Full text of DOI:10.1007/s003390000682

Appl. Phys. A 73, 215–218 (2001) / Digital Object Identifier (DOI) 10.1007/s003390000682
Applied Physics A
Materials
Science & Processing
Structural and magnetic properties of Co-C composite films and Co/C
multilayer films
∗
,∗∗
J. Shi
, M. Azumi, O. Nittono
Department of Metallurgical Engineering, Tokyo Institute of Technology, 2-12-1 Oh-okayama, Meguro-ku, Tokyo, 152-8552, Japan
Received: 11 July 2000/Accepted: 13 July 2000/Published online: 30 November 2000 –  Springer-Verlag 2000
Abstract. Co-C composite films and Co/C multilayer films
have been prepared by a method incorporating ion beam
sputtering and plasma chemical vapor deposition. It has
been found that the structure and magnetic properties of
both the Co-C composite and the Co/C multilayer films de-
pend strongly on the substrate temperature during deposition.
The Co-C composite film deposited at room temperature is
amorphous, with relatively low saturation magnetization and
beam sputtering for application as ultra-high density mag-
netic recording media [12]. In such a material, the incorpo-
rated carbon plays an important role in separating the cobalt
crystallites and magnetically decoupling them, thus obtain-
ing high coercivity. Besides application as magnetic record-
ing media, magnetic films are also used in magnetic head
and other high-frequency magnetic devices. For such ap-
plications, soft magnetic properties, i.e. low coercivity and
good frequency response of the permeability are necessary.
It is known that the frequency response of the permeabil-
ity is limited mainly by eddy current loss and the resonance,
therefore, high film resistivity is of benefit because of the re-
duction in eddy current loss. Recent approaches to reduce
both coercivity and eddy current loss include incorporating
insulating materials during the deposition of magnetic metal
films and lamination of the magnetic thin film with insulating
layers [13–16]. In the present work, amorphous carbon was
used for this purpose. Co-C composite films and Co/C mul-
tilayer films were prepared, and their structure and magnetic
properties were studied.
◦
coercivity. On the other hand, the film deposited at 250 C
is composed of fine Co crystallites separated by amorphous
C or Co-C phase. As a result, both the saturation magneti-
zation and coercivity are increased compared with the film
deposited at room temperature. When deposited at room tem-
perature, the Co/C multilayer film exhibits good periodicity,
with a period of 70 nm (Co: 40 nm, C: 30 nm) and sharp
3
and flat Co-C interfaces. High magnetization (602 emu/cm )
and low coercivity (1.6 Oe) are obtained for such a film.
◦
However, increasing the substrate temperature to 250 C
was found to be detrimental to the magnetic properties
due to the formation of cobalt carbide at the Co-C inter-
face.
1
Experimental procedure
PACS: 75.70.-i; 75.50.Kj; 68.55.Jk
A dual-source deposition system was used to prepare the
films, which included a Kaufman ion source for ion beam
sputtering deposition of Co and a compact electron cyclotron
resonance (ECR) plasma source for chemical vapor deposi-
tion (CVD) of C. During the deposition processes, the Kauf-
man ion source produced an Ar ion beam directed at a Co
target (99.98%) with an energy of 2 keV. The reaction gas
supplied to the ECR plasma source was a pre-mixed me-
thane (CH4) and hydrogen (H2) gas mixture, and the power
supplied to the source was fixed at 245 W. During depo-
sition, the flow rate of CH4 was controlled at 0.2 sccm by
a mass-flow controller. For deposition of Co-C composite
films, the sputtered cobalt flux and the ECR plasma were
directed at the substrate simultaneously. For the deposition
of Co/C multilayer films, the flux and plasma were directed
at two different directions. In this case, cobalt and carbon
layers were deposited sequentially at the two substrate pos-
Carbon-related materials show great application potential due
to their unique structures and physical properties [1–7]. One
of the important characteristics of such materials is that they
can be incorporated with metals to form nanocomposite mate-
rials, of which novel properties are expected [8–11]. Carbon
nanoparticles filled with pure iron, nickel, and cobalt have
been synthesized [10]. Recently, copper nanowire encapsu-
lated by carbon nanotubes has also been formed [11]. In
the field of magnetic applications, cobalt nanocrystals encap-
sulated by graphite-like carbon have been prepared by ion
∗
Corresponding author.
∗∗
Present address: Department of Applied Physics and Chemistry, The Uni-
versity of Electro-Communications, 1-5-1, Chofugaoka, Chofu-shi, Tokyo
82-8585, Japan
Fax: +81-424/435461, E-mail: shi@pc.uec.ac.jp)
1
(

216
itions by rotating the substrate holder periodically. Prior to
the depositions, the system was evacuated to a base pres-
−5
sure of less than 3 ×10 Pa. The chamber pressures were
around 1 Pa during depositions. Corning #7059 glass slides
were used as substrates, and they were ultrasonically cleaned
in acetone before mounting in the vacuum chamber. The
structure of the films was characterized by X-ray diffrac-
tion (XRD) and transmission electron microscopy (TEM).
