Full text of DOI:10.1007/BF02901176

NOTES
apparent difference in crystallographic texture and surface
roughness for both films. That means that there must be
other reasons for the Hex decrease of the film with Ta/Cu
buffer. In this note, the surface composition of Ta/Cu/NiFe
film was studied using X-ray photoelectron spectroscopy
(XPS). We found that Cu atoms can segregate to the NiFe
surface. We believe that this is the main reason for the
decrease of the exchange coupling fields of NiFe/FeMn
films with Ta/ Cu buffer layers.
Effect of Cu surface segrega-
tion on the exchange coupling
field of NiFe/FeMn bilayers
LI Minghua1,2, YU Guanghua1, ZHU Fengwu1,
JIANG Hongwei2 & LAI Wuyan2
1. Department of Materials Physics, Beijing University of Science and
Technology, Beijing 100083, China;
1
Experimental methods
2. Institute of Physics, Chinese Academy of Sciences, Beijing 100080,
China
The multilayers were prepared in a magnetron sput-
tering system. The Ta layers were deposited by rf magne-
tron sputtering while the other layers Cu, NiFe and FeMn
were deposited by dc magnetron sputtering. Ta(8 nm)/
NiFe(13 nm)/FeMn(12 nm)/Ta(6 nm) and Ta(8 nm)/Cu
2.6 nm)/NiFe(13 nm)/FeMn(12 nm)/Ta(6 nm) because the
spacer thickness is 2.6 nm in spin valve) were deposited
on Si (100) substrates in a regular order. The base pressure
was below 2h10ꢀ5 Pa. The deposition rate was 0.12 nm/s
for Ta and Cu films, and 0.1 nm/s for NiFe and FeMn,
respectively. The films were deposited in a uniform mag-
netic field of 20 kA/m (or 250 Oe), which produced an
easy axis in the NiFe film and defined the exchange cou-
pling axis. All layers were deposited at the same argon
pressure of 0.6 Pa. The substrates were cooled by water.
The crystallography of the films was examined by
using X-ray diffraction (XRD). The hysteresis loops were
measured with the vibrating sample magnetometer (VSM).
The surface roughness was characterized by atomic force
microscope (AFM). The interface segregation was studied
using the X-ray photoelectron spectroscopy (XPS).
Abstract
The NiFe/FeMn bilayers with different buffer
layers (Ta or Ta/Cu) were prepared by magnetron sputtering.
Results show that the exchange coupling field of NiFe/FeMn
films with Ta buffer is higher than that of the films with Ta/
Cu buffer. We analysed the reasons by investigating the
crystallographic texture, surface roughness and surface seg-
regation of both films, respectively. We found that the de-
crease of the exchange coupling fields of NiFe/FeMn films
with Ta/ Cu buffer layers was mainly caused by the Cu sur-
face segregation on NiFe surface.
Keywords: NiFe/FeMn, exchange coupling field, texture, surface
roughness, surface segregation.
The phenomenon of exchange anisotropy was dis-
covered by Meiklejo et al. in 1956.[1] The exchange cou-
pling between an antiferromagnet (AF) and a ferromagnet
(FM) has attracted considerable interest in recent years.
The studies are intensified because of the wide application
of FM/AF systems in giant magnetoresistive (GMR) read
heads and magnetoresistive sensors. The NiFe/FeMn
FM/AF system is one of the most extensively studied
model systems, and it was early used in giant magnetore-
sistive (GMR) spin valve.
2
Experimental results and discussion
( i ) Hysteresis loops of the NiFe/FeMn films with
different buffer layers. Fig. 1(a) shows the hysteresis
loops of the NiFe (13 nm) /FeMn (12 nm) film with Ta(8
nm) buffer. The coercivity Hc of the film with Ta buffer
(0.98 kA/m or 12.3 Oe ) is almost the same as that of the
film with Ta /Cu buffer (0.93 kA/m or 11.7 Oe), but the
exchange coupling field in the former (8.88 kA/m or 111.5
Oe) is much higher than that in the latter (7.99 kA/m or
100.5 Oe). Comparing Ta(12 nm)/ NiFe(75 nm)/FeMn(15
nm)/Ta(5 nm) with Ta(12 nm)/Cu(10 nm)/NiFe(7.5 nm)/
FeMn(15 nm)/Ta(5 nm), Fujiwara proposed that a higher
pinning field may be obtained for the former than for the
latter[7]. In other words, the decrease of the exchange cou-
pling fields was caused by the Cu layer between the layers
of Ta and NiFe. We first investigated the crystallographic
texture and interface roughness in order to find the reason
for the decrease of the exchange coupling.
The coupling between the ferromagnetic layer and
the antiferromagnetic layer relates not only with the mag-
netism of the FM layer and the AF layer but also with
the microstructures of the films such as grain size[2],
texture, surface roughness and newly discovered interlayer
diffusion[3] and interface reaction[4]. The processing of
sample preparation [5] can affect the microstructure, and
hence the exchange coupling. In some previous papers it
was found that the exchange coupling field of films with
proper buffer was much higher than that of the films
without buffer[6].
In our experiment, the NiFe/FeMn bilayers with Ta
buffer and Ta/Cu buffer were prepared by magnetron
sputtering. The results show that the exchange coupling
field (Hex) of NiFe/FeMn films with the Ta buffer is
higher than that of the films with the Ta/Cu buffer, which
is accordant with Fujiwara’s results[7]. They suggested that
this phenomenon was caused by the texture difference, i.e.
the film with Ta buffer has a higher oriented (111) texture
of NiFe and FeMn. However, we found that there is no
( ii ) X-ray diffraction of the NiFe/FeMn films with
different buffer layers. Fig. 2 shows the X-ray diffrac-
tion patterns of the NiFe(13 nm)/FeMn (12 nm) films with
different buffers Ta(8 nm)(see fig. 2(a)) and Ta (8 nm)/Cu
1934
Chinese Science Bulletin Vol. 46 No. 22 November 2001

