Eugenio Coronado

Coexistence of ferromagnetism and metallic conductivity in a molecule-based layered compound

Crystal engineering—the planning and construction of crystalline supramolecular architectures from modular building blocks—permits the rational design of functional molecular materials that exhibit technologically useful behaviour such as conductivity and superconductivity, ferromagnetism and nonlinear optical properties. Because the presence of two cooperative properties in the same crystal lattice might result in new physical phenomena and novel applications, a particularly attractive goal is the design of molecular materials with two properties that are difficult or impossible to combine in a conventional inorganic solid with a continuous lattice. A promising strategy for creating this type of ‘bi-functionality’ targets hybrid organic/inorganic crystals comprising two functional sub-lattices exhibiting distinct properties. In this way, the organic π-electron donor bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) and its derivatives, which form the basis of most known molecular conductors and superconductors, have been combined with molecular magnetic anions, yielding predominantly materials with conventional semiconducting or conducting properties, but also systems that are both superconducting and paramagnetic. But interesting bulk magnetic properties fail to develop, owing to the discrete nature of the inorganic anions. Another strategy for achieving cooperative magnetism involves insertion of functional bulky cations into a polymeric magnetic anion, such as the bimetallic oxalato complex [MnIICrIII(C2O4)3]-, but only insoluble powders have been obtained in most cases. Here we report the synthesis of single crystals formed by infinite sheets of this magnetic coordination polymer interleaved with layers of conducting BEDT-TTF cations, and show that this molecule-based compound displays ferromagnetism and metallic conductivity.

MAGPACK1 A package to calculate the energy levels, bulk magnetic properties, and inelastic neutron scattering spectra of high nuclearity spin clusters

M agnetic molecular clusters, i.e., molecular assemblies formed by a finite number of exchange-coupled magnetic moments, are currently receiving much attention in several active areas of research as molecular chemistry, magnetism, and biochemistry. A reason for this interest lies in the possibility to use simple molecular clusters as magnets of nanometer size exhibiting unusual magnetic properties as superparamagnetic like behavior or quantum tunneling of magnetization.2 – 4 Organic molecules of increasing sizes and large number of unpaired electrons are being explored as a means of obtaining building blocks for molecule-based magnets.5 Magnetic clusters of metal ions are also relevant in biochemistry.6 This area between molecule and bulk will require new theoretical concepts and techniques for investigation of their peculiar properties. Still, the theoretical treatment required to understand the magnetic and spectroscopic properties of this wide variety of compounds is a challenging problem in molecular magnetism.7 For a long time, this problem has been mostly restricted to treat comparatively simple clusters comprising a reduced number of exchange-coupled centers and special spin topologies, for which solutions can be obtained either analytically or numerically. However, on increasing the spin nuclearity of the cluster, the problem rapidly becomes unapproachable because the lack of translational symmetry in the clusters. An additional complication is the spin anisotropy of the cluster. Until now only the isotropic-exchange case has been treated, so as to take full advantage of the spin symmetry of the cluster.8 In this article we present a very powerful and efficient computational approach to solve the exchange problem in high nuclearity spin clusters with all kind of exchange interactions (isotropic and anisotropic), including the single-ion anisotropic effects. The clusters are formed by an arbitrary number of exchangecoupled centers that combine different spin values and arbitrary topology. This approach is based on the use of the irreducible tensor operators (ITO) technique.7, 9 – 12 It allows evaluation of both eigenvalues and eigenvectors of the system, and then, calculation of the magnetic susceptibility, magnetization, or heat capacity, and also the inelastic neutron scattering spectra. In the following sections we will present both the theory and the four different implemented FORTRAN programs that integrate a package called MAGPACK . In the last section some examples are presented in order to show the possibilities of the programs.

Mononuclear lanthanide single-molecule magnets based on polyoxometalates.

[ErW10O36]9- is the first polyoxometalate behaving as a single-molecule magnet (SMM). It shows frequency-dependent out-of-phase magnetization and a thermally activated single relaxation process with an effective barrier of 55.8 K. This single lanthanide ion polyoxometalate is the inorganic analogue of the bis(phthalocyaninato)lanthanide SMMs, both exhibiting very similar ligand field symmetries around the lanthanide ion (idealized D4d). It is chemically stable and offers new avenues for organization and processing of single-molecule magnets. Furthermore, it can be made free from nuclear spins and opens the possibility to be used for studies of decoherence on unimolecular qubits.

Magnetic polyoxometalates: from molecular magnetism to molecular spintronics and quantum computing.

In this review we discuss the relevance of polyoxometalate (POM) chemistry to provide model objects in molecular magnetism. We present several potential applications in nanomagnetism, in particular, in molecular spintronics and quantum computing.

Mononuclear Lanthanide Single Molecule Magnets Based on the Polyoxometalates [Ln(W5O18)2]9− and [Ln(β2-SiW11O39)2]13−(LnIII = Tb, Dy, Ho, Er, Tm, and Yb)

The first two families of polyoxometalate-based single-molecule magnets (SMMs) are reported here. Compounds of the general formula [Ln(W(5)O(18))(2)](9-) (Ln(III) = Tb, Dy, Ho, and Er) and [Ln(SiW(11)O(39))(2)](13-) (Ln(III) = Tb, Dy, Ho, Er, Tm, and Yb) have been magnetically characterized with static and dynamic measurements. Slow relaxation of the magnetization, typically associated with SMM-like behavior, was observed for [Ln(W(5)O(18))(2)](9-) (Ln(III) = Ho and Er) and [Ln(SiW(11)O(39))(2)](13-) (Ln(III) = Dy, Ho, Er, and Yb). Among them, only the [Er(W(5)O(18))(2)](9-) derivative exhibited such a behavior above 2 K with an energy barrier for the reversal of the magnetization of 55 K. For a deep understanding of the appearance of slow relaxation of the magnetization in these types of mononuclear complexes, the ligand-field parameters and the splitting of the J ground-state multiplet of the lanthanide ions have been also estimated.

