Molecular nanomagnetism in Florence: Advancements and perspectives

Inorganica Chimica Acta 361 (2008) 3356–3364

Contents lists available at ScienceDirect

Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica

Molecular nanomagnetism in Florence: Advancements and perspectives Roberta Sessoli * Department of Chemistry of the Università degli Studi di Firenze and INSTM U.d.R. Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino, Italy

a r t i c l e

i n f o

Article history: Received 26 February 2008 Accepted 29 February 2008 Available online 10 March 2008 Dedicated to Dante Gatteschi, a far-sighted teacher who drove us towards molecular nanomagnetism.

a b s t r a c t Magnetic materials exhibiting magnetic hysteresis in the absence of magnetic order are fascinating systems for fundamental science and possible applications but the temperature at which the magnetic memory is observed remains rather low. The intrinsic limits of the single molecule magnet and single chain magnet approaches are here briefly discussed as well as possible perspectives in molecular nanomagnetism. Ó 2008 Elsevier B.V. All rights reserved.

Keywords: Clusters Magnetic properties Quantum tunneling Magnetic hysteresis

1. Introduction The ability of magnetic objects to exert a force at distance has been attracting the interest of men since the earliest stages of civilization. At the beginning of the 20th century it became clear that such a property resulted from a collective behavior [1] and only with the coming of quantum mechanics its origin was associated to the chemical bond [2–4]. From that point on it was clear that the bulk magnetic properties of a material are indeed related to local structural parameters that in insulators involve the very close neighbors to the atom carrying the unpaired electron. The investigation of magnetic properties became a powerful tool for inorganic chemists to get information on the structure of coordination compounds when X-ray diffraction was not yet the routine technique for structural characterization now employed in every laboratory. The masses of information collected permitted to establish sound correlations between the observed magnetic properties and the molecular structure [5], thanks also to the collaborative efforts of chemists and physicists. The transition from an analytical use of these magneto-structural correlations towards a creative use to synthesize molecular system with desired magnetic properties marked the transition from magnetochemistry [6] to molecular magnetism [7,8]. The goals achieved by molecular magnetism in the past 30 years are remarkable. They range from the observation of bulk magne-

* Tel.: +39 0554573268; fax: +39 0554573372. E-mail address: roberta.sessoli@unifi.it 0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2008.02.071

tism in pure organic materials at low temperature [9], to magnetic order above room temperature in cyanide based materials [10,11]. Novel types of hysteretic behavior [12] have been observed in spin cross-over compounds, as well as the possibility to control the magnetic state with light [13,14]. A review of recent achievements in the huge domain of molecular magnetism is not the scope of this short article. Moreover interesting examples can be found in the many contributions to this special issue. From our privileged location at the crossroads of many stimulating collaborations we will address in the following some open questions that deserve to be addressed as well as possible developments of molecular nanomagnetism in the near future. This particular class of molecular materials [15], confined in zero dimension (clusters) or in one dimension (chains), has been deeply investigated in Florence thanks to the far-sighted directions of Dante Gatteschi. 2. The memory of molecules The observation of a magnetic hysteresis was traditionally associated to a cooperative effect before its presence was detected in crystals of oligonuclear mixed-valence manganese clusters of formula [Mn12O12(CH3COO)16(H2O)4]  2CH3COOH  4H2O [16]. Such a novel phenomenon has lead Hendrickson and co-workers to name this class of materials with the evocating name of single molecule magnets (SMM) [17,18]. The presence of magnetic hysteresis was immediately associated, thanks to the early use of high frequency electron paramagnetic resonance [19], to the combination

R. Sessoli / Inorganica Chimica Acta 361 (2008) 3356–3364

of easy axis magnetic anisotropy and large spin of the ground state. In Fig. 1 we show the effect that an axial anisotropy associated to the spin Hamiltonian: Han ¼ DS2z

ð1Þ

with negative D has on the energy of the jm> states of the ground S multiplet, where m is the projection of the spin along z. This type of plot has been drawn probably more than a hundred times because it allows an efficient visualization of the mechanism of inversion of the magnetization and the energy barrier to be overcome. In fact the application of a magnetic field stabilizes one of the wells and the return to the equilibrium, after the removal of the field, requires an equal population of the two wells. In analogy with what observed in the kinetics of chemical reaction the rate of reversal of the magnetization is expected to follow the Arrhenius law: s1 ¼ s1 0 expðDE=kB TÞ

ð2Þ

where the exponential factor is related to the height of the barrier and thus equal to DS2 or (DS2  1/4) for integer or half integer spin states, respectively. It is therefore rather surprising that magnetic hysteresis of molecular origin, indeed a predictable behavior, was not detected much earlier. Indeed the use of polynuclear systems have allowed to reach large spin states, allowing for high barrier and for a notable reduction of underbarrier process. These are particularly efficient in destroying the memory of small spins, as discussed in detail in the following. Probably less discussed is the role that the level structure of the barrier of Fig. 1 has played in the slowing down of the magnetization dynamics and in the detection of the magnetic hysteresis. At difference from chemical reactions or from the reversal of the magnetization in nanoparticles a small number of discrete levels is involved in the process. The transition from one state to the next one occurs thanks to coupling with lattice vibrations, i.e. through

Fig. 1. Splitting in zero field of the jm> components of a spin state S according to (1) with negative D. The two wells correspond to opposite direction of the projection of the spin. If a field is applied parallel to z the left well is selectively populated and the removal of the field requires that population is transferred from the left to the right well through a multi-steps spin–phonon based mechanism (gray curled arrows).

3357

the spin–phonon interaction. The process of reversal of the magnetization thus occurs through many steps as phonons can only promote transitions between states having Dm = ±1 and ±2. It is well known among chemists that in a chemical reaction the overall rate is determined by the slowest step. In Fig. 1 this corresponds to transitions on the top of the barrier. These are particularly slow at low temperature because the thermal population is very small, giving rise to the exponential dependence of (2). However there is another important fact. The increase in temperature is not as efficient as in other systems because the characteristic quadratic dependence on m of the level spacing associates the steps on top of the barrier to very small energy differences in comparison with the separation from the ground state. The phonon density for such small energies is very little. In other words a sort of paradoxical situation is encountered: higher temperatures populate the states on top of the barrier, thus promoting the relaxation, but at the same time they do not provide enough phonons for the transitions to occur efficiently. This is a very intuitive explanation of the very small pre-exponential factor in (2) commonly encountered in SMM, a few orders of magnitude smaller than in other magnetic materials, which has been of paramount importance for the easy detection of slow relaxation of the magnetization in SMM [20].

