The Magnetic Properties of Metal-Alloy Glass Composites
Prepared by Ion Implantation
César de Julián Fernándeza, Giovanni Matteia, Claudio Sangregoriob, Cinzia Sadaa,
Chiara Maurizioa, Sara Padovania, Dante Gatteschib and Paolo Mazzoldia
a
b
I.N.F.M. –Dip. of Physics , University of Padova. Via Marzolo 8, Padova 35124 Italy
L.A.M.M., University of Firenze, Via della Lastruccia 3, Sesto Fiorentino, Firenze 50019 Italy
Abstract. The structural and magnetic properties of Co-Ni, Co-Fe and Ni-Cu alloy nanoparticles formed in silica matrix
by sequential ion implantation are presented. These nanoparticles show crystal structure similar to the corresponding
bulk alloys. In the Co-Ni and Co-Fe, magnetization saturation and coercive field depend on the the alloy composition,
crystal structure and size effects. Ferromagnetic resonance studies show that collective magnetic processes are present
and these are determined by the film-like morphology of the implanted region. The temperature dependence of the
magnetization of the NixCu100-x samples indicates that their Curie Temperatures are larger than the corresponding bulk
ones. This feature is discussed considering the composition of the nanoparticles and the size effects.
alloy composition: it decreases linearly from the Ni
value to zero, for the Ni40Cu60 alloy [4]. This
relationship will be used to analyze and discuss the
nanostructural and compositional features of Ni-Cu
implants. The magnetic properties of all materials will
be discussed taking into account their nanostructural
and composition features and the size-effects.
INTRODUCTION
Metal based nanocomposites present peculiar
physical properties, due to size-effects, which are
attractive from both the fundamental and the
technological points of view [1, 2]. In the case of the
magnetic properties, nanoparticulated composites
exhibit enhanced coercivity, thermally activated
demagnetization process, shift in the hysteresis loops
and magnetotransport properties [2,3]. Therefore these
materials are candidates to be applied in magnetic
recording
industry
and
high
frequency
microelectronics. The preparation and study of
bimetallic nanoparticulated composites have a large
interest because for application, their physical
properties can be tailored by combining the alloy
composition and size effects. Recently ion
implantation technique has been applied successfully
to obtain single metal nanoparticles dispersed in silica
matrix [1]. In this work we study the magnetic
properties of different binary metallic composites
prepared by sequential ion-implantation: Co-Ni, Co-Fe
and Cu-Ni. Co-Fe bulk alloys are very soft magnetic
materials and have large magnetic moment [4]. The
CoNi alloys can have soft or hard magnetic properties
depending on the Co:Ni composition [4]. In the Ni-Cu
bulk alloys, the Curie temperature (TC) varies with the
EXPERIMENTAL
Fused silica glass slides (Heraeus) were implanted
at room temperature with either a single ion specie
(Co+ , Fe+, Cu+, or Ni+) or sequentially with two
different species with the INFN-INFM Ion
Implantation Facility at the INFN-Legnaro National
Laboratories. The implantation energy for all the ionic
species was 180 keV so they have comparable
projected range. The dose of each species was adjusted
in order to get the different compositions (4:1, and 1:1
Co:Ni ratio, 4:1 and 1:1 Co:Fe ratio, and 7:3, 3:2, 1:1
and 2:3 Ni:Cu ratio) and the total implanted dose was
30 ·1016 ions/cm2 for Co-Ni and Co-Fe samples and 12
·1016 ions/cm2 for Cu-Ni samples. All the samples are
labeled as AB-nA:nB, where A, B are the species in the
order of implantation; nA:nB is the A:B ion dose ratio.
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
675
the CoNi-1:1, the CoNi-4:1 and CoFe-1:1 samples. In
all the case magnetization curve is saturated over 2 T.
We point out that if the saturation was not reached at
high field, this will indicate the presence of sizeeffects or oxidized nanoparticles in these composites.
In those conditions the magnetization at high field can
not be considered representative of the intrinsic
magnetic properties of the nanoparticles. In a pure-Co
sample (24 ·1016 Co+/cm2) the magnetic moment was
1.6 ± 0.2 µB. Considering the Figure 1, we observe that
the magnetization decreases implanting Ni upon Co
(0.9 ± 0.1 µB and 1.2 ± 0.1 µB for CoNi-1:1 and the
CoNi-4:1, respectively) and implanting Fe upon Co the
magnetization increases (2.2 ± 0.2 µB for CoFe-1:1).
The magnetic moments per atom of the bulk alloys
(0.95 µB ,1.4 µB and 2.2 µB for the Co50Ni50 ,Co80Ni20
and Co50Fe50 alloys, respectively) are similar to those
obtained in the implanted samples. These results are a
further evidence of the formation of the alloy at
nanoscale.
