We have investigated the magnetic relaxation of clusters of eight iron ions characterized by a spin ground state of ten and an Ising anisotropy. Below 400 mK the relaxation rate is temperature independent suggesting that tunneling of the magnetic moment across its anisotropy energy barrier occurs. Using the anisotropy constants derived from EPR data, we can calculate both the crossover temperature T c and the expected tunneling frequency 1͞t. The field dependence of the relaxation shows evidence of resonant tunneling. [S0031-9007(97)03302-4]

Platelet-shaped copper sulfide nanocrystals (NCs) with tunable Cu stoichiometry were prepared from Cu-rich covellite (Cu1.1S) nanoplates through their reaction with a Cu(I) complex ([Cu(CH3CN)4]PF6) at room temperature. Starting from a common sample, by this approach it is possible to access a range of compositions in these NCs, varying from Cu1.1S up to Cu2S, each characterized by a different optical response: from the metallic covellite, with a high density of free carriers and strong localized surface plasmon resonance (LSPR), up to Cu2S NCs with no LSPR. In all these NCs the valency of Cu in the lattice stays always close to +1, while the average -1 valency of S in covellite gradually evolves to -2 with increasing Cu content; i.e., sulfur is progressively reduced. The addition of copper to the starting covellite NCs is similar to the intercalation of metal species in layered transition metal dichalcogenides (TMDCs); i.e., the chalcogen-chalcogen bonds holding the layers are progressively broken to make room for the intercalated metals, while their overall anion sublattice does not change much. However, differently from the TMDCs, the intercalation in covellite NCs is sustained by a change in the redox state of the anion framework. Furthermore, the amount of Cu incorporated in the NCs upon reaction is associated with the formation of an equimolar amount of Cu(II) species in solution. Therefore, the reaction scheme can be written as: Cu1.1S + 2γCu(I) → Cu1.1+γS + γCu(II).

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The synthesis, crystal structure, and magnetic characterization of a novel tetranuclear iron(III) methoxo-bridged cluster of formula Fe 4 (OCH 3 ) 6 (dpm) 6 (where Hdpm ) dipivaloylmethane) is reported. The cluster has a ground spin state of S ) 5, which is selectively populated below 20 K. High-field EPR spectra revealed that the system has a uniaxial magnetic anisotropy, corresponding to a zero field splitting parameter D ) -0.2 cm -1 of the S ) 5. Such anisotropy below 1 K gives rise to the slow relaxation of the magnetization similar to that of super-paramagnets. To investigate the origin of the magnetic anisotropy we have evaluated the projection of the single-ion and dipolar contributions to the zfs of the ground state. The zfs tensors of the three structurally independent iron(III) centers have been calculated from the coordination geometry and spectroscopic data using the angular overlap model. To test the reliability of the approach high-field EPR spectra of the parent monomer Fe(dpm) 3 have been recorded to compare the calculated and experimental zfs parameters.

The structural and magnetic properties of nanocrystalline manganese, cobalt, and nickel spinel ferrites dispersed in a highly porous SiO 2 aerogel matrix were studied. X-ray diffraction and high-resolution transmission electron microscopy indicate that single crystalline ferrite nanoparticles are well dispersed in the amorphous matrix. The cation distribution between the octahedral and tetrahedral sites of the spinel structure was investigated by X-ray absorption spectroscopy. The analysis of both the X-ray absorption near edge structure and the extended X-ray absorption fine structure indicates that the degree of inversion of the spinel structure increases in the series Mn, Co, and Ni spinel, in accordance with the values commonly found in the corresponding bulk spinels. In particular, fitting of the EXAFS data indicates that the degree of inversion in nanosized ferrites is 0.20 for MnFe 2 O 4 , 0.68 for CoFe 2 O 4 , and 1.00 for NiFe 2 O 4 . Magnetic characterization further supports these findings.

The peculiar redox-active character of quinonoid metal complexes makes them extremely appealing to design materials of potential technological interest. We show here how the tuning of the properties of these systems can be pursued by using appropriate molecular synthetic techniques. In particular, we focus our attention on metal polyoxolene complexes exhibiting intramolecular electron transfer processes involving either the ligand and the metal ion or the two dioxolene moieties of a properly designed ligand thus inducing electronic bistability. The transition between the two metastable electronic states can be induced by different external stimuli such as temperature, pressure, light, or pH suggesting the use of these systems for molecular switches.

Below 360 mK, Fe8 magnetic molecular clusters are in the pure quantum relaxation regime and we show that the predicted "square-root time" relaxation is obeyed, allowing us to develop a new method for watching the evolution of the distribution of molecular spin states in the sample. We measure as a function of applied field H the statistical distribution P (ξH) of magnetic energy bias ξH acting on the molecules. Tunneling initially causes rapid transitions of molecules, thereby "digging a hole" in P (ξH) (around the resonant condition ξH = 0). For small initial magnetization values, the hole width shows an intrinsic broadening which may be due to nuclear spins.PACS numbers: 75.45.+j, 75.60Ej Strong evidence now exists for thermally-activated quantum tunneling of the magnetization (QTM) in magnetic molecules such as Mn 12 ac and Fe 8 [1][2][3][4][5]. Crystals of these materials can be thought of as ensembles of identical, iso-oriented nanomagnets of net spin S = 10 for both Mn 12 ac and Fe 8 , and with a strong Ising-like anisotropy. The energy barrier between the two lowest lying spin states with S z = ±10 is about 60 K for Mn 12 ac and 25 K for Fe 8 [6,7]. Theoretical discussion of thermallyactivated QTM assumes that thermal processes (principally phonons) promote the molecules up to high levels, not far below the top of the energy barrier, and the molecules then tunnel inelastically to the other side. The transitions are therefore almost entirely accomplished via thermal excitations.At temperatures below 360 mK, Fe 8 molecular clusters display a clear crossover from thermally activated relaxation to a temperature independent quantum regime, with a pronounced resonance structure of the relaxation time as a function of the external field [5]. This can be seen for example by hysteresis loop measurements (Fig. 1). In this regime only the two lowest levels of each molecule are occupied, and only "pure" quantum tunneling through the anisotropy barrier can cause direct transitions between these two states. It was surprising however that the observed relaxation of the magnetization in the quantum regime was found to be non-exponential and the resonance width orders of magnitude too large [5,8]. The key to understanding this seemingly anomalous behavior now appears to involve the ubiquitous hyperfine fields as well as the (inevitable) evolving distribution of the weak dipole fields of the nanomagnets themselves [9].In this letter, we focus on the low temperature and low field limits, where phonon-mediated relaxation is astronomically long and can be neglected. In this limit, the S z = ±10 spin states are coupled by a tunneling matrix element ∆ tunnel which is estimated to be about 10 −8 K [9]. In order to tunnel between these states, the magnetic energy bias ξ H = gµ B SH due to the local magnetic field H on a molecule must be smaller than ∆ tunnel implying a local field smaller than 10 −9 T for Fe 8 clusters. Since the typical intermolecular dipole fields are of the order of 0.05 T, it seems at first that almost all...

Slow magnetic relaxation and hysteresis effects of dynamic origin have been observed above liquid helium temperature in a chain compound (see picture), comprising CoII centers and organic radicals, without any evidence of phase transition to three‐dimensional magnetic order. These results are the first evidence of the slow dynamics predicted for one‐dimensional magnetic systems with Ising anisotropy, and they open the possibility of storing information in a single magnetic nanowire.