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phys. stat. sol. (c) 4, No. 12, 4384–4387 (2007) / DOI 10.1002/pssc.200777325
© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Giant parallel and perpendicular exchange biases
in MnPd/Co bilayers
Nguyen Thanh Nam*1
, Nguyen Phu Thuy2,3
, Nguyen Anh Tuan2, Nguyen Nguyen Phuoc
1,
and Takao Suzuki1
1 Information Storage Materials Laboratory, Toyota Technological Institute, Nagoya, Japan 2 International Training Institute for Materials Science, Hanoi University of Technology, Vietnam 3 Vietnam College of Technology, Vietnam National University, Hanoi, Vietnam
Received 7 May 2007, revised 5 September 2007, accepted 19 October 2007
Published online 18 December 2007
PACS 75.25.+z, 75.30.Gw, 75.70.Cn
A systematic study of exchange bias in MnPd/Co bilayers has been carried out in both parallel and per-
pendicular directions, where the dependences of blocking temperature, exchange bias and unidirectional
anisotropy constant on the thicknesses of MnPd and Co layers were investigated. One of the particular in-
terests is that the blocking temperature of parallel exchange bias is higher than that of perpendicular ex-
change bias which can be interpreted as the difference of the ordering in parallel and perpendicular direc-
tion. The other is that a huge unidirectional anisotropy constant, JK = 5.5 erg/cm2 was observed, which is
in reasonable agreement with the theoretical prediction based on the model by Meiklejohn and Bean.
© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction
Exchange bias, discovered by Meiklejonh and Bean in 1957 [1], is the phenomenon associated with the
exchange anisotropy created at the interface between antiferromagnet (AF) and ferromagnetic (FM)
layers when these layers are cooled in a magnetic field through the Néel temperature of the AF layer.
Although exchange bias was discovered more than fifty years old, it still has attracted much interest. One
of the reasons is that it can be used in magnetic sensors and high-density magnetic recording system. The
other is that until now the mechanism of this phenomenon is not clear.
It is known that the theoretically predicted exchange bias field is larger than the experimental value by
two orders of magnitude. There are several theoretical works, such as the domain wall models [3, 4] or
spin flopping model [5] proposed to account for this discrepancy. However, there is still a lack of ex-
perimental confirmation for these models. Recently several groups have found a great enhancement of
the exchange bias field in MnIr/CoFe [6, 7] and in MnPd/Co [8, 9] bilayer system, but its physical origin
is still in controversy. The fact that this large exchange bias shows contradicted results to that of the
“normal” exchange bias thus requires a modification of the theoretical works for the quantitative under-
standing of exchange bias coupling. Therefore, from the fundamental viewpoint, studies of materials
with giant unidirectional anisotropy play a vital role on the way to get a better understanding of the
mechanism of exchange bias phenomenon. From the application point of view, the quest for materials
exhibiting giant unidirectional anisotropy is indispensable for the realization of very thin read head sen-
sors used for ultra-high density magnetic recording.
* Corresponding author: e-mail: [email protected]
phys. stat. sol. (c) 4, No. 12 (2007) 4385
www.pss-c.com © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
The present work reports about the observation of a very large unidirectional anisotropy constant in both
parallel and perpendicular directions of MnPd/Co bilayers. The dependences of blocking temperature,
exchange bias and unidirectional anisotropy constant on thicknesses are investigated and discussed.
2 Experimental
Samples with the structure of Co/MnPd/Si(111) were grown at room temperature using the RF sputter-
deposition system with the base pressure of 10–6 mbar. MnPd layers were fabricated from Pd target with
bonded Mn chips. No external field was applied during deposition. The argon pressure during deposition
process is 10-3 mbar. The composition of MnPd layer identified by the energy dispersion X-ray spectro-
scope is Mn30Pd70. The structural properties of the samples were investigated by X-ray diffraction. For
field cooling procedure, all the samples were heated in a vacuum oven (10-5 mbar) to temperature of 570
K and cooled down to 80 K in the field applied in both the parallel and perpendicular directions. The
magnetic properties of samples were characterized by the vibrating sample magnetometer (VSM) at
various temperatures from 80 K to 300 K with the field applied in both the parallel and perpendicular
directions
3 Results and discussion
Figure 1 shows the X-ray diffraction patterns of
MnPd(540 nm) single layer and MnPd(36 nm)/
Co(80 nm) bilayer. It is observed that in the sample
with very thick MnPd single layer, MnPd is fcc
phase of with several peaks. It indicates that the
sample is polycrystalline without texture. However,
for the bilayer of MnPd(36 nm)/Co(80 nm), only the
peak of MnPd(321) appears. This may suggest that
for the bilayer MnPd has the weak texture in (321)
orientation. Another point worthwhile noting is that
there is no peak for Co layers. It may therefore be
concluded that Co is in amorphous state.
