Exchange coupling in CoO-Co bilayer

Article abstract of DOI:10.1016/j.jmmm.2006.01.118

In this work, exchange bias and coercivity enhancement in ferromagnet (FM)-antiferromagnet (AFM) bilayer have been investigated. CoO film (50 nm) was deposited by sputtering with a relatively high oxygen partial pressure. The deposited films were subsequently annealed at varied temperature up to 973 K in the air atmosphere. The CoO film shows a disordered structure in the as-deposited state and an increase of crystallinity after annealing characterized by XRD and Raman spectra. A 40-nm Co film was deposited on the as-deposited CoO and annealed films. The Co-CoO bilayer shows a large exchange bias up to 1600 Oe and relatively high coercivity up to 3200 Oe (H_C-) at 5 K, which is much larger than that of crystalline Co-CoO bilayer films without any treatment. The spin glass behavior combined with increasing crystallinity, surface roughness of CoO after annealing may be attributed to the large exchange bias and high coercivity.

Full text of DOI:10.1016/j.jmmm.2006.01.118

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 303 (2006) e160–e164
www.elsevier.com/locate/jmmm
Exchange coupling in CoO–Co bilayer
Ã
J.B. Yi, J. Ding
Department of Materials Science and Engineering, National University of Singapore, 119260, Singapore
Available online 24 February 2006
Abstract
In this work, exchange bias and coercivity enhancement in ferromagnet (FM)–antiferromagnet (AFM) bilayer have been investigated.
CoO film (50 nm) was deposited by sputtering with a relatively high oxygen partial pressure. The deposited films were subsequently
annealed at varied temperature up to 973 K in the air atmosphere. The CoO film shows a disordered structure in the as-deposited state
and an increase of crystallinity after annealing characterized by XRD and Raman spectra. A 40-nm Co film was deposited on the as-
deposited CoO and annealed films. The Co–CoO bilayer shows a large exchange bias up to 1600 Oe and relatively high coercivity up to
3200 Oe (HCꢀ) at 5 K, which is much larger than that of crystalline Co–CoO bilayer films without any treatment. The spin glass behavior
combined with increasing crystallinity, surface roughness of CoO after annealing may be attributed to the large exchange bias and high
coercivity.
r 2006 Elsevier B.V. All rights reserved.
PACS: 75.50.Lk; 75.50.Ee; 75.50.Kj; 75,70.Cn
Keywords: Exchange coupling; Co/CoO bilayer; Ordering; Spin-glass
1. Introduction
exchange bias could disappear. It is because that the spin
of the disordered structure is difficult to align anti-parallel
to form antiferromagnetic order. However, it is still in
dispute whether antiferromagnetic order exists in an ideal
amorphous (disordered) solid, which has no obvious
subnetworks. A short range of antiferromagnetic order is
supposed to be possible by the spin frustration [13]. In this
study, disordered CoO antiferromagnetic thin film was
deposited by sputtering, subsequently annealed with a
variety of temperature. The annealed CoO films were
deposited with Co film. The exchange coupling between Co
and CoO film with different ordering by annealing is
discussed.
Since the discovery of exchange bias in the oxidized Co
fine powders, exchange coupling of ferromagnet–antiferro-
magnet has been widely investigated for the potential
applications and technical interest [1–4]. CoO has a Neel
´
temperature of 291 K [5], and a large interface energy
density, which makes the exchange bias relatively large and
easily observed in FM–CoO system. The effect of grain
size, thickness, crystallinity and interface disorder of CoO
on the exchange coupling between ferromagnet and
antiferromagnet (FM)/(AFM) has been studied [6–10].
Furthermore, the effect of interaction between CoO phases
on the exchange bias and coercive field were also studied. It
showed that with the decreasing of the interaction (dipolar,
exchange), the exchange bias decreases [11,12]. It is
believed that the exchange interaction of the atoms
becomes weak with less ordering of the antiferromagnet,
the exchange anisotropy of the system may also decrease.