3. A race for high temperature SMM If we compare the evolution of nanostructured magnetic materials, like magnetic multilayers, with that of SMM the difference is striking. Giant magneto resistance, i.e. the large (giant) change in conductivity associated to the magnetization state of the material that characterizes magnetic multilayers discovered at the end of 1980s [21,22], is already employed to read the magnetic state of a data storage medium or in magnetic random access memories [23]. SMM, despite their virtual capability to provide a medium with exceptional storage density, are on the contrary still far from any technological application, mainly for the prohibitive temperatures at which the magnetic memory effect is observable. The record blocking temperature of Mn12 clusters has been only recently surpassed in a hexanuclear MnIII cluster having S = 12 [24]. The first feature to be optimized is therefore the height of the barrier, and therefore according to (2) it follows that we have to maximize the spin of the ground state, S, and the magnetic anisotropy, expressed in (1) by the zero field splitting parameter D of the spin Hamiltonian. The exceptional efforts of many synthetic groups have lead to a significant increase in the spin ground state, with a record value as high as S = 83/2 [25]. Very high spin values can be obtained either though a parallel alignment of identical spins, or antiparallel alignment of two sets of spins differing in either their number of their individual value. This last approach is reminiscent of ferrimagnetism and is of particular value because antiferromagnetic interactions are usually stronger and more commonly encountered than ferromagnetic ones. Moreover the latter often rely on the orthogonality of the magnetic orbitals [26,27], a feature that in some cases is accompanied by orthogonality of the anisotropy axes and thus to an overall reduction of the magnetic anisotropy. The quadratic dependence of (2) on the S value suggests however that any small increase in S could reflect in sizeable effects on the height of the energy barrier. This concept is however intrinsically misleading for multispin SMM, at least if we consider clusters based on transition metal ions that have quenched orbital angular momentum and possess a moderate single ion magnetic anisotropy. Given its relevance in slowing down the race for high temperature SMM it is interesting to further comment it here, even if this point has already been discussed in a book [15] and later addressed in a dedicated communication [28].

3358

R. Sessoli / Inorganica Chimica Acta 361 (2008) 3356–3364

In the assumption of strong isotropic exchange between the metal centers, the coupling of angular momenta does not simply imply a tensorial sum of the single ion magnetic anisotropy tensors but requires taking into account the composition of the wavefunction describing the ground spin state through the use of projection coefficients. Gatteschi and Bencini underlined the relevance of this fundamental property in the electron paramagnetic resonance spectra of exchange coupled systems [29]. In a spin cluster where the isotropic exchange is dominating the possible states that arise from the coupling of the individual spins can be labeled according to their total spin state, here for clarity indicated as ST. However to describe any state we have to select a basis for the representation. Each state of the basis is described by a set of quantum numbers that arise from a recursive coupling of angular momenta. An example, for instance the four spins system of Fig. 2, can clarify this concept. Each state of the basis is given by jw >¼ jS1 ; S2 ; S3 ; S4 ; S12 ; S123 ; ST ; mST >¼ jS12 ; S123 ; ST ; mST >

ð3Þ

where S12 is the intermediate spin that arises from the coupling of the individual spin S1 with S2, and can assume all the values differing by an integer comprised between S1 + S2 and jS1  S2j. The same is valid for the S123, which comes from the coupling of the intermediate S12 spin state with the individual spin S3. This procedure is applied recursively to include all the spins of the cluster and these (n  1) quantum numbers define the states of n coupled spin. In zero field and in the isotropic case the energy does not depend on mST but only on ST. The single spin values are often omitted in the notation as they are the same for all the states. For each state jw> it is possible to mathematically relate the zero field splitting of the state ST to that of the individual spins, where for simplicity the spin–spin contribution is neglected: X Dw;ST ¼ di Di ð4Þ i¼1;n

In this case the relation (4) is much simpler [15] DSmax ¼

ð2Si  1Þ Di ð2Smax  1Þ

ð5Þ

Let us suppose that we are so ingenious to be able to build a sort of dendrimeric SMM, as schematized in Fig. 3, adding a spin at the time with a linker that transmits a ferromagnetic interaction but keeps all the anisotropy tensors parallel to each other. From (5) it obviously follows that the energy barrier D ¼ DS2max only grows linearly with Smax, as shown in Fig. 3 in the case of clusters based on individual spins Si = 2. An increase in the spin of the cluster is therefore not as efficient as could be deduced at a first glance from (2). In Ref. [28] it has been nicely underlined how poorly efficient would have been to make an effort to ferromagnetically couple all the spins of the archetypal SMM Mn12 to reach S = 22 instead of the observed S = 10. The same is true for our hypothetical dendrimeric clusters. In case of AF interaction between successive generations the total spin states are ST = 4 and ST = 8 for four and ten spins, respectively. The energy barriers are calculated to be 10.08 Di and 24.58 Di, which are smaller than the corresponding ones for the same clusters ferromagnetically coupled, as shown in Fig. 3. They are, however, not as smaller as 1/4 or 2/25 of the corresponding ferromagnetic clusters, as one could simply expect on the quadratic dependence on ST and neglecting that the single ion magnetic anisotropy projects more efficiently in ferrimagnetic structures. This makes the ferrimagnetic approach still of great value. Our race for high temperature SMM needs to explore other routes. The use of 4d and 5d metal ions has been indicated as a possible strategy thanks to their larger spin orbit coupling and more diffuse magnetic orbitals that lead to stronger exchange interactions [30,31]. Despite these encouraging premises and some synthetic efforts this approach has not yielded an increase in the blocking temperature. An elucidative example is the coordination of RuIII with pyridine substituted nitronyl-nitroxide radicals. This

the bold character has been used to recall the tensorial nature of the parameters. For the evaluation of di recursive algebra is necessary and the reader is addressed to the original works [29]. The basis can be arbitrarily chosen depending on the order in which pure and intermediate spins are coupled. It must be recalled that all the states characterized by the same ST are admixed and thus any state of the four spin systems is indeed a linear combination of the basis states, making the scenario more complex. There is however an exception: there is only one way of coupling all the spins parallel to each other and thus the ferromagnetic state, where ST has the largest possible value, is unambiguously defined.

Fig. 2. An example of a tetranuclear cluster used here to elucidate the recursive spin-coupling procedure (see text).

Fig. 3. Height of the energy barrier for the reversal of the magnetization (filled circles) and zero field splitting parameter D (open squares) for ferromagnetic clusters based on individual spins Si = 2 with increasing number (nS) of spins in the cluster. The stars represent the energy barrier for two hypothetical dendrimer clusters, sketched in the upper part of the figure, with antiparallel alignment of the spins of successive generations and thus characterized by ST = 4 and ST = 8 for nS = 4 and nS = 10, respectively. In the plot they are referred only to the x-scale on the top.