Shown in Fig. 2 is the temperature dependence of
exchange bias field HE for some of representative
samples of series MnPd(x nm)/Co(20 nm), with x =
7, 14, 18 and 22 nm. It is observed that HE in paral-
lel direction are always larger than those in perpen-
dicular direction. Moreover the blocking tempera-
tures, defined as the temperature beyond which
exchange bias vanishes, in parallel direction are also
larger than those in perpendicular direction. This
effect has been observed by Phuoc and Suzuki in the
FePt/FeMn multilayer [10, 11] and can be explained
by assuming that the large shape anisotropy arising
from demagnetization field at interface of AF and
FM layers forces the spins to lie in the plane of the
films. Therefore the ordering of the antiferromag-
netic spins at the interface in the parallel direction is
larger than that in the perpendicular direction and
consequently the parallel TB and HE are higher than the perpendicular TB and HE. The dependences of
blocking temperature TB, exchange bias HE and the unidirectional anisotropy constant JK on MnPd thick-
ness (tMnPd) are shown in Fig. 3. Here, JK is the unidirectional anisotropy which is defined from the equa-
Fig. 1 (a) XRD pattern of Si(111)/MnPd (540
nm) single layer. (b) XRD pattern of Si(111)/
MnPd(36 nm)/Co(80 nm) bilayer.
20 40 60 80 1000
50
100
150
200
(b)
(a)
(321)
(220)
(200)
Si(111)
(111)
Inte
nsi
ty (
a.u.)
2θ (deg.)
Fig. 2 Temperature dependence of parallel and
perpendicular exchange bias fields for MnPd(x
nm)/Co(20 nm) bilayer (x = 7, 14, 18 and 22 nm.)
80 120 160 200 240 280
0
200
400
600
800
tMnPd
= 14 nm
HE (
Oe)
T (K)
Parallel
Perpendicular
80 120 160 200 240
0
300
600
900
1200
tMnPd
= 7 nm
Parallel
Perpendicular
HE (
Oe)
T (K)
80 120 160 200 240 280
0
200
400
600
800
1000
tMnPd
= 18 nm
HE (
Oe)
T (K)
Parallel
Perpendicular
80 120 160 200 240 280
0
200
400
600
800
tMnPd
= 22 nm
Parallel
Perpendicular
HE (
Oe)
T (K)
4386 N. T. Nam et al.: Exchange biases in MnPd/Co bilayers
© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com
tion JK = HE*MS*tFM, where HE is the exchange bias field, MS the saturation magnetization of the FM
layer and tFM the thickness of FM layer. It can be seen that TB, HE and JK are independent on tMnPd in both
directions. Normally, in exchange bias systems, the dependence of HE and JK on antiferromagnetic
thickness tAF is divided into two regions. When the antiferromagnetic thickness is larger than a critical
thickness, HE and JK are independent on tAF. When tAF is smaller than the critical thickness, as tAF is re-
duced, HE and JK decrease abruptly. But in this case there is only one region, HE and JK are independent
on JK. Therefore the critical thickness in this MnPd/Co system is smaller than 4 nm.
The dependences of blocking temperature TB, exchange bias field HE and the unidirectional anisotropy
constant JK on the thickness of Co layer are shown in Fig. 4. With tCo is larger than 40 nm both parallel
and perpendicular HE decrease, indicating that this is an interfacial effect. This is confirmed by the fact
that JK with the value of about 5.5 erg/cm2 and 3.5 erg/cm2 for parallel and perpendicular respectively, is
nearly constant over this Co thickness range. But when Co thickness is less than 40 nm, JK and HE de-
crease as the thickness of Co decreases. The trend is observed in several systems which were attributed
to the less well crystalline of the FM layer when its thickness is small [12]. However, in this study, this
argument may not be correct since the XRD pattern show that Co layer is amorphous.
