Agustín Camón

Cryogenic Magnetocaloric Effect in a Ferromagnetic Molecular Dimer

Over the last few years, great interest has emerged in the synthesis and magnetothermal studies of molecular clusters based on paramagnetic ions, often referred to as molecular nanomagnets, in view of their potential application as lowtemperature magnetic refrigerants. What makes them promising is that their cryogenic magnetocaloric effect (MCE) can be considerably larger than that of any other magnetic refrigerant, for example, lanthanide alloys and magnetic nanoparticles. The MCE is the change of magnetic entropy (DSm) and related adiabatic temperature (DTad) in response to the change of applied magnetic field, and it can be exploited for cooling applications via a field-removal process called adiabatic demagnetization. Although the MCE is intrinsic to any magnetic material, in only a few cases are the changes sufficiently large to make them suitable for applications. The ideal molecular refrigerant comprises the following key characteristics: 1) a large spin ground state S, since the magnetic entropy amounts to R ln(2S+1); 2) a negligible magnetic anisotropy, which permits easy polarization of the net molecular spins in magnetic fields of weak or moderate strength; 3) the presence of low-lying excited spin states, which enhances the field dependence of the MCE owing to the increased number of populated spin states; 4) dominant ferromagnetic exchange, favoring a large S and hence a large field dependence of the MCE; 5) a relatively low molecular mass (or a large metal/ligand mass ratio), since the nonmagnetic ligands contribute passively to the MCE. Although this last point is crucial for obtaining an enhanced effect, it has beenmostly ignored to date. Molecular cluster compounds tend to have a very low magnetic density because of the large complex structural frameworks required to encase the multinuclear magnetic core. Herein we propose a drastically different approach by focusing on the simple and well-known ferromagnetic molecular dimer gadolinium acetate tetrahydrate, [{Gd(OAc)3(H2O)2}2]·4H2O (1). [4a,b] The structure of 1 (Figure 1) com-

Origin of slow magnetic relaxation in Kramers ions with non-uniaxial anisotropy

Transition metal ions with long-lived spin states represent minimum size magnetic bits. Magnetic memory has often been associated with the combination of high spin and strong uniaxial magnetic anisotropy. Yet, slow magnetic relaxation has also been observed in some Kramers ions with dominant easy-plane magnetic anisotropy, albeit only under an external magnetic field. Here we study the spin dynamics of cobalt(II) ions in a model molecular complex. We show, by means of quantitative first-principles calculations, that the slow relaxation in this and other similar systems is a general consequence of time-reversal symmetry that hinders direct spin-phonon processes regardless of the sign of the magnetic anisotropy. Its magnetic field dependence is a subtle manifestation of electronuclear spin entanglement, which opens relaxation channels that would otherwise be forbidden but, at the same time, masks the relaxation phenomenon at zero field. These results provide a promising strategy to synthesize atom-size magnetic memories.

Single-molecule magnetic behavior in a neutral terbium(III) complex of a picolinate-based nitronyl nitroxide free radical

The terdentate anionic picolinate-based nitronyl nitroxide (picNN) free radical forms neutral and robust homoleptic complexes with rare earth-metal ions. The nonacoordinated Tb(3+) complex Tb(picNN)(3)·6H(2)O is a single-molecule magnet with an activation energy barrier Δ = 22.8 ± 0.5 K and preexponential factor τ(0) = (5.5 ± 1.1) × 10(-9) s. It shows magnetic hysteresis below 1 K.

Molecular prototypes for spin-based CNOT and SWAP quantum gates

We show that a chemically engineered structural asymmetry in [Tb2] molecular clusters renders the two weakly coupled Tb3+ spin qubits magnetically inequivalent. The magnetic energy level spectrum of these molecules meets then all conditions needed to realize a universal CNOT quantum gate. A proposal to realize a SWAP gate within the same molecule is also discussed. Electronic paramagnetic resonance experiments confirm that CNOT and SWAP transitions are not forbidden.

