Chemistry Reference
In-Depth Information
5.1 Introduction
In this chapter, the usefulness of the Mössbauer spectroscopy for studies on
magnetic multilayers and interfaces of magnetic thin films is described.
Since magnetism of condensed matters is originated from electron spins,
magnetic properties are understood by analyzing the behavior of electron spins.
Although spins exist also on nuclei of odd numbers, nuclear spins are too small to
contribute to the bulk magnetic properties. However, they can serve as very useful
probes to investigate fundamental properties of condensed matters, particularly
magnetic properties. Experimental techniques to use nuclei as probes for con-
densed matters are called ''nuclear methods'' and two methods are very popular:
namely NMR (nuclear magnetic resonance) and Mössbauer spectroscopy. As was
already introduced in the first chapter of this topic, Mössbauer effect can be
observed for more than 40 elements but easily measured only for very limited
nuclear species. The most convenient is
57
Fe and the second is
119
Sn. Mössbauer
effect measurements are used as a very simple analytical tool if
57
Fe or
119
Sn is
available as a microprobe in a sample. It is very fortunate for investigators in the
field of magnetism that Fe nucleus is the most suitable one for Mössbauer effect
measurements, since Fe often plays as a leading actor in magnetic materials.
57
Fe
is a stable isotope found in natural Fe with the abundance of 2.2 %. If a sample
includes a certain amount of Fe, Mössbauer absorption (or emission) spectrum is
always able to be obtained. If a sample can be enriched in
57
Fe, measurements
become possible with much less Fe contents. For
57
Fe Mössbauer measurements
generally, it is conventional to use a radioactive isotope
57
Co as a gamma ray
source. On the other hand, instead of radioisotope, Mössbauer experiments with
synchrotron radiation source are making steady progress in recent years but this
chapter is not concerned with synchrotron Mössbauer studies [
1
].
In our daily life, there can be found a great variety of commercial products
containing magnetic materials. Examples are a hard-disk-drive in a personal
computer, small permanent magnets on a bulletin board, credit cards with mag-
netic memory in our pocket, and so on. It is rather usual that the main component
of magnetic materials in commercial products is Fe and therefore we can obtain
Mössbauer absorption spectra from commercial magnetic goods if they have thin
film shapes with such a thickness as gamma-ray can penetrate. In the case that they
have bulky shapes and are too thick for gamma-ray penetration, Mössbauer
measurements with scattering geometry would be appropriate.
Figure
5.1
shows Mössbauer absorption spectra obtained from very familiar
magnetic materials around us; (a) a train ticket and (b) a credit card. Both spectra
are well resolved six line patterns and from the values of hyperfine field, we can
identify the magnetic materials. The spectrum (a) indicates that the surface of the
train ticket is entirely coated by Ba-ferrite and the spectrum (b) shows that
the material of magnetic memory part in the credit card is solely c-Fe
2
O
3
. From the
intensity ratio of the magnetically split lines, in both cases, it is suggested that
the magnetization is preferentially oriented in the plane. These examples may be
