Giant magnetoresistance effect of spin valve Co/Pd/Co/NiO

Article abstract of DOI:10.1016/S0304-8853(99)00494-1

The possibility of the giant magnetoresistive (GMR) effect of Co/Pd multilayer was studied based on the prediction of the first principle band calculation. Anti-parallel magnetization configuration was achieved by fabricating spin valve structure, although the parallel magnetization is stable in the Co/Pd multilayer. Spin valve structures of Co/Pd/Co/NiO and Co/Cu/Co/NiO were fabricated on the glass substrate by dual ion beam sputtering. Anti-ferromagnetic NiO films were prepared at room temperature by oxygen reactive sputtering. The metal layers were deposited on the oxide layer subsequently. It is found that there exist a hexagonal phase and well-known cubic phase of NiO by analyzing XRD pattern. The exchange field of Cu/Co/hexagonal-NiO trilayer is larger than that of Cu/Co/cubic-NiO trilayer. The increase of the exchange field is proportional to the relative oxygen concentration. The GMR effect appeared in spin valve structure of Co/Pd/Co/hexagonal-NiO above critical Pd thickness, although GMR ratio is not so large. Anti-parallel magnetization configuration induced by a strong exchange field from NiO leads the GMR effect in the Co/Pd.

Full text of DOI:10.1016/S0304-8853(99)00494-1

Journal of Magnetism and Magnetic Materials 205 (1999) 136}142
Giant magnetoresistance e!ect of spin valve Co/Pd/Co/NiO
Wataru Gouda, Kazuo Shiiki*
Department of Instrumentation Engineering, Keio University, Yokohama 223-8522, Japan
Received 29 April 1999; received in revised form 21 June 1999
Abstract
The possibility of the giant magnetoresistive (GMR) e!ect of Co/Pd multilayer was studied based on the prediction of
the "rst principle band calculation. Anti-parallel magnetization con"guration was achieved by fabricating spin valve
structure, although the parallel magnetization is stable in the Co/Pd multilayer. Spin valve structures of Co/Pd/Co/NiO
and Co/Cu/Co/NiO were fabricated on the glass substrate by dual ion beam sputtering. Anti-ferromagnetic NiO "lms
were prepared at room temperature by oxygen reactive sputtering. The metal layers were deposited on the oxide layer
subsequently. It is found that there exist a hexagonal phase and well-known cubic phase of NiO by analyzing XRD
pattern. The exchange "eld of Cu/Co/hexagonal-NiO trilayer is larger than that of Cu/Co/cubic-NiO trilayer. The
increase of the exchange "eld is proportional to the relative oxygen concentration. The GMR e!ect appeared in spin
valve structure of Co/Pd/Co/hexagonal-NiO above critical Pd thickness, although GMR ratio is not so large. Anti-
parallel magnetization con"guration induced by a strong exchange "eld from NiO leads the GMR e!ect in the
Co/Pd. ( 1999 Elsevier Science B.V. All rights reserved.
Keywords: GMR; Spin valve; Multilayers; Exchange "eld
1
. Introduction
ever, the Co/Pd multilayer never shows the GMR
e!ect because the parallel magnetization con"gura-
tion between two ferromagnetic Co layers through
Pd layer is stable in zero magnetic "eld. If an
anti-parallel alignment is realized by using a spin
valve structure, the Co/Pd multilayer may show
large GMR e!ect.
In this article, the GMR e!ect of the free-
Co/Pd/pinned-Co spin valve structure is reported.
Anti-ferromagnetic layer with high exchange "eld is
necessary to achieve highly pinned Co ferromag-
netic layer resisting the strong ferromagnetic coup-
ling between two Co layers through the Pd layer.
We used NiO as a pinning layer, because bulk NiO
is an anti-ferromagnetic insulator with high ex-
change "eld at room temperature [4,5]. NiO "lm
In recent years, giant magnetoresistive (GMR)
material with high sensitivity has been studied to
develop high density magnetic storage system.
Among many multilayers so far investigated, the
largest GMR e!ect has been observed in Co/Cu-
based structures [1]. We have been studying the
Co/X magnetic multilayers by using the band cal-
culation [2] to develop a new GMR material with
high GMR e!ect. We predicted that Co/Pd multi-
layer as well as the Co/Cu multilayer has GMR
e!ect by "rst principles band calculation [3]. How-
*
Corresponding author.
0
304-8853/99/$- see front matter ( 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 0 0 4 9 4 - 1