R. Sessoli / Inorganica Chimica Acta 361 (2008) 3356–3364

has indeed lead to a larger ferromagnetic interaction, compared to the MnII analogue, but accidentally to an almost complete cancellation of the RuIII magnetic anisotropy [32] The lower reactivity and the fact that low-spin configurations are more common in heavier ions further limit the use of this approach. Lanthanides, on the contrary, have large magnetic moments and in general a large anisotropy. They have provided examples of slowing down of the magnetization dynamics of one metal ion, as well as slow dynamics even in the presence of a non-magnetic ground state. However they are often characterized by a fast tunneling, which, unless a few exceptions [33], hampers the observation of large hysteresis loops at moderate temperatures. This argument will be the focus of the next section. 4. Controlling the tunneling of the magnetization It is important to recall that if up to now molecular nanomagnetism has failed to provide devices ready for application it has enormously contributed to the understanding of underbarrier mechanisms [34] for the relaxation of the magnetization, also known as quantum tunneling of the magnetization (QTM) [35]. This additional mechanism, which cannot be avoided by reducing the thermal motion, is an unfavorable event responsible of the lost of information. Stability of the information is however not the only key parameter in data storage. This must be associated to a rapid and easy writing of the information on the magnetic medium. Currently the research is focusing on the spin torque exerted by a current that flows through a magnetic medium, as well as on more phenomena in the emerging field of spintronics, where the conduction is manipulated acting on the spin state of the moving electrons. The interplay of charge transport and magnetization dynamics is however not possible in bulk molecular nanomagnets and nanosized magnetic particles. Tunneling could indeed represent an alternative route for fast switching of the magnetization and a deep understanding of its origin is mandatory to achieve its control. If the dominating easy axis anisotropy, parametrized by the D parameter in (1), gives origin to the barrier for the reversal of the magnetization, the transverse anisotropy is the key ingredient for tunneling because terms in Sx and Sy of the spin Hamiltonian, not commuting with Sz, induce admixing of states located on opposite sides of the barrier in the double well potential of Fig. 1. Let us suppose that our SMM has a ground spin state S. If we want to evaluate the tunnel probability in zero field between the lowest pair of almost degenerate levels characterized by m = ±S we have to consider the effect on these states of the transverse terms in the spin Hamiltonian. The simplest one is a transverse field, practically always present if we consider also internal sources like the magnetic nuclei or the dipolar field generated by surrounding magnetic molecules. This term, being linear in S+ and S, is able to admix the m = ±S states only at the (2S)th level of perturbation. Therefore tunneling due to a transverse field is negligible for SMM with large S. More efficient are the transverse components of the zero field splitting tensor D. These couple the ground pair of states only at the Sth order of perturbation. SMM with axial symmetry, like Mn12, do not possess such a term but exhibit anyhow tunneling. It is interesting to recall that the spin Hamiltonian is just an effective Hamiltonian that allows treating orbital contribution in a parametric way. The series expansion in spin operators describing the zero field splitting of a spin S state indeed would consider terms of order up to 2S according to X N N HZFS ¼ Bk Ok ð6Þ N;k

where ONk are the Stevens’ operators [36]. We can neglect their complex formulation, retrievable in the literature, and only retain here

3359

that k is the total order of operator in Sz plus S±, while N indicates the order in S± only. The parameters BNk for high order operators are so little that are hardly detectable in spectroscopy experiments but indeed they play a crucial role in tunneling because the higher is their order (N) the lower is the order of perturbation at which they promote tunneling. Moreover, transverse anisotropy of order higher than 2 is present also in axial molecules like Mn12. The quantification of the parameters describing higher order anisotropy is an important issue but understanding their origin is even more relevant and will be briefly discussed in the following. Higher order anisotropy can be just a projection on the total spin state of higher order terms of the single spins constituting the cluster as shown in (4) for second order ones. These are however very small [37] and moreover they cannot exceed the order 2Si, where again i refers to the single spin. Fourth order anisotropy has indeed been observed in a tetranuclear cluster comprising only NiII centers [38], and therefore is incompatible with a single spin origin. Several authors have pointed out that the combination of magnetic exchange and single ion magnetic anisotropy, both ingredients being present in SMM, leads necessarily to admixing of the states differing by ST [39,40]. This spin-admixing is larger the stronger is the magnetic anisotropy compared to the exchange interaction. In other words ST is no more a good quantum number and the minor changes in the spectrum of the levels are modeled by higher order anisotropy terms, which are small but crucial for the tunneling mechanism. In a recent study it has been shown that spin-admixing is not sufficient to induce transverse anisotropy in axial molecules [41]. The other necessary ingredient is that of non-collinearity of the single ion anisotropy axes. In Fig. 4 two possible arrangements of single ion anisotropy axes, here represented by the elongation of the ellipsoids, are schematized for an ideal SMM with tetragonal symmetry. In the first case (Fig. 4a) the easy axes are all parallel to each other and, independent of the relative strength of magnetic anisotropy and exchange interaction, no transverse high order anisotropy is present. In the second case we see that the elongation axes do not coincide with the fourfold symmetry axis and therefore the four ellipsoids point toward different, but symmetry related, directions. In this case a transverse anisotropy can be present. Is situation two a common case in molecular nanomagnetism? Undoubtedly yes. We can say even more: non-collinearity of anisotropy axes is a feature that characterizes molecular magnetism. Whenever low-symmetry magnetic sites, so common in molecular nanomagnetism, are arranged in more symmetric structures, either idealized or rigorously imposed by the crystal symmetry, the phenomenon of noncollinearity occurs. This is not the case in oxides and metals, where

Fig. 4. In a cluster with tetragonal symmetry the single ion easy axis directions, represented by the elongation of the ellipsoids, are parallel to each other if they coincide with the fourfold symmetry axis as in (a), but in the general case, where they form an angle h with the symmetry axis, they are no more parallel to each other (b).

3360

R. Sessoli / Inorganica Chimica Acta 361 (2008) 3356–3364

the spin carriers often lie in special positions of the crystal unit cell and have a high symmetric environment. By employing a derivative of the Mn12 family, of formula [Mn12O12(tBu–CH2CO2)16(CH3OH)4]  2CH3OH [42,43] with strict tetragonal symmetry it has been possible to analyze in detail the transverse anisotropy through single crystal high frequency EPR and to locate exactly the orientation of intermediate and hard axes in the molecular frame [41]. Hard and intermediate axes, i.e. maxima and minima in the resonance field in Fig. 5 show p/2 periodicity, as indeed expected for a tetragonal system. More interesting is the possibility to identify as hard axes the directions in the plane where are bending the elongated octahedra of those MnIII ions that are more deviating from the tetragonal axis. It has been possible to establish a precise relation between the terms of the spin Hamiltonian that promote tunneling and the molecular structure, indeed not only up to the fourth order but also for the sixth order terms. No more details are presented in this short overview but the interest reader can retrieve them in the original work [41]. The relevance of non-collinearity has also clearly emerged in a study on DyIII based SMM. Several groups have employed the

