degrees of freedom at the molecular scale.

Quantum tunnelling of the magnetization in a monolayer of oriented single-molecule magnets

A fundamental step towards atomic- or molecular-scale spintronic devices has recently been made by demonstrating that the spin of an individual atom deposited on a surface, or of a small paramagnetic molecule embedded in a nanojunction, can be externally controlled. An appealing next step is the extension of such a capability to the field of information storage, by taking advantage of the magnetic bistability and rich quantum behaviour of single-molecule magnets (SMMs). Recently, a proof of concept that the magnetic memory effect is retained when SMMs are chemically anchored to a metallic surface was provided. However, control of the nanoscale organization of these complex systems is required for SMMs to be integrated into molecular spintronic devices. Here we show that a preferential orientation of Fe4 complexes on a gold surface can be achieved by chemical tailoring. As a result, the most striking quantum feature of SMMs—their stepped hysteresis loop, which results from resonant quantum tunnelling of the magnetization—can be clearly detected using synchrotron-based spectroscopic techniques. With the aid of multiple theoretical approaches, we relate the angular dependence of the quantum tunnelling resonances to the adsorption geometry, and demonstrate that molecules predominantly lie with their easy axes close to the surface normal. Our findings prove that the quantum spin dynamics can be observed in SMMs chemically grafted to surfaces, and offer a tool to reveal the organization of matter at the nanoscale.

Electric Field Controlled Magnetic Anisotropy in a Single Molecule

We have measured quantum transport through an individual Fe(4) single-molecule magnet embedded in a three-terminal device geometry. The characteristic zero-field splittings of adjacent charge states and their magnetic field evolution are observed in inelastic tunneling spectroscopy. We demonstrate that the molecule retains its magnetic properties and, moreover, that the magnetic anisotropy is significantly enhanced by reversible electron addition/subtraction controlled with the gate voltage. Single-molecule magnetism can thus be electrically controlled.

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