Mossbauer Analysis of the Atomic and Magnetic Structure of Alloys

269 Pages·2006·4.47 MB·English

by  V V Ovchinnikov

MÖSSBAUER ANALYSIS
OF THE ATOMIC
AND
MAGNETIC STRUCTURE
OF ALLOYS
i
ii
MÖSSBAUER ANALYSIS OF THE
ATOMIC AND
MAGNETIC STRUCTURE
OF ALLOYS
V.V. Ovchinnikov
CAMBRIDGE INTERNATIONAL SCIENCE PUBLISHING
iii
Published by
Cambridge International Science Publishing
7 Meadow Walk, Great Abington, Cambridge CB1 6AZ, UK
http://www.cisp-publishing.com
Translation from Russian of V.V. Ovchinnikov ‘Messbauerovskie
metody analiza atomnoi i magnitnoi struktury splavov. Fizmatlit,
Moscow, 2002, (ISBN 5-9221-0259-1)
English translation published November 2006
© V V Ovchinnikov
© Cambridge International Science Publishing
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British Library Cataloguing in Publication Data
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Library
ISBN 10: 1-904602-13-4
ISBN 13: 978-1-904602-13-2
Cover design Terry Callanan
Printed and bound in the UK by Lightning Source (UK) Ltd
iv
.
Preface
I am pleased to welcome English-language readers and I would like
to say several words about why I have written this book.
Undoubtedly, the Mössbauer effect in the period since its
discovery has acquired a position as one of the most informative
methods of examination of the atomic and magnetic structure of
solids. However, of the large number of original publications, it is
relatively difficult to extract fundamental principles representing the
basis of these investigations. This must be carried out using several
generalisations, and I have undertaken this task. I regarded it as
necessary because it is clearly evident that the discovery of the
Mössbauer effect has indicated a number of completely new
possibilities in the examination of the atomic and magnetic structure
of solids, in comparison with the possibilities provided by the
conventional methods.
I participated in the development of non-diffraction Mössbauer
methods of analysis of the atomic and magnetic structure of alloys
[31,59,63,138,139,213 – FMM (Rus), 203,204, 330 – FTT, Pis’ma
JTF (Rus), 68, 427 (Engl), and others], representing an alternative
to the conventional methods based on the diffraction phenomenon
(x-ray, electron and neutron). However, the above studies, published
mainly in Russian journals, are usually not known to foreign readers.
The justification of the Mössbauer methods of the analysis of the
atomic and magnetic structure of alloys required analysis of the
problems associated with the listing of different non-equivalent
positions of resonant nuclei (atoms) in these systems. In addition
to this, it was necessary to develop methods of the analytical
description of the probabilities of appropriate positions, in relation
to the condition of the investigated objects (with rational restriction
of the radius within which the differences in the interactions are
taken into account). The probabilities of the positions specify the
relative intensities of ‘sub-spectra’ which form the Mössbauer
spectrum. Their number for alloys with a variable composition,
including multiphase alloys, with imperfect long- and short-range
great atomic and magnetic orders, and also of multilayered
superlattices, is relatively large. Finally, it was necessary to
v
describe the electrical and magnetic superfine interactions of
resonant nuclei in the investigated positions (in the presence of
atoms of different type in several closest coordination spheres of
these nuclei) which determine the position of the individual lines in
the composition of the sub-spectra.
The conclusions of the current theory of the interpretation of the
results of indirect investigations are linked most directly with the
problems of obtaining reliable information from the Mössbauer
spectrum. This relates to the problem of well-founded construction
of an entire class of permissible models and of selection, from this
class, of the best model (in agreement with the error of the initial
data). The final result are the optimum estimates of the physical
parameters and of their errors. The most important role is played
here by apriori information (fundamental physical laws, symmetric
considerations, exact knowledge of apparatus functions,
characterising the properties of apparatus, etc, so that it is possible
to construct generalised models). The deviation from the principles
of this theory results in inaccurate information obtained from the
experiment on the structure of investigated objects and also
estimates of the appropriate physical parameters and their errors.
Part of the permissible models may be simply lost. Therefore,
special attention is given to these problems in the monographs.
Being a nuclear-physical model, the Mössbauer effect provides
information averaged out in respect of the ensemble of the
individually absorbing resonant nuclei distributed in the crystal or
amorphous solid.1 This results in many important cases in
considerable advantages of the method in comparison with
diffraction methods (x-ray, electron and neutron diffraction), in
particular, in examination of the initial stages of atomic ordering,
formation of disperse phases, nanocrystalline and amorphous
materials. In these cases, the zones of coherent scattering are
extremely small, and if they are smaller than 8–10 nm, diffraction
effects simply do not form. For the same reason, because of the
absence of measurable diffraction effects, the Mössbauer methods
has advantages in the examination of the atomic and magnetic