bis(2-pyridylcarbonyl)amine anion ligand (pbca) schematized in Fig. 6 to link 3d metal ions and lanthanides [44,45]. If two pbca ligands coordinate a 3d metal ions, they form two outer pockets that can host other metal ions but these outer metal iones have necessarily their coordination polyhedron orthogonal to each other, as shown in Fig. 6 for {[Dy(hfac)3]2[M(bpca)2]} [46]. Comparing the dynamic properties of two trimeric species, where M = NiII or low spin diamagnetic FeII, a weak ferromagnetic interaction has been detected between the terminal DyIII and the central paramagnetic NiII center. A comparison of the ac susceptibility of the two derivatives has shown that the NiII derivative has however a smaller energy barrier for the reversal of the magnetization than the l.s. Fe, II analogue despite the weak ferromagnetic interaction. On the contrary another study on DyIII nitronyl-nitroxide radical units weakly interacting in pairs, thanks to an additional donor atom on the radical, has shown that switching on this pair interaction slows down the relaxation. Interestingly, in this last case the coordination polyhedra of the two DyIII ions are parallel to each other, being related by an inversion center [47]. Non-collinearity of the easy axis has been found to be at the basis of the peculiar magnetic behavior of the triangular DyIII cluster

Fig. 5. (a) Angular dependence of the resonance field Br for the jm = 10> ? jm = 9> transition in the single crystal HF-EPR spectra of [Mn12O12(tBu–CH2CO2)16(CH3OH)4]  2CH3OH when the field is perpendicular to the c axis of the tetragonal space group. (b) View of the molecular cluster along c. The pale grey spheres (green) are MnIV atoms, while the largest dark spheres (blue) are the MnIII atoms with the Jahn–Teller elongation axis, evidenced by the middle size grey spheres (red), forming a larger angle h with c. The dotted lines are the projections on the ab plane of the above mentioned elongation axes and they coincide with the maxima of the resonance field detected in (a). More details can be found in Ref. [41]. (For interpretation of the references to colors in this figure legend, the reader is referred to the web version of this paper.)

Fig. 6. On the left: schematic view of the bis(2-pyridylcarbonyl)amine anion ligand, pbca. On the right the structure of the trinuclear cluster {[Dy(hfac)3]2[M(bpca)2]} is reported, with the shaded planes evidencing the orthogonality of the terminal DyIII coordination octahedra imposed by the two pbca ligands coordinated to the central atom. More details can be found in Ref. [46]. (For interpretation to colors, the reader is referred to the web version of this paper.)

3361

R. Sessoli / Inorganica Chimica Acta 361 (2008) 3356–3364

was indeed theoretically predicted in 1960s by Glauber [56], when he developed the kinetics for the 1-D Ising model: H¼J

N X

Szi Sziþ1  glB Bz

i¼1

N X

Szi

ð7Þ

i¼1

Here again the key ingredient is the easy axis magnetic anisotropy but the magnetic interaction between neighboring spins is indeed the parameter that enters in the barrier hampering the reversal of the magnetization according to 2 s1 ¼ s1 0 expð4jJjS =kB TÞ

ð8Þ

An intuitive explanation of (8) can be found in the scheme below where the passage from a full magnetized state, for instance due to the application of a strong field that polarizes the spins of the chain, to the equilibrium non-magnetic state in zero field occurs through the nucleation of a domain wall. Fig. 7. View of the [Dy3(l3-OH)2L3Cl(H2O)5]3+ cluster, where HL = o-vanillin. The large big spheres are DyIII atoms, while oxygen atoms are shown as small black spheres (blue). The arrows represent the proposed arrangement of the magnetic moments that are forced to lie in the Dy3 plane by the single ion anisotropy of the rare-earth ion. The ground state is non-magnetic but has double degeneration corresponding to clock and anti-clockwise (not shown here) orientation of the magnetic moments. More details can be found in Refs. [48,49]. (For interpretation of the references to colors in this figure legend, the reader is referred to the web version of this paper.)

of formula [Dy3(l3-OH)2L3Cl(H2O)5]3+ cluster, where HL = o-vanillin, schematized in Fig. 7 [48]. This compound shows a maximum in the susceptibility around 7 K, suggesting that the ground state is non-magnetic despite the odd number of electrons in the cluster. Interestingly it shows also slow relaxation of the magnetization in the ac susceptibility. Single crystal investigations of the magnetic properties have recently revealed that the non-magnetic ground state is due to spin non-collinearity [49]. DyIII has in fact Ising type anisotropy and the magnetic moments are lying in the plane of the triangle at 120° from each other (see Fig. 7). The ground state is non-magnetic but degenerate, because the magnetic moment can be oriented clockwise or anti-clockwise. This gives rise to a sort of magnetic chirality that could represent a novel type of magnetic memory. We hope to have highlighted that the control of how the building blocks are spatially arranged in the molecular architecture does not only allow tailoring the exchange interaction and the axial anisotropy but also subtler details involved in tunneling. To achieve an efficient control of the spatial arrangement remains however a challenging task for the ingenuity of synthetic chemists. 5. Beyond zero-dimensionality Molecular chemistry has also played a key role in one-dimensional (1-D) magnetism [50], which in 1980s has constituted a common playground for chemists and physicists consolidating the interdisciplanarity of the field [51]. The interest in 1-D magnetic structures has been renewed after that magnetic hysteresis, again without any evidence of 3-D magnetic order, has been observed in a CoII-radical chains [52]. Metal-radical chains have been studied in Florence for about 20 years [53] but continue to provide fascinating examples of novel phenomena, like the experimental validation in GdIII nitronyl-nitroxide 1-D helimagnets of Villain’s conjecture on the chiral spin liquid phase [54]. In this novel phase translational invariance is broken without violation of rotational invariance. One-dimensional systems showing hysteresis in the absence of long range magnetic order have been named by Clerac et al. [55]. Single chain magnet, SCM, in analogy with their 0-D analogues. The possibility to observe a slowing down of the magnetization

