Selective Excitation Double Mossbauer Research Articles

On the application of selective-excitation double Mössbauer spectroscopy to problems in materials science

Results showing the influence of optical thickness of a scatterer in selective-excitation double Mossbauer spectroscopy are presented on the example of α-Fe and FeBO 3 . Significant transformation of the spectral γ-radiation structure predicted theoretically is demonstrated for single-layered FeBO 3 .

A study of spin dynamics in the a-Fe90Sc10 spin glass

Spin dynamics in a-Fe90Sc10 have been examined with zero-field muon spin relaxation (ZF-μSR) and selective excitation double Mössbauer (SEDM) spectroscopy. Analysis of the μSR data yields Tsg∼105 K in agreement with both χac and conventional Mössbauer data. SEDM shows fluctuation rates in good agreement with μSR, but reveals an abrupt change in relaxation behavior from spin-flip to diffusive above 90 K (0.9Tsg).

Dynamics in fine particle magnets

Several magnetic line particle systems have been studied with selective excitation double Mossbauer (SEDM) spectroscopy. Static disorder, collective excitations, and superparamagnetic spin flips are all detected and theircontributions quantified. Zero-field muon spin relaxation spectroscopy has been used to confirm these observations in one sample. A theoretical description of SEDM spectra in static and dynamic disordered magnetic systems is presented.

Selective Excitation Double Mössbauer Spectroscopy

An improved selective excitation double Mossbauer spectrometer has been used to study both static and dynamic disorder in magnetic materials. Static disorder has been identified in a-Fe80B20 and Fe65Ni35, while magnetic relaxation has been measured in a Fe3O4 ferrofluid. Simultaneous static and dynamic disorder have been established in a-Fe92Zr8.

An improved selective excitation double Mössbauer spectrometer

The design, operation and performance of a selective excitation double Mössbauer (SEDM) spectrometer are described. An innovative drive-mounted conversion electron detector for scattered γ-ray energy analysis provides a three orders of magnitude improvement in counting efficiency over previous SEDM equipment. Simple digital circuitry together with Wissel Mössbauer velocity transducers and electronics supplies energy synchronization for indefinite collection times. Magnetic materials containing resonant Mössbauer nuclei (e.g., Fe57) can now be conveniently studied using SEDM, offering a unique insight into dynamic processes.

Superparamagnetic spin dynamics studied using selective excitation double Mössbauer spectroscopy.

A detailed study of spin dynamics in a 6 nm Fe3O4 ferrofluid using a substantially improved selective excitation double Mossbauer (SEDM) experimental technique has led to the unambiguous separation of static disorder, collective excitations, and moment reversals. Superparamagnetic spin flips have been observed through the appearance of an additional line in the SEDM spectrum, defining the energy transitions during relaxation, with frequencies of 2.5+/-0.3 MHz at 70 K to 9.7+/-1.0 MHz at 110 K. SEDM offers a precise window into the dynamics and blocking behavior of fine particle systems.

Selective excitation double Mössbauer spectroscopy: In search of magnetic relaxation

Selective excitation double Mössbauer (SEDM) spectroscopy has been used to unambiguously separate the influence of chemical disorder from time-dependent effects in two alloys studied at and above room temperature. a-Fe80B20 is a metallic glass with substantial chemical disorder, but no dynamic effects, and the SEDM spectrum can be fitted assuming only disorder broadening. Fe65Ni35 is a disordered alloy which may exhibit magnetic relaxation. The SEDM spectra of Fe65Ni35 are consistent with chemical disorder, and we rule out magnetic relaxation faster than 2.0±1.2 MHz at 496 K (0.99TC).