(2.6 nm) (fig. 2(b)), respectively. A good (111) texture of
J-FeMn is necessary to obtain a high exchange coupling[8].
The film with proper buffer has highly oriented (111) tex-
ture of NiFe. Thus, a stronger FeMn (111) diffraction peak
is expected to form on a highly oriented NiFe (111) tex-
ture film. Comparing fig. 2(a) with fig. 2(b), we found
that the perfect diffraction peaks of NiFe (111) and FeMn
(111) were well grown when the films are deposited on
either Ta or Ta/Cu buffer. The diffraction peaks of NiFe
(111) and FeMn (111) were not apparently influenced by
2.6 nm Cu deposited on the Ta layer. It means that the
texture is not the main factor for the decrease of the cou-
pling field of NiFe/FeMn films with Ta/Cu buffer layer.
Fig. 2. X-ray diffraction of NiFe/FeMn bilayers with buffer Ta (a) and
Ta/Cu (b).
(iv) XPS investigation of surface segregation of
Ta/NiFe and Ta/Cu/NiFe. In order to obtain the reason
for the decrease of the exchange coupling of NiFe/FeMn
with Ta/Cu buffer, we measured element distribution in
different depths by angle-resolved XPS. Ta(3 nm)/NiFe (5
nm) and Ta(3 nm)/Cu(2 nm)/NiFe(5 nm) were deposited
on Si (10) substrates in the same way as the Ta/NiFe/
FeMn/Ta and Ta/Cu/NiFe/FeMn/Ta layers by magnetron
sputtering. The samples were introduced into a MICRO-
Fig. 1. Hysteresis loops of NiFe/FeMn bilayers with buffer Ta (a) and Ta/Cu
(b).
(iii) AFM surface topography of the NiFe/FeMn
films with different buffer layers. We measured the
surface topography by using an atomic force microscope
(AFM). Since Cu, NiFe and FeMn are all of an fcc struc-
ture and their lattice mismatch is smaller than 3%, the
multilayers grow coherently when they are deposited se-
quentially in the same vacuum[4]. In this study we as-
sumed that the surface morphology of the multilayer
likely results from the interface morphology of the
NiFe/FeMn layer. We measured the surface topography of
Ta(8 nm)/NiFe(13 nm)/FeMn(12 nm)/Ta(6 nm) and Ta(8
nm)/Cu(2.6nm)/NiFe(13nm)/FeMn(12 nm)/Ta(6 nm) with
root mean square roughness (Rrms) of 0.237 nm and 0.278
nm respectively by using an atomic force microscope.
Generally, the “smooth” means the Rrms ranges from 0.2 to
0.4 nm, and the “roughness” Rrms ranges from 0.5 to 0.8
nm. We can see from the above that the surface was basi-
cally smooth although there was a little difference be-
LAB MKĊX-ray photoelectron spectroscopy system in-
stantly after being taken out of the deposition system. In
order to detect the composition and chemical states on
NiFe surface, the interfacial layer has to be within the
XPS detectable sampling depth d = 3OsinD[9], where O and
D are inelastic mean-free paths (IMFPs) for photoelec-
trons and a take-off angle for photoelectrons with respect to
the samples surface plane, respectively. The IMFPs can be
obtained by using the table compiled by Tanuma et al.[10]
.
The detectable depths changed from 0.6 to 2.3 nm when
the take-off angle changed from 15e to 90e. The spectra
of Cu and nickel acquired were fitted by the curve fitting
software to determine the atomic fraction of Cu and Ni in
all chemical states. Fig. 3(a) gives Cu 2p high resolution
XPS spectrum for Dꢁ=15e and fig. 3(b) gives relationship
tween Ta buffered Rrms and Ta/Cu buffered Rrms
.
Chinese Science Bulletin Vol. 46 No. 22 November 2001
1935

NOTES
between the Cu/Ni atomic number ratio and photoelectron
emitting angle. In fig. 3(b) we can see that Cu decreased
with the increase of the take-off angle. This means that Cu
atoms segragate to NiFe surface in the Ta/Cu/NiFe films.
However, there was no Ta segragation on NiFe surface for
Ta/NiFe bilayer by the same analysis. As to NiFe/FeMn
bilayer with Ta/Cu buffer layer, Cu segragation on NiFe
surface decreased the effective contacting area between
higher exchange coupling field. Therefore, the decrease of
the exchange coupling fields of NiFe/ FeMn films with
Ta/Cu buffer layers was mainly caused by the Cu segrega-
tion on NiFe surface.
3
Conclusion
The coupling between the ferromagnetic layer and
the antiferromagnetic layer relates not only with crystal
texture and surface roughness but also with the surface
segregation caused by some buffer layers (such as Cu).
The exchange coupling field of NiFe/FeMn films with the
Ta buffer was higher than that of the films with the Ta/Cu
buffer. The XPS analyses indicated that the decrease of
the exchange coupling field of NiFe/FeMn films with
Ta/Cu buffer layers was caused by the Cu surface segre-
gation.
Acknowledgements This work was supported by the National Natural
Science Foundation of China (Grant No. 19890310).
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(Received June 14, 2001)
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