In the pure Glauber model, i.e. in the Ising limit [57], the spins have only one degree of freedom, i.e. up or down orientation, and thus the wall is infinitely narrow. The cost to generate such a wall is proportional to the exchange interaction as two interactions have to be violated. Once the wall is nucleated its propagation occurs at no energy cost, because for the spins at the wall the exchange interaction is cancelled out. However the probabilities for the wall to go forward or come back to the original position are the same, and this is at the origin of the strong similarities between 1-D dynamics and the statistics of gambling. A detailed discussion of the dynamics in 1-D structure is out of the scope of this overview and the reader is addressed to a dedicated contribution in this volume [58]. For a chemist interested in molecular magnetism the first thing that takes one’s eye is that in (8) a parameter appears, the exchange coupling constant J, whose control at the synthetic level is now well established. This could reflect in a significant advance in the increase of the blocking temperature that has been indeed realized. This is even more relevant now that is well established that homo-spin antiferromagnetically coupled chains can also show SCM behavior, provided that the neighboring spins have their anisotropy axes non-collinear to each other. A detailed investigation on the MnIII porphyrin chain, of formula [Mn(TPP)O2PPhH]  H2O (TPP = meso-tetraphenylporphyrin), schematized in Fig. 8, has evidenced the role played by the different parameters like the single ion anisotropy, the isotropic exchange, and the tilting angle of the anisotropy axes [59]. A canted antiferromagnetic approach to SCM is therefore possible, enlarging the zoo of SCM and allowing in principle larger barrier, because antiferromagnetic interactions are in general much stronger than ferromagnetic ones. In this respect (8) can however be misleading. In fact the Ising spin Hamiltonian we have employed to describe SCM behavior is a model one, in which the magnetic anisotropy is taken as infinite. A better representation of real systems should take into account both magnetic exchange and anisotropy at the same level as in [60]: H¼J

N X i¼1

~ Siþ1 þ D Si  ~

N N X X ~ B ~ Si ðSzi Þ2  glB i¼1

ð9Þ

i¼1

The spins can be either classical vectors or quantum operators, but as far as the anisotropy dominates over the exchange the dynamics is still described by a sharp domain wall. In the opposite case the spins assume intermediate orientations to minimize the exchange energy, thus giving rise to a thick domain wall that comprises many

3362

R. Sessoli / Inorganica Chimica Acta 361 (2008) 3356–3364

Fig. 8. Schematic view of the chain structure in [Mn(TPP)O2PPhH]  H2O (TPP = meso-tetraphenylporphyrin). The equatorial coordination planes defined by the porphyrin ligands are shadowed to evidence the alternating orientations they have in the chain. The black arrows (red) represent the spin alignment favored by antiferromagnetic interactions, while the white arrow indicates the direction of the non-compensate magnetic moment. More details can be found in Ref. [59]. (For interpretation of the references to colors in this figure legend, the reader is referred to the web version of this paper.)

spins. It is straightforward that we cannot arbitrarily increase J without giving up the Glauber dynamics. This establishes an intrinsic limitation of the SCM approach as far as high blocking temperatures are concerned. On the other side it has been shown that both exchange and single ion anisotropy contribute to the height of the barrier opposing to the magnetization reversal according to [60] 2 s1 ¼ s1 0 exp½S ð4jJj þ jDjÞ=kB T

ð10Þ

In other words, slow relaxing units, like SMM, if arranged in chains can experience a much higher barrier for the reversal of the magnetization and consequently an increased blocking temperature. Only few examples are now available and this approach deserves to be further exploited [61–63]. The increase in blocking temperature, indeed still modest, is not the only advantage of SCM. Glauber dynamics, associated to 1-D arrangement of strongly anisotropic units, is a very robust one and can persist also if a magnetic phase transition occurs as observed in another member of the CoII nitronyl-nitroxide chains family. Interesting the slowing down of the magnetization typical of the Glauber dynamics seems at the origin of the record coercive field of more than 50 kOe at 5 K [64]. At low temperature this material based on Cobalt-radical chains is thus one of the hardest magnets, as traditional hard magnets do not experience a significant increase of the coercivity on lowering the temperature. 6. Spintronics with SMMs The previous sections have highlighted the limits of the current approach to molecular nanomagnetism, mainly related to the low operative temperatures. However the impact of SMM on our understanding on the magnetization dynamics at the nanoscale has been really impressive. Is something similar expected in the new field of magneto-transport, in particular in quantum tunneling effects in spin-valve magnetoresistance? The answer is still uncertain because molecular nanomagnets in the bulk phase are insulators at the temperature at which their magnetic properties are so exciting [65]. However we can go round this obstacle by directly connecting the SMM to the electrodes or to a conducting surface [66]. What are the advantages of spintronics based on molecules? For sure the perspective of cheap devices based on plastic technology is very attractive but the main interest is now in pushing the control down to the single spin and investigating the interaction between transport properties and spins at a very fundamental level [67].

Research has advanced rapidly in this field and reports on experiments where a Mn12 cluster is inserted in nanometric contacts have recently appeared [68,69]. The results show a weak coupling of the molecule with the electrodes and the appearance of the phenomenon of Coulomb blockade, i.e. once an electron has tunneled from the electrode into the molecule it prevents the tunneling of a another electron resulting in suppression of the current at low bias voltage. Such an observation however does not imply that an integer cluster is present in the nanocontact or that it has retained the SMM behavior in such an environment. Many laboratories are now involved in a more systematic and extensive investigations on the organization of SMMs on surfaces [70–77]. The most studied systems are again derivatives of the Mn12 family, either functionalized to be grafted on gold or silicon substrates through self assembly, or anchored by an exchange reaction on the preformed self assembled monolayer. Physical adsorption on graphite has also been essayed [78,79]. The major challenge we are facing at the moment is the investigation of the dynamics of the magnetization in presence of very small quantities of active material. While very innovative techniques, like the nanoSQUID magnetometry based on carbon nanotube junctions [80,81], are pushed toward the limit of the detection of the single molecule magnetism, more traditional magneto-optic surface techniques are currently employed. First results are somehow disappointing and deserve a deeper analysis. Mn12 clusters functionalized with sulfur terminated ligands to be attached on gold do not show any magnetic hysteresis once they are assembled in the monolayer [82]. X-ray absorption spectra, characterized by element and oxidation state selectivity, have shown a significant presence of MnII and suggest redox instability of the cluster in this environment [83,84]. Such an electron transfer from the gold substrate has been also predicted from DFT calculations [85]. More recent magnetic investigations through X-ray magnetic circular dichroic spectra have confirmed the absence of a magnetic hysteresis [86]. Despite these not fully encouraging preliminary results the research is going on, trying different molecular clusters and different preparation techniques. Evaporation of atoms or molecules in ultra high vacuum condition is indeed the most employed technique in surface science as it allows in situ preparation without significant contamination of the substrate. Unfortunately Mn12 clusters are too fragile and decompose before their sublimation. More gentle techniques, like matrix assisted laser ablation [87], have been also essayed but they still provide a significant degradation of the clusters, evidenced by the formation of the mixed valence Mn3O4 oxide in the Haussmannite phase. We have recently shown that a hybrid

R. Sessoli / Inorganica Chimica Acta 361 (2008) 3356–3364

3363

undoubtedly more complex if large polynuclear molecules are considered, but up to now this route has not been explored at all even if it appears to be feasible with the current vacuum compatible in situ deposition techniques. Molecular nanomagnetism is a mature field of research, which is now entering in a different phase of its evolution. The research is becoming even more interdisciplinary than ever, and significant advancements rely on cross-fertilization from other fields in nanoscience. Nevertheless we are fully convinced that a lot remain to be learnt about matter at the nanoscale from these model systems, which have been up to now an excellent school of physics. Acknowledgments