W. Gouda, K. Shiiki / Journal of Magnetism and Magnetic Materials 205 (1999) 136}142
137
has been applied to spin valve elements and mag-
netoresistance ratio was discussed in relation to the
exchange "eld and crystalline orientation [6].
However, the properties of the NiO thin "lm are
not clear as yet. So, NiO thin "lm fabricated by
dual ion beam sputtering was characterized "rst.
Next the exchange "eld of Cu/Co/NiO trilayer was
investigated. Finally, magnetoresistance of Co/
Pd/Co/NiO and Co/Cu/Co/NiO spin valve struc-
tures were studied.
Oxide thicknesses were monitored by quartz-
crystal oscillator (QCO) and con"rmed by trans-
mission electron microscope (TEM).
All multilayers (included Cu/Co/NiO trilayer
and Co/NM/Co/NiO spin valve) were prepared by
depositing metal layers successively on NiO in
magnetic "elds at about 200 Oe. Co layer with
unidirectional anisotropy induced by the magnetic
"eld was formed on NiO layer. Thicknesses of
metals were monitored by QCO and con"rmed by
interval of XRD satellite peaks of [Co/Cu] and
It is con"rmed that the GMR e!ect appeared in
spin valve structure of Co/Pd/Co/NiO above criti-
cal Pd thickness. We found the possibility of a new
type GMR.
ꢀ
ꢃ
[Co/Pd] "lms. The magnetic properties were
ꢀ
ꢃ
studied by vibrating sample magnetometer (VSM).
The exchange "eld, H , of Cu/Co/NiO trilayer was
%ꢀ
estimated by measuring the shift of the major hys-
teresis loop. The magnetoresistance was measured
by the four-point probe method.
2
. Experiment
NiO "lms were prepared by the dual ion beam
sputtering method [7]. The main gun sputters an
Ni target using Ar (99.999%) beam and a "lm was
deposited on the Corning 7059 glass substrate at
room temperature. The secondary gun irradiated
the growing "lm with a low-energy oxygen beam
3. Results and discussion
3.1. Characteristics of NiO xlm
Fig. 1 shows typical XRD patterns of NiO "lms
prepared at di!erent gas #ows. Film thickness was
generated from a gas #ow of Ar and O (9 : 1)
ꢀ
mixture in order to oxidize the Ni. The base pres-
sure of the chamber was &1;10\ꢁ Torr and the
pressure during deposition was &5;10\ꢂ Torr.
Accelerating voltages of the main gun and the sec-
ondary gun were 1250 and 100 V, respectively.
Crystal structure was analyzed by X-ray di!rac-
tion (XRD) using Cu K radiation. The X-ray tube
a
voltage and current were 40 kV and 150 mA, re-
spectively. The crystal structures of the "lms were
identi"ed by XRD analysis in conjunction with
peak identi"cation using the joint committee on
powder di!raction standards (JCPDS)-Interna-
tional Centre for Di!raction Data. Oxygen concen-
trations and electronic state were measured by
Mg K X-ray photoelectron spectroscopy. The
a
X-ray tube voltage and current were 10 kV and
1
0 mA. Before measuring the oxygen concentra-
tions, the surface of the "lm was etched for 20 s in
order to remove adherent impurities on surface by
an argon beam of ions. The concentrations were
determined from stationary peaks. Resistivity was
measured by van der Paw method [8]. Transmit-
tance was measured in visible radiation.
Fig. 1. XRD patterns of NiO "lms prepared at various gas
#ows.

