Program: K7 Other Techniques and Data Evaluation
K7 Other Techniques and Data Evaluation

K7.1 Mössbauer Diffraction
B. Fultz
California Institute of Technology, Pasadena, USA

For diffraction experiments on materials, three types of waves have proved useful - x-rays, electrons and neutrons [1]. Mössbauer gamma-ray diffraction is a relatively new phenomenon. A Mössbauer diffraction pattern from a single crystal was first measured in 1964 [2]. Much of the subsequent work with Mössbauer diffraction has been performed in the dynamical limit with large, perfect crystals. Unfortunately, dynamical diffraction is of little use for determining atom arrangements in materials. Almost all structural studies of materials by diffraction require kinematical, rather than dynamical conditions.

It is possible to suppress dynamical diffraction by using small, imperfect crystals. Doing so unfortunately suppresses enormously the intensities of Mössbauer Bragg diffractions, presenting severe technical challenges. The first Mössbauer powder diffraction pattern from bcc 57Fe was measured with a Mössbauer diffractometer having a linear position-sensitive detector [3]. This instrument was useful for demonstration experiments, but not real investigations on materials. A new Mössbauer diffractometer employing a Bruker area detector has enabled measurements of from bcc 57Fe diffraction patterns in about 12 hours [4]. The spatial correlations of disordered chemical environments in Fe3Al have been measured, and are under interpretation.

The unique chemical environment selectivity of Mössbauer diffractometry differs from the chemical selectivity often obtained by the anomalous scattering of x-rays. Conventional chemical selectivity modifies the scattering factor of a particular species of atom with respect to other species, whereas chemical environment selectivity picks out different chemical environments of the same species of atom.

To test the calculated diffraction intensities in kinematical theory, Mössbauer diffraction peaks were measured from polycrystalline bcc 57Fe [4] with and without applied magnetic fields. The intensities of Mössbauer diffractions were calculated for polycrystalline samples in the kinematical limit, for the first time including interference with x-ray electronic scattering.

[1] B. Fultz and J. M. Howe, Transmission Electron Microscopy and Diffractometry of Materials (SpringerVerlag, 2001).
[2] P. J. Black and I. P. Duerdoth, Proc. Phys. Soc. 84 (1964) 169.
[3] T. A. Stephens, W. Keune, and B. Fultz, Hyperfine Interact. 92 (1994) 1095.
[4] U. Kriplani, J. Y. Y. Lin, M. Regehr, and B. Fultz, Phys. Rev. B 6502 (2): 4405 (2002).


K7.2 Emission Mössbauer Spectroscopy in CoO
K. Ruebenbauer and U.D. Wdowik
Institute of Physics, Pedagogical University, Cracow, Poland

Emission Mössbauer spectra of (57Co)CoO single crystal source were measured vs. temperature and oxygen partial pressure. Diffusional line broadening at temperatures exceeding ca. 1228 K was used to determine diffusion coefficient and activation energy of daughter iron following 57Co EC decay. Diffusion coefficient follows Arrhenius law with the activation energy 1.32(6) eV. The charge state of 57Fe is temperature and pressure dependent both. Ferric and ferrous lines are present at RT spectra. Divalent Fe tends to convert to trivalent one at high temperatures under oxidizing atmosphere. Fe2+ state was observed in the whole temperature range studied with the oxygen pressure ca. 10-4 atm. No line broadening was seen under low oxygen pressure, which suggests the vacancy mechanism of diffusion with vastly different pre-exponential factor depending upon the oxygen pressure. Metallic Co was obtained applying highly reducing atmosphere. Mössbauer spectra of metal were obtained for the temperature ranging from RT to 1074 K. A spontaneous magnetization is best described either applying spin-wave theory with 2.13 exponent or molecular field theory with S = 7/2. Hence, one can conclude that magnetic hyperfine field on Fe does not follow exactly free electron polarization.


K7.3 Comparative Transmission and Emission Mössbauer Studies on Various Perovskite-Related Systems
Z. Homonnay1, Z. Klencsár2, K. Nomura3, G. Juhász1, E. Kuzmann2, A. Nath4 and A. Vértes1,2
1) Department of Nuclear Chemistry, Eötvös Loránd University, Budapest, Hungary
2) MTA-ELTE Research Group on Nuclear Methods in Structural Chemistry, Budapest, Hungary
3) Graduate School of Engineering, the University of Tokyo, Tokyo, Japan
4) Department of Chemistry, Drexel University, Philadelphia, USA

Emission and transmission Mössbauer spectroscopy measurements are not directly comparable because of several aspects. The best known problem is the after effects of the 57Co(EC)57Fe decay which shows up in emission experiments mostly when insulators are studied. In the case of impurity Mössbauer spectroscopy, the introduction of Fe and Co into the investigated system is usually different: iron is usually added at the beginning of the synthesis, while Co is added by external doping followed by a treatment which is normally a repetition of the last synthesis step. This may affect the localization of the dopant atoms in the target material. Depending on the average atomic number of the host material in the source and absorber, the necessary concentration of the Mössbauer dopant for a feasible measurement may differ by several orders of magnitude. Either the system contains Fe and/or Co or not, the chemical difference between Fe and Co is another factor to keep in mind when the interpretation of the Mössbauer spectra is given.