Fig. 9. Schematic view of the vacuum-spray deposition technique. A microscopic pulsed nozzle injects a solution of Mn12 in a vacuum chamber. The beam goes through a heated tube to reduce the amount of solvent that reaches the target. More details can be found in Ref. [88].

method, based on vacuum spray of a solution of Mn12 clusters on a target, as schematized in Fig. 9, is able to provide thin films of Mn12 clusters with roughness of the order of one or two clusters height without any alteration of the SMM properties [88]. In this case, however, the interaction with the substrate does not play any role. The result is however very promising for further developments, in particular for the vacuum compatibility of the technique. Promising perspectives for molecular nanomagnetism can also be found in quantum computation [89–93]. In a quantum computer the information is not stored in the up or down orientation of the magnetization of the bit but rather in the coefficients that describe the wavefunction of the system in the basis of the entangled qu-bits. To be usable however a qu-bit must be sufficiently isolated from the environment to retain the information in the superposition of the states of the basis set during its operation. Results about long-living spin coherence in antiferromagnetic molecular rings doped to posses a resulting spin have been recently reported thanks to pulsed EPR experiments [94,95]. The observation of Rabi oscillations and their frequency in respect of the coherence time provides the qu-bit figure of merit, i.e. how many oscillations can be performed before loosing the coherence. Promising results have been obtained for isolated rare-earth ions [96], as well as for paramagnetic nitrogen atoms in C60 [97]. The influence of the multispin nature of qu-bits based on molecular clusters deserves to be further investigated, as well as the possibility to control the decoherence through a tailored synthetic approach.

I am indebted to the many students, co-workers, and collaborators I have been honored to work with, and whose original research has inspired this overview. The list will be too long to be reported here but I hope to have correctly quoted their works. A special thank goes to the guest editor, Andrea Dei, for the stimulating discussion and his efficient efforts to realize this special issue. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

[18]

[19] [20]

7. Conclusions Molecular nanomagnetism is undoubtedly suffering from the impasse originated by the difficulties encountered in pushing up the blocking temperatures. We hope to have shown that these difficulties are rather intrinsic in the phenomenon of magnetic memory at the molecular level. Completely revolutionary approaches need to be explored. We have to disappoint the expectations of the guest editor of this volume by saying that we cannot envisage either a winning strategy or a detailed roadmap. However, it is interesting to confront us with what is currently done in neighboring fields. Proximity effects, so relevant in giant magneto resistance of multilayer nanostructures, are currently exploited to control the magnetism of molecules evaporated on magnetic surfaces. In particular a ferromagnetic behavior, with opening of the hysteresis, has been observed by Wende et al. for an iron(III)-octaethylporphyrin deposited on metallic nickel or cobalt substrates by exploiting the element selectivity of XMCD [98,99]. The situation is

[21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]

P. Weiss, J. Phys. 6 (1907) 661. W. Heisenberg, Z. Phys. 38 (1926) 411. P.A.M. Dirac, Proc. Roy. Soc. A 123 (1929) 714. J.H. Van Vleck, The Theory of Electric and Magnetic Susceptibility, Oxford University Press, Oxford, 1932. R.D. Willet, D. Gatteschi, O. Kahn, Magneto-Structural Correlations in Exchange Coupled Systems, Reidel Publishing, Dordrecht, 1983. R.D.L. Carlin, Magnetochemistry, Springer-Verlag, Berlin, 1986. D. Gatteschi, O. Kahn, J.S. Miller, F. Palacio, Magnetic Molecular Materials, NATO ASI, Kluwer, Dordrecht, 1991. O. Kahn, Molecular Magnetism, VCH, Weinheim, 1993. M. Tamura, Y. Nakazawa, D. Shiomi, Y. Nozawa, M. Hosokoshi, M. Ishikawa, M. Takahashi, M. Kinoshita, Chem. Phys. Lett. 186 (1991) 401. J.M. Manriquez, G.T. Yee, R.S. McLean, A.J. Epstein, J.S. Miller, Science 252 (1991) 1415. S. Ferlay, T. Mallah, R. Ouahes, P. Veillet, M. Verdaguer, Nature 378 (1995) 701. O. Kahn, Chem. Br. 35 (1999) 24. P. Gütlich, Y. Garcia, T. Woike, Coord. Chem. Rev. 219 (2001) 839. P. Gütlich, H.A. Goodwin (Eds.), Spin Crossover in Transition Metal Compounds I–III, Springer, Berlin, 2004. D. Gatteschi, R. Sessoli, J. Villain, Molecular Nanomagnets, Oxford University Press, Oxford, UK, 2006. R. Sessoli, D. Gatteschi, A. Caneschi, M.A. Novak, Nature 365 (1993) 141. H.J. Eppley, S.M.J. Aubin, M.W. Wemple, D.M. Adams, H.L. Tsai, V.A. Grillo, S.L. Castro, Z.M. Sun, K. Folting, J.C. Huffman, D.N. Hendrickson, G. Christou, Mol. Cryst. Liq. Cryst. A (1997) 167. G. Aromi, S.M.J. Aubin, M.A. Bolcar, G. Christou, H.J. Eppley, K. Folting, D.N. Hendrickson, J.C. Huffman, R.C. Squire, H.L. Tsai, S. Wang, M.W. Wemple, Polyhedron 17 (1998) 3005. A. Caneschi, D. Gatteschi, R. Sessoli, A.-L. Barra, L.C. Brunel, M. Guillot, J. Am. Chem. Soc. 113 (1991) 5873. J. Villain, F. Hartmann-Boutron, R. Sessoli, A. Rettori, Europhys. Lett. 27 (1994) 159. M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Eitenne, G. Creuzet, A. Friederich, J. Chazelas, Phys. Rev. Lett. 61 (1988) 2472. G. Binasch, P. Grünberg, F. Saurenbach, W. Zinn, Phys. Rev. B. 39 (1989) 4828. C. Chappert, A. Fert, F.N. Van Dau, Nature Mater. 6 (2007) 813. C.J. Milios, A. Vinslava, W. Wernsdorfer, S. Moggach, S. Parsons, S.P. Perlepes, G. Christou, E.K. Brechin, J. Am. Chem. Soc. 129 (2007) 2754. A.M. Ako, I.J. Hewitt, V. Mereacre, R. Clerac, W. Wernsdorfer, C.E. Anson, A.K. Powell, Angew. Chem., Int. Ed. 45 (2006) 4926. J.B. Goodenough, J. Phys. Chem. Solids 6 (1958) 287. J. Kanamori, J. Phys. Chem. Solids 10 (1959) 87. O. Waldmann, Inorg. Chem. 46 (2007) 10035. A. Bencini, D. Gatteschi, EPR of Exchange Coupled Systems, Springer-Verlag, Berlin, 1990. E.J. Schelter, F. Karadas, C. Avendano, A.V. Prosvirin, W. Wernsdorfer, K.R. Dunbar, J. Am. Chem. Soc. 129 (2007) 8139. E.J. Schelter, A.V. Prosvirin, W.M. Reiff, K.R. Dunbar, Angew. Chem., Int. Ed. 43 (2004) 4912. F. Pointillart, K. Bernot, L. Sorace, R. Sessoli, D. Gatteschi, Dalton Trans. (2007) 2689. C. Aronica, G. Pilet, G. Chastanet, W. Wernsdorfer, J.F. Jacquot, D. Luneau, Angew. Chem., Int. Ed. 45 (2006) 4659. D. Gatteschi, R. Sessoli, Angew. Chem., Int. Ed. 42 (2003) 268.