When antiferromagnet is in full disordered state, the
2. Experimental procedure
A 99.99% Co target was used to deposit Co and CoO
film by DC sputtering (Discovery 18, Denton Vacuum Inc.
USA). The films were deposited on glass substrate. During
deposition of CoO, an oxygen partial pressure of
0.15 mTorr was introduced by a precise flow meter. The
base pressure of the sputtering is 2 ꢁ 10ꢀ6 Torr. The
deposited pressure is approximately 7 mTorr. The thickness
Ã
Corresponding author. Tel.: +65 68744317; fax: +67763604.
E-mail address: msedingj@nus.edu.sg (J. Ding).
0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2006.01.118

ARTICLE IN PRESS
J.B. Yi, J. Ding / Journal of Magnetism and Magnetic Materials 303 (2006) e160–e164
e161
of CoO film is approximately 50 nm taken by a profil-
ometer (P-12, Tencor Instruments, USA). The sputtered
CoO films were annealed in air atmosphere with tempera-
tures of 373, 573, 773, and 973 K for 30 min. After
annealing, the sample was immediately put into sputtering
chamber for the deposition of Co film, subsequently
capped with a 2-nm SiO2 layer by RF sputtering to avoid
film oxidation. The thickness of Co is approximately 40 nm
and the deposition parameters are same as that of CoO film
except no oxygen partial pressure during sputtering. The
surface morphology of CoO films with different annealing
temperature were taken by atomic force microscopy
(AFM) scanning (Nano III, DI, USA). X-ray diffracto-
metry (XRD, D8 Advance, Bruker, USA) and Raman
spectrometry (LabRam HR800, Jobin Yvon Horiba Inc.
USA) were utilized for the structural and phase character-
ization. The magnetic properties of Co–CoO films were
taken by superconductor quantum interference device
(SQUID, MPMS XL5, Quantum Design, USA) and
vibrating specimen magnetometer (VSM, Maglab, Oxford,
England).
CoO(111)
CoO(200)
973 K
773 K
573 K
373 K
As-deposited
25
(a)
30
35
40
45
50
2 Theta (deg)
3. Results and discussion
Fig. 1a shows XRD spectra of CoO films. In the as-
deposited state, no peaks but only a hump can be observed,
indicating disordered structure of the film. After annealing
at an elevated temperature, CoO peaks indexed to (1 1 1)
and (2 0 0) were seen in the spectra. The intensity is
stronger with higher annealing temperature, showing an
increase of crystallinity of the films. The weak peaks in the
XRD spectra suggest the films may not have a well
crystalline structure. Raman spectrometry is one of the
effective ways to identify disordered structure of metal
oxide. Fig. 1b shows the Raman spectra of CoO films
annealed at different temperatures. It can be seen that all
the films show typical curves of CoO spectra. With increase
in annealing temperature, intensity of the spectra increases,
indicating an increasing crystallinity of films, as shown in
XRD spectra. It is to note that no Co3O4 phase could be
identified from XRD and Raman spectra.
Fig. 2a shows the AFM micrograph of CoO in the as-
deposited state. From the graph, the surface is smooth and
none of the particles can be observed. It may be due to a
large amount of disordered structure inside the film, as
shown in our XRD spectra. After annealing at 973 K, as
shown in Fig 2b, particles can be seen from the
micrograph, indicating an increase of crystallinity after
annealing. The image shows the particle size is 4–6 nm.
AFM images of CoO film annealed at other temperatures
973 K
773 K
573 K
373 K
298 K
400
(b)
500
600
Wavenumber (cm-1)
700
Fig. 1. XRD, (a) Raman spectrum, (b) of as-deposited CoO and CoO
after annealing at 373, 573, 773, and 973 K.