In the past several years, we have encountered these problems in many different perovskite-related systems. Compounds including high temperature superconductors like (Y,Pr)Ba2Cu3O7-d and the Tl(Pb,Bi)1223 system, potential industrial CO2-absorbers as (Sr,Ca,Ba)(Fe,Co)O3-d, and lately (Ln,Sr)(Fe,Co)O3-d-type (Ln = La, Eu) materials showing colossal magnetoresistance have been investigated by both methods. The interpretation of the Mössbauer spectra will be reviewed in the light of the above-mentioned problems and new results will be presented on CMR materials.


K7.4 Trends in Polarimetry
K. Szymanski1, L. Dobrzynski1, S. Satula1 and B. Kalska-Szostko2
1) Institute of Experimental Physics, University of Bia³ystok, Bia³ystok, Poland
2) Institut für Experimentalphysik, Freie Universität Berlin, Berlin, Germany

It is known that the Mössbauer spectroscopy sensitive to the angular anisotropy of the polarised radiation presents a tool in determination of principal axes of the electric field gradient, as well as a direction and sign of the magnetic hyperfine field. We present construction of the source of monochromatic circularly polarised radiation, used for the aforementioned purposes. Its use is illustrated on two examples. First, the orientation of the 57Fe hyperfine magnetic field in Cr rich, bcc Cr-Fe-Mn disordered alloys is analysed. The experiments show that two types of Fe magnetic moments with various local magnetic susceptibility are present in the system. In another system, namely Er-Fe amorphous alloy, specific properties of antiferromagnetic orientation of Fe with respect to the Er moments are presented.

The powerfulness of nuclear local probe techniques is generally strongly limited by presence of distributions of the hyperfine fields. We have shown, that for the Mössbauer spectrum, in thin absorber approximation, averages of the integer powers of the velocity (so called velocity moments) are functions of the hyperfine fields only and do not depend on the structure of nuclear levels. This general result allows one to determine some averages of the hyperfine fields without a need of neither specification of the type of the transition nor determination of the shape of the distribution. The results are strict for mixed magnetic dipolar and electric quadrupolar interactions. This is illustrated by a number of examples in which determination of the projection of the average local magnetic field on the applied field direction, and determination of the sign of the electric field gradient is carried out.


K7.5 New Trends in Mössbauer Spectroscopy of Nanostructured Magnetic Materials: Evaluation, Theory and Methodology
M. Chuev1, A.M. Afanas'ev1, J. Hesse2 and O. Hupe2
1) Institute of Physics and Technology, Russian Academy of Sciences Moscow, Russia
2) Institut für Metallphysik und Nukleare Festkörperphysik, Technische Universität Braunschweig, Braunschweig, Germany

Several theoretical approaches recently developed to treat the Mössbauer spectra of nanocrystalline magnetic alloys (NCMA) will be discussed. First of them is a new method for analysis of complex Mössbauer spectra (called as [1]) within which the spectral model is not prescribed in advance, but is derived directly in the course of fitting the spectrum so that the number of lines appears to be an adjustable parameter. Along with the conventional representation of spectra as a superposition of single lines (quadrupole doublets or magnetic sextets) an additional Gaussian-type broadening of spectral lines (the Voigt profile) is used to describe continuous distributions of hyperfine parameters. In contrary to conventional methods for searching the distributions this approach allows one to evaluate the resulting hyperfine field distributions in NCMA with indication of their mean-square deviations [2]. The analysis results in a detailed information about the iron nanograins, the amorphous residual phase and the so-called interface zone in NCMA.

An alternative approach for analysis of Mössbauer spectra of NCMA is to treat them within relaxation effects. However, the conventional two-level relaxation model almost everywhere fails to describe experimental spectra even qualitatively. A generalisation of the two-level relaxation model recently suggested for description of the relaxation in a system of superparamagnetic particles [3] accounts for the diversity of relaxation Mössbauer spectra (including NCMA ones) allowing all the nonstandard features in the spectra to be described qualitatively. The generalized two-relaxation model seems to be rather efficient in describing specific shapes of Mössbauer spectra of NCMA, especially when the distribution over nanoparticle's sizes is simultaneously taken into account.

However, it may be difficult in some cases to distinguish between contributions of both the above-mentioned factors into Mössbauer spectra (even if the spectra are taken at different temperature). One of possible ways to solve the problem for NCMA is methodological: Mössbauer spectroscopy under radiofrequency (rf) field excitation proved to be rather informative [4], using which the transformation of the spectra could be traced as a function of the rf field amplitude and frequency as well as the relaxation parameters. Facilities of the method are strongly enhanced with prediction of new type of resonant phenomena: relaxation-stimulated resonance (RSR) in Mössbauer spectra of NCMA in rf magnetic fields [5]. Resonant effects of the kind are still unknown in physics, they are realized not at frequencies of transitions between the energy sublevels of the ground or excited nuclear states, which takes place in the conventional nuclear magnetic resonance, but at frequencies being a combination of the frequencies of hyperfine transitions. Non-trivial effects of a static magnetic field on the RSR frequency will be also discussed.

