Applied Physics A manuscript No.
(will be inserted by the editor)
Mass-filtered cobalt clusters in contact with epitaxially ordered metal
surfaces
J. Bansmann1 , M. Getzlaff2 , A. Kleibert1 , F. Bulut2 , R.K. Gebhardt2 , K.H. Meiwes-Broer1
1
2
Institut für Physik, Universität Rostock, Universitätsplatz 3, D-18051 Rostock, Germany
Institut für Angewandte Physik, Universität Düsseldorf, Universitätsstrasse 1, D-40225 Düsseldorf, Germany
e-mail: [email protected], fax: +49 381 498-6802
Received: date / Revised version: date
Abstract Mass-filtered cobalt clusters with a size between 5 nm and 12 nm have been deposited in-situ under
soft-landing conditions onto epitaxially ordered iron and
nickel films. The spin and orbital moments of both the
clusters as well as the substrate films have been investigated using the element-specific method of X-ray magnetic circular dichroism in photoabsorption. Here, the
ferromagnetic films with in-plane magnetic anisotropies
have been used to magnetize the clusters remanently
without applying external magnetic fields during the measurements. Experimental results from the cobalt clusters
are discussed with respect to the different substrates.
Furthermore, the influence of oxygen exposure on the
spin and orbital moments of cobalt clusters has been
investigated in in-situ oxidation experiments.
Key words
PACS: 73.22.-f 75.70.-i 75.75.+a 81.07.-b
1 Introduction
In recent years, experiments on metal clusters and nanoparticles have gained rising interest in fundamental research as well as for technical applications. These materials cannot be considered as smaller parts of a solid, their
physical properties clearly differ from those of the respective bulk. The high surface to bulk ratio and the lower
coordination of atoms are the driving forces that lead to
size-dependent characteristics. Even the structural parameters such as the shape of the particles or the interatomic distances are affected by these processes. Many
investigations have been carried out on small particles
in the gas phase which dramatically show a variation
of electronic, optical, chemical and magnetic properties
as a function of the cluster size. An overview on cluster
physics in the gas phase as well as on surfaces in given
in severeal review articles [1–4].
In this contribution, we will focus on the magnetic properties of preformed large metal clusters that are deposited
onto surfaces from a beam. Usually, the interesting size
regime in cluster physics, where large variations occur
when changing the number of atoms in each cluster, is
clearly limited by less than 1000 atoms per cluster. Nevertheless, a significant deviation in the magnetic spin
and orbital moment could even be observed for much
larger systems being in contact with surfaces. In the size
regime up to 12 nm (or about 100,000 atoms per cluster),
the magnetic properties such as the magnetic anisotropy
and the spin as well as the orbital moments clearly deviate from the respective bulk material.
However, interesting magnetic phenomena are not restricted to alloy clusters. Even mass-filtered Fe clusters
in the size regime from 6 to 12 nm show enhanced orbital moments when being deposited on a ferromagnetic
cobalt layer [5]. Enhanced orbital moments arise mainly
from the high number of atoms sitting in surface positions. Of course, these enhanced orbital moments observed for Fe clusters open the question whether such
effects may also be observed for other 3d metal clusters
deposited on surfaces. Much effort has been employed to
produce size-selected particle beams in this size regime
by different techniques [6–9]. Today, binary clusters with
sizes up to 12 nm (e.g., FePt and CoPt clusters) are considered as prototypes for new magnetic storage materials
due to their unique magnetic properties such as tuneable
magnetic anisotropies. In this article we report on investigations related to mass-filtered cobalt particles being
in contact with different ferromagnetic surfaces.
2 Experimental
Experiments on exposed mass-filtered metal clusters on
ferromagnetic supports require in-situ cluster deposition
and element-specific analysis techniques. We have applied the method of X-ray magnetic circular dichroism
which is based on resonant photoabsorption at the 2p
core levels of 3d-transition metals. The technique offers
2
J. Bansmann et al.
Fig. 1 Schematic drawing of the arc cluster ion source
(ACIS) including the mass-separation unit.
the possibility to determine the magnetic orbital and
spin moments separately (see Sect. 2.2). The complete
set-up with the cluster source has been installed at the
helical undulator beamline UE46-PGM1 of BESSY for
in-situ deposition experiments. The spectroscopy chamber at the beamline is equipped with a LEED system
for the characterization of epitaxially ordered substrates
and thin films as well as a magnetization coil close to
the sample for reversing the magnetization at each data
point by short current pulses with about 100 A.
