Pulsed electrodeposition and magnetism of two-dimensional assembly of controlled-size Co particles on Si substrates

Article abstract of DOI:10.1016/j.susc.2006.03.007

We present an extremely simple and inexpensive way to obtain controlled-size and density Co metallic particles on Si(1 1 1) using electrodeposition. When unpatterned substrates are used, the particle density and size can be controlled by adjusting the pul

Full text of DOI:10.1016/j.susc.2006.03.007

Surface Science 600 (2006) 2178–2183
www.elsevier.com/locate/susc
Pulsed electrodeposition and magnetism of two-dimensional
assembly of controlled-size Co particles on Si substrates
*
M.V. Rastei , S. Colis, R. Meckenstock, O. Ersen, J.P. Bucher
Institut de Physique et Chimie des Mate´riaux, Universite´ Louis Pasteur, UMR 7504, 23 rue du Loess, BP 43, F-67034 Strasbourg Cedex 2, France
Received 19 January 2006; accepted for publication 7 March 2006
Available online 29 March 2006
Abstract
We present an extremely simple and inexpensive way to obtain controlled-size and density Co metallic particles on Si(111) using elec-
trodeposition. When unpatterned substrates are used, the particle density and size can be controlled by adjusting the pulse frequency and
the total deposition time. Randomly arranged cobalt particles with diameters of few tens of nanometres are obtained for short deposition
times. Continuing the deposition, the particle size and density can be increased until coalescence. Magnetic force microscopy images
show magnetically coupled/uncoupled particles depending on the size and distance between them. For small decoupled particles, no
in-plane uniaxial anisotropy is found, in agreement with transmission electron microscopy observations which show randomly oriented
single crystal particles. As the particle coalescence increases, the in-plane anisotropy evaluated from magnetization loops increases as
well. When deposited on focused ion beam patterned substrates, well organized nanoparticles with adjustable magnetic anisotropy
are obtained. Ferromagnetic resonance measurements performed on these samples reveal that the magnetic anisotropy originates mainly
from the particle shape.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Metal–semiconductor thin films structures; Magnetic nanoparticles; Pulsed electrodeposition
1. Introduction
is mostly due to its low cost but also to the fact that it al-
lows the growth of high aspect ratio structures like nano-
wires [1], pillars [2,3], well-organized nanoparticles [4–6],
or metal/semiconductor Schottky barriers [7]. The pro-
cesses involved in electrodeposition are quite well under-
stood nowadays, but many aspects related to the early
stage of the metal electrodeposition, especially on semicon-
ductor surfaces, are still under investigation. It is generally
found that electrodeposition on semiconductor starts from
surface defects and that the growth itself follows a Volmer–
Weber mode [6,8–17].
In this work, we present a simple and inexpensive way to
grow metallic nanoparticles with controlled-size and den-
sity on patterned and unpatterned Si(111) using pulsed
electrodeposition. First, we show that the particle size
and the density in the randomly arranged nanoparticles
deposited on unpatterned substrates can be modified by
varying the total deposition time and the pulse frequency
applied during growth. Second, we show that the particle
The morphology, structure and magnetic properties of
small metallic particles present an increased interest due
to their potential applications in the field of magnetic
recording and magneto-electronics. In most cases specific
magnetic properties are required which can be obtained
by controlling the particle size, density and shape. An inter-
esting and delicate issue in the field of magnetism is to
understand how these parameters influence the magnetic
properties in such systems. From the experimental point
of view, a simple method of growing and arranging metallic
particles is of vital importance.
Electrodeposition is one of the most attractive tech-
niques in many areas needing metal deposition. This fact
*
Corresponding author. Tel.: +33 388107021.
E-mail address: rastei@ipcms.u-strasbg.fr (M.V. Rastei).
0039-6028/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2006.03.007

