[Show abstract][Hide abstract] ABSTRACT: We have investigated Magnetic Nanoparticles (MNPs) of spinel type iron oxide (of approximately 8 nm) mineralized in the internal cavity of the bioreactor ferritin nanocage. In particular, we have used Electron Magnetic Resonance, EMR, spectroscopy and taken advantage of the capacity of the protein shells to control the size of the MNPs. EMR measurements in perpendicular and parallel configurations have been recorded at various temperatures. A model based on the giant spin is used to interpret the experimental results. The analysis indicates that the observed quantum behaviour has to be ascribed to the whole MNP and that the thermal population of excited spin states has a strong influence in the EMR behaviour of MNPs.
Full-text · Article · Jan 2016 · Physical Chemistry Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: The first chapter of the book contain a short history of magnetism opening on the discovery of magnetic phenomena up to the discovery in the magnetic properties of molecular systems. A first glance on the different of magnetic behaviors in the field of molecular magnetism is offered by analyzing the basic properties of p, d, and f magnetic orbitals. The organization of the book contents is widely explained with a particular attention to the relevance of magnetic properties of derivatives containing 4f metal ions. The last part of the chapter is devoted to the description of the applications of Lanthanides.
[Show abstract][Hide abstract] ABSTRACT: The strategy to design molecular ferromagnets is briefly resumed stressing the role of the magnetic building blocks and the importance of the overlap of the magnetic orbitals. Some considerations are made on the importance of the magnetic dimensionality on the properties. The key factor is the description of the magnetic interactions. At this level it is described by the spin Hamiltonian formalism which, instead of using true operators, SH uses effective spin operator. The interaction has a double nature, the dipolar and the exchange interaction. In many cases, the coupled states can be described by the total spin and experimental techniques like temperature dependent magnetic susceptibility provide a deep insight in the nature of the low-lying coupled levels. The coupling between the spins can be analyzed by the HDVV or the Ising approach. The dipolar interaction provides a tool for measuring the distance between the spins. Orbital degeneracy is introduced by using the Lines mode formalism. The effect of non collinearity of the individual spins is introduced.
[Show abstract][Hide abstract] ABSTRACT: A survey about the characterization of Molecular Magnets adsorbed on surfaces by density functional methods studied in our laboratory is presented. Several computational issues afflict the description of the different players involved and, therefore, a discriminating analysis of the single players, first, and the scenario in which they interact, afterwards, is presented. An assessment of the applicable computational protocols describing the evolution of the magnetic properties of some molecular magnets from the bulk to the surface adsorbed scenario are presented. The limits of the approaches and their applicability are also discussed. In the framework proposed, the role of the surface and its effects on the geometry and the magnetic properties on the adsorbed molecular magnets is enlightened
No preview · Article · Jan 2015 · Coordination Chemistry Reviews
[Show abstract][Hide abstract] ABSTRACT: One of the distinctive properties of transition-metal compounds is the presence of unpaired electrons in the partially filled d-shell which give rise to magnetic properties. The metal ions are therefore paramagnetic, which means that they are attracted by an applied magnetic field. The paramagnetism has its origins in both the spin and orbital moments of the unpaired electrons. In general, the passage from isolated ions to coordination compounds is associated with the quenching of the orbital moment, and the paramagnetic properties can be described to a first approximation by considering the orbital contribution as a perturbation on the spin term. It is the presence of the unquenched orbital contribution that makes the measurement of the paramagnetic properties an important tool for the structural characterization of transition-metal coordination compounds. In fact magnetic measurements have long been used in order to obtain structural information. Magnetochemistry (Carlin, R. L. Magnetochemistry; Springer-Verlag: Berlin, 1986; O'Connor, C. J. Magnetochemistry - Theory and Experimentation; Wiley: New York, 1972) was extensively developed in the 1950s in conjunction with the first impact of quantum mechanics in transition-metal chemistry as a fundamental tool for the characterization of coordination compounds. In the eighties there was a gradual shift from simple paramagnetic properties to all kinds of cooperative magnetic phenomena, with the investigation of complex zero-, one-, two-, and three-dimensional molecule-based magnetic materials (de Jongh, L. J.; Miedema, A. R. Adv. Phys.1974, 23, 1-260; Kahn, O. Molecular Magnetism; VCH: Weinheim, 1993; Gatteschi, D.; Kahn, O.; Miller, J. S.; Palacio, F., Eds. Magnetic Molecular Materials; NATO ASI; Kluwer: Dordrecht, The Netherlands, 1991). This shift of interest was accompanied by a progressive increase in the complexity of the magnetic techniques used to investigate transition-metal compounds. In the early stages the measurement of the effective magnetic moment at room temperature was enough, now it is almost routine to measure down to the liquid helium region and below, in both d.c. and a.c. mode. Sophisticated techniques are also used for the investigation of single crystals.