As substrates for the deposited clusters, we have prepared epitaxial bcc(110) iron and fcc(111) nickel films
on a clean W(110) crystal under UHV conditions. The
cleanliness and quality of both the tungsten surface as
well as the thin ferromagnetic films has been checked by
LEED. Both films grow in the layer-by layer mode on
the W(110) surface at room temperature. In the case of
Fe(110) on this tungsten surface, the easy magnetization
axis is pointing along the in-plane W[110]-direction. For
Ni(111) films, the in-plane W[001] axis has to be chosen
for a remanent magnetization. Details on the growth and
magnetic properties of these films have been studied in
the past by several groups: Fe: [10,11], and Ni: [12–14]).
2.1 Cluster source and sample characterization
For the experiments described here, pure cobalt clusters
have been produced in the arc cluster ion source (ACIS),
cf. Fig. 1. This continuously working source has been developed for a high flux of mass-filtered metal clusters in
the size regime from 4 nm to 15 nm for in-situ cluster deposition experiments [15]. The cluster source including
the mass-filtering unit is fully ultrahigh vacuum compatible and small in size to enable an easy transport to synchrotron light sources such as the storage ring BESSY
in Berlin.
The hollow cathode as central part of the source consists of the target material, in our case cobalt with a
purity of 99.9 %, where the cluster material is eroded
from. The operation is based on an arc discharge in
the presence of noble seeding gases (here: argon with
a pressure of about 20 mbar), where the nucleation of
Fig. 2 Top part: TEM image from Co clusters deposited
onto commercial TEM grids with pass energies of 440 eV (left
part) or 3500 eV (right part) at the quadrupole deflector, cf.
Fig. 1. Lower part: corresponding size distribution of Co clusters with a mean size of 6.5 nm (left) and 12 nm Co clusters
(right).
nanoparticles starts. The aggregates are kinetically accelerated by collisions in the nozzle and in a weak supersonic expansion to almost the velocity of the noble
seeding gas. Two pumping stages reduce the large background of noble gas by several orders of magnitude. An
electrostatic quadrupole serves as a size selector by deflecting the charged particles by 90◦ with respect to the
ratio of the elementary charge and the mass of the cluster. The major part (more than 50%) of the cluster beam
is charged, both positively and negatively with nearly
equal contributions. Due to the nearly constant velocity,
the kinetic energy of the clusters is directly related to
their mass and allows a separation by using an electrostatic energy dispersive element. Moreover, the kinetic
energy of the nanoparticles prior to deposition is clearly
below the threshold for fragmentation, usually less than
0.1 eV per atom. The size distribution of the particles has
been investigated by ex-situ transmission electron microscopy (TEM). After passing the mass-filtering unit,
the cluster beam has been deposited onto commercial
TEM grids covered with amorphous carbon, cf. Fig. 2.
Evaluation of the data yields a narrow size distribution
of the deposited clusters with a sharp cut-off to smaller
sizes without any hint for significant fragmentation. The
results displayed in Fig. 2 correspond to cobalt clusters
with a mean size of 6.5 and 12 nm. A small amount of
larger clusters may occur due to multiple charged particles and a certain velocity slippage in the cluster beam
known for large clusters.
Mass-filtered cobalt clusters in contact with epitaxially ordered metal surfaces
3
2.2 X-ray magnetic circular dichroism in
photoabsorption
All experiments described here have been carried out using the technique of X-ray magnetic circular dichroism
(XMCD) in photoabsorption at the spin-orbit split 2p
core levels of the respective 3d materials. In the following
we will briefly outline this technique [16,17] that allows
us to determine element-specifically the orbital and spin
moments of both the deposited clusters as well as the
underlying ferromagnetic film independently. For the 3d
transition metals Fe, Co, and Ni, the density of states
(DOS) can be regarded as a composition of two separate
DOS with opposite spin character (majority spin and minority spin) being energetically shifted by the exchange
interaction in the 3d valence band (typically 0.6-2 eV),
cf. fig. 3. The spin-orbit split 2p core levels (binding energies of 708 and 721 eV for Fe) are the initial states in
the photoabsorption process. The use of circularly polarized radiation leads to an effective spin polarization
of the excited electrons due to the Fano-effect [18]. By
taking into account the conservation of the spin of these
electrons, the exchange-split unoccupied states near the
Fermi level EF act as a spin detector.
