Dante Gatteschi

Large clusters of metal ions: The transition from molecular to bulk magnets

Clusters of metal ions are a class of compounds actively investigated for their magnetic properties, which should gradually change from those of simple paramagnets to those of bulk magnets. However, their interest lies in a number of different disciplines: chemistry, which seeks new synthetic strategies to make larger and larger clusters in a controlled manner; physics, which can test the validity of quantum mechanical approaches at the nanometer scale; and biology, which can use them as models of biomineralization of magnetic particles.

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.

Magneto-structural correlations in exchange coupled systems

This book contains 19 chapters. Some of the chapter titles are: Optical Spectroscophy; The Basis of Spin-Hamiltonian Theory; Inelastic Neutorn Scattering From Clusters; Magneto-structural Correlations in Bioinorganic Chemistry; and Magnetic Exchange Interactions Propagated by Multi-Atom Bridges.

Electron paramagnetic resonance of exchange coupled systems

Contents: Exchange and Superexchange.- Spin Hamiltonians.- Spectra of Pairs.- Spectra of Clusters.- Relaxation in Oligonuclear Species.- Spectra in Extended Lattices.- Selected Examples of Spectra of Pairs.- Coupled Transition-Metal Ions-Organic Radicals.- Biological Systems.- Low Dimensional Materials.- Excitons.- Appendix A: Second Quantization.- Appendix B: Properties of Angular Momentum Operators and Elements of Irreducible Tensor Algebra.

Magnetic molecular materials

The strategies currently developed for the synthesis of molecular based magnetic materials are reviewed. After a brief introduction resuming the conditions which are required to observe spontaneous magnetization, the organic, inorganic, organometallic approaches are illustrated. Particular attention is devoted to the metal-radical approach, in which the materials are assembled using transition metal complexes and stable organic radicals as building blocks.

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.

HIGH-FREQUENCY EPR SPECTRA OF A MOLECULAR NANOMAGNET : UNDERSTANDING QUANTUM TUNNELING OF THE MAGNETIZATION

EPR spectra have been recorded in very high field, up to 25T, and at high frequency, up to 525 GHz, on a polycristalline sample of Mn12ac (see paper for detailed formula), the first example of molecular cluster behaving like a nanomagnet. The simulation of the spectra has provided an accurate determination of the parameters of the spin hamiltonian (see paper for formula and values of the various parameters). The presence of the fourth order term in the total spin justifies the irregularities in the spacing of the jumps, recently observed in the hysteresis loop of Mn12ac and attributed to acceleration of the relaxation of the magnetization due to Quantum Tunneling between degenerate M states of the ground S=10 multiplet of the cluster. The term in (S_+^4 + S_-^4) is responsible of the transverse magnetic anisotropy and plays a crucial role in the mechanism of Quantum Tunneling. The HF-EPR spectra have for the first time evidenced its presence and quantified it.

Water-Dispersible Sugar-Coated Iron Oxide Nanoparticles. An Evaluation of their Relaxometric and Magnetic Hyperthermia Properties

Synthesis of functionalized magnetic nanoparticles (NPs) for biomedical applications represents a current challenge. In this paper we present the synthesis and characterization of water-dispersible sugar-coated iron oxide NPs specifically designed as magnetic fluid hyperthermia heat mediators and negative contrast agents for magnetic resonance imaging. In particular, the influence of the inorganic core size was investigated. To this end, iron oxide NPs with average size in the range of 4-35 nm were prepared by thermal decomposition of molecular precursors and then coated with organic ligands bearing a phosphonate group on one side and rhamnose, mannose, or ribose moieties on the other side. In this way a strong anchorage of the organic ligand on the inorganic surface was simply realized by ligand exchange, due to covalent bonding between the Fe(3+) atom and the phosphonate group. These synthesized nanoobjects can be fully dispersed in water forming colloids that are stable over very long periods. Mannose, ribose, and rhamnose were chosen to test the versatility of the method and also because these carbohydrates, in particular rhamnose, which is a substrate of skin lectin, confer targeting properties to the nanosystems. The magnetic, hyperthermal, and relaxometric properties of all the synthesized samples were investigated. Iron oxide NPs of ca. 16-18 nm were found to represent an efficient bifunctional targeting system for theranostic applications, as they have very good transverse relaxivity (three times larger than the best currently available commercial products) and large heat release upon application of radio frequency (RF) electromagnetic radiation with amplitude and frequency close to the human tolerance limit. The results have been rationalized on the basis of the magnetic properties of the investigated samples.

The molecular approach to nanoscale magnetism

Abstract Molecular clusters of paramagnetic metal ions have been widely investigated as model for magnetism at the nanoscale, especially for quantum effects like the tunneling of the magnetic moment. We present here some recent results obtained on derivatives of the well-known Mn12 cluster, especially on the half-integer spin compounds. The role of the transverse anisotropy in the dynamics of the magnetization is here elucidated through the comparison of the tunneling rate of the magnetization in two Fe8 cluster compounds, which differ only in the transverse anisotropy. Local dipolar fields and nuclear hyperfine fields have also revealed to strongly affect the relaxation in the pure tunneling regime and recent experiment has allowed to determine the intrinsic linewidth of the tunneling resonance. The transverse field dependence of the relaxation rate of Fe8 has revealed oscillations that are analog to the topological constructive–destructive interference of the spin phase (Berry phase) and we review here some very recent results. The magnetic behavior of antiferromagnetic ring-shaped clusters is also discussed for their potential interest as models for antiferromagnetic particles. Some recent results obtained by other chemists in the synthesis of large spin clusters are also reviewed.