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.

We study the origin of the strong difference in the resistivity of focused-electron- and focused-Ga-ion-beam-induced deposition (FEBID and FIBID, resp.) of Pt performed in a dual beam equipment using(CH3)3Pt(CpCH3)as the precursor gas. We have performed in-situ and ex-situ resistance measurements in both types of nanodeposits, finding that the resistivity of Pt by FEBID is typically four orders of magnitude higher than Pt by FIBID. In the case of Pt by FEBID, the current-versus-voltage dependence is nonlinear and the resistance-versus-temperature behavior is strongly semiconducting, whereas Pt by FIBID shows linear current-versus-voltage dependence and only slight temperature dependence. The microstructure, as investigated by high-resolution transmission electron microscopy, consists in all cases of Pt single crystals with size about 3 nm embedded in an amorphous carbonaceous matrix. Due to the semiconducting character of the carbon matrix, which is the main component of the deposit, we propose that the transport results can be mapped onto those obtained in semiconducting materials with different degrees of doping. The different transport properties of Pt by FEBID and FIBID are attributed to the higher doping level in the case of FIBID, as given by composition measurements obtained with energy-dispersive X-ray microanalysis.

We investigate the mechanism of spin-lattice relaxation of Er ions encapsulated in polyoxometalate clusters, which below 4 K can only reverse its spin via quantum tunneling processes. The temperature-independent rate −1 is, at zero field, ten orders of magnitude larger than the rates predicted for direct phonon-induced processes. In addition, we observe that −1 is suppressed by external magnetic bias and hyperfine interactions but enhanced by increasing the concentration of Er ions. The observed relaxation agrees with predictions for pure quantum tunneling, showing that this phenomenon drives the thermalization of electronic spins. A possible link between these two phenomena is discussed, involving the collective emission of phonons from particular spin configurations attained via quantum tunneling. The fundamental equations of magnetism, including Curie's law, rely on the ability of magnetic moments to attain thermal equilibrium with the solid lattice. In spite of the progress achieved in studying and manipulating individual spins in solids, 1 the spin-lattice relaxation ͑SLR͒ mechanisms are not well understood yet. An intriguing situation arises, near zero field, for strongly anisotropic spins, e.g., magnetic molecular clusters or rare-earth ions. When thermally activated tunneling processes 2 die out, at sufficiently low temperatures ͑typically T Շ 1 K͒, spins can only flip by pure quantum tunneling ͑QT͒ across the anisotropy energy barrier. Theoretical descriptions 3-5 of QT in the presence of hyperfine couplings and dipolar spin-spin interactions account well for experiments that measure the time-dependent magnetization under such conditions. 6-8 Concerning SLR, a major difficulty arises. It stems from the fact that QT modifies the magnetization but conserves the energy of the ensemble of nuclear and electronic spins. Therefore, equilibrium states might well be reached long after the characteristic time scales of QT. However, a few experiments suggest otherwise. Specificheat studies 10 indicate that Mn 4 and Fe 8 single-molecule magnets ͑SMM͒ attain thermal equilibrium at rates comparable to those found in magnetization relaxation experiments. In addition, NMR experiments on Mn 12 clusters 11 show that the nuclear spin and bath temperatures remain the same down to the neighborhood of absolute zero.In order to elucidate the nature of the SLR mechanism and its relationship with QT, direct measurements of the SLR rates as a function of temperature, magnetic field, concentration of spins, etc., are clearly desirable. Studying the SLR of molecular nanomagnets at very low temperatures and under weak magnetic fields is, however, a demanding experimental task because tunneling time scales are on the order of days even for clusters made of a few atoms. In order to overcome this difficulty, simpler molecules need to be studied. In the present work, we report the SLR rates of polyoxometalate ͑POM͒ clusters containing individual lanthanide ions. The results evidence that the thermalization of electronic spins is dictat...

The polyoxometalate clusters with formula [Gd(W(5) O(18) )(2) ](9-) and [Gd(P(5) W(30) O(110) )](12-) each carry a single magnetic ion of gadolinium, which is the most widespread element among magnetic refrigerant materials. In an adiabatic demagnetization, the lowest attainable temperature is limited by the presence of magnetic interactions that bring about magnetic order below a critical temperature. We demonstrate that this limitation can be overcome by chemically engineering the molecules in such a way to effectively screen all magnetic interactions, suggesting their use as ultra-low-temperature coolers.