[Show abstract][Hide abstract] ABSTRACT: The selective replacement of the central iron(III) ion with vanadium(III) in a tetrairon(III) propeller-shaped single-molecule magnet has allowed us to increase the ground spin state from S=5 to S=13/2. As a consequence of the pronounced anisotropy of vanadium(III), the blocking temperature for the magnetization has doubled. Moreover, a significant remnant magnetization, practically absent in the parent homometallic molecule, has been achieved owing to the suppression of zero-field tunneling of the magnetization for the half-integer molecular spin. Interestingly, the contribution of vanadium(III) to the magnetic anisotropy barrier occurs through the anisotropic exchange interaction with iron(III) spins and not through single ion anisotropy as in most single-molecule magnets.
No preview · Article · Oct 2014 · Chemistry - A European Journal
[Show abstract][Hide abstract] ABSTRACT: Molecular magnetism is considered an area where magnetic phenomena that are usually difficult to demonstrate can emerge with particular clarity. Over the years, however, less understandable systems have appeared in the literature of molecular magnetic materials, in some cases showing features that hint at the spontaneous emergence of global structures out of local interactions. This ingredient is typical of a wider class of problems, called complex behaviours, where the theory of complexity is currently being developed. In this perspective we wish to focus our attention on these systems and the underlying problematic that they highlight. We particularly highlight the emergence of the signatures of complexity in several molecular magnetic systems, which may provide unexplored opportunities for physical and chemical investigations
No preview · Article · Jul 2014 · Physical Chemistry Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: Two non-perturbative numerically exact methods: exact diagonalization and quantum transfer matrix are applied to computationally complex Heisenberg-like spin models of ring shaped molecular nanomagnets and implemented in the high performance computing environment. These methods are applicable to the wide class of ring-shaped nanomagnets. For the hypothetical antiferromagnetic nanomagnet Ni12 the influence of single-ion anisotropy on the ground states is investigated. For Cr8 it is demonstrated that the alternation of the nearest-neighbor bilinear exchange couplings leads to small changes in the magnetic torque with respect to the uniformly coupled system. Specific heat and entropy for Cr8 are showed to be good indicators of crossing fields. The applicability of the Lande rule to both systems is checked.
Full-text · Article · May 2014 · Lecture Notes in Computer Science
[Show abstract][Hide abstract] ABSTRACT: Molecular magnetism is producing new types of materials which cover up to date aspects of basic science together with possible applications. This article highlights recent results from the point of view of lanthanides which are now intensively used to produce single molecule magnets, single chain and single ion magnets. After a short introduction reminding the main steps of development of molecular magnetism, the basic properties of lanthanides will be covered highlighting important features which are enhanced by the electronic structure of lanthanides, like spin frustration and chirality, anisotropy and non collinear axes in zero and one dimensional materials. A paragraph of conclusions will discuss what has been done and theperspectives to be expected.
Preview · Article · Jan 2014 · The European Physical Journal Conferences
[Show abstract][Hide abstract] ABSTRACT: The quest for efficient ways of modulating localized surface plasmon resonance is one of the frontiers in current research in plasmonics; the use of a magnetic field as a source of modulation is among the most promising candidates for active plasmonics. Here we report the observation of magnetoplasmonic modes on colloidal gold nanoparticles detected by means of magnetic circular dichroism (MCD) spectroscopy and provide a model that is able to rationalize and reproduce the experiment with unprecedented qualitative and quantitative accuracy. We believe that the steep slope observed at the plasmon resonance in the MCD spectrum can be very efficient in detecting changes in the refractive index of the surrounding medium, and we give a simple proof of principle of its possible implementation for magnetoplasmonic refractometric sensing.
[Show abstract][Hide abstract] ABSTRACT: Here we critically discuss the use of electron paramagnetic resonance (EPR) technique in the investigation of anion doped TiO2 based photocatalyst. Starting from a brief general overview of the potentiality of the EPR technique to investigate local properties of a given material, we used selected applications on the characterization of bare, C-, N-, B-, F-, and some anion co-doped TiO2 to highlight the power of this approach and to discuss its limitations and future challenges. In particular, we emphasize the need of other complementary techniques in addition to the EPR characterization, such as high-resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, and computational calculations.
[Show abstract][Hide abstract] ABSTRACT: Single-chain magnets are molecular spin chains displaying slow relaxation of
the magnetisation on a macroscopic time scale. To this similarity with
single-molecule magnets they own their name. In this chapter the distinctive
features of single-chain magnets as opposed to their precursors will be
pinpointed. In particular, we will show how their behaviour is dictated by the
physics of thermally-excited domain walls. The basic concepts needed to
understand and model single-chain magnets will also be reviewed.