Fig. 3 shows the coercivity and exchange bias of the
Co–CoO bilayer with different annealing temperature of
CoO. The coercivity and exchange bias both increase with
the scaling of annealing temperature. For CoO in the as-
deposited state, the exchange bias is almost zero, suggest-
ing CoO in the as-deposited state may not have anti-
ferromagnetic order (disordered structure). With the
increasing of annealing temperature, the coercivity and
exchange bias increase quickly. This is due to the long-
range order of antiferromagnetic coupling by annealing.
The increasing ordering has been shown in the XRD,
Raman spectra and AFM images. AFM images have
shown that the roughness of the films increases with the
increase of annealing temperature. The increasing rough-
ness may also affect the exchange bias and coercivity [14].
The large roughness may result in an increase of exchange
bias.
(373–773 K) indicate
a surface morphology between
Figs. 2a and b. The particles appear gradually and particle
size increases. The roughness of the film is shown in Fig. 2c.
It can be seen that with the increasing of annealing
temperature, the surface roughness increases. The rough-
ness can be fitted linearly.

ARTICLE IN PRESS
J.B. Yi, J. Ding / Journal of Magnetism and Magnetic Materials 303 (2006) e160–e164
e162
2.00
20.0 nm
10.0 nm
1.00
0.0 nm
8
(a)
6
0
2.00
2.00
0
1.00
4
50.6 nm
25.3 nm
0.0 nm
2
1.00
200
400
600
800
1000
(c)
Annealing temperature (K)
(b)
0
2.00
0
1.00
µm
Fig. 2. AFM micrograph of CoO of as-deposited films (a) and 973 K annealed film, (b). The surface roughness of CoO as a function of the different
annealing temperature, (c). The line is a guide to eye.
600
5K
Coercivity
Exchange bias
1200
800
400
0
80K
300K
400
200
0
-8
-6
-4
-2
0
2
4
6
8
200
400
600
800
1000
(a)
Field (kOe)
Substrate annealing temperature (K)
Fig. 3. Coercivity and exchange bias of Co–CoO bilayer as a function of
CoO annealing temperature taken at 80 K.
3600
3200
2800
2400
2000
1600
1200
800
1600
HC-
HE
Fig. 4a shows the hysteresis loop of Co–CoO (973 K
annealing) taken at different temperature. At 300 K, the
hysteresis loop is of small coercivity and no exchange bias
1200
800
400
0
could be observed. It is reasonable since the Ne
´
el
temperature of CoO is 291 K. Above Neel temperature,
´
CoO becomes paramagnetic. Hence, the exchange bias is
zero. At 80 K, coercivity greatly increases and an obvious
exchange bias can be observed (385 Oe). At 5 K, the
coercivity in the negative field direction (HCꢀ) is approxi-
mately 3200 Oe, which is much larger than that at 80 and
300 K. The hysteresis loop shows an exchange bias as large
as 1600 Oe. Fig. 4b shows the temperature dependence of
coercivity and exchange bias of CoO–Co bilayer, in which
CoO was annealed at 973 K for 30 min. From 300 to 120 K,
400
0
0
50
100 150 200 250 300
Temperature (K)
(b)
Fig. 4. Hysteresis loops of Co–CoO bilayer (CoO annealed at 973 K) at
the temperature of 5, 80 and 300 K, (a). Coercivity and exchange bias of
Co–CoO (973 K) bilayer as a function of temperature, (b).

