Matteo Mannini

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.

Organizing and addressing magnetic molecules.

Magnetic molecules ranging from simple organic radicals to single-molecule magnets (SMMs) are intensively investigated for their potential applications in molecule-based information storage and processing. The goal of this Article is to review recent achievements in the organization of magnetic molecules on surfaces and in their individual probing and manipulation. We stress that the inherent fragility and redox sensitivity of most SMM complexes, combined with the noninnocent role played by the substrate, ask for a careful evaluation of the structural and electronic properties of deposited molecules going beyond routine methods for surface analysis. Detailed magnetic information can be directly obtained using X-ray magnetic circular dichroism or newly emerging scanning probe techniques with magnetic detection capabilities.

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.

X‐Ray Detected Magnetic Hysteresis of Thermally Evaporated Terbium Double‐Decker Oriented Films

Fabrication of molecular nanostructures and control of the molecular properties at the nanoscale is at the basis of the development of innovative single molecule devices. [ 1 ] Particularly active is the research for the organization of single molecule magnets (SMMs) that have been proposed as ideal candidates for the development of molecular spintronics and data storage devices. [ 2 , 3 ] These molecules are a well known class of compounds characterized by the peculiar presence of a strong axial magnetic anisotropy that induces a slow relaxation in the magnetization and a magnetic hysteresis of molecular origin showing spectacular quantum effects. [ 4 ] Thanks to the surface sensitivity of synchrotron-based techniques it has been possible to provide the proof of concept that SMM behavior is observable in a single layer of magnetic molecules. [ 5 ] First attempts to control at the nanoscale the SMM assembling have been made by opportune functionalization promoting their grafting on specifi c surfaces in order to form monolayer deposits from solution. [ 6 , 7 ] However, cleaner processes, e.g. thermal evaporation, [ 8 ] are required for the development of real devices or to extend the investigation to reactive surfaces, e.g. ferromagnetic metals, and

Strong magneto-chiral dichroism in a paramagnetic molecular helix observed by hard X-rays

Magneto-chiral dichroism (MχD) is a non-reciprocal, i. e. directional, effect observed in magnetised chiral systems featuring an unbalanced absorption of unpolarised light depending on the direction of the magnetisation. Despite the fundamental interest in a phenomenon breaking both parity and time reversal symmetries, MχD is one of the least investigated aspects of light-matter interaction because of the weakness of the effect in most reported experiments. Here we have exploited the element selectivity of hard X-ray radiation to investigate the magneto-chiral properties of enentiopure crsytals of two isostructural molecular helicoidal chains comprising Cobalt(II) and Manganese (II) ions, respectively. A strong magneto-chiral dichroism, with Kuhn asymmetry of the order of a few percent, has been observed in the Cobalt chain system, while it is practically absent for the Manganese derivative. The spectral features of the XMχD signal differ significantly from the natural and magnetic dichroic contributions and have been here rationalized using the simple multipolar expansion of matter-radiation interaction.

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.

Soft‐X‐ray‐Induced Redox Isomerism in a Cobalt Dioxolene Complex

Valence tautomerism (VT) defines reversible interconversions between two or more redox isomers. It is established that these interconversions can be stimulated by temperature and light irradiation. For example, the diamagnetic [Co(Me2tpa)(DBCat)]PF6·C6H5CH3 complex (1) (Me2tpa = bis(6-methyl-(2-pyridylmethyl)) (2-pyridylmethyl)amine, DBCat = 3,5-di-tert-butylcatecholato) was found to undergo a thermally induced interconversion in the solid state yielding the redox isomer characterized by the high-spin Co-semiquinonato (hs-Co-SQ) charge distribution (see Scheme 1). 3] The observed transition can be formally described as the result of an entropy-driven intramolecular electron transfer involving the donor catecholato and the cobalt(III) acceptor. At cryogenic temperatures, laser irradiation of the solid compound at 904 nm, where a ligand-tometal charge transfer (LMCT) occurs, was found by bulk magnetic measurements to induce the same process, affording the hs-Co-semiquinonato species as a metastable phase in 90% yield with a rather long lifetime (two weeks at 9 K). Soft X-ray absorption spectroscopy (XAS) is an elementsensitive synchrotron-based technique and provides a powerful tool to study the electronic and chemical structure of a specific atom and its coordination environment. It is particularly powerful in the magnetic study of 3d metal complexes. With the additional asset of very high detection sensitivity, XAS has been effectively used in the characterization of systems with multiple quasi degenerated electronic states, including very diluted and nanostructured systems. We have found that for 1 this technique not only yields this important information, but also intrinsically provides the perturbation for inducing interconversion between the two redox isomers. This is an unprecedented result and we believe it to be particularly important for the study of all the complexes exhibiting photochromism. Figure 1 shows the temperature dependence of the cobalt L3-edge X-ray absorption spectra of 1 (the L2,3 spectra are shown in Figure S1 of the Supporting Information). The spectra were obtained with a X-ray flux of 10 photons s 1 on Scheme 1. The two different electronic configurations involved in VT process.

Thermal Deposition of Intact Tetrairon(III) Single‐Molecule Magnets in High‐Vacuum Conditions

A tetrairon(III) single-molecule magnet is deposited using a thermal evaporation technique in high vacuum. The chemical integrity is demonstrated by time-of-flight secondary ion mass spectrometry on a film deposited on Al foil, while superconducting quantum interference device magnetometry and alternating current susceptometry of a film deposited on a kapton substrate show magnetic properties identical to the pristine powder. High-frequency electron paramagnetic resonance spectra confirm the characteristic behavior for a system with S = 5 and a large Ising-type magnetic anisotropy. All these results indicate that the molecules are not damaged during the deposition procedure keeping intact the single-molecule magnet behavior.

Spin noise fluctuations from paramagnetic molecular adsorbates on surfaces

The measurement of spin noise in nuclei was demonstrated on bulk samples more than two decades ago. An ensemble of spins can produce a coherent signal at the Larmor frequency of a static magnetic field, known as spin noise, an effect due to the statistical polarization of small ensembles. The difficulty of these measurements is that the signal is extremely small—even if electron spins are detected. Although the statistical polarization of N spins dominates the Boltzmann statistics if N approaches unity, a more sensitive tool is requested to measure the polarization of the magnetic moment of a single spin. In this paper we report on the verification of recent results on the detection of spin noise from paramagnetic molecules of α,γ-bisdiphenylene-β-phenylallyl (BDPA) by Durkan and Welland [Appl. Phys. Lett. 80, 458 (2002)]. We also present results on a second paramagnetic specie 1,1-diphenyl-2-picrylhydrazyl (DPPH), deposited on Au(111) surfaces. Electron spin resonance spectra from ultrathin films of DPPH...