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TP A2 UP III: Scanning tunneling microscopy and UHV-SQUID magnetometry
(K. Lenz,
R. Nünthel, J. Lindner, K. Baberschke)
This part describes absolute
measurements of magnetic moments on ultrathin films performed by
a novel in situ (UHV-compatible) high-TC-SQUID
magnetometer and real images of the surface morphology of ultrathin
films as they are recorded by Scanning Tunneling Microscopy (STM).
A novel UHV SQUID magnetometer was recently built up. A commercial SQUID magnetometer with a high-TC SQUID sensor was installed inside a liquid nitrogen dewar at a flange of an ultrahigh vacuum chamber. By scanning the sample, i.e. an ultrathin film evaporated on a single crystalline substrate like Cu(001), parallel to the SQUID plane the stray field is directly measured and the magnetization of the film is determined. The SQUID is calibrated once with the help of Helmholtz coils and it does not need any reference sample. Therefore, it is an absolute magnetometry. The basic schematic setup and the principle diagram of operation are shown in Fig. 1. Together it is presented the stray field of a 2.9 ML Co/Cu(001) at room temperature. A very high signal to noise ratio of 40:1 is achieved with a single scan. The chamber offers the opportunity to use all conventional techniques for the characterization of thin films (LEED, AES etc.) and has capabilities of cooling with Liquid Helium resulting in an optimum temperature of 40 K at the sample position. In the inset of Fig. 1 we see temperature-dependent measurements for 2 ML Co/Cu(001). Extrapolating to T=0 and dividing by the number of magnetic atoms one obtains the magnetic moment per atom [Ref. 251] . This new SQUID magnetometry was applied to study the thickness dependence of the magnetic moments of ultrathin Co/Cu(001) films and the effects of capping. The results are plotted in Fig. 2. One may see a linear dependence of the magnetic moments with the inverse thickness. At the large thickness limit the samples behave bulk-like while at the limit of 2 ML an enhancement (reduction) of the magnetic moment is encountered for the uncapped (capped) films. Linear regression of the two data sets may provide a bulk-like volume contribution, enhanced surface and reduced interface moments as they are indicated in Fig. 2 [Refs. 238, 254] .
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Fig. 5 |
Fig. 6:
(a) 24x24 nm 2 STM image of 9 ML Ni/Cu(001) and (b) the
corresponding
configuration of P. Bruno's model
[Ref.
224]
STM can be used to observe mechanisms which improve the growth of
ultrathin film structures. Such a mechanism is provided by using surfactants
during the film deposition. In many cases gases can be used for this purpose.
In case of magnetic thin films the growth with the help of surfactants may
– besides the structural changes – also lead to changes of the magnetic properties.
Figures 7-9 show the result of using oxygen as surfactant during the
growth of Ni on Cu(001). In Fig. 7 a STM images of the bare Cu(001) surface
together with its 1x1 LEED pattern is presented. The right picture shows
the corresponding hard sphere model of the fcc (001) surface which can
be imaged by STM with atomic resolution. Fig. 8 shows the Cu(001) surface
after dosing 1200L of oxygen at elevated temperatures. LEED as well as
STM reveal the well-known
missing row reconstruction. The missing rows and
the oxygen atoms can be resolved within the STM image from which the
hard sphere model can be derived. The saturation coverage of oxygen is
0.5 ML-equivalents, so that the amount of oxygen acting as surfactant can
be well controlled. The result after a 5.5 ML thick Ni film has been deposited
onto the preoxidized Cu(001) surface is presented in the Fig. 9. The LEED
pattern reveals a structure resulting from the oxygen atoms that have detached
from the Cu substrate and successively swim on top of the growing Ni film.
This is shown by the STM image where the c(2x2) structure can be observed.
The scenario suggested by the STM and LEED investigations could be confirmed
by AUGER electron spectroscopy, element-specific NEXAFS results and also
by theoretical calculations, for details see Ref.[270].
Fig. 7 |
Fig. 8 |
Fig.
9 |
The influence on the magnetic properties of the Ni films grown with
the help of oxygen has been investigated with MOKE and FMR. Some results
are shown in Fig. 10. Fig. 10a shows results from polar MOKE measurements
which measures the out-of-plane component of the film magnetization during
the film growth, i.e. as a function of the film thickness for Ni/Cu(001)
(green symbols) as well as Ni on
reconstructed Cu(001) (black symbols). Complete
hysteresis loops for the films on the preoxidized surface taken for specific
thicknesses (indicated by the arrows in (a)) are plotted in Fig. 10b.
Compared to Ni films grown without oxygen the films grown with the use
of O as surfactant show an out-of-plane magnetization already for smaller
film thicknesses. This implies that the reorientation thickness which is
known to occur for the Ni/Cu(001) system between 8-11ML is reduced for Ni
films deposited on the preoxidized surface. The influence of oxygen is also
found in FMR shown in the upper right panel of Fig. 10 by the example of
a 6 ML thick Ni films grown on either the clean (red signal at lower fields)
or the preoxidized surface. The occurrence of the resonance below (above)
3kOe for the Ni film on the clean (preoxidized) Cu surface indicates that
the easy axis of magnetization lies in (out-of) the film plane, thus confirming
the MOKE results. For a 5 ML thick Ni film on the preoxidized surface the
transition from in-plane at lower temperatures towards out-of-plane at
higher temperature could be detected via FMR as presented by Fig. 10d.
A detailed analysis of the thickness dependence of the FMR signal for a
constant reduced temperature reveals that the oxygen atoms mainly reduce
the surface anisotropy KS1 (see schematical drawing in
Fig .10d) which for Ni/Cu(001) favors the in-plane orientation of the magnetization,
whereas the strain induced volume part KV favoring
an out-of-plane alignment is almost unaffected (Ref.[268]). The reduction of KS1
thus is the reason for the shift of the reorientation. The fact that the
strain in the film stays the same with and without oxygen can be directly
seen from I/V-LEED scans as shown in Fig. 11. While the tetragonal distortion
of the Ni films can be derived from the shift of the high energy Bragg
peaks towards higher energies, no difference between the peak positions
for Ni films grown with and without oxygen are observed.
Research activities:
backward
(last update: 21.08.2003)