1
38
W. Gouda, K. Shiiki / Journal of Magnetism and Magnetic Materials 205 (1999) 136}142
about 1400 A
s
. The overall feature of the XRD pat-
tern does not change by the "lm thickness ranging
from 250 to 2000 A, although the thickness depen-
s
dence of the concentration cannot be con"rmed.
Three sharp Bragg di!raction peaks of NiO are
observed, and there exist (1 1 1) and (2 0 0) peaks
from FCC Ni in the "lm prepared at a gas #ow of
2
sccm. It is di$cult to determine the crystalline
structure of NiO, since the three peaks are almost
the same for cubic NiO and hexagonal NiO [9,10].
Fig. 2 shows the interplanar spacings d and d
ꢁ$ ꢄꢂ$
calculated from the 2nd and the 3rd peak, respec-
,
ꢀ
tively. The relationship between d
be expressed as,
and d can
ꢀ
ꢁ$ ꢄꢂ$
d
ꢀꢁ$"(2dꢄꢂ$ for cubic,
cꢀ
(1)
Fig. 3. Lattice constants, a and c of the hexagonal phase.
3
dꢀꢁ$"
d
for hexagonal. (2)
ꢀ
cꢀ#12(dꢄꢂ$)ꢀ ꢄꢂ$
Eqs. (2) and (3) lead to the following relationship
between d
and d
:
The broken line shows theoretical relationship for
cubic NiO estimated from Miller indices and lattice
constant. Plots in the elliptic region seem to be of
hexagonal NiO because they are out of theoretical
line of cubic NiO. Fig. 3 shows the estimated rela-
tionship between lattice constants a and c for plots
in the elliptic region assuming the hexagonal struc-
ture. The experimental relationship can be ex-
pressed as
ꢀ
ꢁ$ ꢄꢂ$
d
ꢀꢁ$"(21.31!9.26d ) d
ꢄꢂ$ ꢄꢂ$
3
;
.
(4)
ꢀ
(21.31!9.26dꢄꢂ$)ꢀ#12(dꢄꢂ$)ꢀ
The curve for the hexagonal phase "ts the experi-
mental values plotted in the elliptic region as in-
dicated in Fig. 2. We can distinguish cubic phase
and hexagonal phase.
c"! 4.63a#21.31.
(3)
It is concluded that (a) a mixture of cubic NiO
and Ni is obtained at gas #ow of about 2 sccm, (b)
and (c) the cubic NiO is obtained at 3}4 sccm and
(d) the hexagonal NiO is obtained at about 5 sccm.
Fig. 4 shows the XPS spectra of four NiO "lms
prepared at di!erent gas #ows from 2 to 5 sccm.
The spectrum of the "lm fabricated at 2 sccm is
identi"ed as Ni2p and Ni2p of Ni. The spectra
ꢄꢅꢀ
ꢆꢅꢀ
of "lms fabricated at (c) 4 and (d) 5 sccm are
understood as the Ni2p peak of NiO. According
ꢄꢅꢀ
to Alders [11], the main peak is c3dꢁL, where c and
L refer to a hole in the 2p core level and the ligand
band, respectively. The creation of a hole in the 2p
core level of the transition metal site is accom-
panied by a strong Coulomb repulsion with holes
in the localized 3d orbitals. The satellite peaks at
high binding energies are assigned to the un-
screened cdꢇ and screened cdꢆꢃLꢀ "nal state. It is
Fig. 2. Relationship between lattice distance d ꢁ$ and dꢄꢂ$
.
ꢀ

W. Gouda, K. Shiiki / Journal of Magnetism and Magnetic Materials 205 (1999) 136}142
139
known that the low-energy peak appeared due to
an oxygen vacancy at surfaces generated by sput-
tering. The spectrum of the "lm fabricated at
intensity ratio of O1s peak to Ni2p peak in
ꢆꢅꢀ
ꢄꢅꢀ
order to estimate relative oxygen concentration for
conventional sake. With increasing relative oxygen
concentration, lattice constant, a, of the cubic NiO
increases. This change is generally thought as
a martensite transformation. The lattice constant,
3
sccm is understood as a cubic NiO with oxygen
de"ciency. The main peak splits into satellite peaks.
Fig. 5 shows the relationship between the oxygen
concentration and the lattice constant, a, in the
cubic and the hexagonal phase. Here, the relative
oxygen concentration, R, is de"ned as integral
a, of the cubic NiO is larger than 4.176 As of the
bulk. On the other hand, lattice constant, a, of the
hexagonal NiO decreases, although plots are dis-
persive.
Fig. 6 shows the relative oxygen concentration
dependence of the "lm resistivity. Mixture phase of
Fig. 4. XPS spectra of NiO thin "lms.
Fig. 6. Resistivity of NiO "lms.
Fig. 5. Relationship between relative oxygen and lattice constant a.