[1] A.M. Afanas'ev and M.A. Chuev, JETP 80 (1995) 560.
[2] M.A. Chuev, O. Hupe, H. Bremers, J. Hesse, A.M. Afanas'ev, Hyp. Interact. 126 (2000) 407.
[3] A.M. Afanas'ev and M.A. Chuev, JETP Lett. 74 (2001) 112.
[4] J. Hesse, T. Graf, M. Kopcewicz, A.M. Afanas'ev, M.A. Chuev, Hyp. Interact. 113 (1998) 499.
[5] A.M. Afanas'ev, M.A. Chuev, J. Hesse, J. Phys: Condens. Matter 12 (2000) 623.


K7.6 Dose Measurements 'in' Mössbauer Spectroscopy and Dose Measurements 'by' Mössbauer Spectroscopy
G. Pedrazzi1, E. Papotti2, S. Vaccari2, M. Ghillani1 and I. Ortalli1
1) Department of Public Health, Section of Physics and INFM Parma, Parma, Italy
2) Health Physics Service, University of Parma, Parma, Italy

The present work deals with two different aspects of Mössbauer spectroscopy and radiation dose. In a first context (radiation safety) Mössbauer spectroscopy is considered as a "producer" of radiation dose since it makes use of radioactive sources to obtain the measurement of some quantity of interest. Therefore, we must account for it in radiation protection. On the other side, we will show that Mössbauer spectroscopy can also be used to "measure" the effective radiation dose that has been absorbed by a medium after irradiation by x-rays of gamma-rays.

Concerning the first aspect of the technique we have considered in some details the radiation dose that is involved in the normal laboratory practice and during uncommon or special situations like, for instance, the arrival of a new strong source that needs to be mounted on the spectrometer bench. The absorbed dose has been calculated for different geometries, operations and accidents. Talking with colleagues and friends we have indeed maturated the conviction that notwithstanding all of them have a general idea of radiation risks and dose, very few had a correct perception of the real amount of dose involved in the laboratory procedures.

Regarding the measurements of absorbed dose by Mössbauer spectroscopy we have investigated the oxidation of ferrous ions (Fe2+) to ferric ions (Fe3+) as the result of the irradiation of a solution of ferrous ammonium sulphate (Fricke solution). Fe2+ ions act like scavengers for the active products of the radiation chemistry of water. The G value (number of molecules formed or destroyed as the result of the deposition of 100 eV from secondary electrons) is known for this reaction therefore the relative amount of Fe2+ and Fe3+ can be used to estimate the effective dose absorbed by the medium.


O7.1 Mössbauer Effect in Iron-Based Nanocrystalline Alloys: An Attempt to Evaluate the Spectra in a Generalized Two Level Relaxation Model
O. Hupe1, M. Chuev2, H. Bremers1, J. Hesse1, A. Afanasev2, K.G. Efthimiadis3 and E.K. Polychroniadis3
1) Institut für Metallphysik und Nukleare Festkörperphysik, Technische Universität Braunschweig, Braunschweig, Germany
2) Institute of Physics and Technology, Russian Academy of Sciences, Moscow, Russia
3) Department of Physics, Aristotle University, Thessaloniki, Greece
The most popular approach for evaluation of complex Mössbauer spectra collected on nanostructured ferromagnetic alloys is taking into consideration continuous distributions of the hyperfine field Hhf, quadrupole splitting and isomer shift. The results obtained within the distribution method suffer often some ambiguousness. The width of Hhf distribution requires for explanation a rather wide distribution over particle's size, which is often not confirmed by direct observation like transmission electron microscopy (TEM). TEM results show that the sizes of nano-grains are rather well defined with a narrow distribution. From the grain size it can be expected that the particles should demonstrate superparamagnetic relaxation at finite temperatures.

An alternative to interpret the Mössbauer spectra bears the thermally driven fluctuation of the nano-particles magnetic moment, which is called relaxation. The simplest way to describe the relaxation effects on the Mössbauer lineshape is to use the well-known two-level relaxation model. This model is dealing with only two energy states corresponding to opposite directions of the particle's magnetic moment along the easy magnetization axes, so that jumps from one state to the other are determined by only one energy barrier U0 between them.

In this contribution we apply the recently developed generalized two-level relaxation model for the evaluation of the Mössbauer spectra. We present examples from the ferromagnetic Fe(86-x)Cu1NbxB13 (x = 4, 5, 7) nanostructured alloys studied in a wide temperature range, below and above the Curie temperature of the amorphous matrix. This model allows one to take into account the interparticle interaction in a simpler form and to describe the often observed asymmetric shape of Mössbauer lines with sharp outer and smeared inward sides. This approach is actually an alternative way in order to evaluate the Mössbauer spectra of nanostructured ferromagnetic alloys without taking into consideration a rather wide and diverse distribution over the particle sizes.