The magnetic orbital mL and spin moments mS (in units
of µB ) can be derived by integrating the XMCD difference and the sum spectra recorded over the energy range
enclosing both absorption peaks, cf. Eqs. 1, 2 and Fig. 4.
The orbital moment is then proportional to the difference of the two areas A and B indicated in the difference
spectrum, whereas the spin moment is proportional to
the sum of area A and two times area B.
nh
mL ∝
· (A − B)
(1)
cos θ · Pcirc
nh
· (A + 2 · B)
(2)
cos θ · Pcirc
The number of holes nh of the material (cf. the unoccupied states), the degree of circular polarization Pcirc
(with Pcirc =0.9 at UE-46 beamline) and the angle of incidence θ enter both equations (here, cos θ denotes the
projection of the photon spin on the in-plane magnetization direction). It should be mentioned that the determination of the spin moments is influenced by a smaller
contribution from the magnetic dipole term mT [17,19,
20]. In the following, we will refer to mS + 7mT as an
effective spin moment denoted for simplicity as mS . Details on the evaluation of the orbital and spin moments
can be found in the original work on the magneto-optical
sum rules by Thole et al. [21] and Carra et al. [22] as
well as in several other articles (e.g., Chen et al. [23] and
Stöhr et al. [17]). All photoabsorption spectra presented
here have been measured by means of total electron yield
(TEY). In this case, self-absorption effects may result
in underestimated spin and orbital moments. For thin
films, a procedure for correcting self-absorption can be
applied [24]. Details on self-absorption effects for clusters
on surfaces have recently been studied by Fauth [25].
mS + 7mT ∝
Fig. 3 Schematic drawing of the mechanism of X-ray magnetic circular dichroism in photoabsorption at 3d metals with
exchange split valence bands. The incoming circularly polarized photons excite transitions from the core electrons into
the unoccupied 3d bands. Due to spin conservation in photoabsorption, the probability of this process depends on the
spin polarization of the excited electrons (Fano-effect) and
the level occupation in the exchange split valence band.
3 Results and Discussion
The investigations concentrate on cobalt clusters being
in contact with ferromagnetic layers grown epitaxially
on a tungsten single crystal surface. Recently, we have
shown that even large clusters can be magnetized remanently by exchange interaction with the underlying magnetic film [5]. This section is organized as follows. First,
we characterize the magnetic properties of the ferromagnetic Fe(110) and Ni(111) films before discussing the results on clean cobalt clusters on surfaces in Sect. 3.2.
Investigations on cobalt clusters on Ni(111) exposed to
oxygen are presented later in Sect. 3.3.
3.1 Fe(110) and Ni(111) films as substrates
We will start with the magnetic characterization of an
Fe(110) film with a thickness of 15 ML using the XMCD
technique. The sample is oriented with the easy magnetization axis (W[110]) parallel to the plane of incidence. In the upper panel, Fig. 4 shows photoabsorption
spectra from the Fe 2p levels close to 700 eV (see spinorbit split Fe 2p3/2 and 2p1/2 peaks) up to 825 eV, i.e.,
behind the energetic position of the cobalt absorption
edges. The two curves correspond to spectra taken with
opposite magnetization directions at a fixed helicity of
circular polarized radiation. Different intensities in the
Fe peaks reflect the magnetization in the Fe film. The
lower panel displays the respective intensity difference
(i.e., the XMCD signal) with the shaded areas A and B
close to the Fe 2p levels. Both insets show those parts of
the spectra enlarged, where the Co 2p features will appear upon cobalt cluster deposition. In the upper panel,
4
Fig. 4 Top: photoabsorption spectra for opposite magnetization directions taken at a 15 ML iron film on W(110) with
a grazing angle of incidence of 30◦ . Bottom: corresponding
XMCD signal (intensity difference, curve with shaded area).
The insets in both panels show the part of the spectrum
related to the energetic position of the Co 2p core levels enlarged.
the inset displays the oscillating behaviour of the absorption curve well behind the absorption edges of Fe.