RESULTS AND DISCUSSION
The Co-Ni and Co-Fe systems
Magnetic moment per implanted atom (µB)
TEM results show that in all the samples the
morphology in the implantation region is similar: 6 ± 3
nm spherical particles are dispersed in a in-depth
narrow film two hundred nanometers thick. RBS
studies indicate that the species in- depth concentration
is almost gaussian and, considering that all the ions are
forming metal clusters, the maximum metal volume
concentration is around 30%. Selected Area Electron
Diffraction spectra of the Co-Ni samples shows that
the crystal structure of the nanoparticles depends on
the metal composition: the 4:1 sample has hcp
structure whereas Co-Fe nanoparticles have always
bcc structure. Samples prepared implanting only Co
(24 1016 Co+/cm2) show also hcp structure. In a
previous work [5,6], we had studied the structural
features of Co-Ni nanoparticles prepared implanting an
half total dose. In this case, the nanoparticles were
smaller, but the structure of the nanoparticles of 4:1
sample was hcp and the structure of the 1:1 sample
was fcc. So, it is observed a critical change of the
nanoparticle structure depending on the implanted ions
and the respective composition. Moreover the
observed crystal structures agree with the respective
bulk alloys [4]. Both results indicate clearly that
sequential ion implantation allows to obtain alloybased nanoparticles.
2
CoFe-1:1
CoNi-1:1
CoNi-4:1
1
0
2
Magnetic moment
The samples for transmission electron microscopy
(TEM) were prepared and examined at CNR-IMM
Institute in Bologna with a Philips CM30T
Microscope, operating at 300 kV. Rutherford
Backscattering Spectrometry (RBS) measurements
were performed using 4He+ ions at the energy of 2.2
MeV at the Van der Graaff Accelerator of INFN
Legnaro Laboratory. The magnetic characterization
was carried out using a Cryogenic S600 SQUID
magnetometer. The angular dependence of
Ferromagnetic Resonance (FMR) spectra was
measured using a spectrometer VARIAN E9 operating
at X-band (9.25 GHz) at 300 K.
-1
-2
1
0
-1
-2
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
µ0Hapl (T)
-2
-1
0
1
2
Applied Field (Tesla)
FIGURE 1. Hysteresis loops of the CoNi-4:1, CoNi
1:1 and CoFe-1:1. Inset: Details at low field of the
hysteresis loops
In the inset of the Figure 1 the hysteresis loops in
the low field region are shown. It can be observed the
large difference in the coercive field (HC) of the CoNi
4:1 sample (0.17 T) and the CoNi 1:1 and CoFe 1:1
samples (0.028 T and 0.02 T, respectively). Pure-Co
sample ( 24 ·1016 Co+/cm2) shows also large coercivity
(0.11 T). As these nanoparticles are single domain,
their coercive fields can be analyzed using the Stoner –
Wohlfarth model [2,3]. This model proposes that the
theoretical maximum coercive field, HCSW, of an
ensemble of randomly oriented, single domain and non
interacting particles is given by the relation µ0HCSW =
2α K/MS, where MS is the magnetization saturation, K
is the magnetic anisotropy constant, µ0 is 9.27 10-24
Tm/A and α is a factor that depends of the anisotropy
type. K includes the contribution of the
Their magnetic properties were studied by
measuring the hysteresis loops at 3 K. These loops
shows a large diamagnetic contribution, related to the
silica slide, which was subtracted. Then, the magnetic
moment per implanted atom was calculated taking into
account the ion dose obtained from RBS studies. In
Figure 1 are represented the final hysteresis loops of
676
the plane and perpendicularly to the plane of the slide.
The observed shift of the ferromagnetic resonance
(∆H) is typically explained with the presence of an
effective field related to textured anisotropies in the
particles (typically the shape of the particles) and to
the demagnetization field in the film. The first
possibility is excluded considering the structural and
morphological data which do not show any texture or
non-spherical shape of the particles. In a rough
approximation the demagnetization field will be
<MS>ε, where <MS> is the average magnetization outof–plane and ε is the volume concentration. Taking
into account the MS and ε values of the FeCo sample,
the observed shift of 0.8 T is in a good agreement with
the estimated demagnetizing field. However the
different shape between the spectra can not be
understood considering that the only difference
between the measurements in- and out-of-plane is the
demagnetization field. We believe that this is a
fingerprint that the collective magnetic behavior are
different in both directions. Concluding, we observe
that the magnetic characteristics of Co-Ni and Co-Fe
composites are determined by the magnetic features of
the individual nanoparticles, linked to compositional
and size effects, and collective behaviors, which are
driven by the film-like morphology of the implanted
region.