ARTICLE IN PRESS
J.B. Yi, J. Ding / Journal of Magnetism and Magnetic Materials 303 (2006) e160–e164
e163
0.0008
0.0007
0.0006
0.0005
0.0004
0.0003
0.0007
FC
ZFC
0.0006
FC
0.0005
0.0004
0.0003
0.0006
500 Oe
400 Oe
ZFC
500 Oe
0.0008
0.0006
0.0004
0.0005
0.0004
0.0003
400 Oe
300 Oe
0.0002
0.0005
0.0004
0.0003
0.0002
0.0001
0.0000
0.0005
0.0004
0.0003
300 Oe
200 Oe
0.00040
0.00035
0.00030
0.00025
0.0005
0.0004
0.0003
0.0002
0.00048
200 Oe
100 Oe
0.00032
0.00028
0.00024
0.00044
0.00040
0.00036
100 Oe
0
50
100
150
200
250
300
350
0
50
100
150
200
250
300
350
(a)
Temperature (K)
(c)
Temperature (K)
3500
65
60
55
50
45
40
35
3000
2500
2000
1500
1000
500
30
120
160
200
240
280
320
160
200
240
280
320
Temperature (K)
(b)
Temperature (K)
(d)
Fig. 5. ZFC and FC curve of CoO–Co bilayer with different applied fields (CoO annealed at 973 K for 30 min), (a). Field dependence of spin glass like
transition temperature TF, showing AT line, i.e., dTFpH2/j, j ¼ 3, (b). ZFC and FC curve of Co–CoO bilayer with different applied fields (as-deposited
CoO), (c). Field dependence of Co–CoO (as-deposited CoO), transition temperature TFpH2/j, j ¼ 1.2, (d).
the coercivity and exchange bias increase slowly with the
decreasing of the temperature. Below 120 K, the coercivity
and exchange bias increases quickly, suggesting that the
block temperature of antiferromagnet in this system may
be approximately at 120 K. The temperature is much lower
finite size effects [15]. At 31 K, there is a cusp of coercivity.
However, the exchange bias still monotonically increases.
The phenomenon may be due to the exchange coupling
Co3O4–Co inside the film since CoO has been annealed at
high temperature. Co3O4 phase may be formed during
annealing, though it has not been observed in XRD and
than Neel temperature of bulk CoO (291 K). It is due to the
´

ARTICLE IN PRESS
J.B. Yi, J. Ding / Journal of Magnetism and Magnetic Materials 303 (2006) e160–e164
e164
Raman spectra. The Ne
´
el temperature of Co3O4 is 33 K
shows an increase of surface roughness with increase of
[16]. After Co3O4 becomes antiferromagnet below 30 K, the
strong interface exchange coupling between AFM–FM
may make ferromagnet reversal drag antiferromagnet
interface spins more difficult, resulting in a decrease of
coercivity. The strong exchange coupling of Co3O4–Co
may also enhance the exchange anisotropy, showing a
continuous increase of exchange bias. In addition, it has
shown that CoO is not a well crystalline structure. The
decreasing temperature may enhance the ordering of CoO
film. The increasing ordering of CoO may result in an
increase of exchange bias and a reduction of coercivity
since the dragging of CoO spin by Co spins in the interface
during magnetic reversal becomes more difficult.
annealing temperature. Spin-glass-like behavior has been
observed in the 973 K annealing sample. The long-range
ordering, surface roughness and spin-glass-like magnetic
order may be attributed to the relatively large exchange
anisotropy.
Reference
[1] W. Meiklejohn, C.P. Bean, Phys. Rev. 102 (1956) 1413.
[2] R.D. Hempstead, S. Krongelb, D.A. Thompson, IEEE Trans. Magn.
14 (1978) 521.
[3] J. Fujikata, K. Ishihara, K. Hyashi, H. Yamamoto, K. Yamada,
IEEE Trans. Magn. 31 (1995) 3939.
[4] T. Lin, C. Tsang, R.E. Fontana Jr., J.K. Howard, IEEE Trans.
Magn. 31 (1995) 2584.
