Cooling-field dependence of exchange bias in Mg-diluted Ni1-xMgxO/Ni granular systems

Article abstract of DOI:10.1016/j.jmmm.2008.12.016

The exchange-bias (EB) properties of Mg-diluted Ni_1-xMg_xO/Ni (0≤x≤0.3) granular systems have been investigated. Magnetic dilution with Mg greatly affects the EB field and the coercivity. The temperature dependence of the EB field and

Full text of DOI:10.1016/j.jmmm.2008.12.016

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Journal of Magnetism and Magnetic Materials 321 (2009) 1943–1946
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Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Cooling-field dependence of exchange bias in Mg-diluted Ni1 Mg O/Ni
ꢀx
x
granular systems
Ã
W.B. Cui, W.J. Hu, Y.J. Zhang, X.H. Liu, S. Guo, F. Yang, W. Liu , Z.D. Zhang
Shenyang National Laboratory for Materials Science, Institute of Metal Research, International Center for Materials Physics, Chinese Academy of Sciences,
Shenyang 110016, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
The exchange-bias (EB) properties of Mg-diluted Ni1 Mg O/Ni (0pxp0.3) granular systems have been
ꢀx
x
Received 1 September 2008
Received in revised form
investigated. Magnetic dilution with Mg greatly affects the EB field and the coercivity. The temperature
dependence of the EB field and the coercivity can be explained in terms of formation of domain states.
The value of the EB field increases and shows a maximum value with increasing cooling field, which can
be explained by the competition between the field-dependent Zeeman energy and the exchange
interaction at the interface.
2
3 October 2008
Available online 24 December 2008
Keywords:
Exchange bias
Nanocomposites
&
2009 Elsevier B.V. All rights reserved.
1.
Introduction
AFM CoO is diluted by Mg substitution [9]. However, it was also
found that the deposition sequence of Co and CoO has great
influence on the EB effect [10]. Besides in oxide systems, a similar
The exchange-bias (EB) effect, i.e. a shift of the hysteresis loop
along the magnetic-field axis, results from the interfacial
magnetic interaction between ferromagnetic (FM) and antiferro-
magnetic (AFM) materials and has been found for the first time in
CoO/Co [1]. EB occurs upon cooling of an AFM system in an
applied magnetic field from above the N e´ el temperature to lower
temperatures. The application of EB in spin valves [2–4] and in
giant-magnetoresistive sensors for high-recording-density hard-
disk devices [5] is very promising.
x 2
effect was also observed in Zn-diluted Fe1ꢀxZn F thin films with
Co overlayer, where the EB effect was observed to be independent
on the Zn concentration for 1.0pxp0.35, in contrast with other
reports [11]. This indicated that there are still unsolved problems
in diluted AFM/FM EB systems.
Nickel oxide, NiO, which is isostructural to CoO, is miscible
with MgO at all concentrations and the non-magnetic Mg atom
can be used to dilute the Ni-sublattice in NiO magnetically.
Recently, some interesting results have been reported on oxide
nanocomposites prepared by mechanical alloying [12,13]. In the
present article, the effect of Mg dilution on the exchange-coupled
0 E
It is generally considered that the EB field (m H ) is related to
the coupling between moments in the FM layer and uncompen-
sated interfacial moments in the AFM layer [6]. The density of
uncompensated interfacial AFM moments has been proposed to
determine the magnitude of exchange field, suggesting that we
can tune the EB by changing this moment density. Recently, non-
magnetic elements have been used as additives to tune the
density of the uncompensated AFM interfacial moments. The
effect of minor amounts of metals M (M ¼ Pt, Pd, Rh, Ir and Cu) as
dilution elements in AFM CrMn film has been investigated. Only a
Ni1ꢀxMg
The EB field
dilution and a maximum of
increased.
x
O/Ni (0pxp0.3) granular systems has been investigated.
and the coercivity decrease upon Mg
is found if the cooling field is
m
0
H
E
0 c
m H
0 E
m H
2
.
Experiments
small increment of
could be achieved by small addition of Pd, which was found to be
0 E b
m H and of the blocking temperature (T )
Mg-doped Ni1ꢀxMg O/Ni (0pxp0.3) granular systems have
x
been prepared by co-precipitation and subsequent mechanical
alloying. In a typical procedure, the raw materials NiSO
ꢁ 6H
and MgSO O were dissolved in distilled water according to
ꢁ 2H
the desired composition of the Ni1ꢀxMg
due to the change of the Mn–Mn distance [7]. Similarly, a
2
O
maximum of
0 E
m H as function of concentration of added Cu has
4
been reported for CoFe/IrMn bilayers [8]. More recently, investi-
gations have been focused on diluted AFM oxides. The EB field
could be increased by 300% in expitaxially grown Co/CoO if the
4
2
x
O (0pxp0.3) precipi-
tates. Then NaOH was added to the solution and a green
precipitate was obtained. The precipitate was filtered off, washed
with distilled water and ethanol for several times. Finally, after
Ã
annealing in air for 12 h at 1073 K, Ni1ꢀxMg
was obtained, as confirmed by means of X-ray diffraction (XRD).
O (0pxp0.3) powder
Corresponding author. Tel.: +86 24 83978856; fax: +86 24 23891320.
x
E-mail address: wliu@imr.ac.cn (W. Liu).
0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2008.12.016

