Pervasiveness of cluster excitations as seen in the Mössbauer spectra of magnetic materials

Magnetic cluster excitations in various physical systems (e.g., soliton bearing one-dimensional solids, metallic alloys, amorphous materials, small particle aggregates, magnetically ordered substances near T_C, transition metal di-chloride graphite intercalation compounds, etc.) are described. Use of Fe-57 Mössbauer effect spectroscopy as a probe of the spin dynamics for inverse autocorrelation times between 10⁷Hz to 10¹⁰ Hz is emphasized. Particular attention is given to systems which exhibit local or long range magnetic order and whose Mössbauer spectra must therefore be described by more than one autocorrelation (or dwell) time for fluctuations between different allowed hyperfine field directions on a given site.     

Applications of thermoelectrometry to metal-ion modified polyimide films

As part of a research program to decrease the electrical resistivity of polyimide films with ionic additives a variable temperature three probe electrical resistivity measurement system has been designed and constructed. Sample temperature, electrification time, atmosphere, and measurement mode are computer controlled. As a data interpretation aid, temperature cycled analysis can be routinely performed. Surface resistivities in the range 10³–10¹⁵ ohm and volume resistivities in the range 10⁵–10¹⁸ ohm-cm are theoretically measurable under well controlled experimental conditions from room temperature to 250°. The electrical resistivity measurement system is useful for the evaluation of polymer films or films in general. Application of the system for analysis of cobalt, lithium and tin ion-modified polyimide films and some experimental considerations are presented. Correlation of the electrical measurements with differential scanning calorimetry, thermomechanical analysis, and thermogravimetric analysis is demonstrated.

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Zusammenfassung Als Teil eines Forschungsprogramms zur Herabsetzung des elektrischen Widerstandes von Polyimidfilmen mit ionischen Additiven wurde ein 3-Proben-Meßsystem zur Ermittlung des elektrischen Widerstandes in Abhängigkeit von der Temperatur entworfen und konstruiert. Probentemperatur, Elektrisierungszeit, Atmosphäre und Meßmethoden waren computergesteuert. Als Dateninterpretationshilfe kann eine hinsichtlich der Temperatur zyklische Analyse routinemäßig ausgeführt werden. Die Oberflächenwiderstände sind im Bereich von 10³–10¹⁵ Ohm und die Volumenwiderstände im Bereich von 10⁵–10¹⁸ Ohm-cm unter gut kontrollierten experimentellen Bedingungen von Raumtemperatur bis 250° theoretisch meßbar. Das elektrische Widerstandmeßsystem ist nützlich für die Bewertung von Polymerfilmen oder Filmen im allgemeinen. Die Anwendung des Systems zur Analyse von mit Co-, Li- und Sn-Ionen modifizierten Polyimidfilmen und einige experimentelle Hinweise werden angegeben. Es wird gezeigt, daß die mit dem beschriebenen System erhaltenen Resultate mit durch DSC, thermomechanische Analyse und TG erhaltenen Werten korrelieren.

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, , . , , . . 10³–10¹⁵ , 10⁵–10¹⁸ 250°. , . , , , . , .

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The financial support of the National Aeronautics and Space Administration is gratefully appreciated. The assistance of John Swartzentruber in performing many of the measurements is recognized.     

Invar Behavior in Fe-Ni Alloys is Predominantly a Local Moment Effect Arising from the Magnetic Exchange Interactions Between High Moments

I argue that the main models that have been advanced to explain Invar behavior in Fe-Ni alloys (the original, classical, Invar system) can all be shown to be critically deficient, except one: The local moment frustration model of Rancourt and Dang ( Phys. Rev. B , 54 , 12225, 1996). The latter model explains all the measured structural, magnetic, and magnetovolume features of the Fe-Ni alloys with 0-65 apc (atomic percent) Fe, based on the assumptions that these systems are predominantly high-moment in character at the temperatures of interest and that the Fe-Fe pairs have large inter-atomic separation dependencies of their magnetic exchange parameters. The large magnetovolume Fe-Fe couplings are understood (based on ab initio electronic structure calculations) as a precursor effect of the low-moment/high-moment (LM/HM) transition that has recently been observed to occur at larger Fe concentrations, as a continuous transition occurring in the range , 65-75 apc Fe (Lagarec, Ph.D. thesis, 2001).     