For TEM plan-view observation, the samples were deposited
on SiO supporting films directly. For the cross-section views,
the samples were thinned by mechanical grinding and sub-
sequent ion milling. The in-plane magnetic properties of the
films were measured at room temperature by using a vibra-
tion sample magnetometer. The resistivity was measured by
the four-probe method.
Fig. 2a,b. Electron micrographs for the Co-C composite films deposited at
room temperature (a) and at 250 C (b). Insets show the electron diffraction
patterns of the films
◦
Figures 2a and b show TEM bright field images of these
two films respectively, and the insets show the electron
diffraction patterns. For the film deposited at room tempera-
ture, although the TEM image reveals a grain-like structure,
the halo diffraction pattern proves the amorphous nature of
the film, consistent with the XRD result. The film deposited
2
Results and discussion
◦
at 250 C reveals a kind of granular structure [13], and the
2
.1 Microstructure of Co-C composite films
inset diffraction pattern confirms the formation of an ε-Co
(
hcp) phase. The Co crystallites are round, and their aver-
Figure 1 shows the XRD profiles of Co-C films deposited
at room temperature and at 250 C. For the film deposited
age size is about 8 nm in diameter as estimated from the
TEM observation. Since Co has almost no solid C solubil-
ity near room temperature [18], the probable location of most
of the incorporated C is at the inter-grain regions. We noted
that the inter-grain regions are about 1–2 nm in thickness
and are characterized by light contrast, therefore we consider
that such regions are composed of C or a Co-C amorph-
ous phase. Similar structure has also been reported on Cu-C
using a co-sputtering method [19], and the formation of such
a structure is ascribed to the de-mixing of Cu and C during
the deposition process. According to this formation mechan-
ism, the de-mixing is caused by surface diffusion of the two
species, which depends strongly on the energy of the incident
species arriving at the surface of the substrate. This mechan-
ism can be applied to explain our results. When the Co-C film
was deposited at room temperature, Co and C species were
trapped on the substrate surface due to their low energies and
surface diffusivities, preventing the de-mixing process from
occurring and leading to the formation of a Co-C amorph-
◦
at room temperature, the profile shows only a broad peak
covering a wide range of angular positions where several re-
flections from the ε-Co phase and the cobalt carbides are ex-
pected to appear, indicating the formation of a Co-C amorph-
◦
ous phase in the film. For the film deposited at 250 C, the
◦
relatively sharp peak located at 44.5 , is identified as the
0
0 · 2 reflection from ε-Co (hexagonal close packed struc-
◦
ture, hcp) phase, and the weak peak located at 51.5 is due
to the α-Co (face-centered cubic structure, fcc) phase. This
result indicates that the film is composed mainly of an ε-
Co phase, with a strong 00 ·1 preferred orientation. In add-
ition, small amount of α-Co phase is also formed in the
film. No reflections from crystalline carbon phases were ob-
served in the XRD profile, indicating the carbon incorpo-
rated in the film is still amorphous. Furthermore, according
to our previous results, the amorphous carbon in the film
is believed to be in a glassy carbon state, which is com-
2
◦
posed mainly of sp hybridization in the form of disordered
ous phase. When the Co-C film was deposited at 250 C,
graphite [17].
de-mixing occurred due to the enhanced surface diffusivities,
leading to the formation of Co crystallites and an amorphous
C phase.
