Giant Magnetoresistance Phenomenon in Manganates

Abstract

The cover picture shows various stages of the mineralization/coagulation process in the kinetically controlled formation of gold nanoparticles (yellow spheres) in a microemulsion of polystyrene-block-poly(2-vinylpyridine) (green-and-white strands) in toluene. The transmission electron micrographs and the superimposed UV/Vis absorbance spectra correspond to the states with one gold particle per micelle (top) and to aggregated micelles containing two (middle) or three (bottom) gold particles. More about this novel method of stabilizing defined metal particles in 1499-1504 Giant Magnetoresistance Phenomenon in Manganates Ferromagnetism Insulator -MetalThe many complex, interrelated phenomena that contribute to giant magnetoresistance (GMR) i n rare earth manganates Ln, -,A,MnO, (Ln = rare earth, A = divalent cation) are discussed. GMR is observed when these manganates become ferromagnetic and transform from an insulating state to a metallic state close to the Curie temperature. The essential features of magnetoresistance can be understood on the basis of the double-exchange mechanism, but this is too simplistic to account for all the observed data. 1509-1513Oxygen and Nitrogen in Competitive Situations: Which Is The Hydrogen-Bond Acceptor?By utilizing a rapid redox reaction between liquid MOO, and "N3-" [Eqs. (1) and (2)], we have been able to perform the first high-temperature oxide melt solution calorimetry on ternary metal nitrides. The energetics of the Ca/Zn/N system indicate that Ca,N, acts as a basic nitride analogous to CaO in oxide systems, while Zn,N, acts as an acidic nitride (''N3-" acceptor). Further study of ternary nitride systems should enable classification of binary nitrides into those of acidic or basic character, and possibly lead to the design and synthesis of new materials. AS2-AS1 AS3-ASZ-ASI AS4-AS3-AS2-ASlA matched set of isomers-porphyrin (l), porphycene (Z), and corrphycene (3)-allow various issues in porphyrin chemistry to be explored. In this paper the doubly protonated forms of these macrocycles, studied as their tetraethyl-tetramethyl derivatives, are compared. as the formal oxidant has been developed. In this mild ruthenium-catalyzed procedure, which utilizes catalyst/substrate ratios as low as I/lOOO, alcohols with oxidation-sensitive functionalities can be selectively oxidized to ketones (above). The cleavage of an amide bond catalysed aspartylglucosaminidase (AGA) has been investigated with ab initio quantum mechanical calculations (model used shown on the right). It is shown that catalytic mechanisms similar to those of serine proteases are feasible for AGA and other N-terminal nucleophilic (Ntn) amidohydrolases. It is also suggested that the protonated cr-amino group of the substrate may be important for the catalytic activity of AGA. Kinetically controlled formation of gold nanoparticles in a microemulsion of polystyrene-block-poly(2-vinylpyridine) in toluene is described for the first time. Various stages of the mineralization/coagulation process could be stabilized, namely,...

Polyoxometalates: Very Large ClustersNanoscale Magnets

“…It has long been known that heteropoly anions can be reduced to mixed-valence species (heteropoly blues and browns) in which the excess electrons can be fully or partially delocalized, giving rise to delocalized−delocalized, delocalized−localized, and localized−localized interactions . The problem of the interplay of exchange and delocalization effects is another popular topic at the moment, the interest ranging from the magnetic properties of metallo enzymes and metallo proteins, like iron−sulfur proteins and the water oxidizing center of photosystem II of bacterial photosynthesis, to colossal magnetoresistance effects observed in lanthanide manganites . Again the possibility of fine-tuning the structural features of the polyoxometalates controlling the relative ratios of mixed-valence species and the nature of the interaction, corresponding to class II and class III of the Robin and Day classification, makes these materials ideal candidates for modeling these interactions and providing a testing grounds for theories.…”

Manganese clusters: a common ground for photosynthesis, quantum tunnelling of the magnetization and colossal magnetoresistance *

“…5 Further these systems have been shown to undergo quantum tunnelling of the magnetization, 6 which is currently much investigated, also with the aim of finding suitable new materials for the development of future quantum computers. Finally mixedvalence Mn III ᎐Mn IV perovskites are actively investigated 7 because they have been found to change dramatically their electric resistance in the presence of an applied magnetic field, giving rise to the so called colossal magnetoresistance. 8 We feel that an overview of the relevance of simple manganese clusters to such diverse fields can be of interest and potentially cross fertilizing.…”

“…(Ϫ81/4 ϩ 1/4)D ≈ 6 cm Ϫ1 . The relaxation time of the magnetization is expected to depend on temperature as shown in equation (7),…”

Electrochemistry in Magnetic Fields

“…The magnetoresistance phenomenon may change the catalysts' resistance, thus influencing the overall catalytic performance. [66] However, it should not affect powder catalysts substantially because conductive carbon is usually added to the catalyst ink to conduct electrons.…”