These EXAFS oscillations are well known in the literature. The inset in the lower panel shows a non-vanishing
magnetic effect in the EXAFS regime. Magnetic EXAFS
(MEXAFS) phenomena at the 2p absorption edge of 3d
metals are difficult to analyse due to the strongly overlapping signals from the L3 and L2 edges [26–29] and
are far beyond the scope of this article. However, the
magnitude of the EXAFS oscillations strongly depends
on the angle of incidence and increases with grazing incidence. Since these features overlap with typically very
weak cluster-induced signals from the cobalt 2p levels
(see Fig. 6), they make a thorough data analysis difficult at small angles of incidence (cf. Sect. 3.2).
The values for the spin and orbital moments given in
Table 1 have been obtained by applying the XMCD sum
rules with the number of holes nh = 3.39 according to
the procedure outlined by Chen et al. [23]. For the spin
moment of Fe(110) films on W(110), we found mS =
2.01 ± 0.05µB independent from the angle of incidence
between 30◦ and 55◦ , whereas the ratio of orbital to spin
moment mL /mS changes from about 0.048 to 0.064. The
latter reduction of the ratio for the smaller angle of incidence is only caused by the determination of the orbital
J. Bansmann et al.
Fig. 5 Upper and lower part: photoabsorption and XMCD
spectra from clean Ni(111) film on W(110) taken analoguously to Fig. 4. The upper inset shows the background at
the cobalt 2p related photon energy range together with a
spectrum from 9.5 nm cobalt clusters. Lower inset: vanishing XMCD background (enlarged) in the cobalt 2p energy
regime.
moments being very sensitive to self-absorption effects
[24]. Hence, the orbital to spin moment mL /mS = 0.064
obtained at 55◦ should be close to the true value and
is clearly larger than the corresponding Fe bulk value of
0.043. Enlarged values have also been observed for thin
Fe films on different substrates, e.g., ultrathin films on
copper surfaces [30,31].
Fig. 5 shows photoabsorption and XMCD spectra for a
clean Ni(111) film (15 ML) taken analoguously to the
Fe(110) film. The sample has been oriented with the
easy magnetization axis, i.e., the W[001] axis, parallel
to the plane of incidence. The Ni spectra exhibit an additional feature, a satellite located 6 eV towards higher
energies for both levels, cf. Fig. 5. This satellite, a final state effect, is well-described in the literature (see
e.g., Ref. [32]). When calculating the orbital and spin
moments separately, we obtain mL /mS = 0.125 ± 0.005
for the ratio of orbital to spin moment and mS = 0.7 ±
0.005 µB for the spin moment (see Table 1) by using
nh = 1.45 for the number of holes [33]. The data for
the ratio of orbital to spin moment as well as for the spin
moment in nickel films are close to the bulk Ni data and
in good agreement with results on thin films from other
groups [34,35]. Due to the smaller absorption coeffecient
of nickel when compared to iron, self-absorption plays a
Mass-filtered cobalt clusters in contact with epitaxially ordered metal surfaces
30◦
55◦
bulk
mL /mS
Fe
0.048 ± 0.004
0.064 ± 0.004
0.043
mS [µB ]
Fe
2.02 ± 0.03
1.99 ± 0.04
1.99
mL /mS
Ni
0.125 ± 0.005
0.127 ± 0.005
0.11
mS [µB ]
Ni
0.706 ± 0.005
0.705 ± 0.005
0.6-0.65
5
Table 1 Ratio of the magnetic orbital and spin moment
mL /mS and the spin moment mS per atom for 15 ML Fe(110)
and Ni(111) films on W(110) for two angles of incidence. The
bulk values (last column) have been taken from Chen et al.
[23] for bcc Fe and Dhesi et al. [33, 34] for fcc Ni.
less dominant role. Thus, significant differences in the
magnetic moments have not been observed between 30◦
and 55◦ . The inset in the upper part of Fig. 5 shows the
smooth photoabsorption spectrum of nickel in the energy range of the Co 2p edges. Additionally, a spectrum
after cobalt cluster deposition is displayed in order to
show the small magnitude of the cobalt absorption signal with respect to the substrate. The lower inset reveals
the absence of any magnetic background signal (XMCD
spectrum enlarged) before depositing cobalt clusters.
