Magnetoresistance and collective Coulomb blockade in super-lattices of ferromagnetic CoFe nanoparticles

We report on transport properties of millimetric super-lattices of CoFe nanoparticles surrounded by organic ligands. R(T)s follow R(T) = R_0.exp(T/T_0)^0.5 with T_0 ranging from 13 to 256 K. At low temperature I(V)s follow I=K[(V-V_T)/V_T]^ksi with ksi ranging 3.5 to 5.2. I(V) superpose on a universal curve when shifted by a voltage proportional to the temperature. Between 1.8 and 10 K a high-field magnetoresistance with large amplitude and a strong voltage-dependence is observed. Its amplitude only depends on the magnetic field/temperature ratio. Its origin is attributed to the presence of paramagnetic states present at the surface or between the nanoparticles. Below 1.8 K, this high-field magnetoresistance abruptly disappears and inverse tunnelling magnetoresistance is observed, the amplitude of which does not exceed 1%. At this low temperature, some samples display in their I(V) characteristics abrupt and hysteretic transitions between the Coulomb blockade regime and the conductive regime. The increase of the current during these transitions can be as high as a factor 30. The electrical noise increases when the sample is near the transition. The application of a magnetic field decreases the voltage at which these transitions occur so magnetic-field induced transitions are also observed. Depending on the applied voltage, the temperature and the amplitude of the magnetic field, the magnetic-field induced transitions are either reversible or irreversible. These abrupt and hysteretic transitions are also observed in resistance-temperature measurements. They could be the soliton avalanches predicted by Sverdlov et al. [Phys. Rev. B 64, 041302 (R), 2001] or could also be interpreted as a true phase transition between a Coulomb glass phase to a liquid phase of electrons

Magnetic hyperthermia in single-domain monodisperse FeCo nanoparticles: Evidences for Stoner-Wohlfarth behaviour and large losses

We report on hyperthermia measurements on a colloidal solution of 15 nm monodisperse FeCo nanoparticles (NPs). Losses as a function of the magnetic field display a sharp increase followed by a plateau, which is what is expected for losses of ferromagnetic single-domain NPs. The frequency dependence of the coercive field is deduced from hyperthermia measurement and is in quantitative agreement with a simple model of non-interacting NPs. The measured losses (1.5 mJ/g) compare to the highest of the literature, though the saturation magnetization of the NPs is well below the bulk one.Comment: 14 pages, 3 figure

Tuning complex shapes in Pt(0) nanoparticles : from cubic dendrites to five-fold stars

A platinum star performance: Quasi-single-crystalline Pt nanoparticles with peculiar morphologies—cubic dendrites, planar tripods, and fivefold stars—were synthesized in high yield. Shape selectivity was achieved by finely tuning the growth kinetics under a dihydrogen atmosphere

Chemical Ordering in Bimetallic FeCo Nanoparticles: From a Direct Chemical Synthesis to Application As Efficient High-Frequency Magnetic Material

Single-crystalline FeCo nanoparticles with tunable size and shape were prepared by co-decomposing two metal-amide precursors under mild conditions. The nature of the ligands introduced in this organometallic synthesis drastically affects the reactivity of the precursors and, thus, the chemical distribution within the nanoparticles. The presence of the B2 short-range order was evidenced in FeCo nanoparticles prepared in the presence of HDAHCl ligands, combining 57 Fe Mössbauer, zero-field 59 Co ferromagnetic nuclear resonance (FNR), and X-ray diffraction studies. This is the first time that the B2 structure is directly formed during synthesis without the need of any annealing step. The as-prepared nanoparticles exhibit magnetic properties comparable with the ones for the bulk (M s = 226 Am 2 ·kg -1 ). Composite magnetic materials prepared from these FeCo nanoparticles led to a successful proof-of-concept of the integration on inductor-based filters (27% enhancement of the inductance value at 100 MHz)

Modeling Molecular Interactions in Water: From Pairwise to Many-Body Potential Energy Functions.

Almost 50 years have passed from the first computer simulations of water, and a large number of molecular models have been proposed since then to elucidate the unique behavior of water across different phases. In this article, we review the recent progress in the development of analytical potential energy functions that aim at correctly representing many-body effects. Starting from the many-body expansion of the interaction energy, specific focus is on different classes of potential energy functions built upon a hierarchy of approximations and on their ability to accurately reproduce reference data obtained from state-of-the-art electronic structure calculations and experimental measurements. We show that most recent potential energy functions, which include explicit short-range representations of two-body and three-body effects along with a physically correct description of many-body effects at all distances, predict the properties of water from the gas to the condensed phase with unprecedented accuracy, thus opening the door to the long-sought "universal model" capable of describing the behavior of water under different conditions and in different environments

Insight into the mechanism of decarbonylation of methanol by ruthenium complexes; a deuterium labelling study

The authors thank Lucite international for funding.In the reaction of [RuHClP3] (P = PPh3) with NaOMe in methanol, the product is [RuH2(CO)P3]. Short reaction times show that the final product is formed through [RuH4P3] as the major intermediate. Using NaOCD3 in CD3OD, the first formed product is [RuH4P'3] (P' is PPh3 partially deuterated in the ortho positions of the aromatic rings). Further reaction leads to a mixture of [RuHnD2-n(CO)P3] (n = 0, 22 %; n = 1, 2 isomers each 28 %; n = 2, 22 %). Mechanistic aspects of both steps of the reaction are explored and, together with previously published calculations, they provide definitive mechanisms for both dehydrogenation and decarbonylation in these interesting systems.PostprintPeer reviewe

Chemical control of structural and magnetic properties of cobalt nanoparticles

Oxide-free cobalt nanoparticles (NPs) of 2.5?3 nm diameter have been synthesized in the presence of di-isobutyl aluminum hydride (DiBAH), a potential alumina source, from two different precursors:? Co(?3C8H13)(?4C8H12) and Co[N(SiMe3)2]2. The NPs display very different structural and magnetic features, which are related to the difference in chemical environment at the NPs surface

Catalytic mechanism of 4-oxalocrotonate tautomerase: significances of protein-protein interactions on proton transfer pathways

4-oxalocrotonate tautomerase (4-OT), a member of tautomerase superfamily, is an essential enzyme in the degradative metabolism pathway occurring in the Krebs cycle. The proton transfer process catalyzed by 4-OT has been explored previously using both experimental and theoretical methods, however, the elaborate catalytic mechanism of 4-OT still remains unsettled. By combining classical molecular mechanics with quantum mechanics, our results demonstrate that the native hexametric 4-OT enzyme, including six protein monomers, must be employed to simulate the proton transfer process in 4-OT due to protein-protein steric and electrostatic interactions. As a consequence, only three out of the six active sites in the 4-OT hexamer are observed to be occupied by three 2-oxo-4-hexenedioates (2o4hex), i.e., half-of-the-sites occupation. This agrees with experimental observations on negative cooperative effect between two adjacent substrates. Two sequential proton transfers occur: one proton from the C3 position of 2o4hex is initially transferred to the nitrogen atom of the general base, Pro1. Subsequently, the same proton is shuttled back to the position C5 of 2o4hex to complete the proton transfer process in 4-OT. During the catalytic reaction, conformational changes (i.e., 1-carboxyl group rotation) of 2o4hex may occur in the 4-OT dimer model but cannot proceed in the hexametric structure. We further explained that the docking process of 2o4hex can influence the specific reactant conformations and an alternative substrate (2-hydroxymuconate) may serve as reactant under a different reaction mechanism than 2o4hex