Alloying at the Cr/Fe Interface
While much progress has been made using epitaxial growth of Fe/Cr/Fe(001)
structures to study
magnetic exchange coupling a number of magnetic anomalies have
arisen in this model system.
For example, scanning electron microscopy with polarization analysis
(SEMPA) measurements of Cr/Fe(001)
[EPG Pub# 580] show anomalous surface
magnetizations for the first few layers of Cr deposition including: 1) a
strong decrease in the surface magnetization at sub-monolayer Cr coverages;
2) a lack of exchange coupling oscillations for the first 3-4 layers of Cr
deposition; and 3) the phase of the exchange coupling opposite to that
which is expected. In addition to the SEMPA measurements, there is a lack of
consistency in measurements of the average magnetic moment of Cr overlayers on
Fe. In order to explain these anomalous effects we have carried out detailed
structural studies of the Cr/Fe interface with scanning tunneling microscopy.
In this work we have identified a potential structural cause of these anomalies
that is due to the formation of an alloy at the Cr-Fe interface, instead of the
usually assumed chemically abrupt interface.
Figure 1: STM images of Cr growth on Fe(001) at a sample bias of
-1.1 V. (a) Large area scan showing the layer-by-layer quality of
the growth of 0.4 ML Cr deposited at 290 ± 10° C.
(b) Small area scan of the surface shown in (a). The island levels are
surrounded by the thick black lines and the Fe substrate level by the thin
white line. (c) Rendered perspective of the image in (b). The atomic steps
are 0.14 nm high.
We observe the alloy with STM as a distribution of single Cr impurity atoms
imbedded in the Fe substrate. Figure 1 shows STM images of 0.4 ML of
Cr deposited on Fe(001) at 300°C. A large area scan that shows many
single-atomic step islands resulting from the Cr deposition, is shown in
Figure 1 (a). The alloying can be seen in a high resolution image of the
surface, as shown in Figure 1 (b). A grey-scale range has been wrapped
twice through the z-range of this image to emphasize the fine structure on both
the substrate level (central region) and the islands. When imaging the filled
states of the sample, as in Figure 1, single atomic impurities in the
surface appear as atomic-scale white
dots.
The presence of a Cr and Fe bcc(001)
surface state near the Fermi energy allowed us to use tunneling
spectroscopy to chemically identify the atoms in the surface alloy. This
surface state leads to a sharp conductance peak at a sample bias of
-0.05 V for Cr(001) and +0.17 V for Fe(001), as shown by the dashed
curves in Figure 2.
Figure 2: Tunneling conductance versus tunneling bias. The top two
dashed curves are representative tunneling conductance spectra from clean
Cr(001) and Fe(001) surfaces. The
solid-line conductance spectra are representative of spectra taken on the
submonolayer Cr/Fe(001) alloyed surface. Both the substrate and island levels
have these Fe and impurity spectroscopic characteristics.
Spectra taken on the alloyed surface are also shown in the figure. When the
tip is away from an impurity, a strong conductance peak is present at the
voltage corresponding to the Fe surface state. When the tip is over an impurity
atom, the Fe surface state peak is reduced and a weaker broad peak appears at
-0.3 V. For submonolayer Cr coverages where the single impurities are
clearly resolved, these two types of spectra are seen on both the substrate
level and the islands.
The spectroscopy tells us that the smooth gray regions in the images on both
levels are Fe and that the impurity atoms must therefore be Cr. As the
schematic in Figure 3 shows, for Cr deposition at 300° ,
most of the deposited Cr atoms have replaced Fe atoms, resulting in a growth
layer that is mostly Fe instead of pure Cr. These observations of alloying at
the Cr/Fe interface may explain many of the anomalies seen in the magnetic
properties of Cr/Fe structures.
Figure 3: Schematic of Cr-Fe alloyed surface after submonolayer
deposition of Cr on Fe(001) at 300°.
Atomic-scale Observations of Alloying at the
Cr-Fe(001) Interface
Tunneling Spectroscopy of bcc(001) Surface States
Magnetic Moments in Cr Thin films on Fe(100)
Influence of Cr Growth on Exchange Coupling in
Fe/Cr/Fe(100)
Observation of Two Different Oscillation
Periods in the Exchange Coupling of Fe/Cr/Fe(100)
Joseph A. Stroscio
Daniel T. Pierce
Robert J. Celotta
Angela Davies - University of North Carolina (Charlotte)
Supported in part by the Office of Naval Research
Online: May 1996
Last Updated: February 2008
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