2
.2 Structure of Co/C multilayer films
Figure 3 shows the XRD profiles of two Co/C multilayer
◦
films deposited at room temperature and at 250 C respec-
tively. For the film deposited at room temperature, the XRD
profile shows a broad peak located around the angular pos-
◦
ition of 43 . The other broad peak at the left is due to the
glass substrate. No perceivable reflection from crystalline car-
bon, such as graphite, can be observed on the profiles. These
results indicate that both the cobalt and carbon layers are
◦
amorphous. For the film deposited at 250 C, reflections from
ε-Co and cobalt carbide (Co3C, hcp) are observed in the
profile, indicating that Co layers are composed of polycrys-
talline Co and the carbide was formed at the Co-C inter-
faces. In order to evaluate the period of the Co/C multilayer
Fig. 1. X-ray diffraction profiles for the Co-C composite films deposited at
room temperature and at 250 C
◦

217
Fig. 3. X-ray diffraction profiles for the Co/C multilayer films deposited at
◦
room temperature and at 250 C
Fig. 5. Electron micrograph showing the cross-section of the Co/C multi-
layer film deposited at room temperature. Inset shows the electron diffrac-
tion pattern of the same film
2
.3 Magnetic properties of Co-C composite and Co/C
multilayer films
The saturation magnetization (Ms), coercivity (Hc) and resis-
tivity (ꢀ) of the Co-C composite films and Co/C multilayer
films are listed in Table 1. It is seen that the Co-C composite
film deposited at room temperature shows the lowest satura-
tion magnetization, relatively low coercivity and the highest
resistivity among the tested films. This is due to the amorph-
ous nature of the film, and we consider that the cluster-scale
mixing of Co and C also contributes to the low saturation
magnetization and high resistivity. Increasing the substrate
◦
temperature to 250 C resulted in significant changes in the
magnetic and electrical properties. The saturation magnetiza-
tion increased by more than a factor of two and the coercivity
increased by more than five times the values for the film
deposited at room temperature. Furthermore, the resistivity
decreased to one percent of the value for the film deposited
at room temperature. These changes are apparently due to
the de-mixing of Co and carbon by surface diffusion and the
formation of Co crystallites. According to Fig. 2b, the Co-C
Fig. 4. Low-angle x-ray diffraction profile for the Co/C multilayer film
deposited at room temperature
films, low-angle X-ray diffraction measurements and cross-
sectional TEM observations were performed on the Co/C
film deposited at room temperature. Figure 4 shows the low-
angle X-ray diffraction profile. It contains a broad peak cen-
◦
◦
tered at 3.7 , which is superimposed by an oscillation. The
composite film deposited at 250 C reveals small crystallites
broad peak represents a spacing of 2.4 nm, which corresponds
to the cluster size in the amorphous film. The oscillation is
caused by the interference of X-rays reflected by the Co-C
(sized about 8 nm) and small physical gaps (less than 2 nm)
among the Co crystallites, which keep the Co crystallites
magnetically interacted with each other [20]. Although mag-
netic films of such a structure are generally expected to yield
good soft magnetic properties [13–21], in our experiment,
the film reveals relatively large coercivity (63 Oe). A possible
reason for this may be the growth of Co crystallites along
the direction perpendicular to the film surface. This means
◦
interfaces. The period of the oscillation, 0.125 , represents
a spacing of 70 nm, which is considered to be the period of the
multilayer film. The good X-ray reflectivity also implies that
the film has a smooth surface, and has flat and sharp interfaces
within the film. Furthermore, the cross-sectional TEM image
of the Co/C film is shown in Fig. 5. The dark layers are cobalt
layers with a thickness of about 40 nm, and the bright layers
are the carbon layers with a thickness of about 30 nm. The
image also clearly shows the flat and sharp interfaces between
cobalt and carbon layers. It is seen that these TEM results are
in good agreement with the low-angle X-ray diffraction re-
sults. Moreover, the inset electron diffraction pattern shows
a streak along the direction perpendicular to the film surface,
which is composed of periodically arranged satellite reflec-
tions, indicating the good periodicity of the Co/C multilayer
film.
Table 1. The saturation magnetization (Ms), coercivity (Hc) and resis-
tivity (ꢀ) of Co-C composite films and Co/C multilayer films
Ms (emu/cm 3) Hc (Oe)
−
ꢀ (Ωcm)
1
Co-C (R.T.)
Co-C (250 C)
293
611
602
579
12.0
63.5
1.6
5.4×10−
◦
−3
5.0×10
2.1×10
−
−6
Co/C (R.T.)
◦
Co/C (250 C)
349.2

218
that the crystallites are not spherical but columnar in shape.
Such growth behavior has been observed for Cu-C films [19].
Therefore, different magnetic domains may exist in the crys-
tallites in spite of their small in-plane size, and this prevents
the film from exhibiting smaller coercivity [21].
(Co: 40 nm, C: 30 nm), the Co-C interfaces are sharp and flat.
Such a film exhibits soft magnetic properties (Hc = 1.6 Oe)
with relatively high saturation magnetization. For the Co/C
◦
film deposited at 250 C, Co layers are crystalline. A cobalt
carbide phase was formed in the film due to the interfacial
reaction. As a result, the saturation magnetization was de-
creased, and the coercivity was increased significantly.
The Co/C multilayer film deposited at room temperature
shows relatively high saturation magnetization and low co-
ercivity. Considering the volume fraction of Co in the film,
3
the Co layers give an effective Ms of 1053 emu/cm , which
3
is close to the bulk value of cobalt (1400 emu/cm ). The
coercivity of the film, 1.6 Oe, is comparable to that of Co-
base granular soft magnetic films [13] and Fe-base multilayer
films [14]. Moreover, there is a good possibility of further
reducing the coercivity by optimizing the thickness of the
cobalt and carbon layers. The film shows the lowest resistivity
amongst the tested films because of the formation of continu-
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