Miscibility of blends of poly(2,6-dimethyl 1,4-phenylene oxide) with polystyrene-based ionomers and copolymers

Blends of poly(2,6-dimethyl 1,4-phenylene oxide) (PPhO) with the copolymer poly(styrene-co-methacrylic acid) (PS-MAA) and the ionomer poly(styrene-co-sodium methacrylate) (PS-MAA-Na), up to 10 mol% co-unit content, were investigated by dynamic mechanical thermal measurements. The PPhO/PS-MAA-Na blends are compared with PS/PS-MAA-Na blends. The blends of PPhO with PS-MAA are no longer miscible at 10 mol% acid content; this is attributed to a copolymer effect induced by the reduction of PS-PPhO interactions due to the presence of the MAA group which does not interact favorably with PPhO. The blends of PPhO with the ionomer are already immiscible at the lowest ion content studied (2.4 mol%), but become increasingly so as ion content is increased. Despite favorable PS-PPhO interactions, these blends are only a little more miscible than the PS/PS-MAA-Na blends. This is attributed to a combination of the increasing importance of the ionomer cluster phase (from which the homopolymer chains presumably are excluded) as ion content is increased, and of a copolymer effect between the homopolymers and the unclustered phase of the ionomer. These results are compared with published data indicating that blends of PPhO with another biphasic ionomer, zinc sulfonated polystyrene, are miscible. The contrasting behavior is rationalized in part by the suggestion that the copolymer effect between PPhO and the unclustered phase of the latter ionomer, but not of the former, is absent; this is related to multiplet structure and sizes. The analysis made of the above systems is extended to predict what might be the miscibility behavior between PPhO and other PS-based ionomer and related copolymer systems. © 1993 John Wiley & Sons, Inc.     

Failure of the direct HFD extraction method

It is well known that, in principle, hyperfine field distributions can be obtained from Mössbauer spectra by direct extraction using simple matrix inversion. We explore the limits of this method in terms of the spectral signal/noise ratio, elemental sextet parameter uncertainties and matrix dimensions.On leave from: Department of Materials Physics, University of Science and Technology Beijing, Beijing 100083, PR China.     

Determination of cis and trans Fe2+ populations in 2M1 muscovite by Mössbauer spectroscopy

Specimens of muscovite from Siluro-Devonian Appalachian granites of the Gander zone in New Brunswick were studied by ⁵⁷Fe Mössbauer spectroscopy, microprobe analysis and X-ray powder diffractometry. Chemical compositions, corresponding structural formulae and powder patterns indicate that they are dioctahedral true micas of 2M₁ polytype. Mössbauer spectroscopy shows that these muscovites fall into two groups having distinct spectra, despite an absence of systematic differences in their chemical compositions, X-ray patterns, unit-cell parameters, and Fe³⁺/Fe_total ratios. In the first group, two distinct and well-resolved ^viFe²⁺ spectral contributions occur whereas, in the second group, a single but broader ^viFe²⁺ contribution occurs. All spectra from both groups have ^viFe³⁺ contributions. These observations are confirmed by quadrupole splitting distribution (QSD) analyses of the spectra. Spectra from the first group clearly show a bimodal distribution of quadrupole splittings for Fe²⁺, with a dominant contribution at ~3.0 mm/s and a minor one at ~2.1 mm/s. In the second group, the spectra show a broad unimodal distribution of QSDs for Fe²⁺. We attribute the 3.0 and 2.1 mm/s QSD components to Fe²⁺ in cis and trans octahedral sites, respectively. Muscovites from our second group may have Fe²⁺ in both cis and trans sites but these cannot be resolved, as is usually the case, for example, with trioctahedral micas. In group one, cis/trans populations provide measures of the degree of cation order and of the density of vacancies on the cis sites. Simple models based on average unit cell site dimensions are found not to hold. Local effects seem to dominate, with Fe²⁺ showing no systematic preference for cis or trans sites.     

Magnetically induced electric field gradients

Several investigations of the Mössbauer spectrum of Fe₂As have yielded results which do not agree with those obtained by neutron diffraction studies. We report here a new investigation, and we show that the earlier disagreement is caused by a lack of appreciation of several factors. These include fluctuating hyperfine magnetic fields (relaxation) and an asymmetry of the electric field gradient produced by the spontaneous magnetization.On leave of absence at Laboratoire de Spectroscopie Hertzienne de l'Ecole Normale Superieure, 24 Rue Lhomond, 75005 Paris, France     

Absolute quantitative analysis by Mössbauer spectroscopy

We develop a method for measuring the Fe-57 Mössbauer absorber thickness parameter t_a ≡ n_af_aσ_o for the case of Zeeman split hyperfine patterns with well-separated lines. The method is independent of experimental conditions such as: counting efficiency, electronic window settings, absorber thickness, temperature, source lineshape, background level, velocity calibration, absorber orientational texture, source strength, ordinary mass absorption, etc. Only the raw spectral data are needed. We have tested the method in detail with metallic foils having various thicknesses between t_a=2 and 35. It works well and can be generalized to other types of spectra.