M.V. Rastei et al. / Surface Science 600 (2006) 2178–2183
2179
random nucleation on pre-existing defects can be avoided if
one generates focused ion beam (FIB) patterned structures.
This induces the growth of organized nanoparticles on well
defined arrays. For each system the morphologic and mag-
netic properties including anisotropy and local magnetic
correlations between particles are discussed.
measurements were performed in a standard X-band
microwave cavity. The technique gives information on
the anisotropy distribution of the samples in the saturated
state [20]. By analyzing the resonance line position as a
function of the external magnetic field magnitude and
direction, it is possible to find the easy magnetization axis.
This magnetization axis corresponds to the lowest line po-
sition in the FMR spectrum, since in this direction the
magnetic moments are easier to saturate. The FMR mea-
surements were performed at room temperature in a field
comprised between 0 and 300 mT.
Transmission electron microscopy (TEM) observations
were performed using a TOPCON 002B microscope oper-
ating at 200 kV. Small quantities of Co nanoparticles/Si
were removed by scraping the substrate and then dispersed
on a copper grid covered with an amorphous carbon mem-
brane. The samples were investigated in high resolution
transmission microscopy (HRTEM) mode.
2. Experimental details
Negatively doped Si (111) substrates with a resistivity of
ꢀ0.1 X cm, were first cleaned in sonically agitated acetone
and washed several times with ultra-pure water. The back
side of the substrate was electrically contacted and then
covered by an insulating varnish in order to delimit at
the front side a 3 · 3 mm2 area for electrochemical deposi-
tion. The native silica layer was etched in 5% HF solution
and then the pure Si was immediately transferred into the
electrodeposition cell.
The electrochemical deposition was performed using a
solution of CoSO4 (0.1 M) + CoCl2 (0.1 M) + H3BO3
(0.64 M) with a pH of about 3.4. This mixed sulphate–chlo-
ride electrolyte assures a completely random crystallo-
graphic orientation of the grains [18], which is especially
important in the case of organized particles. In this case
the shape anisotropy imposed by the FIB pattern’s shape
has a stronger effect on the magnetization state of the par-
ticles. The reference electrode was Hg/Hg2SO4 and a Pt
wire was used as counter electrode. The electrodeposition
of cobalt was carried out in galvanostatic mode by apply-
ing a duty cycle (50% on and 50% off) of 1, 3, and 5 Hz.
The cycle off-time corresponds to zero current in the cell,
while during the on-time, the galvanostat was set to deliver
a constant current density of about À0.5 mA/cm2. This
current value was set low enough to avoid hydrogen gener-
ation. Such phenomena can lead to uncovered areas on the
substrate surface which can strongly influence the magnetic
properties of the sample.
3. Results and discussion
3.1. Randomly arranged nanoparticles
To obtain low-density Co nanoparticles, relatively short
deposition times were used. In the Fig. 1 we show AFM (a),
(b), (c) and the corresponding MFM (d), (e), (f) images
taken on samples deposited for 12 s deposition time and
for pulse frequencies of 1, 3 and 5 Hz, respectively. As
expected for the weak silicon–cobalt ions interaction,
the growth was performed in a 3D Volmer–Weber mode
The topography of the deposits was investigated using a
Nanoscope Dimension 3100 atomic force microscope
(AFM) working in tapping mode. The local magnetism
of the deposits was investigated by means of magnetic force
microscopy (MFM), which allows the visualization of the
magnetic domain structure and the magnetic correlations
between the particles. As MFM probe, a commercially
available silicon pyramidal tip coated with a 50 nm thick
CoCr layer was used. The tip was magnetized along its axis
in ‘‘down’’ direction (perpendicular to the sample plane)
using a strong magnetic field of about 3 kOe.
The average magnetism was investigated using an alter-
nating gradient field magnetometer (AGFM) [19]. The
magnetization loops were recorded by applying the mag-
netic field parallel and perpendicular to the film surface
in order to estimate the deposit anisotropy. In the case of
organized particles on the FIB patterned substrates, due
to the limited number of particles (about 62500), the
anisotropy analysis was performed with a more sensi-
tive technique: ferromagnetic resonance (FMR). These
Fig. 1. 1 · 2 lm2 atomic force microscopy (AFM) (a)–(c) and the
corresponding (d)–(f) magnetic force microscopy (MFM) images recorded
on samples grown during 12 s. Different pulse frequencies were applied
during growth: 1 Hz for sample (a), 3 Hz for sample (b), and 5 Hz for
sample (c). No magnetic field was applied prior or during the MFM
experiments.

