Influence of adsorbates and substrate interdiffusion on the spin-dependent electron scattering
of Fe/Ag„001…: Exploration of the parameter space for an electron spin detector
A. Scholl, L. Baumgarten, and W. Eberhardt
IFF der KFA Ju ¨lich, D-52425 Ju ¨lich, Germany
Received 7 November 1996
The elastic low-energy electron scattering of ferromagnetic Fe001 films on Ag001 exhibits a strong spin
dependency, an effect that has been utilized for spin detection in electron spectroscopy. The quality of the
substrate crystal, the morphology of the Fe film, and the exposure to adsorbates all have a strong influence on
the spin-dependent electron scattering. These effects are especially prominent at low electron energies. An-
nealing of the Fe layer leads to interdiffusion. We observe that Ag migrates through the Fe film and forms a
thin layer at the surface. Since the exchange asymmetry does not decrease, protection of the Fe film against
residual gases by a Ag layer promises longer lifetimes for a spin detector without a significant loss of detection
efficiency. S0163-18299703726-0
I. INTRODUCTION
The technological demand for improved magnetic record-
ing media as well as new magnetic sensors has spawned an
intense interest in the study of magnetic thin-film systems.
Compared to bulk solids, thin films and multilayers exhibit
novel magnetic properties. These differences are due to the
reduced dimensionality of the thin-film systems on one hand
and also due to specific coupling effects in multilayer sys-
tems. Over the last decade this has lead to the development
of a new magnetic sensor and readout technique based upon
magnetic multilayer systems. Simultaneous to the techno-
logical advancement, a quite intense basic science effort de-
veloped in trying to understand these new magnetic proper-
ties and coupling effects on the basis of the microscopic
electronic interactions in magnetic thin-film systems.
On the experimental side this has lead to an increased
effort to develop better spin-detection devices for electrons.
The classic high-energy Mott detector, based on the spin-
orbit interactions of high-Z elements, was improved through
a retardation technique, which allows in combination with
lower scattering energies a more compact layout.
1
Other
spin-detection techniques work with diffuse scattering
2
or
with diffraction
3
of low-energy electrons, again based on the
spin-orbit interaction. Common to all these spin detectors for
electrons is the very low efficiency, as only a small part of
the incoming electrons is scattered into the electron detec-
tors. Progress was achieved by a new concept for a spin
detector, utilizing the exchange interaction of electrons in
magnetized materials, developed by Kisker and
co-workers.
4,5
This detector uses the reflection of electrons
with very low energies at magnetized thin Fe films grown in
situ on a Ag001 crystal. It has a superior figure of merit,
because of the high reflectivity of Fe at low energies.
The figure of merit F =( I / I
0
) A
2
, a measure for the effi-
ciency of the spin analyzer, is used to compare different
detectors. A is the normalized intensity difference that would
be measured for a fully polarized electron beam, and I / I
0
is
the reflectivity. A figure of merit more than one order of
magnitude grater than for conventional Mott detectors was
reported for the new detector.
4,5
The Fe detector works as
follows: The spin-polarized electron beam with a kinetic en-
ergy of about 11 eV is reflected by the Fe film near normal
incidence, and a channeltron is used to detect the elastic
fraction of the reflected beam. The normalized intensity dif-
ference upon reversal of the magnetization direction, the so-
called exchange asymmetry A
ex
=( I
+
-I
-
)/( I
+
+I
-
), is
then proportional to the beam polarization.
6
As the Fe spin
detector is rather new, some questions have not yet been
thoroughly investigated. First there are uncertainties con-
cerning the effect of film quality on the scattering properties
of the Fe surface. Here we will show that a good film quality
is necessary to achieve a high figure of merit and that espe-
cially the cleaning procedure by ion sputtering deteriorates
the exchange asymmetry after some time, since the quality of
the Ag substrate deteriorates.
The second question is, how do adsorbates affect the mag-
netization of the thin-Fe layer as well as its reflectivity, and
what can be done to prolong the effective lifetime of the spin
analyzer. Since the spin-dependent reflectivity for electrons
is caused by the electronic structure of the Fe film near the
surface, a change of the electronic structure caused by
chemisorption of adsorbates will affect the performance of
the detector. Therefore a very good vacuum is needed to
minimize the adsorption of residual gases. Here we want to
show that two other possible solutions exist to enhance the
stability of the detector performance. This can be achieved
either by using a higher electron energy or by covering the
Fe surface with a very thin Ag layer by interdiffusion from
the substrate.
II. DESCRIPTION OF THE EXPERIMENT
AND FILM CHARACTERIZATION
We used angle-resolved Auger electron spectroscopy
AES and low-energy electron diffraction LEED to exam-
ine the structure of the substrate and the thin Fe films, which
are grown on a Ag001 single crystal, as well as SPLEED,
the spin-polarized variant of LEED, to investigate the prop-
erties of the Fe films as spin detector.
The vacuum system consists of three chambers, the scat-
PHYSICAL REVIEW B 1 JULY 1997-II VOLUME 56, NUMBER 2
56 0163-1829/97/562/7476/$10.00 747 © 1997 The American Physical Society