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 .
                                                                    s and      2
the coercive force H                                                                Y. Liou, J. C. A. Huang, Y. D. Yao, C. H. Lee, K. T. Wu, C. L. Lu, S. Y.
                          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.
and 300 K are plotted in Figs. 4 b  and 4 c , respectively.                         Appl. Phys. 76, 6516  1994 .
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
a slight shift in these data, perhaps again explainable by the                      M. N. Baibich, J. M. Broto, A. Fert, F. Nguyen van Dau, F. Petroff, P.
                                                                                    Etienne, G. Creuzet, and J. Chazelas, Phys. Rev. Lett. 61, 2472  1988 .
presence of demagnetizing fields, exists in the observed val-                  5 Y. Liou, J. C. A. Huang, Y. D. Yao, S. F. Lee, W. T. Yang, S. Y. Liao, and
ues for the Co(11Ŋ00)/Cr 211 /MgO 110  samples. All H                               C. P. Chang, IEEE Trans. Magn. 31, 3927  1995 .
                                                                      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
                                                                                    Y. M. Hu, Phys. Rev. B 52, R 13 110  1995 .
mately 150 K.                                                                  7 A. Berger and H. Hopster, Phys. Rev. Lett. 73, 193  1994 .



J. Appl. Phys., Vol. 79, No. 8, 15 April 1996                                                                                           Yao et al.       6535

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