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M.V. Rastei et al. / Surface Science 600 (2006) 2178–2183
on randomly distributed surface defects. Interestingly, a
strong dependence of the particle density and size can be
observed with respect to the pulse frequency applied during
growth (see also Fig. 3), having strong influence on the
magnetic correlations. A maximum density of about 100
particles/lm2 and small particle sizes, less than 30 nm, were
obtained for the deposit performed at 1 Hz pulse frequency
(Fig. 1(a)). We observed that the nucleation density in-
creases when the pulse frequency decreases. This effect
may be related to the shorter on/off deposition time (high
pulse frequency), which limits the diffusion of Co ions on
the Si surface. Additionally, the positive nucleation ramp
in the beginning of the deposition process is only partly fol-
lowed for each frequency pulse of the deposition sequence,
for high pulse frequencies. Since during the on-time inter-
val the number of the nuclei is enhanced, it is expected that
low frequency duty cycles generate high particle densities.
In contrast, for very low frequencies (<1 Hz) the tendency
is reversed due to the beginning of the diffusion limited
growth during the on-time interval of the duty cycle.
The strong dark-bright dipole contrasts in the MFM im-
age (Fig. 1(d)) reveals that many Co particles are magnet-
ically coupled. Groups of such particles can be observed in
both AFM and MFM images (Fig. 1). These high resolved
MFM contrasts can be well observed by a carefully choice
of the lift scan height. We found that the appropriate lift is
50 nm for the best MFM signal. The lateral resolution ob-
tained by this approach is of about 15 nm, which is good
enough to visualize if the particles are magnetically corre-
lated, but also to investigate the magnetization state of a
single particle. By increasing the pulse frequency at 3 Hz,
the particle size increases and a strong decrease of the par-
ticle density is observed. Less particles are magnetically
coupled due to the large distances between them. However,
as noticed in the Fig. 1(b) and (e) even for these relatively
large particles some magnetic correlations can be found,
typically observed for coalesced particles. The deposition
performed at 5 Hz produces particles up to 100 nm in
diameter and density of only ꢀ11 particles/lm2. In this
case, no magnetic coupling was found by MFM measure-
ments, as observed in Fig. 1(f).
Fig. 2. Idem as Fig. 1, but for samples grown during 24 s.
Fig. 3. The variation of the particle density for 12 s and 24 s deposition
times as a function of pulse frequency applied during growth.
Fig. 2(a)–(c) and (d)–(f) shows the AFM and MFM
images for samples grown for 24 s deposition time, respec-
tively. Much dense cobalt particles (see also Fig. 3) were
obtained, with a slight increase of the particle size. Whereas
the sample grown at 1 Hz reveals strong magnetic coupling
between particles, much less magnetic correlations are ob-
served for the samples grown at 3 and 5 Hz, due to the lar-
ger particle size. The strong correlations within the particle
assembly made at 1 Hz during 24 s were evidenced by the
in-plane large magnetic domains up to 9 · 104 nm2 size.
Such domains contain typically few tens of correlated Co
particles. The in-plane orientation of the magnetization in
these domains is due to the shape anisotropy which in-
creases drastically when the particles become coalesced.
For 40 s deposition time and for all three pulse frequen-
cies the Co particles are highly coalesced and the samples
Fig. 4. AFM image taken on the film edge after the removing of the
insulating varnish. For this sample the deposition time was 40 s and the
pulse frequency 1 Hz.
can be considered as thin films. As an example we show
in Fig. 4 an AFM image taken at the film edge. The film
thickness can be easily measured after removing the insu-
lating varnish. In this case homogeneous film thickness of
ꢀ30 nm was measured in different edge areas of the sample.