3.2 Mass-filtered clean Co clusters on Fe(110) and
Ni(111) surfaces
Mass-filtered cobalt clusters have first been deposited
onto thin Fe(110) films. Fig. 6 displays the corresponding
photoabsorption and XMCD (lower curve in each panel)
spectra of the remanently magnetized cluster sample
(size: 12 nm) taken at three different angles of incidence
(10◦ , 30◦ and 55◦ with respect to the surface plane). The
shape of the absorption curves strongly differs with the
angle of incidence. The EXAFS wiggles from the Fe(110)
shown in Fig. 4 strongly disturb the absorption signal
from the cobalt clusters. At 10◦ (lower panel in Fig. 6),
the amplitude of the EXAFS is even comparable to the
cobalt peaks. The MEXAFS background visible in the
difference signal is present up to an angle of 30◦ and
prevents a reliable XMCD analysis. Thus, we choose an
angle of 55◦ where, on the one hand, the EXAFS oscillation vanish, and, on the other hand, the XMCD signal
still has a good signal to noise ratio.
Mass-filtered cobalt clusters have also been deposited
onto thin Ni(111) films on tungsten. The data evaluation
of cobalt clusters on this surface is much easier since the
strong absorption edges of the nickel films are at higher
binding energies and thus do not overlap with the cobalt
features, cf. fig. 5. Thus, we were able to analyse the angular dependence (10◦ - 55◦ ) of the spectra in detail.
When averaging the two absorption data for opposite
magnetization, we could not observe any difference in
the shape of the spectra (not shown here). This hints
Fig. 6 Photoabsorption spectra for three different angles
of incidence taken with circularly polarized radiation from
12 nm Co clusters deposited on a bcc(110) Fe film on W(110).
The system has been magnetized remanently in-plane along
the easy magnetization axis of the Fe(110) films. Lower part:
XMCD difference spectra.
at a nearly spherical shape of the clusters on the Ni surface. Self-absorption effects of clusters strongly deformed
upon deposition would induce an angular dependence in
the spectra. Also the magnetic moments obtained from
our data do not show a significant angular dependence.
In Fig. 7 we display the magnetic moments of cobalt
clusters on Ni(111) and Fe(110) surfaces in the size regime
between 5 nm and 12 nm. The coverage of cobalt clusters
on the surfaces is very small (average distance larger
than 50 nm), magnetic dipolar interactions and agglomeration of clusters to larger units can be neglected. Cobalt
clusters on Fe(110) and Ni(111) do not show significantly
enhanced orbital moments as observed for iron clusters
on a hcp(0001) Co film [5]. In the case of cobalt clusters
on Ni(111), the ratio of orbital to spin moment is even
below the corresponding bulk value, cf. upper panel in
Fig. 7. The data of Co clusters on Fe(110) show slightly
higher ratios. The spin moments of cobalt cluster larger
than 6 nm (see middle of Fig. 7) are close to the bulk Co
value (fcc and hcp) of 1.55 µB , only the data for clusters
with a size of 5 nm show a significantly smaller value.
We suppose that, in this case, the exchange coupling to
the Ni film is too small to suppress the onset of thermal
6
J. Bansmann et al.
3.3 Influence of oxygen exposure on the magnetic
properties of cobalt clusters deposited on Ni(111)
Fig. 7 Magnetic spin and orbital moments of mass-filtered
cobalt clusters on ferromagnetic Ni(111) and Fe(110) surfaces
as a function of cluster size. Top: ratio of orbital to spin moment; middle: spin moments; lower panel: orbital moments.
Dashed lines represent the data for bulk hcp cobalt (from
ref. [23]).
fluctuations in the magnetization of cobalt clusters at
room temperature. Finally, the orbital moments of Co
clusters on Ni(111) are generally smaller than the bulk
hcp Co value and the data for Co clusters on Fe(110). A
possible explanation could be that the structure of the
cobalt particles on the surface changes from the hexagonal closed packed (hcp) structure to the closely related
fcc structure on the epitaxially ordered fcc Ni surface.