H II plane
FMR signal (arb. units)
magnetoelastic,
shape
and
magnetocristalline
anisotropies. TEM studies show that the nanoparticles
are spherical so shape anisotropy is negligible. The
magnetoelastic contribution could be important for
FeCo case, however to simplify, it will be considered
only its magnetocristalline anisotropy. Typically this
anisotropy is related to the crystal structure: hexagonal
structure gives rise to uniaxial anisotropy, while cubic
anisotropy is related both the bcc and fcc structures. K
is then characteristic of each material. α is 0.49 and
0.25 when the anisotropy is uniaxial (K>0) and cubic
with K<0, respectively [3]. Considering the values of
magnetocristalline anisotropy at room temperature of
the Co80Ni20 (4.8 ·105Jm-3) Co50Ni50 (-1.5 ·104 Jm-3)
and Co50Fe50 (-6.4 ·103 Jm-3) bulk alloys [4], HCSW
values are 360 mT, 7 mT and 2 mT, respectively. The
Co80Ni20 alloy is uniaxial and the K value is larger
than the value of the another alloys. Thus the larger HC
of the CoNi-4:1 is related to its crystal structure. We
can also conclude that the nanoparticles of the CoNi1:1 sample must have fcc structure. However the
previously calculated HCSW values are underestimated
because we have used the values of the magnetic
anisotropy at room temperature which are smaller than
the value at 3 K. Thus the measured HC of the CoNi4:1 sample will be smaller than the theoretically
expected, while those of the CoNi-1:1 and CoFe-1:1
could be larger. Size – effects are typically considered
in order to explain such differences [2,3,7]. They are
related to the contribution of the surface which
increases the total anisotropy of the nanoparticle and
therefore its coercive field. Our results suggest that the
effect of the surface can depend on the structure of the
nanoparticle, increasing the anisotropy in the case of
fcc nanoparticles and decreasing in the case of hcp
nanoparticles.
In this discussion, we have not taken into account
the effect of the interparticle interactions. The volume
particle concentration in the most populated region is
around 0.3, near the percolation limit. TEM studies
(not shown) indicate that nanoparticles do not touch so
dipolar interactions are dominant. As the
magnetization of the CoNi and CoFe nanoparticles are
very large (larger than 1 T) the magnetic fields around
the particles must be very intense. In this case, dipolar
interactions give rise to a collective magnetic behavior
whose consequence is the increase or decrease of the
coercive field [3]. Also we point out that the
morphology of the implanted region is similar to that
of a film. In this case if the magnetization is forced to
be perpendicular to the film a demagnetizing field
appears reducing the local magnetic field in a particle.
Consequently the magnetization remains preferentially
in the in-plane region. In the Figure 2 are represented
the FMR spectra of the CoFe-1:1 sample measured in
∆H
H ⊥ plane
0.0
0.2
0.4
0.6
0.8
Magnetic field (T)
1.0
FIGURE 2. FMR spectra of the FeCo-1:1 measured
applying the magnetic field in the plane of the slide (dotted
line) and perpendicular to this plane (full line).
The Ni-Cu system
In all the cases investigated, spherical nanoparticles
dispersed into a 100 nm thick region were formed. The
particle size was between 2 - 5 nm and the crystal
structure was always fcc.
The Curie temperature of these composites was
studied by measuring the temperature dependence of
677
Normallized magn. moment
the magnetization at high field (6 T). In the Figure 3
are represented the results for the 3:7 and 1:1 samples
but normalized respect to the value measured at 3 K.
The magnetization moment of the sample 4:6 was very
small, so the corresponding measurements are not
reported. In all the cases the magnetization decreases
almost continuously with temperature indicating a
thermal demagnetizing effect associated to the order
transition. The TC of 1:1 sample is near room
temperature while that of the 3:7 is at larger
temperatures. The TC of the Cu30Ni70 and Cu50Ni50
bulk alloys are 285 K and 75 K, respectively.
Considering the described relationship between TC and
alloy composition, we can conclude that the
nanoparticles are Ni-enriched with respect to the
nominal implanted composition. In a previous work
[5] we have studied the structural and magnetic
properties of a Cu-Ni implanted sample, whose only
difference with the 1:1 sample was that the
implantation energy was smaller (120 keV). In this
case, the nanoparticles were more concentrated near
the surface than in the samples studied in this work,
but the morphological features of the nanoparticles
were similar. However, the sample was not
ferromagnetic at any temperature and we concluded
that the nanoparticles were Cu-enriched. This was
confirmed by structural studies [5]. These results
suggest that the diffusion of the second implanted
element in the nanoparticle is not complete. We
conclude that the implantation conditions, in particular
the implantation energy, can critically determine the
features of the alloy nanoparticles prepared by ionimplantation.
relate these steps to the phase transition from
ferromagnetic to paramagnetic states of different
nanoparticles with different compositions. However
size effects could give rise to similar features. For
example, the TC of nanostructured materials depends
on particle size [2,7], being typically smaller. Also we
point out that in the Cu-Ni alloys the magnetic
properties depend on the electronic structure and, in
particular, on the transfer charge between Ni and Cu.
As the electronic structure is strongly modified in the
nanoparticles due to size effects it can be expected the
Curie temperature will change. Considering these
observations the magnetic behavior can be related both
to compositional inhomogeneities and size-effects
which are peculiar to alloy-based clusters.
ACKNOWLEDGMENTS
The technical assistance of M. Parolin at INFNINFM Ion Implantation Laboratory-Legnaro is
gratefully acknowledged. This work has been partially
supported by MURST National University Research
Project and by CNR National Project MSTA II.
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6
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