We show that a crystal of mesoscopic Fe(8) single-molecule magnets is an experimental realization of the quantum Ising model in a transverse field, with dipolar interactions. Quantum annealing has enabled us to explore the quantum and classical phase transitions between the paramagnetic and ferromagnetic phases, at thermodynamical equilibrium. The phase diagram and critical exponents that we obtain agree with expectations for the mean-field universality class.

We show that the dynamic magnetic susceptibility and the superparamagnetic blocking temperature of an Fe8 single molecule magnet oscillate as a function of the magnetic field Hx applied along its hard magnetic axis. These oscillations are associated with quantum interferences, tuned by Hx, between different spin tunneling paths linking two excited magnetic states. The oscillation period is determined by the quantum mixing between the ground S = 10 and excited multiplets. These experiments enable us to quantify such mixing. We find that the weight of excited multiplets in the magnetic ground state of Fe8 amounts to approximately 11.6 %. PACS numbers:High-spin molecular clusters [1, 2] display superparamagnetic behavior, very much as magnetic nanoparticles typically do. Below a time-(or frequency-)dependent blocking temperature T b , the linear magnetic response "freezes" [3,4] and magnetization shows hysteresis [5]. The slow magnetic relaxation of these single-molecule magnets (SMMs) arises from anisotropy energy barriers separating spin-up and spin-down states. Because of their small size, the magnetic response shows also evidences for quantum phenomena, such as resonant spin tunneling [3,[6][7][8]. In the case of molecules that, like Fe 8 (cf Fig. 1A and [4]), have a biaxial magnetic anisotropy, tunneling between any pair of quasi-degenerate spin states ±m can proceed via two equivalent trajectories, which, as illustrated in Fig. 1B, cross the hard anisotropy plane close to the medium anisotropy axis. A magnetic field along the hard axis changes the phases of these tunneling paths, leading to either constructive or destructive interferences. This phenomenon is known as Berry phase interference [9,10].Experimental evidences for the ensuing oscillation of the quantum tunnel splitting ∆ m , shown in Fig. 1C, were first observed in Fe 8 [11,12] and then in some other SMMs [13][14][15][16][17][18][19] by means of Landau-Zener magnetization relaxation experiments. Interference patterns measured on Fe 8 at very low temperatures, which correspond to tunneling via the ground state doublet m = ±10, are reproduced by the following spin Hamiltonian C/k B = −2.9 × 10 −5 K are magnetic anisotropy parameters, and g = 2. The sizeable fourth-order parameter C reflects not only the intrinsic anisotropy but, mainly, it parameterizes quantum mixing of the S = 10 with excited multiplets (S-mixing) and how it influences quantum tunneling via the ground state [20].In the present paper, we study the influence of Berry phase interference on the ac magnetic susceptibility χ and T b of Fe 8 , that is, on those quantities that characterize the standard SMM (or superparamagnetic) behavior. Close to T b , magnetic relaxation is dominated by tunneling near the top of the anisotropy energy barrier, thus also near excited multiplets with S = 10. In this way, we

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III : Tb, Dy, Ho, Er, Tm, and Yb). -The compounds (III), (V), and (VIII) are characterized by static and dynamic magnetic measurements. (IIIc), (V), (VIIIb-d), and (VIIIf) show magnetic relaxation properties typical of single molecule magnets. As shown by single crystal XRD (V) crystallizes in the triclinic space group P1 with Z = 2 and compounds (VIIId) and (VIIIe) in the monoclinic space group P21/c with Z = 4. (V) is formed by two anionic [W5O18] 6moieties sandwiching the central Er 3+ ion. (VIIId) and (VIIIe) are composed of a central Ln 3+ encapsulated by two (β2-SiW11O39) 8units in a distorted square-antiprismatic coordination. Compounds (IIIa-c) and (V), and (VIIIa-f) are isostructural, respectively. -(ALDAMEN, M. A.; CARDONA-SERRA, S.; CLEMENTE-JUAN, J. M.; CORONADO*, E.; GAITA-ARINO, A.; MARTI-GASTALDO, C.; LUIS, F.; MONTERO, O.; Inorg.