Chemistry Reference
In-Depth Information
the existence of minor fraction with a reduced hyperfine field. This fraction with a
reduced hyperfine field is attributed to the interface of Fe layer contacting with Mg
layer. In the case of bcc Fe-alloy including a non-magnetic metal impurity of a
minor amount, the hyperfine field of Fe, having one non-magnetic neighbor atom
(Mg in this case), must be reduced by around 10 %. It is apparent that the inter-
mixing of Fe and Mg does not take place and the Fe layers with the thickness of
1.5 nm are regarded as pure bcc Fe sandwiched by Mg layers.
The profiles of spectra for Fe thinner than 0.8 nm are entirely different. At
300 K, spectra have no magnetically split structure but show only a doublet with a
small separation due to a quadrupole effect. This result indicates that the Curie
temperatures of these very thin Fe layers are lower than room temperature. The
reason is the crystallographic structural change from bcc to amorphous. At 4.2 K,
all spectra show magnetically split spectra but the each line width is very broad.
Such broad six lines are a characteristic for ferromagnetic amorphous Fe alloys,
and therefore from the observed line profile, we can judge that the structure of Fe
layers thinner than 0.8 nm is amorphous. Amorphous phase of Fe alloys in a bulk
form can be obtained by mixing some non-magnetic elements (typically B, N, etc.)
but amorphous form of ''pure'' Fe is known to be unstable and the Curie tem-
perature of amorphous pure Fe is not able to estimate experimentally, although it
has been suggested to be remarkably low. The spectra for Fe thinner than 0.8 nm
therefore furnish us information on the magnetic properties of amorphous ''pure''
Fe in ultrathin layer forms. According to the results of magnetization measure-
ments by using a SQUID magnetometer, as a function of temperature, the Curie
temperatures show a variation from about 110 K for 0.8 nm Fe layer to about 35 K
for 0.1 nm Fe layer. It is to be noted that the Mössbauer spectrum for 0.1 nm Fe
layer is entirely ferromagnetic at 4.2 K and non-magnetic fraction is not visible in
the spectrum. The nominal thickness of 0.1 nm corresponds to be less than one
perfect monoatomic layer and cannot cover whole area of Mg surface. However, in
case that isolated Fe atoms exist in the sample of 0.1 nm film, the Mössbauer
spectrum must exhibit a non-magnetic fraction. The fact that there is no non-
magnetic fraction indicates that the Fe atoms form fractional monolayers but
diffusion or mixing of Fe into Mg layers is negligible. Another interesting feature
is seen in the intensity ratio of six lines in the magnetically split Mössbauer
pattern. The intensity ratio of the spectrum for 1.5 nm Fe is almost ideally
3:4:1:1:4:3, indicating the magnetization is perfectly oriented in the film plane. If
the thickness is 0.8 nm, the structure has changed to be amorphous and the six
lines become broader, but the intensity ratio is still close to 3:4:1:1:4:3. In contrast,
with decreasing of the thickness, the intensities of No. 2 and 5 lines are reduced
gradually and become very weak in the spectra for 0.1 and 0.2 nm. This result
means that the magnetization direction of Fe monolayer (perhaps 0.2 nm also) is
preferentially oriented to be perpendicular to the plane. It is usual that magneti-
zation in a thin film is oriented in the plane because of the shape anisotropy. On the
other hand, another kind of anisotropy, to orient magnetization perpendicularly to
the plane can exist in the surface (and interface) atomic layers. In magnetic
materials of bulky shape, the influence of such interface anisotropy is negligible
