Chemistry Reference
In-Depth Information
place. In contrast, the spectrum for Sample B, where the
57
Fe probing layer was
deposited on the surface of Mn layer prior to the thick
56
Fe deposition (Fe-on-Mn)
was found to have a considerably different profile. A fairly large non-magnetic
fraction was observed, which means diffusion of Fe atoms into Mn layers ranging
over a significant depth. Thus, interface diffusion (or mixing) depends not only on
the combination of the materials but also the procedure of sample preparation. In
the process of vapor deposition, an interface is formed with combining a cold
substrate and hot vapor atoms. In the present case, a significant reaction occurs
between cold Mn and hot Fe, but not so much between cold Fe and hot Mn.
In a multilayer structure built by successive depositions, it can happen that the
structures at the top and bottom interfaces of each layer have different chemical
profiles. In other words, the compositional modulation may have a unidirectional
profile with respect to the film growth direction. Mössbauer spectroscopy is a
unique method to clarify the difference of interface chemical profile in two
interfaces; top and bottom. In general, if a Mössbauer probe is located at the
interface of magnetic layer, observed hyperfine field distribution is useful infor-
mation for the estimation of degree of intermixing at the interface.
Multilayers consisting of Fe and Cr layers are a particularly interesting system.
It is well known that the giant magnetoresistance (GMR) was first found in Fe/Cr
multilayers [
11
] and therefore Fe interfaces in Fe/Cr multilayers have attracted
much attention [
2
]. The GMR phenomenon means the great difference of resis-
tance between the states of spin-parallel and antiparallel and the reason is attrib-
uted to the spin-dependent scattering. Since the spin-dependent scattering is
considered to occur at interface sites, the relation between spin-dependent scat-
tering probability and the situation of interface is an important issue. Mössbauer
spectroscopy is a useful method for the analysis of interface roughness on an
atomic scale. Before the detailed argument of interface roughness, however, the
difference of interface properties between two kinds of interface, Fe-on-Cr and Cr-
on-Fe, has to be clarified, with using a similar procedure as the case of Fe/Mn
interfaces. The result of CEMS measurements for the two kinds of Fe layer
interface contacting with Cr layer is shown in Fig.
5.7
. The nominal structures are
Cr 20 ML/natural Fe 12 ML/
57
Fe 2 ML/Cr 8 ML (upper interface) and Cr 20
ML/
57
Fe 2 ML/natural Fe 12 ML/Cr 8 ML (lower interface). Both of two obtained
spectra are ferromagnetic and the profiles are similar to the previous case of Mn-
on-Fe, suggesting that the intermixing at the Fe/Cr interface is being fairly
suppressed.
The difference of spectra for the two interfaces can be revealed by analyzing the
distribution of hyperfine field as shown in the inserted figure. However, before
arguing the degree of mixing between Fe and Cr, an additional problem has to be
considered, that is the different degrees of mixing between
57
Fe probe and natural
Fe matrix at the two interfaces. In the case that hot
57
Fe atoms of a minor amount
are deposited on a cold Fe substrate, the interdiffusion may be fairly limited, while
in the other case, more diffusion might be plausible. Therefore in such a case that
the degree of intermixing is small, a quantitative discussion is very difficult.
Although there have been several investigations on the relation between interface