In this case the ratio of orbital to spin moment mL /mS
is supposed to be smaller since the corresponding fcc
bulk Co value of mL /mS = 0.078 [30,36] is clearly reduced when compared to hcp cobalt. However, the situation may be different on the quasihexagonal bcc(110)
surface of the iron film. This would explain the clearly
visible differences in the orbital moments. The discussion on the structure of cobalt clusters demonstrates the
importance of further structural investigations in order
to understand the experimental results on the magnetic
properties of nanoparticles. It should be mentioned that
the spin and orbital moments have also been analyzed
for the underlying films. The magnetization in all films
was saturated (not shown here).
Several groups have investigated chemically prepared cobalt clusters with an oxide shell resulting from the fabrication process (e.g., [36,37]). They observed a core-shell
state with metallic cobalt in the center and antiferromagnetic cobalt mono-oxide (CoO) in the outer shell. In
this section, we present an in-situ investigation on the
oxidation of initially pure cobalt clusters on the Ni surface.
When exposing the sample to oxygen also the Ni(111)
film will be oxidized and change its magnetic properties. In the present case the system has been exposed
to 500 L oxygen. With respect to the photoabsorption
spectra in Fig. 5 for clean Ni(111), only the intensity
of the L3 peak increases slightly, further changes in the
shape of the spectra are not visible. The effect is discussed by charge transfer from the pure Ni 3d states
to the 2p-derived oxygen states NiO and leads to a decrease of the number of occupied states (and thus to
an increase of the number of holes nh ). Similar results
have been observed in other groups, e.g., for Ni films on
Cu(100) [38,35]. With respect to magnetic properties, a
small reduction in the XMCD intensity difference was
observed (not shown here). The ratio of orbital to spin
moment is nearly unaffected by the oxygen adsorption
and oxide formation, the value is still mL /mS = 0.11.
However, when calculating the spin and orbital moments
assuming a nearly un-changed value for the number of
holes, one obtains a spin moment of mS = 0.5 µB , i.e.,
a reduction of about 25% with respect to the clean surface. Similar effects had been observed in the past, e.g.,
in spin resolved photoelectron spectroscopy at iron [39],
cobalt [40] and nickel films and single crystals [38,41].
An exposure to oxygen leads to a reduction of the spin
polarization in the 3d band close to the Fermi energy
EF of these itinerant magnets indicating a strong reduction in the magnetization. The adsorption of of 500 L
oxygen on Ni(111) gives rise to an oxidation to NiO in
the upper layers and hence, to a formation of about two
”magnetically dead” layers (vanishing spin moment).
The exposure to oxygen has a much larger effect
on the spin and orbital moments of Co clusters on the
Ni(111) surface. Photoabsorption spectra from 9.5 nm
Co clusters after an exposure to 500 L oxygen are displayed in the lower part of Fig. 8 in comparison to clean
cobalt clusters (upper part). The data clearly show smaller intensity differences in both cobalt peaks, especially
the XMCD in the 2p1/2 peak is strongly reduced. This
effect hints at a reduction of the spin moment. Moreover, the spectra after exposure to oxygen show some
additional features, which are known from cobalt oxide
(CoO), cf. inset (Co 2p3/2 peak) in the lower part (from
[42]). A comparison of the Co 2p3/2 peak shape after an
exposure to 500 L oxygen with the reference data (see
inset) clearly shows the peaks (A and C) in the left and
Mass-filtered cobalt clusters in contact with epitaxially ordered metal surfaces
7
of the outer 2-3 layers is confirmed. It should be mentioned that the information depth of electrons escaping
from Co and CoO is clearly different (Co: 22 Å, CoO
30 Å[43]). On the other hand, the larger penetretation
depths in CoO is more or less cancelled by the opening
of the cobalt lattice upon oxidation (1 Å in cobalt corresponds to 1.75 Å in cobalt oxide CoO [43]). The results
are discussed in [36] by uncompensated spins of Co2+ at
the core-shell interface. Ghiringhelli et al. [44] calculated
a spin moment of mS = 2.62 µB and an orbital moment
of mL = 1.36 µB at a temperature of 0 K. This refer
to an ratio mL /mS = 0.6 at 0 K; at room temperature
mL /mS still amounts to 0.36. Finally, we will discuss
the results for the spin moment. After oxygen treatment
a strong reduction in the cobalt spin moments to about
mS = 0.7 µB is observed. Generally, a reduction in mS
is not surprising since CoO is known to be antiferromagnetic and does not contribute to the spin moments.
