Magnetoresistance Effect and Magnetic Properties
of Strain Induced Co/Cu Multilayer Films
C. Rizal1* and Y. Ueda2
Abstract: We have produced Co(tCo)/Cu multilayer and alloy
films on a polyimide substrate (1 cm2) with layers grown in
the atomic scale using a pulse generator circuit. The induced
uniaxial magnetic anisotropy was observed due to the effect of
strain in all the multilayer films. The multilayer [Co 1.0
nm/Cu 1.5 nm]50 showed a minimum hysteresis loss. The
maximum magnetoresistnace (MR) ratio observed was 3.4%
at 1 kOe. A remarkable difference of the magnetic field
dependence of the MR ratio was observed corresponding to
the orientation of the magnetization curves.
INTRODUCTION
Research on the metallic multilayer and alloy films is
a typical example that has attracted much attention in view
of the fundamental science and applications [1]. Recent
studies on giant magnetoresistnace in the thin films are
based on films grown mainly from the vapor phase. The
phenomena exhibit different properties depending on the
growth conditions [2] [3]. One of the possible factors that
is expected to cause different properties of the
electrodeposited film is due to the existence of the electric
field in the order of 107 V/cm, between the electrode and
ions in the double layer. Another difference is the charged
nature of the particles arriving at the surface during the
growth process. Therefore, it provides the possibility of
depositing film structures different from those being
produced from the vapor phase.
We produced many Co(tCo)/Cu multilayered films,
systematically regulating the electrode potential wave
(pulse amplitude), current density, and deposition time
(pulse width). In multilayers, by varying the thicknesses of
the individual layers, choosing appropriate material
composition and inducing strain, it appears to be possible
to tailor the magnetic anisotropy.
electrical resistivity and the magnetization curves were
measured by the four-probe method and vibrating sample
magnetometer (VSM), respectively. The magnetic field
dependence of MR ratio was examined by varying the
relative direction between the field, H, and current, I. The
composition of Co and Cu in the film was determined by
energy dispersive X-ray analysis. A strain gauge was used
to measure the strain in the multilayer film.
RESULTS AND DISCUSSIONS
Fig. 1 illustrates the magnetic field dependence of the
MR ratio and the corresponding magnetization curves (at
the magnetic field of ± 1 kOe). Notice the high sensitivity
at the low magnetic field and the magnetization curves of
the as-deposited and the strain-induced multilayer films.
The evidence reveals the isotropic character of the MR
ratio prior to introducing strain.
Fig. 2 exhibits the field dependence of
magnetoresistnce (MR) ratio at the magnetic field range of
EXPERIMENTAL
The electrolytic bath for the Co/Cu multilayer film
deposition was composed of CoSO4.7H2O, CuSO4.2H2O,
Na3C6H5O7, and NaCl. The solution was prepared in
double distilled water and the pH was adjusted in the range
of 3.0 to 5.0. Prior to electrodeposition, Cu buffer layers
each 15 nm thick, were vapor deposited on the polyamide
substrate. In our previous work, we used glass [4][5]. The
1
Author is with the Department of Electrical and Computer Engineering,
University of British Columbia, Vancouver, Canada.
2
Author is with the Department of Electrical and Electronic Engineering,
Muroran Institute of Technology, Hokkaido, Japan.
*Contacting Author: Conrad Rizal (Phone: +1 604 822 6268;
e-mail:[email protected])
978-1-4244-3544-9/10/$25.00 ©2010 IEEE
Fig. 1 Field dependence of the MR ratio and the magnetization curves
for the [Co 10 Å /Cu 15 Å]50 films for (i) randomly oriented (isotropic)
and (ii) anisotropic films
0 to ± 20 kOe for the randomly oriented (isotropic) and
uniaxially oriented (anisotropic) multilayer films. The MR
ratio has two components: (i) isotropic, and (ii) anisotropic.
The slight difference in shape of the MR-H curve was
observed by changing the direction of an applied field
against the measuring current. This is shown in the upper
side in Fig. 2. Since the tendency of the field dependence
shows a negative field dependence in the lower side of the
of Fig. 2, this effect seems to be due to the giant magnetoresistance (GMR) effect. The MR ratio depends on the
Uniaxially oriented
Randomly oriented
0.5
(a)
AMR
0
1
AMR
0
AMR
Co/Cu
GMR
Co/Cu
-1
-2
GMR
-3
-3
(c)
0
MR ratio (%)
MR ratio (%)
MR ratio (%)
(b)
Co/Cu
-1
-2
Uniaxially oriented
0.5
-1
GMR
-2
-3
-4
Current // easy axis
Current//hard axis
-3.5
I
-20
-10
0
H [kOe]
10
20
-20
-10
0
H [kOe]
10
20
-20
-10
0
10
20
H [kOe]
Fig. 2 Magnetic field dependence of the MR ratio of [Co 10 Å /Cu 15 Å]50
multilayers. (a) randomly oriented film. (b) and (c) uniaxially oriented
film: (b) measuring current // magnetic easy direction. (c) measuring
current // magnetic hard direction.
Fig. 3 Ferromagnetic layer thickness dependence of MR ratio for the
[Co t Å /Cu 15 Å]50 multilayer. (open square) the randomly
RULHQWHG ILOPƔ FORVHG FLUFOHV DQG ż (open circles) are uniaxially
oriented films, where (b) measuring current // magnetic easy
direction (c) measuring current // magnetic hard direction
The dependence of the absolute value of MR ratio on
the ferromagnetic Co layer thickness, tCo, at the constant
non-magnetic layers thickness (tCu=15 Å) is studied for
both the isotropic and anisotropic multilayer films. The
absolute value of MR ratio (|MR% (H = 20 kOe)|) against
the Co layer thickness for tCu =15 Å is plotted for three
different sets of samples (e.g., isotropic and anisotropic
multilayer films). Fig. 3 illustrated the ferromagnetic layer
thickness dependence of MR ratio for the [Co t Å /Cu 15
Å]50 multilayer: The (open square) indicates the MR
ratio of the randomO\RULHQWHGILOPƔFORVHGFLUFOHDQGż
(open circle) indicate MR ratio of uniaxially oriented films
where (b) is the measuring current // magnetic easy
direction, and (c) is the measuring current // magnetic hard
direction. Solid lines are drawn as a direction for the eyes.
Here, the ferromagnetic layer thickness, tCo, is varied in the
range of 5 to 20 Å. The MR ratio reaches a maximum of
3.4 % at tCo = 10 Å. The results suggest that the adequate
thickness of the Co (ferromagnetic) and Cu (non-magnetic)
layers are required to create the high sensitivity and large
MR ratio. The MR ratio for the anisotropic multilayer films
along the hard axis shows the value larger than that of the
easy axis.
Field dependence of magnetization curves for the
series of strain-induced ( H =1.5%) Co/Cu multilayer films
with varying Co layer thickness, tCo, were carried out to
investigate magnetic anisotropy properties in the films.
Fig. 4 illustrates he set of magnetization curves as a
function of the Co layer thickness. Magnetization curves
were measured with magnetic field perpendicular (h.a.) and
parallel (e.a.) to the measuring current direction. The
maximum MR ratio in Fig. 3 is corresponding with the Co
layer thickness of 10 Å in Fig. 4, at H 1.5% .
Due to the high sensitivity at the low magnetic field,
these Co/Cu multilayer films are considered useful in the
application as a magnetic sensor
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Fig. Co layer thickness dependence of the magnetization for the
Co/Cu multilayer films at 1kOe ( = 1.5%).
H
relative orientation of the current and the magnetic field,
and shows an anisotropic character.
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