Direct observation of local Mn-Mn distances in the paramagnetic compound CsMnxMg1-xBr3

We introduce a novel method for local structure determination with a spatial resolution of the order of 0.01 Angstroem. It can be applied to materials containing clusters of exchange-coupled magnetic atoms. We use neutron spectroscopy to probe the energies of the cluster excitations which are determined by the interatomic coupling strength J. Since for most materials J is related to the interatomic distance R through a linear relation dJ/dR={\alpha} (for dR/R<<1), we can directly derive the local distance R from the observed excitation energies. This is exemplified for the mixed one-dimensional paramagnetic compound CsMnxMg1 xBr3 (x=0.05, 0.10) containing manganese dimers oriented along the hexagonal c-axis. Surprisingly, the resulting Mn-Mn distances R do not vary continuously with increasing internal pressure, but lock in at some discrete values.Comment: 16 pages, 2 tables, 3 figure

Determination Of The Orientational Order Parameters In Polysilane Lb-Films By Measuring Third-Order-Susceptibility Tensor Components

We demonstrate a new method for investigating the orientational distribution of the rod-like poly[bis(m-butoxyphenyl) silane] molecules in Langmuir-Blodgett (LB) films. The first two non-trivial in-plane orientational order parameters C2 and C4 could be easily deduced by measuring the independent tensor components of χ(3)(- approximately ega 3; approximately ega1, approximately ega1, approximately ega2). Using this method, we show that the annealing process improves the molecular alignment along the dipping direction. We were also able to get information about the domain morphology of the film from the off-diagonal components of χ(3)

Preparation Of Oriented Multilayers Of Poly(Silane)S By The Langmuir-Blodgett Technique

The spreading behavior of poly(silane)s at the air-water interface of a Langmuir trough was investigated. Force-area diagrams of poly(silane)s with various side groups and different molecular weights were recorded. Stable monomolecular layers were not obtained for alkyl substituents. In the case of p-alkoxy-phenyl and, even better, m-alkoxyphenyl residues attached to the silicon backbone, stable monolayers were obtained that could be transferred to solid hydrophobic substrates by applying the Langmuir-Blodgett technique. These transferable poly(silane)s have a rodlike shape and form dense rafts at the water surface. Multilayers consisting of up to 600 monolayers of this raftlike structure with the rods preferentially oriented into the dipping direction have been obtained. The order parameter describing the degree of orientation of the rods with regard to the dipping direction was determined from the dichroic ratio of the band of the silicon-silicon chromophores at 397 nm for the m-alkoxyphenyl-substituted poly (silane). The order parameter could be improved substantially by annealing the samples subsequent to deposition of the layers at temperatures up to 150 °C. Samples thus obtained have the character of a monodomain nematic liquid crystal of molecularly defined thickness. Details of the structure were further elucidated by X-ray reflection and polarized IR spectroscopy. © 1991, American Chemical Society. All rights reserved

Determination Of The Two First Non-Trivial Orientational Order Parameters In Lb Films Of Rod-Like Molecules By Third-Order Sum Frequency Mixing

We demonstrate a new method for investigating orientational order in Langmuir-Blodgett films built from rod-like poly [bis(m-butoxyphenyl) silane] molecules. Two laser beams with different input frequencies ω1 and ω2 were collinearly overlapped to generate mixed light at the frequency 2ω1+ω2 using all possible permutations of the beam polarizations. From the independent tensor components of χ(3) (- ω3; ω1, ω1, ω2) the first non-trivial in-plane order parameters C2 and C 4 could be deduced with high accuracy. The orientational order could be well described by a two-dimensional diffusion model. © 1993

Characterization of the Grain Size in Ferromagnetic Colloids: Comparing Torsional-Pendulum Measurements with Standard Complementary Methods

A recently introduced shear-flow-free method for measuring the rotational viscosity of a resonantly forced torsional pendulum is used to determine the transverse magnetic relaxation time in magnetite and cobalt-based ferrofluids. From these data the average size of the ferromagnetic grains and their hydrodynamic diameter (core plus surfactant coating) are deduced under in-situ conditions, i.e. without diluting the sample. The reliability of the method is demonstrated by comparing the results with those of the complementary techniques of magneto-granulometry, X-ray diffraction, electron microscopy, and photon-correlation spectroscopy