A moving microfoil conversion electron detector for selective excitation double Mössbauer experiments

A lightweight microfoil conversion electron detector has been built for use as a detector/analyser for selective excitation double Mossbauer experiments. The detector weighs approximately 40 g; it can be moved with constant acceleration by a conventional electromechanical drive, with peak velocities up to ±15 mm/s. Conversion electrons from resonant absorption events are collected from both sides of two 0.3 μm thick57Fe-enriched stainless steel foils inside the detector. The detector's performance (signal to noise per unit time) has been compared with a potassium ferrocyanide absorber and proportional counter, and it is found to be about ten times better for a given source strength and sampling angle. The overall detector linewidth is 0.42 (2) mm/s, compared with 0.37 (1) mm/s for the PFC absorber.

Selective excitation double Mössbauer measurements on an Fe−Ni invar alloy

Selective excitation double Mossbauer (SEDM) measurements on an Fe-34.0 at.% Ni invar alloy show narrow lines, indicating that time-dependent effects do not make a major contribution to the line broadening observed in conventional Mossbauer spectra of these alloys.

THE APPLICATION OF SELECTIVE EXCITATION DOUBLE MÖSSBAUER TECHNIQUES TO THE STUDY OF RELAXATION IN FERRICHROME A

The selective excitation double Mossbauer (SEDM) technique has been applied to the study of electronic relaxation in the polypeptide ferrichrome A. Calculations of the SEDM lineshape using the superoperator formalism were performed and compared with the experimental spectra at 300 K and 5.4 K. These results do not agree with the theoretical model of the relaxation mechanism in this material proposed by other investigators based on information obtained from transmission experiments. The study by the Mossbauer effect of time dependent hyperfine interactions such as those induced by electronic relaxation has become increasingly important to the understanding of the properties of biological and physical systems. Most of these studies are performed in transmission geometry. The advantages of using selective excitation double Mossbauer (SEDM) techniques for these investigations have been discussed / ] / and the technique has already been used to study a variety of phenomena /2,3/. However, no investigation of the line shape with relaxation occuring in the scatterer in a well characterized system, has been performed. Such a study is required for the complete understanding of SEDM spectra and is a prerequisite for the application of the SEDM technique to complex dynamic Fe environments. This is especially timely because recently several theoretical calculations of the SEDM lineshape have appeared in the literature /4,5,6/. These calculations were done for arbitrary electronic relaxation rates and general interaction Hamiltonians but were restricted to thin scatterers. We have extended this calculation to more realistic scatterers of arbitrary thickness 8, ($ = n0 f), so that our •experimental results could be compared with the theory. The thickness correction for scatterers without relaxation has previously been discussed (Balko, Hoy) /l/. Details of the calculation for SEDM lineshapes with relaxation in the scatterer On leave from George Mason University, Fairfax, VA 27030, U.S.A. will be presented in a future publication. In the present paper we will concentrate on the application of this theory to the study of electronic relaxation in ferrichrome A. Ferrichrome A is an iron containing polypeptide which has been studied in detail by transmission techniques 111 and thus serves as an appropriate model for our investigation. In the transmission geometry between IK and 300K it exhibits a Mossbauer relaxation spectrum characteristic of a paramagnetic material. At low temperatures the spectrum shows a split six line pattern. As the temperature increases the lines broaden and collapse to form a single line at 300K. This behavior as a function of temperature was inferred by Wickman et al. to occur because of electronic relaxation. They assumed that the Fe nucleus is subjected to a time dependent randomly varying effective hyperfine field. We show in this paper that such an adiabatic spin flip model is inadequate for a paramagnet like ferrichrome A. However, it does explain some general features of the relaxation lineshape obtained in the transmission geometry. To reveal more detailed information about the relaxation mechanism a more specific and descriminating technique needs to be employed. Our experimental arrangement is shown in figure 1. The constant velocity drive (CVD) was set to excite the ferrichrome A scatterer a particular energy. We used a 90 mCi Co source mounted in a Pd matrix and an enriched polycrystalline ferrichrome A (70 mg of Fe) scatterer. The scattered Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979204 c218 JOURNAL DE PHYSIQUE radiation was analyzed by a single line absorber (with 8 = 14) mounted on a constant acceleration drive (CAD) in the transmission geometry. ( DRIVE I 1 CO DRIVE 2 r l