Chemistry Reference
In-Depth Information
Fig. 5.4 Temperature
dependence of the relative
intensity of the No. 2 and 5
lines of the Mössbauer
spectra (X) and the
corresponding direction of
the Fe magnetic moments
relative to the plane normal
(h) for an Fe (3.9 nm)/Nd
(2.8 nm) multilayer [
9
]
that Fe and RE magnetic moments are coupled through the interface and anisot-
ropy of RE atoms plays a dominant role to determine the easy direction of mul-
tilayer magnetization. The key to obtain perpendicular magnetic anisotropy is to
choose RE with ''pancake-like'' 4f electron distribution. The crystal field at the
interface tends to align the 4f pancakes parallel to the interface and eventually
the magnetic moments of RE, which are parallel to the axis of each pancake, are
aligned perpendicularly to the interface. The magnetic moments of Fe, which
are magnetically coupled with those of RE, also align perpendicularly. When the
temperature increases, the interface magnetic anisotropy caused by the interfacial
RE decreases and the in-plane shape anisotropy of the film becomes dominant, so
that the direction of magnetic moments turns to the film plane. This situation is
observed as an increase of the intensity of No. 2 and 5 lines of Mössbauer spectra
as the temperature increases (Fig.
5.4
)[
9
]. This phenomenon is called ''spin
reorientation''. The spin reorientation temperature increases when the RE layer
thickness is reduced to 1.0 nm, resulting in a realization of perpendicular mag-
netization at room temperature [
10
].
When the Fe layer thickness is reduced to 1.3 nm, the
57
Fe Mössbauer spectra
change drastically as shown in Fig.
5.5
[
9
]. The spectra at 4.2 K show a broad line
profile, and those at 300 K have more paramagnetic-like features. As in the case of
Fe/Mg multilayers already shown in
Sect. 5.3.1
, thin Fe layers become amorphous
with reduced magnetic transition temperature and wide distribution of hyperfine
fields. The perpendicular magnetic anisotropy appears at 4.2 K in these multilayers
as well, which is clearly evidenced from the intensity ratio of the No. 2 and 5 lines
in the spectra.
Please refer Ref. [
2
-
4
] for the results of investigations on the multilayers
with combinations other than Fe/Mg and Fe/RE.