Hybrid Magnetic/Superconducting Materials Obtained by Insertion of a Single‐Molecule Magnet into TaS2 Layers

N Among the hot topics in materials chemistry, the design of novel materials in which two or more physical properties of interest can be rationally combined into the same solid is currently attracting important attention. With this regard, the versatility of the molecular building-block approach, which relies on the combination of different molecular entities carrying specifi c functionalities into an extended molecule-based solid, has proven to be a very fruitful synthetic route. [ 1–5 ]

A hexacarboxylic open-shell building block: synthesis, structure and magnetism of a three-dimensional metal-radical framework

An octahedral hexacarboxylic polychlorotriphenylmethyl radical (PTMHC) and its hydrogenated precursor (αH-PTMHC) have been reacted with Cu(II) and 4,4′-bipyridine to prepare two isomorphic three-dimensional (3-D) coordination polymers of formula [Cu6(PTMHC)2(4,4′-bipy)3(H2O)12]n (1) and [Cu6(αH-PTMHC)2(4,4′-bipy)3(EtOH)6(H2O)6]n (2). Both 3-D structures can be described as two interpenetrating primitive cubic nets connected through bipyridine linkers, which defines an unusual topology with Schlafi symbol of (62·81)·(66·89). Magnetic properties of both metal–organic frameworks have been studied in detail. 2 shows weak antiferromagnetic interactions between Cu(II) ions at low temperature. In contrast, 1 reveals unexpected metal–radical ferromagnetic interactions (θ = 2.1 (2) K). Information on the existence of magnetic ordering and the nature of the ordered phase for 1 has been investigated via very low temperature magnetic susceptibility measurements. Surprisingly, the experimental data indicate the occurrence of 3-D antiferromagnetic ordering below 0.5 K. This latter phenomenon has been explained with specific heat measurements. Experimental results reveal the coexistence of relatively strong ferromagnetic interactions with weaker antiferromagnetic ones, mediated through the bipyridine linkers, which finally determine the low temperature magnetic structure. A deeper study of the data allows the analysis of the magnetic behavior of 1 as a 3-D antiferromagnet, with TC = 0.39 K, with ferromagnetic exchange interactions that do not propagate with equal strength along the three crystallographic directions above this temperature.

Size and dimensionality effects in superconducting Mo thin films

Molybdenum is a low Tc, type I superconductor whose fundamental properties are poorly known. Its importance as an essential constituent of new high performance radiation detectors, the so-called transition edge sensors (TESs) calls for better characterization of this superconductor, especially in thin film form. Here we report on a study of the basic superconducting features of Mo thin films as a function of their thickness. The resistivity is found to rise and the critical temperature decreases on decreasing film thickness, as expected. More relevant, the critical fields along and perpendicular to the film plane are markedly different, thickness dependent and much larger than the thermodynamic critical field of Mo bulk. These results are consistent with a picture of type II 2D superconducting films, and allow estimates of the fundamental superconducting lengths of Mo. The role of morphology in determining the 2D and type II character of the otherwise type I molybdenum is discussed. The possible consequences of this behaviour on the performance of radiation detectors are also addressed.

1:30000 cryogenic current comparator with optimum SQUID readout

We developed a 1:30000 Cryogenic Current Comparator for SET current amplification. A dedicated low-noise, directly-coupled SQUID was used for the readout, which allowed reaching a sensitivity close to ideal. The ratio error was <9/spl times/10/sup -9/. The CCC-SQUID equivalent current input noise was 3.0 fA/Hz/sup 1/2/, measured down to 0.1 Hz.

Effects of Stress and Morphology on the Resistivity and Critical Temperature of Room-Temperature-Sputtered Mo Thin Films

We report on the structural and electrical characterization of Mo thin films deposited at room temperature by RF magnetron sputtering. The effect of RF power on the morphology and residual stress of the films is analyzed. The films are under compressive stress and consist of densely packed columns with a lateral size on the order of 20 nm. The stress, critical temperature, and resistivity of the films are found to rise when increasing the ejected ion energy during the sputtering process. The changes in critical temperature and resistivity are discussed in terms of the observed morphology and stress changes.

Mo-Based Proximity Bilayers for TES: Microstructure and Properties

We report on the fabrication and characterization of Mo films, Mo/Au and Mo/Cu bilayers for Transition Edge Sensors (TES). The fabrication conditions (at room temperature) have been varied to achieve layers with the required properties for TES applications. The dependence of their functional properties (i.e. electrical resistivity and superconducting critical temperature) on microstructure (grain size, stress) is investigated.