Enhancement of magnetoresistance in Co(11Ŋ00)/Cr(211) bilayered films
on MgO(110)
Y. D. Yaoa) and Y. Liou
Institute of Physics, Academia Sinica, Taipei 115, Taiwan, Republic of China
J. C. A. Huang
Department of Physics, National Cheng Kung University, Tainan 701, Taiwan, Republic of China
S. Y. Liao, I. Klik, W. T. Yang, C. P. Chang, and C. K. Lo
Institute of Physics, Academia, Sinica, Taipei 115, Taiwan, Republic of China
Epitaxial Co/Cr bilayered films have been successfully grown on the MgO 100 and MgO 110
substrates by molecular-beam epitaxy. According to the reflection high-energy electron-diffraction
and x-ray-diffraction measurements the crystal structure of the film depends on orientation of the
buffer and substrate. Epitaxial growth of biaxial Co(112Ŋ0)/Cr 100 on MgO 100 substrate and of
uniaxial Co(11Ŋ00)/Cr 211 on MgO 110 substrate has been confirmed. The anisotropy
magnetoresistance AMR is strongly influenced by the orientation of the Cr buffer. In
Co(112Ŋ0)/Cr 100 on MgO 100 AMR is isotropic for all in-plane fields. However, for
Co(11Ŋ00)/Cr 211 on MgO 110 we observed enhancement of AMR along the easy axis for
temperatures below 150 K, while along the hard axis AMR has a local maximum at about 150 K.
The easy axis data suggest that the longitudinal spin density wave of Cr and the crystal anisotropy
of Co on Cr 211 plane dominate the enhancement of the AMR. Đ 1996 American Institute of
Physics. S0021-8979 96 57808-1
In previous studies of the Co/Cr multilayer system13 its tion XRD . The magnetic properties of all samples were
magnetoresistance MR was shown to be quite small in studied using a superconducting quantum interference device
comparison with other giant MR GMR multilayer SQUID magnetometer. The AMR measurements were car-
systems.4 Recently it was realized5,6 that the magnetic prop- ried out by standard four-probe technique in a magnetic field
erties of the Co/Cr multilayer system are sensitive to anisot- up to 5 T at temperatures ranging between 10 and 300 K.
ropy and to the orientation of the applied magnetic field with Typical area of the film sample was roughly 1.5 6.0 mm2.
respect to crystallographic axes and MR as high as 18% as The structure arrangement of both Co and Cr layers in
has been observed in the Co/Cr 211 superlattice system. the Co/Cr bilayered films is considerably affected by the
However, the mechanism of this effect is presently not clear choice of the interface direction of the MgO substrate. In this
and this motivated us to investigate the simpler case of an-
isotropy of MR AMR in the epitaxial bilayered Co/Cr film study, the epitaxial Co/Cr bilayer films were simultaneously
system. grown on MgO 100 and on MgO 110 substrates. Their
Epitaxial Co/Cr bilayer films have been simultaneously crystalline orientation and quality were determined by
prepared on MgO 110 and MgO 100 substrates by using an RHEED and XRD. Figures 1 a and 1 b show schematic
Eiko EL-10A molecular-beam-epitaxy MBE system with diagrams of the 3D geometry of the Co 11Ŋ00)/
base pressure of 2 10 10 Torr. To enable the growth of Cr 210 /MgO 110 and Co(112Ŋ0 /Cr 100 /MgO 100 bilayer
high-quality film samples, polished and epitaxial grade films. These epitaxial relationships were also confirmed by
MgO 110 and MgO 100 substrates were chemically pre- both RHEED and XRD studies. Part of the structural analy-
cleaned and rinsed in an ultrasonic cleaner. They were then sis related to the Co/Cr superlattice films will be published
outgassed at 9001000 °C for at least 0.5 h under ultrahigh elsewhere,5,6 in this study only the bilayer case is discussed.
vacuum in the MBE chamber. High-purity Co and Cr ele- In Fig. 1 a the lattices of the Co and Cr layers appear to be
ments 99.99% were evaporated from two independent rectangular, in acccordance with the 4.21 2.98 Å2 unit cell
e-beam evaporators. During deposition of the films, the sub- of the MgO 110 surface. The unit cell of Co(11Ŋ00), 4.07
strate temperature was kept between 300 and 350 °C, the 2.51 Å2, matches perfectly that of Cr 211 , 4.07 2.50 Å2,
growth pressure was controlled at below 5 10 9 Torr, and and even though the match between Cr 211 and MgO 110
the deposition rate kept at 0.1 Å/s. The film thickness and
deposition rate were measured by a quartz-crystal thickness is poorer, we did experimentally observe a twofold symmetry
monitor. The crystallographic structure of the surface of the for the whole system and confirmed that the c axis of Co is
films was in situ examined throughout the growth by 15 keV in the film plane and in the Cr 01Ŋ1 direction only. On the
reflection high-energy electron diffraction RHEED . The other hand, for the Co(112Ŋ0 /Cr 100 /MgO 100 system, be-
crystal orientation was ex situ characterized by x-ray diffrac- cause the bcc Cr 100 plane has fourfold symmetry with unit
cell of 2.88 2.88 Å2, the hcp Co(112Ŋ0) plane possesses
a only pseudotwofold symmetry with unit cell of
Also with: Department of Physics, National Chung Cheng University, Chi-
ayi 621, Taiwan, Republic of China. 4.34 4.07Å2. This suggests that the Co(112Ŋ0) layers behave
J. Appl. Phys. 79 (8), 15 April 1996 0021-8979/96/79(8)/6533/3/$10.00 Đ 1996 American Institute of Physics 6533
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FIG. 2. a The magnetization M and b and c the surface resistivity for
Co200 Å 11Ŋ00 /Cr6 Å 211 /MgO 110 films at T 10 K as functions of mag-
netic field applied parallel to the film surface.
varied between 6 and 100 Å. As an example, Fig. 2 shows
the magnetic hysteresis loops and the corresponding surface
resistivity of the Co200 Å(11Ŋ00)/Cr6 Å(211)/MgO(110)
sample at 10 K for field applied parallel to the film surface.