It is of great interest to see that the maximum parallel and perpendicular JK is 5.5 erg/cm2 and 4.2
erg/cm2. These JK are several orders of magnitude larger than those observed on most of the metallic
exchanged bias thin films [2]. For example, in MnPd/Fe bilayers, the JK value is only 0.032 erg/cm2 [13]
and 0.017 erg/cm2 [14], in MnIr/Co bilayers, the JK value is 0.14 erg/cm2 [15] and MnFe/Co bilayers, the
JK value is 0.059 erg/cm2 [15]. In the literature, there are few systems, which exhibit such large unidirec-
tional anisotropy constant. The largest JK ever reported in the literature is 2.11 erg/cm2 in the system of
Fe3O4/CoO bilayers measured at T = 5 K [16]. Therefore, it may be concluded that the present obtained
JK of 5.5 erg/cm2 is the largest value ever found in the literature.
Fig. 3 Thickness (MnPd) dependences of
blocking temperature TB, exchange bias field
HE and unidirectional anisotropy constant JK
for MnPd(x nm)/Co(20 nm).
0
200
400
600
800
1000
1200 P a ra lle l
P e rp en d icu la r
HE (
Oe)
0 10 20 30 40 50 600 .0
0 .5
1 .0
1 .5
2 .0
2 .5
3 .0
3 .5 P ara lle l
P erp en d icu la r
J K(erg/cm
2
)
M nP d th ickness (nm )
0
50
100
150
200
250
300
P ara lle l
P erpend icu lar
TB (
K)
Fig. 4 Thickness (Co) dependences of block-
ing temperature TB, exchange bias field HE and
unidirectional anisotropy constant JK for
MnPd(36 nm)/Co(x nm).
0
200
400
600
800
1000
1200
HE (
Oe)
P a ra lle l
P e rp en d icu la r
0 10 20 30 40 50 60 70 80 900
1
2
3
4
5
6
7
P aralle l
P erpend icu lar
C o th ickness (nm )
J K(erg/cm
2
)
0
50
100
150
200
250
300
TB (
K)
P ara lle l
P erp en d icu lar
phys. stat. sol. (c) 4, No. 12 (2007) 4387
www.pss-c.com © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
It is interesting to find the physical origin of giant exchange bias energy. Recently, Tsunoda et al. [6, 7]
reported a very large value of the unidirectional anisotropy constant JK of MnIr/CoFe bilayers up to 1.3
erg/cm2 and found a strong correlation between exchange bias anisotropy and chemical ordering. In the
present study, it is difficult to find such a correlation since the giant exchange bias is strongly dependent
on the Co thickness. Based on the XRD patterns, it can only be concluded that Co is in amorphous state
and consequently, one cannot find any structural change when changing the thicknesses of Co layers.
In this paper, the simple model put forward by Meiklejohn and Bean [1] is employed to compare the
theoretical value with the obtained experimental result. According to this model, the AF spins at the
interface are uncompensated and fixed during the FM magnetization rotation. Hence, the unidirectional
anisotropy constant can be calculated as follows: JK = Jex SiSj/a2. Here, Jex is the exchange integral at the
interface, which is believed to be in the range of the exchange integrals of the AF and FM materials, Si
and Sj the spins of the interfacial atoms, and a the lattice parameter. If using Jex from the exchange inte-
gral of bulk materials, then JK is in the range from 1.8 erg/cm2 to 14.1 erg/cm2. Therefore, roughly speak-
ing, one may conclude that the present result lies in the range of theoretical prediction by the model of
Meiklejohn and Bean [1]. Even though the present result is consistent with the theoretical prediction, it
may not be considered as a firm support for the simple model by Meikljohn and Bean [1]. A more com-
plicated model which is able to cover for all the cases should therefore be developed.
4 Conclusion
In summary, the present study shows an interesting result in MnPd/Co bilayers that the parallel blocking
temperature is higher than the perpendicular one, which can be interpreted as the difference of the order-
ings in parallel and perpendicular directions. The present study also reported on the largest unidirectional
anisotropy constant ever found in the literature up to 5.5 erg/cm2 in MnPd/Co bilayers. This huge unidi-
rectional anisotropy constant is found to be in reasonable agreement with the simple model proposed by
Meiklejohn and Bean, which predicts that JK in MnPd/Co system is in the range from 1.8 erg/cm2 to 14
erg/cm2. Although the present system of MnPd/Co bilayer has a low blocking temperature, suggesting
that this system cannot be applied for spin-valve sensors, the finding of giant exchange bias may provide
some useful information for better understanding of the mechanism of exchange bias.
Acknowledgements This work is partially supported by the Vietnamese Fundamental Research Grant #4.049.06
(2006-2008). Also, the supports from the Academic Frontier Center for Future Storage Materials Research [MEXT
HAITEKU (2004-2008)] and from the Japanese Storage Research Consortium are gratefully acknowledged.
References
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