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   Int. J. Nanotechnology, Vol. x, No. x, xxxx  1 Molecular Nanomagnets: towards molecularspintronicsWolfgang Wernsdorfer Institut N´eel, CNRS & Universit´e J. Fourier, BP 166,25 rue des Martyrs, 38042 GRENOBLE Cedex 9, FranceFax: + 33 476 88 1191E-mail: Abstract:  Molecularnanomagnets, oftencalledsingle-moleculemagnets, haveattracted much interest in recent years both from experimental and theoreticalpoint of view. These systems are organometallic clusters characterized by a largespin ground state with a predominant uniaxial anisotropy. The quantum nature of these systems makes them very appealing for phenomena occurring on the meso-scopic scale, i.e., at the boundary between classical and quantum physics. Belowtheir blocking temperature, they exhibit magnetization hysteresis, the classicalmacroscale property of a magnet, as well as quantum tunneling of magnetizationand quantum phase interference, the properties of a microscale entity. Quan-tum effects are advantageous for some potential applications of single-moleculemagnets, e.g. in providing the quantum superposition of states for quantum com-puting, butareadisadvantageinotherssuchasinformationstorage. Itisbelievedthat single-molecule magnets have a potential for quantum computation, in par-ticular because they are extremely small and almost identical, allowing to obtain,in a single measurement, statistical averages of a larger number of qubits. Thisreview introduces few basic concepts that are needed to understand the quantumphenomena observed in molecular nanomagnets and discusses new trends of thefield of molecular nanomagnets towards molecular spintronics. Keywords:  Single-moleculemagnets, molecularnanomagnets, molecularspin-tronics, magnetic hysteresis, resonant quantum tunneling, quantum interference,spin parity effect, decoherence, quantum computation, qubit, exchange-bias,spin-Hamiltonian, micro-SQUID, magnetometer. Biographical notes: Dr. Wolfgang Wernsdorfer , born in W¨urzburg, Ger-many, in 1966, received his education in Physics in W¨urzburg, Lyon, and thenGrenoble, where he is at present Research Director at the Centre National dela Recherche Scientifique. During his PhD in the low-temperature laboratory(CNRS, Grenoble) Wolfgang Wernsdorfer and collaborators developed a uniquedevice(micro-SQUID)for measuringmagneticpropertiesofnanostructureswitha billiontimes higher sensitivitythan commercial magnetometers (BronzeMedalfrom CNRS, 1998). His instrument allows observation of the magnetic behaviorof nanomagnets containing less than a thousand magnetic centers, which is stilla world record. Using the unique advantages of this device, Wolfgang Werns-dorfer has studied a variety of peculiar phenomena in depth, such as tunnellingof magnetization in molecular clusters, leading to the Agilent Europhysics Prizein 2002 and the International Olivier Kahn Award in 2006.Over the years, the innovative approach to such studies combined with the rec-ognized superiority of this micro-SQUID have led to worldwide collaborationwith most other notorious groups working on synthesizing molecular magnetsto investigate single-molecule magnet behavior in more than 350 systems. TheCopyright c  200x Inderscience Enterprises Ltd.  2  Wolfgang Wernsdorfer  leading work of Wolfgang Wernsdorfer and collaborators in this field is at theheart of today’s knowledge on molecular magnetism. 1 Introduction A revolution in electronics is in view, with the contemporary evolution of two noveldisciplines, spintronics and molecular electronics. A link between these two fields can beestablished using molecular magnetic materials and, in particular, single-molecule mag-nets, which combine the classic macroscale properties of a magnet with the quantum prop-erties of a nanoscale entity. The resulting field, molecular spintronics aims at manipulatingspins and charges in electronic devices containing one or more molecules [1, 2, 3].The contemporary exploitation of electronic charge and spin degrees of freedom is aparticularly promising field both at fundamental and applied levels. This discipline, calledspintronics, has already seen some of its fundamental results turned into actual devices in arecord time of 10 years and it holds great promises for the future [4, 5]. Spintronic systemsexploit the fact that the electron current is composed of spin-up and spin-down carriersthat carry information encoded in their spin state and interact with magnetic materialsdifferently. Information encoded in spins persists when the device is switched off; it can bemanipulated with and without using magnetic fields and can be written using little energy,to cite just a few advantages of this approach.New efforts are now directed towards spintronic devices that preserve and exploit quan-tum coherence, so that fundamental investigations are shifting from metals to semiconduct-ing [4, 5], and organic materials [6], which potentially offer best promises for cost, integra-tion and versatility. For example, organic materials are already used in applications suchas organic light-emitting diodes (OLED), displays and organic transistors. The concomi-tant trend towards ever-smaller electronic devices (having already reached the nano-scale),and the tailoring of new molecules possessing increased conductance and functionalitiesare driving electronics to its ultimate molecular-scale limit [7], and the so-called molecularelectronics is now being intensively investigated.In experiments of molecular electronics, the measuring devices are usually constitutedby two nanoelectrodes and a bridging molecule in between, allowing the measurementof electron transport through single molecules. As the measurement is performed at themolecular level, the observables are connected to molecular orbitals and not to Blochwaves as in bulk materials. Hence, new rules are found for these systems and it becomespossible to probe the quantum properties of the molecule directly. The electron tunnellingprocesses in the electrode-molecule-electrode system can show the presence of Kondo orCoulomb-blockade effects, depending on the binding strength between the molecule andthe electrodes, which can be tuned by selecting the appropriate chemical functional