3364

R. Sessoli / Inorganica Chimica Acta 361 (2008) 3356–3364

[35] L. Gunther, B. Barbara, Quantum Tunneling of Magnetization-QTM ’94, Kluwer, Dordrecht, 1995. [36] A. Abragam, B. Bleaney, Electron Paramagnetic Resonance of Transition Ions, Dover, New York, 1986. [37] F. Hartmann-Boutron, P. Politi, J. Villain, Int. J. Mod. Phys. B 10 (1996) 2577. [38] A. Wilson, J. Lawrence, E.C. Yang, M. Nakano, D.N. Hendrickson, S. Hill, Phys. Rev. B 74 (2006). art. no. 140403. [39] S. Carretta, E. Liviotti, N. Magnani, P. Santini, G. Amoretti, Phys. Rev. Lett. 92 (2004). art. no. 207205. [40] O. Waldmann, H.U. Gudel, Phys. Rev. B 72 (2005). art. no. 094422. [41] A.L. Barra, A. Caneschi, A. Cornia, D. Gatteschi, L. Gorini, L.P. Heiniger, R. Sessoli, L. Soracet, J. Am. Chem. Soc. 129 (2007) 10754. [42] N.E. Chakov, S.C. Lee, A.G. Harter, P.L. Kuhns, A.P. Reyes, S.O. Hill, N.S. Dalal, W. Wernsdorfer, K.A. Abboud, G. Christou, J. Am. Chem. Soc. 128 (2006) 6975. [43] W. Wernsdorfer, M. Murugesu, G. Christou, Phys. Rev. Lett. 96 (2006) 057208. [44] T. Kajiwara, M. Nakano, Y. Kaneko, S. Takaishi, T. Ito, M. Yamashita, A. IgashiraKamiyama, H. Nojiri, Y. Ono, N. Kojima, J. Am. Chem. Soc. 127 (2005) 10150. [45] A.M. Madalan, K. Bernot, F. Pointillart, M. Andruh, A. Caneschi, Eur. J. Inorg. Chem. 35 (2007) 5533. [46] F. Pointillart, K. Bernot, R. Sessoli, D. Gatteschi, Chem. Eur. J. 13 (2007) 1602. [47] G. Poneti, K. Bernot, L. Bogani, A. Caneschi, R. Sessoli, W. Wernsdorfer, D. Gatteschi, Chem. Commun. (2007) 1807. [48] J.K. Tang, I. Hewitt, N.T. Madhu, G. Chastanet, W. Wernsdorfer, C.E. Anson, C. Benelli, R. Sessoli, A.K. Powell, Angew. Chem., Int. Ed. 45 (2006) 1729. [49] J. Luzon, K. Bernot, R. Sessoli, I. Hewitt, C.E. Anson, A.K. Powell, submitted for publication. Manuscript available at http://arxiv.org/abs/0804.1272. [50] P. Delhaes, M. Drillon (Eds.), Organic and Inorganic Low Dimensional Crystalline Materials, NATO ASI Series B, vol. 168, Plenum, New York, 1987. [51] L.J. de Jongh, A.R. Miedema, Adv. Phys. 50 (2001) 947. [52] A. Caneschi, D. Gatteschi, N. Lalioti, C. Sangregorio, R. Sessoli, G. Venturi, A. Vindigni, A. Rettori, M.G. Pini, M.A. Novak, Angew. Chem., Int. Ed. 40 (2001) 1760. [53] A. Caneschi, D. Gatteschi, R. Sessoli, P. Rey, Acc. Chem. Res. 22 (1989) 392. [54] F. Cinti, A. Rettori, M.G. Pini, M. Mariani, E. Micotti, A. Lascialfari, N. Papinutto, A. Amato, A. Caneschi, D. Gatteschi, M. Affronte, Phys. Rev. Lett. 100 (2008). art. no. 057203. [55] R. Clerac, H. Miyasaka, M. Yamashita, C. Coulon, J. Am. Chem. Soc. 124 (2002) 12837. [56] R.J. Glauber, J. Math. Phys. 4 (1963) 294. [57] E. Ising, Z. Phys. 31 (1925) 253. [58] A. Vindigni, Inorg. Chim. Acta, doi:10.1016/j.ica. 2008.02.058. [59] K. Bernot, J. Luzon, R. Sessoli, A. Vindigni, J. Thion, S. Richeter, D. Leclercq, J. Larionova, A. van der Lee, J. Am, Chem. Soc. 130 (2008) 1619. [60] C. Coulon, R. Clerac, L. Lecren, W. Wernsdorfer, H. Miyasaka, Phys. Rev. B 69 (2004). art. no.-132408. [61] M. Ferbinteanu, H. Miyasaka, W. Wernsdorfer, K. Nakata, K. Sugiura, M. Yamashita, C. Coulon, R. Clerac, J. Am. Chem. Soc. 127 (2005) 3090. [62] L. Lecren, O. Roubeau, Y.G. Li, X.F. Le Goff, H. Miyasaka, F. Richard, W. Wernsdorfer, C. Coulon, R. Clerac, Dalton Trans. (2008) 755. [63] H. Miyasaka, M. Yamashita, Dalton Trans. (2007) 399. [64] N. Ishii, Y. Okamura, S. Chiba, T. Nogami, T. Ishida, J. Am. Chem. Soc. 130 (2008) 24. [65] H. Hiraga, H. Miyasaka, K. Nakata, T. Kajiwara, S. Takaishi, Y. Oshima, H. Nojiri, M. Yamashita, Inorg. Chem. 46 (2007) 9661. [66] L. Bogani, W. Wenrsdorfer, Nature Mater. 7 (2008) 179. [67] S. Sanvito, J. Mater. Chem. 17 (2007) 4455. [68] H.B. Heersche, Z. De Groot, J.A. Folk, H.S.J. Van Der Zant, C. Romeike, M.R. Wegewijs, L. Zobbi, D. Barreca, E. Tondello, A. Cornia, Phys. Rev. Lett. 96 (2006). art. no. 206801. [69] M.H. Jo, J.E. Grose, K. Baheti, M.M. Deshmukh, J.J. Sokol, E.M. Rumberger, D.N. Hendrickson, J.R. Long, H. Park, D.C. Ralph, Nano Lett. 6 (2006) 2014. [70] R.V. Martinez, F. Garcia, R. Garcia, E. Coronado, A. Forment-Aliaga, F.M. Romero, S. Tatay, Adv. Mater. 19 (2007) 291. [71] V. Corradini, R. Biagi, U. Del Pennino, V. De Renzi, A. Gambardella, M. Affronte, C.A. Muryn, G.A. Timco, R.E.P. Winpenny, Inorg. Chem. 46 (2007) 4937. [72] Z. Salman, K.H. Chow, R.I. Miller, A. Morello, T.J. Parolin, M.D. Hossain, T.A. Keeler, C.D.P. Levy, W.A. Macfarlane, G.D. Morris, H. Saadaoui, D. Wang, R. Sessoli, G.G. Condorelli, R.F. Kiefl, Nano Lett. 7 (2007) 1551.