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1944
W.B. Cui et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 1943–1946
No impurities were detected. After this, Ni1ꢀxMg
and Ni powder were mixed in a molar ratio of 4:1. The mixture of
g was sealed under Ar atmosphere in a hardened stainless-steel
x
O (0pxp0.3)
5
vial with stainless-steel balls of 12 mm diameter. Mechanical
milling of the powders was performed in a high-energy ball mill,
which was rotated in two dimensions perpendicular to the
horizontal plane [14,15]. The rotation speed of the mill was
chosen to be 400 rpm, and the ball-to-powder weight ratio was
4
0:1. After milling, the powders were annealed for 30 min at 673 K
in a vacuum furnace that was directly connected to a closed glove
box. XRD analysis was conducted using Cu-K radiation. The grain
size and microstructure of the powders were studied using a JEOL
010 transmission electron microscope (TEM). The magnetic
a
2
properties were measured in a superconducting quantum inter-
ference device (SQUID).
3.
Results and discussion
Fig. 2. Magnetic hysteresis loops of Ni1ꢀxMg
measured at 10 K after cooling from room temperature at
x
O/Ni (0pxp0.3) granular systems,
cool ¼ 1 T.
0
m H
Fig. 1 shows the morphology images of annealed (a) NiO/Ni
and (b) Ni0.7Mg0.3O/Ni nanocomposites and the insets show their
TEM diffraction patterns. It is seen that the typical particle size in
the granular systems is about several hundred nanometer.
However, the discontinuous diffraction circles in the two figures
indicate that the particle is polycrystalline.
The magnetic hysteresis loops at 10 K of Ni1ꢀxMg
0pxp0.3) nanocomposites recorded after cooling in 1 T
cool ¼ 1 T) from room temperature to 10 K are shown in
x
O/Ni
(
(
0
m H
Fig. 2. The shift in the hysteresis loops is equal to the exchange
field (Hright+Hleft)/2, whereas the coercivity is
¼ ꢀ
defined as right and left being the
m
0
H
E
m
¼
0
0 c
m H
m
0
H
c
m
0
(HrightꢀHleft)/2,
m
0
H
0
m H
points where the loop intersects the field axis. The powder can be
easily magnetically saturated at a low magnetic field. It is seen
that a shift of the loop occurs in all granular systems. The largest
values
m
0
H
E
¼ 87 mT and
m
0
H
c
¼ 155 mT are obtained in the NiO/
Ni granular system. When Ni is partially substituted by the non-
magnetic Mg, both
dependencies of
Fig. 3. It is clear that
m
E
0
H
E
and
m
0
H
c
decrease. The temperature
m
0
H
and
m
m
0
H under
and
c
m
0
H
cool ¼ 1 T are shown in
0
H
E
m
0
H
c
decrease rapidly at low
temperature and then slowly at elevated temperature. When Ni
atoms in the NiO lattice are replaced by Mg atoms, a statistical
imbalance of the number of antiparallel moments on the two AFM
sublattices within a finite region is supposed to cause the
formation of domain states [16]. Such a domain state is frozen
at low temperatures after field cooling and the net magnetization
is coupled with the moments of the FM phase, exhibiting
Fig. 3. Temperature dependence of
m
0
H
E
and
0
m H
c
with
0
m H
cool ¼ 1 T for
Ni1ꢀxMg O/Ni (0pxp0.3) granular systems
x
unidirectional anisotropy and EB, as shown in Fig. 2. However,
the exchange coupling at the interface of the two phases will also
be weakened by the substitution of non-magnetic Mg for Ni, so
that
0 E
m H is reduced in Mg-substituted nanocomposites compared
with the NiO/Ni nanocomposite at the same temperature, as
illustrated in Fig. 3. The domain-wall motion can be thermally
activated and will be easy at an elevated temperature, leading to a
decrease of
and are different from the results obtained on thin films [10].
This may be due to the different interfacial situation in
m
0
H
E
. However, the temperature dependencies of
m
0 E
H
Fig. 1. TEM images of annealed (a) NiO/Ni and (b) Ni0.7Mg0.3O/Ni granular
0 c
m H
systems. The insets show the respective TEM diffraction patterns. The reflections
(311), (4 0 0), (4 4 0) are from NiO and (111), (2 2 0) from Ni.