M.V. Rastei et al. / Surface Science 600 (2006) 2178–2183
2181
For 3 and 5 Hz deposits, rougher films were obtained with
thicknesses of about 40 and 50 nm, respectively. This slight
increase of film thickness is due to the different rms error
related to samples with a large particle size.
The in-plane magnetic anisotropy was measured by
angle-dependent magnetization measurements. By rotating
the samples in the film plane, no difference in magnetiza-
tion curves was observed, indicating no in plane uniaxial
anisotropy. This suggests that the particles have a ran-
domly crystalline orientation.
In order to have an insight into the crystalline structure
of the deposits, TEM observations have been performed.
HRTEM images reveal a well defined crystalline structure
of Co particles. Most of them are single crystals without
stacking faults and other defects. Fig. 5 shows a HRTEM
image of two adjacent nanoparticles scraped off from the
sample imaged in Fig. 1(a). Within each particle a single
crystal lattice can be observed, which consists of one set
of lattice fringes with a spacing of 0.205 nm. This is consis-
tent with the theoretical hcp {002} Co interplanar distance
of 0.2035 nm. The c axes of two individual lattices, repre-
sented by white arrows in the figure, show the random ori-
entation of two analyzed nanoparticles. Therefore, the
absence of the uniaxial in-plane anisotropy of these sam-
ples can indeed be attributed to the random orientation
of the single-crystal particles.
The magnetization measurements performed with the
field applied perpendicular to the sample plane, reveal for
all samples, larger saturation fields compared to those ob-
tained in the in-plane configuration measurements, which
speaks in favor of an in-plane anisotropy induced by the
particle coalescence. For samples grown during 12 s, both
in-plane and out-of-plane magnetization measurements re-
veal low values of remnant magnetization, which confirms
the random orientation of the crystalline structure of the
particles. The random character of the crystallographic
axes seems to be maintained in all three space directions,
as revealed by transmission electron microscopy and con-
firmed by a close inspection of the MFM images performed
on single particles. To summarize the magnetization mea-
surements, we plotted in Fig. 6 the effective anisotropy
factor Keff as a function of the deposition time. These vari-
ations show a continuous increase of Keff with the deposi-
Fig. 6. The effective anisotropy Keff as a function of the deposition time
for samples grown at 1, 3 and 5 Hz pulse frequencies. The lines are only a
guide for the eyes.
tion time, for all pulse frequencies, in agreement with the
AFM images which show an increasing number of coa-
lesced particles with the deposition time. Moreover, the sig-
nificant increase of Keff was observed for the deposits made
at 1 Hz, indicating that the coalescence of these small par-
ticles increases significantly the in-plane anisotropy. In con-
trast, Keff of the Co particles made using higher pulse
frequencies (3 and 5 Hz) seems to be nearly unaffected up
to 24 s deposition time, albeit the particle density increases.
The drastic increase of Keff with the further increase of the
deposition time at 40 s is due to the formation of compact
films. In this case, the reduction of the Keff value with the
increasing pulse frequency can easily be explained by the
increased film thickness for high pulse frequency and by
the reduced correlation between the large particles.
3.2. Organized nanoparticles
The random character of the Co particles nucleation on
natural defects can be avoided by creating focused ion
beam (FIB) small defects in the Si substrate, covered previ-
ously with a 200 nm thick SiO2 layer. Square arrays of
holes, 400 nm deep and distant by 2 lm, were etched on
different substrates. As shown in the scanning electron
microscopy (SEM) images of Fig. 7(a)–(c), three hole diam-
eters (550, 400 and 200 nm) were chosen in order to inves-
tigate the effect of the particle shape on the magnetic
properties.
AFM observations performed after the Co electrodepos-
ition at a rate of 1 Hz, show that the holes are homoge-
neously filled and protrude by a maximum of 10 nm out
of the surface (Fig. 7(d)–(f)). The current density and the
deposition time were about À0.5 mA/cm2 and 180 s,
respectively. The deposition time presented limited varia-
tions from one sample to another due to the confined
growth conditions. The choice of 1 Hz pulse frequency
was revealed to be the most adequate to grow organized
particles, since, as observed in the case of particle growth
on free silicon surface, this pulse sequence assures the
smallest grain size and the highest nucleation density. This
Fig. 5. TEM image reflecting the different orientation of the particle c-
axes. The particles were scraped from the sample grown at 1 Hz pulse
frequency and during 12 s.