Magnetic clusters from polyoxometalate complexes

Abstract The present article highlights the increasing interest of polyoxometalates in molecular magnetism, providing at the same time a perspective of the state-of-the-art in this area. The main focus is the polyoxotungstates. The first aspect we discuss is that of the coordination chemistry of these metal–oxide ligands. We show that this chemistry leads to remarkable examples of well-insulated magnetic clusters of controlled nuclearity and topology. In these clusters detailed information on the nature of the magnetic exchange interactions can be extracted by using, in addition to the classical magnetic techniques (magnetic susceptibility, magnetization and EPR spectroscopy), other physical techniques as the inelastic neutron scattering (INS) spectroscopy, which provides more direct information on the lower lying energy levels of the magnetic cluster. The second aspect we discuss is that of the interplay between electron delocalization and exchange interactions in the mixed-valence polyoxometalates. We show that these high-nuclearity multielectronic clusters are model systems for the development of new theories in the mixed valence area.

Spin qubits with electrically gated polyoxometalate molecules

Spin qubits offer one of the most promising routes to the implementation of quantum computers. Very recent results in semiconductor quantum dots show that electrically-controlled gating schemes are particularly well-suited for the realization of a universal set of quantum logical gates. Scalability to a larger number of qubits, however, remains an issue for such semiconductor quantum dots. In contrast, a chemical bottom-up approach allows one to produce identical units in which localized spins represent the qubits. Molecular magnetism has produced a wide range of systems with properties that can be tailored, but so far, there have been no molecules in which the spin state can be controlled by an electrical gate. Here we propose to use the polyoxometalate [PMo12O40(VO)2]q-, where two localized spins with S = 1/2 can be coupled through the electrons of the central core. Through electrical manipulation of the molecular redox potential, the charge of the core can be changed. With this setup, two-qubit gates and qubit readout can be implemented.

Room‐Temperature Electrical Addressing of a Bistable Spin‐Crossover Molecular System

The design of switchable nanodevices based on magnetic molecules has therefore remained a theoretical topic. [ 10–12 ] Here, we report a switchable molecular device made by contacting individual nanoparticles based on spin-crossover molecules between nanometer-spaced electrodes. This nanoscale device exhibits switching and memory effects near room temperature as a consequence of the intrinsic bistability of the spin-crossover nanoparticle. Thus, a sharp increase in the conductance is observed upon heating above ca. 350 K, together with the presence of a thermal hysteresis as large as 30 K for a single-particle device, after which the conductance switches back to the original value. This is a long-sought-for result, as it confi rms the existence of hysteretic spin crossover effects in a single nanoobject. [ 13–17 ] Interestingly for molecular spintronics, the spin crossover in this molecular nanodevice can also be induced by applying a voltage, showing that its magnetic state is controllable electrically. Spin-crossover metal complexes are one of the paradigmatic examples of magnetic molecular materials showing switching and bistability at the molecular level. [ 18 ] In these systems lowspin to high-spin transitions can be triggered through a variety of external stimuli (temperature, illumination, or pressure) [ 19 ]

Lanthanoid single-ion magnets based on polyoxometalates with a 5-fold symmetry: the series [LnP5W30O110]12- (Ln3+ = Tb, Dy, Ho, Er, Tm, and Yb).

A robust, stable and processable family of mononuclear lanthanoid complexes based on polyoxometalates (POMs) that exhibit single-molecule magnetic behavior is described here. Preyssler polyanions of general formula [LnP(5)W(30)O(110)](12-) (Ln(3+) = Tb, Dy, Ho, Er, Tm, and Yb) have been characterized with static and dynamic magnetic measurements and heat capacity experiments. For the Dy and Ho derivatives, slow relaxation of the magnetization has been found. A simple interpretation of these properties is achieved by using crystal field theory.

Rational Design of Single-Ion Magnets and Spin Qubits Based on Mononuclear Lanthanoid Complexes

Here we develop a general approach to calculating the energy spectrum and the wave functions of the low-lying magnetic levels of a lanthanoid ion submitted to the crystal field created by the surrounding ligands. This model allows us to propose general criteria for the rational design of new mononuclear lanthanoid complexes behaving as single-molecule magnets (SMMs) or acting as robust spin qubits. Three typical environments exhibited by these metal complexes are considered, namely, (a) square antiprism, (b) triangular dodecahedron, and (c) trigonal prism. The developed model is used to explain the properties of some representative examples showing these geometries. Key questions in this area, such as the chemical tailoring of the superparamagnetic energy barrier, tunneling gap, or spin relaxation time, are discussed. Finally, in order to take into account delocalization and/or covalent effects of the ligands, this point-charge model is complemented with ab initio calculations, which provide accurate information on the charge distribution around the metal, allowing for an explanation of the SMM behavior displayed by some sandwich-type organometallic compounds.