Oxydation of Fe/Cu and Fe/Ag multilayer
films prepared by DC magnetron sputtering
EMSL Project ID
3670
Abstract
Thin Fe films exhibit a variety of properties important for practical applications such as strong perpendicular magnetic
anisotropy, spin reorientation transition, and magnetic moment
enhancement for surface atoms. By growing films on a suitable
substrate (Cu) it is possible to obtain gamma-Fe with fcc structure
at room temperature (RT), while temperatures well exceeding 1000 C
are needed for bulk fcc Fe. Moreover, due to sensitive dependence
of ground state magnetic properties of fcc gamma-Fe on the atomic
volume, a phase diagram with thickness and growth temperature as
parameters is rather complex. Its main feature is strong, bcc-Fe
like ferromagnetism (FM) for thicknesses of Fe of about several
atomic layers (AL) that is in contrast to antiferromagnetism (AF)
of bulk fcc-Fe. Growing Fe layers sandwiched between template layers
of Cu allows not only for multiplication of the studied volume, but
also provides the possibility to study the role of interfacial
effects.
Magnetron sputtering is one of the fastest methods of growing
thin films and hence is of primary importance for industrial
applications. Here the growth process takes place not in the
ultrahigh vacuum, but in the atmosphere of an inert gas (Ar) at
pressures in the mTorr range thus requiring its high purity.
In addition, the gas pressure has notable effect on the roughness
and alloying (intermixing) of the interfaces.
We prepared a series of Fe/Cu multilayers using this method with
the Fe layers thickness in the range 11.0 - 7.3 A and with different
width of the interfaces. For reference we also prepared Fe/Ag
multilayers with the bcc alpha-Fe layers thickness in the range
2.4 - 8.8 A. In all cases protecting Cu layer of about 20 A thick
was deposited on top. All samples were characterized by low-angle
X-ray reflectivity and high-angle X-ray diffraction and were found
to have a coherent crystalline lattice along the direction of growth
- fcc for Fe/Cu, and bcc for Fe/Ag.
Magnetic properties were studied by Mossbauer spectroscopy which
is the local probe. In short, at RT Fe/Cu multilayers show only
central poorly resolved doublet (due to quadrupole splitting
originating from lattice distortion and the presence of interfaces),
but develop broad Zeeman splitting at the expense of this central
doublet upon cooling. More careful analysis shows that another
doublet, with much smaller intensity, but much larger splitting is
present in every spectrum of Fe/Cu films, with spectral contribution
of about 10-15% and independent of temperature.
There is no common opinion in the Mossbauer literature on such
feature. Doublet with similar parameters was observed previously
for small Fe precipitates in Cu and was ascribed to break of cubic
symmetry due to lattice distortion and interface. At the same time
both its isomer shift and quadrupole splitting are of the same
range as for iron oxides - +0.42-0.49 mms/s (in respect of alpha
bcc-Fe at RT), and 0.54-0.85 mm/s respectively.
Hence it is very important to check for a possible oxidation
of Fe using one or several analytical methods and preferably
non-destructive ones. Our goal for this proposal is to investigate
the presence of oxygen, its oxidation state, and possible
concentration with depth in five representative samples
(three Fe/Cu, and two Fe/Ag films). The most valuable methods in
this respect will be Photo Electron Spectroscopy (with depth
profiling using ion sputtering or angle resolved mode), as well
as deuterium beam nuclear reaction - d(O,N)He.
Project Details
Project type
Exploratory Research
Start Date
2003-09-12
End Date
2004-09-15
Status
Closed
Released Data Link
Team
Principal Investigator
Related Publications
Kouprine AP, L Cheng, Z Altounian, DU Ryan, and MH Engelhard. 2005. "Magnetic Properties of Cu/Fe Multilayers upon Transition to Island Structure of Fe layers." Physical Review. B, Condensed Matter.