Chemistry Reference
In-Depth Information
because the volume ratio of interface fraction to the total is very small. Only in the
case that the magnetic layer thickness is extremely thin, the interface anisotropy
becomes dominant and the whole magnetization may be oriented perpendicularly.
Such a situation can occur, in the present case of Fe/Mg multilayers, if the Fe layer
thickness is one or two monoatomic layers. The existence of such interface
anisotropy that magnetization is oriented perpendicularly to the plane has been
reported for many multilayer systems including very thin Fe or Co layers. An
example of magnetic films with perpendicular magnetization caused by the
interface anisotropy, being stable even at room temperature, is Co/Pt multilayers
including very thin Co layers, which is regarded to have potential as a magnetic
recording media material [
6
].
The spectra shown in Fig.
5.2
were obtained from independent samples
including different Fe layer thicknesses but can be interpreted as a sequence during
the growing of Fe layer thickness. Namely, Fe atoms deposited on Mg layer
surface form fractional monolayers in the initial stage, until the thickness reaches
0.2 nm. The crystal structure is close to amorphous but a stable ferromagnetic
order exists in the Fe layer whose direction is perpendicular to the film plane. By
depositing more Fe atoms, the thickness of amorphous Fe layer increases up to
0.8 nm, and then the magnetization direction gradually turns to the film plane.
A drastic change of the structure occurs in between 0.8 and 1.5 nm, from amor-
phous to crystalline bcc structure [
7
]. The Curie temperature jumps up to much
higher than room temperature with associating the crystallographic transformation.
The spectra for 1.5 nm are perfectly ferromagnetic and the observed hyperfine
field indicates that the sample is pure bcc Fe. If the bcc Fe layer includes some
percentages of Mg as impurities, the absorption lines should have wider line
widths and the average hyperfine field should be smaller than that of pure bcc Fe.
If Mg impurities are dissolved in Fe layers, amorphous phase should have been
more stabilized and the transformation to the bcc crystalline phase could not occur.
Thus, the Fe layer with the thickness of 1.5 nm, sandwiched in Mg layers, is
concluded to be pure bcc Fe which does not include Mg impurities. This result is
consistent with the phase diagram in the thermal equilibrium, which indicates that
Fe and Mg are insoluble with each other.
Concerning the process of film growth, information obtained from the result in
Fig.
5.2
is summarized as follows: In the beginning stage of Fe layer growing, the
structure is amorphous and if the thickness becomes larger than a critical thick-
ness, which lies between 0.8 and 1.5 nm, the structure as a whole transforms into a
crystalline structure (bcc). The composition of ultrathin amorphous Fe layers is
supposed to be pure, not including much Mg impurities, since the composition of
bcc Fe layer after the transformation from amorphous to crystalline is pure Fe. It
has been known that amorphous structure is stable for a mixture of Fe and Mg (i.e.
amorphous Fe-Mg alloys). If an amorphous Fe-Mg layer is formed from the initial
stage, however, such a transformation from amorphous to crystalline should not
occur afterwards since the amorphous alloy phase is always stable. The observed
Curie temperature being low is reasonable for pure Fe amorphous layer. This is in
contrast
to
amorphous
Fe-Mg
alloys
which
can
have
rather
high
Curie