When carrying out a simple model calculation for the
ratio of the orbital to the spin moment based on an exponential decay of electrons escaping from the clusters
we find a thickness of the oxide layer of approximately
2-3 layers.
4 Conclusions
Fig. 8 Photoabsorption spectra with circularly polarized radiation from cobalt clusters on Ni(111) with XMCD signal
in the lower part of each panel. Top: clean Co clusters on
Ni(111) surface, bottom: after exposure to 500 L oxygen after deposition. The inset shows the 2p3/2 peak of CoO [42]
for comparison. Please note a small energy shift (2-3 eV) between our results and the data shown in the inset.
right shoulder of the main cobalt peak B (Co 2p3/2 ). The
peaks D and E are not visible in our spectra. The weak
presence of oxide-related features in the Co 2p3/2 peak
indicates that the oxidation has probably just started in
the outer layers at an exposure of 500 L O2 . A very high
exposure of oxygen is required to oxidize cobalt layers
well below the surface.
The question for cobalt cluster now is: What happens to the spin and orbital moments upon exposure
to oxygen? In the case of the underlying nickel layer,
the ratio of orbital to spin moment mL /mS does not
change significantly when exposed to oxygen. Experimental data from Wiedwald et al. [36] and Flipse et al.
[37] show a strong enhancement in the ratio of orbital
to spin moment mL /mS between 0.24 and 0.3 at room
temperature for chemically prepared cobalt clusters on
surfaces. When analyzing the ratio of orbital to spin moment of the data presented in Fig. 8, we obtain a value
of mL /mS = 0.16 ± 0.02. This value is clearly larger
than the value before oxidation (0.08) or the respective
bulk value of 0.099 for hcp cobalt [23]. Compared to the
much larger values reported by Wiedwald et al. [36] and
Flipse et al. [37], our estimation about a partial oxidation
In conclusion, magnetic properties of mass-filtered cobalt
nanoparticles on ferromagnetic surfaces have been investigated in in-situ deposition experiments on epitaxially
ordered ferromagnetic surfaces using element-specific photoabsorption spectroscopy in the soft X-ray regime. The
technique of X-ray magnetic circular dichroism has been
applied in order to determine the spin and orbital moments in the cobalt nanoparticles in the size regime from
5 to 12 mm and in the underlying iron and nickel films.
Cobalt clusters on Fe(110) and on Ni(111) films do not
show enhanced orbital to spin moments mL /mS , which
is in contrast to results for iron clusters on hcp(0001)
cobalt surfaces. Co clusters being in contact with the
Fe(110) surface show a behaviour close to the cobalt
bulk. On a nickel film, both the spin and the orbital moments decrease with respect to hcp cobalt. We assume
that especially cobalt clusters on Ni(111) exhibit an fccstructure which leads to a smaller value for mL /mS as
observed in our investigations. Finally, we have studied the influence of an exposure to oxygen on cobalt
clusters on a Ni(111) surface. Nickel films clearly show
a reduced spin moment whereas the ratio of orbital to
spin moments is not changed. In contrast cobalt clusters
display a strongly enhanced ratio of orbital to spin moment similar to those observed by other groups. At the
same time the spin moment clearly decreases to about
half its value when exposed to 500 L oxygen. The strong
increase in the ratio mL /mS can be explained by uncompensated spins at the core shell interface of the cobalt
cluster. In order to understand the results on the magnetic properties in detail additional in-situ investigation
8
on the structural order of clusters on surfaces are required. These will be performed in the near future using
UHV scanning tunneling microscopy on these systems.
Moreover, an advanced theory related to the spin and orbital moments in large metal nanoparticles on surfaces
would be highly desirable.
Acknowledgements We would like to thank our co-workers
and colleagues R.-P. Methling (now INP Greifswald), V. Senz,
J. Passig, K.L. Jonas and G. Holzhüter (Univ. Rostock). Furthermore, we are indebted to E. Holub-Krappe, H. Maletta
and D. Schmitz (Hahn-Meitner-Institut Berlin) for their help
during the experiments at the HMI beamline UE-46PGM
and to K. Fauth with respect to many discussions on selfabsorption and saturation effects in clusters. We gratefully
acknowledge technical support by the staff of BESSY in Berlin, and financial support by the Deutsche Forschungsgemeinschaft (DFG) within the priority program 1153 Clusters in
Contact with Surfaces.
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