groups.In this context, a new field of molecular spintronics is emerging that combines theconcepts and the advantages of spintronics and molecular electronics [1, 8] which requiresthe creation of molecular devices using one or few magnetic molecules. Compounds of theSingle-Molecule Magnets (SMMs) class seem particularly attractive: their magnetizationrelaxation time is extremely long at low temperature reaching years below 2 K with recordanisotropy barriers approaching 100 K [9]. These systems, combining the advantages of molecular scale with the properties of bulk magnetic materials, look attractive for high-density information storage and also, owing to their long coherence times [10, 11, 12],   Molecular spintronics  3 Figure1  Representativeexamplesoftheperipheralfunctionalizationoftheouterorganicshellof the Mn 12  SMM. Different functionalizations used to graft the SMM to surfaces are displayed [1, 3].All structures are determined by X-ray crystallography, except d, which is a model structure. Solventmolecules have been omitted. The atom color code is reported in the figure, as well as the diameterof the clusters. for quantum computing [13, 14, 15]. Moreover their molecular nature leads to appealingquantum effects of the static and dynamic magnetic properties. The rich physics behindthe magnetic behaviour produces interesting effects like negative differential conductanceand complete current suppression [16, 17], which could be used in electronics. Anotheradvantage is that the weak spin-orbit and hyperfine interactions in organic molecules islikely to preserve spin-coherence over time and distance much longer than in conventionalmetals or semiconductors. Last but not least, specific functions (e.g. switchability withlight, electric field etc.) could be directly integrated into the molecule.SMMs possess the right chemical characteristics to overcome several problems asso-ciated to molecular junctions. They are constituted by an inner magnetic core with a sur-rounding shell of organic ligands [18] that can be tailored to bind them on surfaces or into junctions [19, 20, 21, 22] (Fig. 1). In order to strengthen magnetic interactions betweenthe magnetic core ions, SMMs often have delocalized bonds, which can enhance their con-ducting properties. SMMs come in a variety of shapes and sizes and permit selective sub-stitutions of the ligands in order to alter the coupling to the environment [18, 19, 20, 23].It is also possible to exchange the magnetic ions, thus changing the magnetic propertieswithout modifying the structure and the coupling to the environment [24, 25]. While graft-ing SMMs on surfaces has already led to important results, even more spectacular resultswill emerge from the rational design and tuning of single SMM-based junctions.From a physics viewpoint, SMMs are the final point in the series of smaller and smallerunits from bulk matter to atoms (Figure 2). They combine the classic macroscale proper-ties of a magnet with the quantum properties of a nanoscale entity. They have crucialadvantages over magnetic nanoparticles in that they are perfectly monodisperse and can be  4  Wolfgang Wernsdorfer  Figure 2  Scale of size that goes from macroscopic down to nanoscopic sizes. The unit of thisscaleisthenumber of magneticmoments inamagnetic system (roughly correspondingtothe numberof magnetic atoms). At macroscopic sizes, a magnetic system is described by magnetic domainsthat are separated by domain walls. Magnetization reversal occurs via nucleation, propagation, andannihilation of domain walls (hysteresis loop on the  left  ). When the system size is of the order of magnitude of the domain wall width or the exchange length, the formation of domain walls requirestoo much energy. Therefore, the magnetization remains in the so-called single-domain state, and themagnetization reverse by uniform rotation or nonuniform modes ( middle ). SMMs are the final pointin the series of smaller and smaller units from bulk matter to atoms and magnetization reverses viaquantum tunneling ( right  ). Mesoscopic physicsMacroscopic Nanoscopic PermanentmagnetsMicronparticles Nanoparticles Clusters Molecular clusters Individual spins S = 10 20  10 10  10 8  10 6  10 5  10 4  10 3  10 2  10 1  Multi-domain Single-domain Magnetic moments Nucleation, propagation andannihilation of domain wallsUniform rotationCurlingResonant tunneling, quantization,quantum thermodynamics -101-40 -20 0 20 40      M     /     M      S µ 0 H(mT) -101-100 0 100      M     /     M      S µ 0 H(mT) -101-1 0 1      M     /     M      S µ 0 H(T) Fe 8 1K 0.1K0.7K studied in molecular crystals. They display an impressive array of quantum effects (thatare observable up to higher and higher temperatures due to progress in molecular designs),ranging from quantum tunnelling of magnetization [26, 27, 28, 29] to Berry phase inter-ference [30, 31] and quantum coherence [10, 11, 12] with important consequences on thephysics of spintronic devices. Although the magnetic properties of SMMs can be affectedwhen they are deposited on surfaces or between leads [23], these systems remain a stepahead of non-molecular nanoparticles, which show large size and anisotropy distributions,for a low structure versatility.This review introduces the basic concepts that are needed to understand the quantumphenomena observed in molecular nanomagnets and shows the new trends towards molec-ular spintronics [1] using junctions [3] and nano-SQUIDs [2]. 2 Overview of molecular nanomagnets Molecular nanomagnets or single-molecule magnets (SMMs) are mainly organic mole-cules that have one or several metal centers with unpaired electrons. These polynuclearmetal complexes are surrounded by bulky ligands (often organic carboxylate ligands). Themost prominent examples are a dodecanuclear mixed-valence manganese-oxo cluster withacetate ligands, short Mn 12  acetate [32], and an octanuclear iron(III) oxo-hydroxo clusterof formula [Fe 8 O 2 (OH) 12 (tacn) 6 ] 8+ where tacn is a macrocyclic ligand, short Fe 8  [33].Both systems have a spin ground state of   S   = 10  and an Ising-type magnetic anisotropy,which stabilizes the spin states with  m  =  ± 10  and generates an energy barrier for the
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