Magnetic properties of ball milled nanocrystalilne Ni and Fe

Thiolate-protected Ni39 and Ni41 nanoclusters: synthesis, self-assembly and magnetic properties

Thiolate-protected soluble nickel clusters, Ni(39)(SC(2)H(4)Ph)24 and Ni(41)(SC(2)H(4)Ph)25, were synthesized via a wet chemical method. The cluster formulae were identified by MALDI-TOF. Possible structures of the clusters were discussed. These clusters exhibit ferromagnetism with hysteresis loops in the 1.8-300 K range. By solvent evaporation, the clusters can self-assemble into simple cubic structured crystals with a width in the range of 1-10 μm and length up to 300 μm. These properties shed light on their application potentials in nanomagnetics working at room temperature.

Study of the magnetic microstructure of Ni/NiO nanogranular samples above the electric percolation threshold by magnetoresistance measurements

Magnetoresistance measurements have been exploited to gain information on the magnetic microstructure of two Ni/NiO nanogranular materials consisting of Ni nanocrystallites (mean size of the order of 10 nm) embedded in a NiO matrix and differing in the amount of metallic Ni, ~33 and ~61 vol%. The overall conductance of both samples is metallic in character, indicating that the Ni content is above the percolation threshold for electric conductivity; the electric resistivity is two orders of magnitude smaller in the sample with higher Ni fraction (10(-5) Ωm against 10(-3) Ωm). An isotropic, spin-dependent magnetoresistance has been measured in the sample with lower Ni content, whereas both isotropic and anisotropic magnetoresistance phenomena coexist in the other material. This study, associated with magnetization loop measurements and the comparison with the exchange bias effect, allows one to conclude that in the sample with lower Ni content neither the physical percolation of the Ni nanocrystallites nor the magnetic percolation (i.e., formation of a homogeneous ferromagnetic network) are achieved; in the other sample physical percolation is reached while magnetic percolation is still absent. In both behaviors, a key role is played by the NiO matrix, which brings about a magnetic nanocrystallite/matrix interface exchange energy term and rules both the direct exchange interaction among Ni nanocrystallites and the magnetotransport properties of these nanogranular materials.

Domain structures of ultrafine grained ferromagnets achieved by severe plastic deformation or melt quenching

Lorentz electron microscopy was used to analyse the domain structure in some ferromagnets having an ultrafine grained structure produced by high pressure torsion (pure Ni, Fe, Co, Mn(55.5)Al(45.5) C(0.5)) and by melt quenching (Fe(83)Nd(13)B(4)). For low anisotropy ferromagnets (Ni, Fe, Co), it has been established that the domain structure does not depend strongly on grain size. In high anisotropy ferromagnets alloy (Mn(55.5)Al(45.5) C(0.5) and Fe(83)Nd(13)B(4)), grain refinement causes changes in the domain structure configuration and affects the magnetization mechanism.

Nanocrystalline Metals Prepared by Electrodeposition

An overview is presented about the preparation of nanocrystalline metals by pulsed electrodeposition out of aqueous electrolytes and ionic liquids. Appropriate selection of the bath chemistry and the plating parameters provide the flexibility to control the crystallite size. For selected examples we demonstrate how these electrochemical methods can be used for the preparation of nanostructured bulk metals and nanoparticle for fuel cell applications. Electrochemical analytical methods like cyclic voltammetry and chronoamperometry are used to fundamentally understand the deposition mechanisms, the effect of bath additives on the micro- and nanostructure of the deposits and to characterize the catalytic activity of the electrocatalysts. Some improved physical properties of electrodeposited nanocrystalline metals are discussed, namely thermal stability of n-Fe, magnetisation of n-Ni and hardness of n-Al.