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M.V. Rastei et al. / Surface Science 600 (2006) 2178–2183
Fig. 7. (a)–(c) SEM images of FIB-etched holes with different geometries on SiO2/Si substrates. All holes are 400 nm deep and have diameters of 550, 400
and 200 nm, respectively. (d)–(f) AFM images taken after the electrodeposition of Co within the holes. (g)–(i) The MFM images recorded for the
perpendicular ‘‘down’’ remnant state.
confirms a compact and homogeneous growth avoiding
empty volumes or other defects inside the particle which
can lead to a large magnetic properties distribution. We be-
lieve that the Co particles within the holes are made of crys-
talline grains randomly oriented. This is supported by
AFM images that reveal a granular nature of the upper
part of the Co particles.
To give an overview on the magnetic properties of the
differently shaped Co particles from Fig. 7, FMR and
MFM investigations were performed.
The Co particles with an aspect ratio of 0.7 have a mul-
tidomain structure, as shown by the MFM observations
(Fig. 7(g)). This is in agreement with the in-plane to out-
of-plane angle-dependent FMR measurements which show
the lowest resonance line for the in-plane applied field of
about 0.10 T as presented in Fig. 8. The large line width
of the spectrum is due to the random crystalline distribu-
tion of the Co particles in the sample plane. The in-plane
angle dependent FMR measurements do not reveal any dif-
ference in the resonance line position, confirming the ran-
dom orientation of the magnetization from magnetic
domains.
(Fig. 8). The shifted line position to higher resonant fields
and the narrow width of the resonance peak show an in-
crease of the shape anisotropy of the Co particles. The
in-plane to out-of-plane angle-dependent FMR measure-
ments show the lowest resonance line position at about
0.20 T for 9.4 GHz which clearly proves that the easy mag-
netization axes of the particles lie in the sample plane.
Moreover, the in-plane angle-dependent FMR measure-
ments show no in-plane uniaxial anisotropy.
The MFM experiments performed on the Co particles
with an aspect ratio of 2 showed single domain states with
the magnetization oriented perpendicular to the sample
plane (Fig. 7(i)). As for the previous samples, this is in
agreement with the FMR measurements given in the
Fig. 8. In this case, the lowest resonance line position is
found when the field is perpendicular to the sample plane,
at about 0.12 T, and exhibits a narrow resonance, indicat-
ing that nearly all particles are in the same magnetic state
with their easy magnetization axis strictly aligned along
the particle axis.
4. Conclusion
For particles with an aspect ratio of 1, the MFM mea-
surements indicate a single domain state of the Co particles
with a magnetization randomly distributed in the plane
(Fig. 7(h)). This is confirmed by the FMR spectrum
Frequency-dependent pulsed electrodeposition with an
equal on- and off-times was used in order to produce con-
trolled-size and density nanometric Co particles on Si
(111) surface. The morphology, structure and magnetic
properties of the Co nanoparticles were investigated. We
found that for randomly distributed particles the magnetic
anisotropy is dominated by the magnetic inter-grain corre-
lations. MFM images indicated magnetically coupled/
uncoupled particles depending on the distance between
them. Small uncoupled particles are found to have no in-
plane uniaxial anisotropy, in agreement with TEM obser-
vations which show randomly oriented single-crystal
particles. The magnetization measurements showed that
the in-plane magnetic anisotropy increases faster for small
particles, when the particle density was increased.
Fig. 8. FMR spectra of different aspect ratios Co nanoparticles (height/
diameter of 0.7, 1.0, and 2.0) electrodeposited on FIB-structured Si
substrates.

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