structure of materials with similar scattering properties of the
components, low-symmetry superstructures and diluted solid
solutions.
1At reduced temperatures T < (0.4–0.6)T the position of the large
melt
majority of atoms in solids during the lifetime of the excited state of the
Mössbauer nucleus (10–11–10–6 s) remains unchanged.
vi
To help the readers not concerned professionally with the
Mössbauer effect, and also students and investigators starting their
work, I have provided in the book a brief introduction to the theory
of the Mössbauer effect.
The metallic alloys which are the main subject in the monograph
are very specific objects of investigation because of collectivisation
of the electrons, the presence of variable composition phases,
multiphase structure, etc. This is reflected in the complex nature
of describing a large number of non-equivalent positions of
Mössbauer nuclei (taking into account the variable atomic
environment in several closest coordination spheres) and of the
description of appropriate variations of their electrical and magnetic
state. Consequently, these problems, starting with the general
formulation to final equations, are examined in detail in chapters 2-
4 of the book. Chapter 5 is concerned with the description and
analysis of a large number of experimental investigations of the
atomic and magnetic structure of alloys using the Mössbauer effect.
Chapter 6 examines investigations of the Mossbauer spectroscopy
of ion-doped metals and alloys (and also of silicon and some
compounds). In these studies, attention was given to examining
defect formation and other diverse and complicated processes
induced by ion radiation.
The Appendices 1 and 2 are concerned with the methods of
processing (‘decoding’) of the Mössbauer spectra. In Appendix 3
the accurate results, obtained by the author and confirming the
excellence of the homogeneous (probability) and microdomain
(topological) methods of description of the short-range order are
described. The topological approach makes it possible to transfer
from the short-range to long-range order with an increase of the
dimensions of the ordered domains.
The latter is an important argument in discussing the justification
of the application of the Mössbauer effect for examining not only
the short-range but also long-range atomic order (regardless of the
fact that each of the resonant nuclei, distributed in the crystal,
‘senses’ only its nearest environment).
I hope that the book will attract the attention of readers to these
problems and will stimulate to a certain extent further development
of the applications of the Mössbauer effect in the area of
examination of the atomic and magnetic structure of the condensed
media.
In the English edition, I have corrected a number of errors made
in the original Russian version, and also made some additions and
vii
improvements in terminology.
In conclusion, I would like to express my gratitude to
Cambridge International Science Publishing and, in particular, to Mr
Riecansky, for the interest in my monograph and excellent work in
publishing it.
V.V. Ovchinnikov
viii
Preface to the Russian Edition
The method of nuclear gamma resonance, or the Mössbauer effect,
discovered several decades ago, has been transformed into one of
the most powerful methods of investigation of the structure of
matter. Its experimental possibilities are continuously expanding.
This relates to the selective indepth analysis of solids with
registration of conversion and Auger electrons, optical, x-ray and
gamma radiation. The applications, associated with the utilisation of
synchrotron radiation, are being efficiently developed. Coherent
effects are studied. The method is used widely for fundamental and
applied investigations in chemistry, biology and solid state physics
in order to examine the dynamics of atoms and the atomic and
electronic structure of matter.
At the same time, analysis of the publications of the recent years
indicates a crisis in the development of the method. This relates
directly to the analysis of the atomic and magnetic structure of
solids and is associated, in particular, with the absence in many
cases important in practice of clear understanding of how to
interpret unambiguously the results of Mössbauer experiments. Some
fundamental problems relating to, for example, the relationship of
the values of hyperfine magnetic fields on the atom nuclei with the
values of the individual atomic magnetic moments have not as yet
been explained. Theory bypasses many key problems relating to the
formulation and interpretation of Mössbauer experiments. The
experimenters often do not use efficiently theoretical results. So,
unique information on the structure of condensed matter remains
outside the examined areas. For example, almost all Mössbauer
investigations of the long-range atomic order in the crystals are of
qualitative nature, whereas the precision methods of analysis of
superstructures using the Mössbauer effect were developed
approximately 30 years ago. This is also combined with the crisis
of classical methods of interpreting the results of investigations.
The Mössbauer effect provides no visual pattern of the structure
of the crystal in the direct or reciprocal space, unlike a series of
other methods. Nevertheless, in practice, the speculative interpret-
ation of the obtained information is often used. This means that the
individual special features of the Mössbauer spectra (for example,
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