1
40
W. Gouda, K. Shiiki / Journal of Magnetism and Magnetic Materials 205 (1999) 136}142
Ni and NiO has the lowest resistivity due to Ni
metal. The cubic NiO with oxygen de"ciency has
the low resistivity, which acts as a semiconductor.
Cubic NiO is an insulator and has high resistivities.
Resistivities of some cubic NiO "lms are more
than 10ꢉ ) cm and cannot be measured. The resis-
tivity of hexagonal NiO is lower than that of cubic
NiO.
3.2. Exchange xeld between Co and NiO
Fig. 8 shows relationship between the relative
oxygen concentration of NiO and the exchange
"eld. The exchange "eld was determined by the
shift of the major loop of the Cu(50 A
NiO(500 A) trilayer from that of the Cu(50 A
Co(50 A
s
)/Co(50 A
s
s
)/
)/
s
s
) bilayer, in order to reduce the experi-
Fig. 7 shows visible absorption spectra for NiO
mental error. The exchange "eld of the hexagonal
phase is larger than that of the cubic phase. The
exchange "eld was reported to be about 70 Oe
[12] for Co/cubic-NiO. It is concluded that the
hexagonal phase is more desirable as the pinning
layer in the spin valve element than the cubic phase.
"
lms of di!erent phases. These results correspond
to the resistivity of the phase. Transmittance of
cubic NiO is the highest one of all.
Characteristics of NiO "lms made by dual ion
beam sputtering are summarized in Table 1.
Fig. 7. Visible absorption spectrum of NiO "lms.
Fig. 8. Exchange "eld of Co/NiO multilayers.
Table 1
Characteristics of NiO "lm made by ion beam sputtering
Phase
Ni and NiO mixing
NiO with defective oxygen
Cubic (II)
Cubic NiO
Cubic (II)
Hexagonal NiO
Crystal structure
Ni cubic and NiO
cubic (I)
*
Hexagonal
c"!4.63a#21.31
3.01}3.05 (7.17}7.35)
0.139}0.175
Lattice constant a (c) (A
Relative oxygen
s
)
4.21
0.1}0.124
4.21}4.27
0.123}0.151
0.008}0.054
concentration (a.u.)
Peak integral intensity ratio
Grain size (A
*
*
0.6}0.4
63}64
852.1}852.7
0}5
0.3}3.9
71}51
853.1}857.7
30&
10ꢂ&
5.3&
55}46
853.1
10}20
10ꢃ}10ꢄ
s
)
Ni2p binding energy (eV)
ꢄꢅꢀ
Transmittance (%)
*
Resistivity () cm)
10\ꢈ}10\ꢄ
10\ꢆ

W. Gouda, K. Shiiki / Journal of Magnetism and Magnetic Materials 205 (1999) 136}142
141
The exchange "eld, H , increases as the relative
%
ꢀ
oxygen concentration increases.
3
.3. Giant magnetoresistance of Co/Pd/Co
multilayer
E!ect of nonmagnetic layer thickness of the
spin valve Co(50 A
s )/NM/Co(50 As )/hexagonal NiO
(
500 A) [NM : Pd,Cu] was studied. Fig. 9 shows the
s
sheet conductance of two series of specimens versus
the thickness of the nonmagnetic layer. The sheet
conductance decreases with decreasing thickness of
the nonmagnetic layer. This is because that the
conduction in the nonmagnetic layer is dominant.
Interfacial scattering is not important in our case.
There is a disadvantage for GMR ratio of Co/Pd
multilayer. The speci"c resistivity of Pd is larger
than that of Cu.
Fig. 10 shows MR transfer curves of Co(50 A
Pd(x)/Co(50 A)/NiO(500 A) spin valve elements for
various Pd thicknesses. Pd thickness ranges from
0 to 74 A. Only AMR e!ect can be observed at
Pd thickness less than 42 A. It is thought that
s )/
s
s
2
s
s
the magnetizations of both Co layers rotate si-
multaneously in the external magnetic "eld. The
ferromagnetic coupling due to the exchange
interaction between Co layers is strong enough to
resist the pinning "eld from NiO. As the thickness
of Pd decreases, the exchange interaction between
Fig. 10. MR transfer curve of Co(50 A
s
)/Pd(x)/Co(50 A
s
)/hexag-
onal-NiO(500 A) spin valve.
s
Co layers decreases. The pinning "eld, which is
stronger than the exchange "eld, realizes the anti-
parallel magnetization con"guration. When the Pd
layer thickness is larger than 48 A
appears. GMR ratio decreases with increasing the
Pd thickness more than 53 A, although anti-paral-
s , GMR e!ect
s
lel alignment can be achieved.
Fig. 11 shows GMR ratio versus thickness of
nonmagnetic layer. With increasing the Pd thick-
ness, GMR ratio of the Co/Pd/Co/NiO spin valve
jumps suddenly at a critical value and then de-
creases monotonously. On the other hand, GMR
ratio of the Co/Cu/Co/NiO spin valve decreases
monotonously. There exists an anti-parallel mag-
netization con"guration at a small Cu thickness
and the exchange interaction drops rapidly with
increasing the Cu thickness.
We found that GMR e!ect can appear in the spin
valve structure, even if the multilayer specimen has
the parallel magnetization con"guration in zero
Fig. 9. Sheet conductance of spin valve elements.

1
42
W. Gouda, K. Shiiki / Journal of Magnetism and Magnetic Materials 205 (1999) 136}142
Cu/Co/NiO always shows the GMR e!ect. Anti-
parallel magnetization con"guration achieved by
the strong exchange "eld from NiO leads the GMR
e!ect in the Co/Pd.
Acknowledgements
This work was supported in part by a Grant-in-
Aid from the Storage Research Consortium of
Japan.
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Spin valve structures of Co/Pd/Co/NiO and
Co/Cu/Co/NiO were fabricated. The GMR e!ect
appeared in spin valve structure of Co/Pd/Co/NiO
above a critical Pd thickness, although Co/