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W.B. Cui et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 1943–1946
1945
mechanically alloyed nanocomposites, in which the interface is
probably rougher and more grain defects are expected.
Together with the formation of domain states in the diluted
AFM/FM exchange-bias systems, an irreversible surplus magneti-
zation after field cooling is expected. This irreversible surplus
magnetization is considered to control the EB, which is deter-
mined by the irreversible domain state magnetization (mIDS). They
+
ꢀ
are equal to mIDS ¼ mAFM(H )+mAFM(H ) and characterize the
vertical shift of the hysteresis loops of diluted AFM. Here,
AFM(H ) and mAFM(H ) denote the magnetization of the AFM at
+
ꢀ
m
the positive and negative maximum field, respectively [16].
However, in the Mg-diluted AFM/FM granular systems, it is
+
ꢀ
difficult to directly measure mAFM(H ) and mAFM(H ) of the AFM
phase. Because the moments at the AFM–FM interface are
coupled, it is reasonable to quantify mIDS as the vertical shift of
the loops of the nanocomposites (mIDS
¼
D
M
loop(H) ¼
+
ꢀ
+
ꢀ
mloop(H )+mloop(H )) [17,18]. Here, mloop(H ) and mloop(H ) stand
for the magnetization of the nanocomposites at the positive and
negative maximum field, respectively. Because is strongly
related with M, it is necessary to first investigate the dependence
of M(H) on the cooling field, which is shown in Fig. 4. The
increase of M with larger cooling field suggests that more
0 E
m H
D
D
D
irreversible domain states are formed, which means the net
magnetization of the nanocomposite is gradually enlarged by the
increased applied field. It has been argued that a larger net
magnetization leads to larger EB fields [19]. However, opposite
experimental results are demonstrated in Fig. 5, where
a
maximum appears in the plots of and versus the
m
0
H
E
0 c
m H
cooling fields. Similar anomalous behavior has also been reported
for Fe-oxide systems [20].
Since, in the annealed state, the magnetic domains randomly
Fig. 5. Dependence of
x
DM on the cooling field for Mg-diluted Ni1ꢀxMg O/Ni
(0pxp0.3) granular systems.
oriented, there is no resulting interaction. When the cooling field
max
is smaller than
0 E
m Hcool below which the maximum value of H is
obtained, the degree of alignment of the magnetic domains in
both the FM and the AFM phase is enhanced. The positive Zeeman
energy (
m
0 1
M Hcool) of the interfacial moments of the AFM phase in
cooling field is not strong enough to compete with the negative
Fig. 6. Sketch for the moments at the interface of diluted AFM/FM systems under
a) a small field and (b) a larger field. The dots mark the defects or dilution atoms.
(
exchange-interaction energy (ꢀJAFM
1 2
M M ) between the interfacial
AFM moments with the moments of the AFM matrix (see
Fig. 6(a)). This positive Zeeman energy would be negligible
compared with the effect of the cooling field on the enhanced
alignment of uncompensated moments at the interface, which has
been confirmed by the increased irreversible domain magnetiza-
tion (Fig. 4).
m
0
H
E
and
0 c
m H will increase to a maximum value
upon increasing the cooling field due to better coupling between
moments of AFM and FM. With increasing cooling field (Fig. 6(b)),
the competition between the Zeeman energy and the exchange-
interaction energy will result in a change of the moment direction
at the interface of the AFM phase from opposite to along the
direction of the cooling field. This leads to an increment of the
irreversible domain magnetization, as can be seen in Fig. 4.
1 2
However, because M and M are parallel, the system is frozen in a
metastable configuration. In the demagnetization process, the
moments in such a configuration can be rotated relatively easy by
a small negative field, leading to a lower value of
larger.
0
m H
E
if
0
m Hcool is
Fig. 4. Dependence of
m
0
H
E
and
0 c x
m H of Ni1ꢀxMg O/Ni (0pxp0.3) granular
systems at 10 K on the cooling field.

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W.B. Cui et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 1943–1946
Further increased substitution of Mg for Ni would weaken the
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This work has been supported by the National Nature Science
Foundation of China under Project nos. 50331030 and 50571098.