Chemistry Reference
In-Depth Information
polarization in magnetic materials, which furnish us fruitful information to study
magnetic properties of materials.
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Sn Studies on Magnetic Multilayers and Interfaces
5.6
5.6.1 Spin Polarization in Nonmagnetic Layers
After the discovery of the GMR effect in Fe/Cr multilayers, similar phenomena were
found in other ferromagnetic/nonmagnetic multilayer systems. In the systems called
''exchange coupling type'', antiferromagnetic coupling appears between the ferro-
magnetic layers through the intervening nonmagnetic layer, resulting in an anti-
parallel magnetic configuration between the ferromagnetic layers at zero external
field. The electric resistance changes drastically when the magnetic configuration
turns parallel by applying an external magnetic field. The antiferromagnetic cou-
pling is thought to be mediated by the electron spin polarization in the nonmagnetic
layer. By doping the nonmagnetic layers with Sn impurities, and using Mössbauer
spectroscopy, such spin polarization can be detected through the induced hyperfine
field at the
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Sn nuclear sites. Actually, finite hyperfine fields were observed in the
Sn impurity sites in the nonmagnetic layers of Co/Au and other multilayer systems,
showing that the conduction electrons in the nonmagnetic layer are really spin-
polarized due to the contacting ferromagnetic layers [
16
]. In this way the
119
Sn
nuclei can be a sensitive detector of electron spin polarization in nonmagnetic
materials.
5.6.2 Magnetism of Cr Layers
Cr, which has a bcc structure as Fe, is an antiferromagnet with the magnetic
moments parallel in one (001) atomic plane but antiparallel between the adjacent
(001) atomic planes. Besides, the size of the magnetic moments shows a sinusoidal
modulation along the \001[ direction. This magnetic structure is called ''spin
density wave''. It is also known that the magnetic structure is sensitively influenced
by impurities and strains. On the other hand, it is in general not easy to investigate the
magnetism of thin antiferromagnetic films experimentally. Therefore, the investi-
gation on magnetism of thin Cr films, including those in Fe/Cr multilayers where the
magnetoresistance effect was first discovered, is a challenging subject [
17
-
19
].
One attractive method is the use of Sn nuclear probes as a tool to detect electron
spin polarization through Mössbauer spectra. When an atomic layer of Cr is
replaced by an atomic layer of Sn, the magnetic moments of neighboring Cr atoms
tend to be enhanced, and at the same time the Sn nuclei feel a large hyperfine field
because of an effect from the magnetic Cr atoms [
20
]. It was found that the