[73] G.G. Condorelli, A. Motta, I.L. Fragala, F. Giannazzo, V. Raineri, A. Caneschi, D. Gatteschi, Angew. Chem., Int. Ed. 43 (2004) 4081. [74] F. Pineider, M. Mannini, R. Sessoli, A. Caneschi, D. Barreca, L. Armelao, A. Cornia, E. Tondello, D. Gatteschit, Langmuir 23 (2007) 11836. [75] B. Fleury, L. Catala, V. Huc, C. David, W.Z. Zhong, P. Jegou, L. Baraton, S. Palacin, P.A. Albouy, T. Mallah, Chem. Commun. (2005) 2020. [76] A. Naitabdi, J.P. Bucher, P. Gerbier, P. Rabu, M. Drillon, Adv. Mater. 17 (2005) 1612. [77] A.N. Abdi, J.P. Bucher, P. Rabu, O. Toulemonde, M. Drillon, P. Gerbier, J. Appl. Phys. 95 (2004) 7345. [78] R.W. Saalfrank, A. Scheurer, I. Bernt, F.W. Heinemann, A.V. Postnikov, V. Schunemann, A.X. Trautwein, M.S. Alam, H. Rupp, P. Muller, Dalton Trans. (2006) 2865. [79] A.M. Ako, H. Maid, S. Sperner, S.H.H. Zaidi, R.W. Saalfrank, M.S. Alam, P. Muller, F.W. Heinemann, Supramol. Chem. 17 (2005) 315. [80] J.P. Cleuziou, W. Wernsdorfer, V. Bouchiat, T. Ondarcuhu, M. Monthioux, Phys. Status Solidi B 244 (2007) 4351. [81] J.P. Cleuziou, W. Wernsdorfer, V. Bouchiat, T. Ondarcuhu, M. Monthioux, Nature Nanotechnol. 1 (2006) 53. [82] L. Bogani, L. Cavigli, M. Gurioli, R.L. Novak, M. Mannini, A. Caneschi, F. Pineider, R. Sessoli, M. Clemente-Leon, E. Coronado, A. Cornia, D. Gatteschi, Adv. Mater. 19 (2007) 3906. [83] S. Voss, M. Burgert, M. Fonin, U. Groth, U. Rudiger, Dalton Trans. (2008) 499. [84] S. Voss, M. Fonin, U. Rudiger, M. Burgert, U. Groth, Y.S. Dedkov, Phys. Rev. B 75 (2007) 045102. [85] S. Barraza-Lopez, M.C. Avery, K. Park, J. Appl. Phys. 103 (2008). art. no. 07B907. [86] M. Mannini, F. Pineider, R. Sessoli, A. Cornia, P. Sainctavit, in preparation. [87] J. Means, V. Meenakshi, R.V.A. Srivastava, W. Teizer, A. Kolomenskii, H.A. Schuessler, H. Zhao, K.R. Dunbar, J. Magn. Magn. Mater. 284 (2004) 215. [88] R. Moroni, R. Buzio, A. Chincarini, U. Valbusa, F.B. De Mongeot, L. Bogani, A. Caneschi, R. Sessoli, L. Cavigli, M. Gurioli, J. Mater. Chem. 18 (2008) 109. [89] M.N. Leuenberger, D. Loss, Nature 410 (2001) 789. [90] F. Meier, J. Levy, D. Loss, Phys. Rev. Lett. 90 (2003). art. no. 047901. [91] M. Affronte, F. Troiani, A. Ghirri, A. Candini, M. Evangelisti, V. Corradini, S. Carretta, P. Santini, G. Amoretti, F. Tuna, G. Timco, R.E.P. Winpenny, J. Phys. D: Appl. Phys. 40 (2007) 2999. [92] M. Affronte, F. Troiani, A. Ghirri, S. Carretta, P. Santini, V. Corradini, R. Schuecker, C. Muryn, G. Timco, R.E. Winpenny, Dalton Trans. (2006) 2810. [93] J. Lehmann, A. Gaita-Arino, E. Coronado, D. Loss, Nature Nanotechnol. 2 (2007) 312. [94] A. Ardavan, O. Rival, J.J.L. Morton, S.J. Blundell, A.M. Tyryshkin, G.A. Timco, R.E.P. Winpenny, Phys. Rev. Lett. 98 (2007). art. no. 057201. [95] W. Wernsdorfer, Nature Mater. 6 (2007) 174. [96] S. Bertaina, S. Gambarelli, A. Tkachuk, I.N. Kurkin, B. Malkin, A. Stepanov, B. Barbara, Nature Nanotechnol. 2 (2007) 39. [97] J.J.L. Morton, A.M. Tyryshkin, A. Ardavan, K. Porfyrakis, S.A. Lyon, G.A.D. Briggs, Phys. Rev. Lett. 95 (2005). art. no. 200501. [98] H. Wende, M. Bernien, J. Luo, C. Sorg, N. Ponpandian, J. Kurde, J. Miguel, M. Piantek, X. Xu, P. Eckhold, W. Kuch, K. Baberschke, P.M. Panchmatia, B. Sanyal, P.M. Oppeneer, O. Eriksson, Nature Mater. 6 (2007) 516. [99] D. Gatteschi, Nature Mater. 6 (2007) 471.

Roberta Sessoli received the Ph.D. in Chemistry in 1992 working on Molecular Magnetism. She is now an assistant professor at the University of Florence. Her research activity first involved the structural and magnetic characterisation of molecular magnets based on metal ions and stable nitronyl-nitroxides, while the present major interest is the study of magnetic bistability in molecular materials. She has been a pioneer in the field of molecules behaving like nanomagnets and in the investigation of quantum effects in the dynamics of the magnetization of these materials. More recently she has become interested in the organization of molecular magnets on surfaces.