[Show abstract][Hide abstract] ABSTRACT: BACKGROUND: The ferroxidase center of DNA-binding proteins from starved cells (Dps) is a major player in the iron oxidation/detoxification process that leads to a decreased reactive oxygen species production. The possible Mn(II) participation in this process has been studied in Dps from Kineococcus radiotolerans, a radiation-resistant bacterium with a high cytosolic Mn/Fe ratio and a high capacity to survive ionizing and stress conditions. METHODS: The X-ray structure of recombinant K. radiotolerans Dps loaded with Mn(II) has been solved at 2.0 Å resolution. Mn(II) binding to K. radiotolerans Dps and its effect on Fe(II) oxidation have been characterized in spectroscopic measurements. RESULTS: In K. radiotolerans Dps, the Fe-Fe ferroxidase center can have also a Mn-Fe composition. Mn(II) binds only at the high affinity, so-called A site, whereas Fe(II) binds also at the low affinity, so-called B site. The Mn-Fe and Fe-Fe centers behave distinctly upon iron oxidation by O(2). A site-bound Mn(II) or Fe(II) play a catalytic role, while B site-bound Fe(II) behaves like a substrate and can be replaced by another Fe(II) after oxidation. When H(2)O(2) is the Fe(II) oxidant, single electrons are transferred to aromatic residues near the ferroxidase center and give rise to intra-protein radicals thereby limiting OH(•) release in solution. The presence of the Mn-Fe center results in significant differences in the development of such intra-protein radicals. GENERAL SIGNIFICANCE: The results provide a likely molecular mechanism for the protective role of Mn(II) under oxidative stress conditions as it participates in redox cycling in the hetero-binuclear ferroxidase center.
Full-text · Article · Feb 2013 · Biochimica et Biophysica Acta
[Show abstract][Hide abstract] ABSTRACT: Nitronyl nitroxides (NitR) are a family of persistent radicals widely used in molecular magnetism and recently suggested as potential candidates for spintronic applications. In this paper we characterize by X- and W- band Electron Paramagnetic Resonance (EPR) spectroscopy the new radical S-4-(nitronyl nitroxide) benzyl ethanethioate (NitSAc) designed for assembling on Au surfaces. We determined the radical magnetic tensors and studied by X-band pulse EPR its spin relaxation behaviour in fluid and glassy solutions of toluene. A comparison with the well known nitroxide 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-oxyl (CTPO) is afforded. The advantages of using NitSAc in technological applications are discussed on the basis of the slow spin relaxation demonstrated by this study.
No preview · Article · Oct 2012 · Journal of Materials Chemistry
[Show abstract][Hide abstract] ABSTRACT: The comparison of the structural and magnetic properties of molecular nanomagnets (MNM) and magnetic nanoparticles (MNP) can be instructive to get a deeper understanding of the magnetic behavior on the intermediate scale between molecular and bulk objects. In this respect iron oxo based clusters are particularly interesting, since they provide an increasing number of molecular systems with sizes close to that of iron oxide MNP. In this Minireview we report a survey of literature data aimed at improving our understanding of the emergence of MNP properties from MNM ones.
Full-text · Article · May 2012 · Angewandte Chemie International Edition
[Show abstract][Hide abstract] ABSTRACT: Ein Vergleich der strukturellen und magnetischen Eigenschaften von molekularen Nanomagneten und magnetischen Nanopartikeln kann hilfreich sein, um ein tieferes Verständnis des magnetischen Verhaltens im Zwischenbereich zwischen molekularen und Bulkobjekten zu gewinnen. In dieser Hinsicht sind Eisen-Oxo-Cluster besonders interessant, weil hier eine Reihe von molekularen Systemen existiert, die der Größe von magnetischen Eisenoxid-Nanopartikeln nahekommen. In diesem Kurzaufsatz geben wir einen Überblick über Literaturdaten, mit dem Ziel, ein besseres Verständnis davon zu erlangen, wie die Eigenschaften molekularer Nanomagnete in magnetischen Nanopartikeln in Erscheinung treten und umgekehrt.
No preview · Article · May 2012 · Angewandte Chemie
[Show abstract][Hide abstract] ABSTRACT: Highly versatile coordinating ligands are designed and synthesized with two β-diketonate groups linked at the carbon 3 through a phenyl ring. The rigid aromatic spacer is introduced in the molecules to orient the two acetylacetone units along different angles and coordination vectors. The resulting para, meta, and ortho bis-(3-acetylacetonate)benzene ligands show efficient chelating properties toward Cu(II) ions. In the presence of 2,2'-bipyridine, they promptly react and yield three dimers, 1, 2, and 3, with the bis-acetylacetonate unit in bridging position between two metal centers. X-ray single crystal diffraction shows that the compounds form supramolecular chains in the solid state because of intermolecular interactions. Each of the dinuclear complexes shows a magnetic behavior which is determined by the combination of structural parameters and spin polarization effects. Notably, the para derivative (1) displays a moderate antiferromagnetic coupling (J = -3.3 cm(-1)) along a remarkably long Cu···Cu distance (12.30 Å).
No preview · Article · Apr 2012 · Inorganic Chemistry