In order to study the effect of annealing temperature of
CoO film on the magnetic ordering of this system, zero field
cooling (ZFC) and field cooling (FC) measurement by
applying different magnetic field were performed, as shown
in Fig. 5. It shows that the blocking temperature of the
bilayer strongly depends on the applied field. The larger the
applied field, the lower the blocking temperature. For Fig. 5a,
by plotting the blocking temperature with applied field H2/j
(here j ¼ 3), a linear relationship can be obtained. It is a
typical characteristic of spin-glass-like behavior [17,18]. The
spin-glass-like materials have been shown an exchange bias
and enhancement of coercivity [19,20]. Therefore, the
enhancement of coercivity and exchange bias in this study
may be due to the exchange coupling of long-range
antiferromagnetic order of CoO after annealing and spin-
glass-like behavior as well. For the ZFC and FC curve of
CoO in the as-deposited state (Fig. 5c), the blocking
temperature of the bilayer as a function of applied field
roughly shows a relationship of TpH2/j (Fig. 5d). j is 1.2,
which is smaller than 2, as observed in the random field
system [21]. Other ZFC and FC curves of the bilayers with
CoO annealed at 373, 573 and 773 K cannot be fitted with
random field model or spin-glass-like model. It may be a
combination of spin-glass-like and random field behavior.
In summary, exchange bias and coercivity enhancement
has been investigated in Co–CoO bilayer by annealing CoO
at different temperature. With increase in annealing
temperature, the ordering of CoO increases. Exchange
bias and coercivity also increase. In addition, AFM image
[5] T. Ambrose, C.L. Chien, Phys. Rev. Lett. 76 (1996) 1743.
[6] A.N. Dobrynin, D.N. Ievlev, K. Temst, P. Lievensa, J. Margueritat,
J. Gonzalo, C.N. Afonso, S.Q. Zhou, A. Vantomme, E. Piscopiello,
G. Van Tendeloo, Appl. Phys. Lett. 87 (2005) 012501.
[7] T.J. Moran, J.M. Gallego, I.K. Schuller, J. Appl. Phys. 78 (1995)
1887.
[8] P.J. van der Zaag, R.M. Wolf, A.R. Ball, C. Bordel, L.F. Feiner, R.
Jungblut, J. Magn. Magn. Mater. 148 (1995) 346.
[9] P.J. van der Zaag, A.R. Ball, L.F. Feiner, R.M. Wolf, J. Appl. Phys.
79 (1996) 5103.
[10] D.L. Peng, K. Sumiyama, T. Hihara, S. Yamamuro, T.J. Konno,
Phys. Rev. B 61 (2000) 3103.
[11] E. Girgis, R.D. Portugal, H. Loosvelt, M.J. Van Bael, I. Gordon, M.
Malfait, K. Temst, C. Van Haesendonck, Phys. Rev. Lett. 91 (2003)
187202-1.
[12] J.B. Yi, J. Ding, B.H. Liu, Z.L. Dong, T. White, Y. Liu, J. Magn.
Magn. Mater. 285 (2005) 224.
[13] T. Kaneyoshi (Ed.), Introduction to Amorphous Magnets, World
Scientific, Singapore, 1992 p. 110.
[14] A.P. Malozemoff, Phys. Rev. B 35 (1987) 3679.
[15] D. Lederman, C.A. Ramos, V. Jaccarino, J.L. Cardy, Phys. Rev. B 48
(1993) 8365.
[16] P. Miltenyi, M. Gierlings, J. Keller, B. Beschoten, G. Guntherodt,
´
¨
Phys. Rev. Lett. 84 (2000) 4224.
[17] H. Maletta, W. Zinn, in: K.A. Gschneidner Jr., L. Eyring, (Eds.),
Handbook on the Physics and Chemistry of Rare Earths, vol 12,
North-Holland, Amsterdam, 1989 (Chapter 84)
[18] R.L. de Almeida, D.J. Thouless, J. Phys. A 11 (1978) 983.
[19] H. Wang, T. Zhu, K. Zhao, W.N. Wang, C.S. Wang, Y.J. Wang,
W.S. Zhan, Phys. Rev. B 70 (2004) 092409.
[20] R.K. Zheng, H. Liu, X.X. Zhang, V.A.L. Roy, A.B. Djuris
Phys. Lett. 85 (2004) 589.
[21] D.P. Belanger, A.P. Young, J. Magn. Magn. Mater. 100 (1991) 272.
ˇ
i, Appl.