For H parallel to the easy axis of the Co layers a square
M H hysteresis loop and small variation of resistivity have
been observed. We interpret the magnetization reversal pro-
cess as being mainly due to domain-wall motion. On the
other hand, for H applied parallel to the hard axis of Co
layers the magnetization varies slowly with increasing H
while electric resistivity decreases very fast below roughly
H 1 T and saturates then at H 1 T. The very small val-
ues of coercivity deduced from the M H loop are consistent
with the peak positions on the resistivity curve.
In Fig. 3 we plot the magnetization M and surface resis-
tivity as functions of applied field H for a
Co200 Å(112Ŋ0)/Cr6 Å(100)/MgO 100 sample at 10 K. Ac-
cording to our structure analysis above, the easy and hard
axes of the Co layers are randomly distributed in both
MgO 001 and MgO 010 directions. The M H curves for
H applied parallel or perpendicular to the long axis of the
FIG. 1. Schematic diagram of the 3D geometry, unit cell indicated by bold sample are roughly the same, apart from a difference due
lines , and epitaxial relationships for a Co(11Ŋ00)/Cr 211 /MgO 110
films, and b Co 112Ŋ0 /Cr 100 /MgO 100 films. presumably to demagnetization factors see the geometry
of Fig. 1 b . The coercive force obtained from the M H
curves coincides with the location of a minimum maximum
like a bicrystalline structure; i.e., that the Co 0001 easy axis of the resistivity with current parallel perpendicular to the
can either be parallel to MgO 001 or to MgO 010 as shown applied field.
in Fig. 1 b . The temperature dependence of the AMR of the satura-
The thickness of the Co layer for all the samples in this tion magnetization MS and of the coercive force Hc between
study is fixed at 200 Å and the thickness of the Cr layer is 10 and 300 K for both Co200 Å /Cr6 Å /MgO 110 and
6534 J. Appl. Phys., Vol. 79, No. 8, 15 April 1996 Yao et al.
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FIG. 3. a The magnetization M and b and c the surface resistivity for
Co200 Å 112Ŋ0 /Cr6 Å 100 /MgO 100 films at T 10 K as functions of mag- FIG. 4. a The AMR, b the saturation magnetization, and c the coercive
netic field applied parallel to the film surface. force as functions of the temperature between 10 and 300 K for L: long side
of film : Co200 Å /Cr6 Å /MgO 110 with L easy axis of Co surface;
Co200 Å /Cr6 Å /MgO 110 with L hard axis of Co surface; Co200 Å /
Cr6 Å /MgO 110 with L MgO 001 direction; and Co200 Å /Cr6 Å /
MgO 110 with L MgO 010 direction.
Co200 Å /Cr6 Å/MgO 100 are presented in Fig. 4. For
samples of Co200 Å /Cr6 Å /MgO 110 with their long axis
parallel to the easy axis of Co, i.e., in the direction of
Co 0001 as shown in Fig. 1 a , we observed a significant In conclusion, this is the first time that AMR enhance-
enhancement of AMR for temperatures roughly below 150 K ment roughly below the spin-flip temperature was observed
in Fig. 4 a while AMR in Fig. 4 a decreases with in epitaxial Co(11Ŋ00)/Cr 211 bilayered films on MgO 110 ,
decreasing temperature below roughly 150 K for samples with current in the Co 0001 direction.
with long axis parallel to the hard axis of Co, i.e., in the We are grateful for the financial support by the National
direction of Co 112Ŋ0 as shown in Fig. 1 a . Our data points Science Council of the R.O.C. under Grant Nos. NSC85-
between 100 and 150 K are not sufficiently dense, however, 2112-M-001-020, NSC85-2112-M-001-019, and NSC85-
this characteristic temperature Tf 150 K may be very 2112-M-006-006.
close to the spin-flip temperature Tsf 123 K of Cr and we
conjecture that magnetization reversal in these samples may
be explained by a mechanism similar to that reported for the
Fe/Cr system.7 By contrast, in Co200 Å /Cr6 Å /MgO 100
samples the AMR as shown in Fig. 4 a is almost indepen- 1 S. S. P. Parkin, R. Bhadra, and K. P. Roche, Phys. Rev. Lett. 64, 2304
dent of temperature. The saturation magnetization M 1990 .
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c as functions of temperature between 10 Liao, Y. Y. Chen, N. T. Liang, W. T. Yang, C. Y. Chen, and B. C. Hu, J.
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Both the M 3
s vs T and the Hc vs T curves completely coin- Y. D. Yao, Y. Liou, J. C. A. Huang, S. Y. Liao, C. H. Lee, K. T. Wu, Y. Y.
cide for the Co(112Ŋ0)/Cr 100 /MgO 100 samples; however, Chen, C. L. Lu, and W. T. Yang, Chin. J. Phys. 32, 863 1994 .
4
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c vs 6
T curves show a marked change in their slope at approxi- J. C. A. Huang, Y. Liou, Y. D. Yao, W. T. Yang, C P. Chang, S. Y. Liao, and
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J. Appl. Phys., Vol. 79, No. 8, 15 April 1996 Yao et al. 6535
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