The exchange anisotropy of NiO/CoFe bilayers

Chong Yan *, Jun Yu, Wen-Li Zhou, Ji-Fan Xie, Jun-Xiong Gao, Dong-Xiang Zhou

Department of Electronic Science and Technology, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China

Received 14 January 2002; accepted 20 August 2002

Abstract

The antiferromagnetic/ferromagnetic NiO/CoFe bilayers are prepared by radio frequency magnetron sputtering method.

Magnetization hysteresis curves were measured by a vibrating sample magnetometer (VSM). The exchange bias fields Hex of NiO/

CoFe bilayers are studied by using different substrates and sputtering Ar gas pressures. When the glass, Si (100), Si (110) and Si

(111) substrates are used, the exchange bias fields of the bilayers are different. The Hex is also influenced because of different

sputtering Ar gas pressures. The crystal texture and surface roughness of the samples were analyzed by using X-ray diffraction

(XRD) and atom force microscope (AFM). It is found that the exchange bias field strongly depends on the NiO/CoFe interface

roughness. With the increase of the interface roughness, the exchange bias field Hex of NiO/CoFe bilayers decreases. It is not

dependent on the existence of NiO (111) texture which is the spin uncompensated plane, believed to strongly correlate with exchange

bias field according to the traditional understanding for the anisotropic exchange biasing mechanism. These results cannot be

explained by the ideal interface model and Mauri et al.’s interfacial antiferromagnetic domain wall model, but the random-field

model can interpret the results.

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: Exchange anisotropy; NiO/CoFe bilayers; Random-field model

1. Introduction

Investigations of exchange coupling between antifer-

romagnetic and ferromagnetic thin films have become of

considerable experimental and theoretic interest in

recent years because of the application of spin valves

for magnetic sensors [1�/5]. However, the researches of

theories are so slow that there are many questions

concerning exchange anisotropy remaining unanswered

[6]. The simple ideal model predicts exchange coupling

field (Hex) two-to-three orders greater than that ob-

served in experiments. In fact, the Hex has been known

to be very sensitive to crystallographic orientation and

interface roughness of the antiferromagnetic layers. But

the experimental results are very different from one

paper to another. For example, some researchers found

that the stronger the (111) crystal orientation of the

antiferromagnetic layer the greater the Hex [7] and other

papers reported that the Hex does not depend on the

(111) crystal orientation at all [8]. In this paper, we will

report how the crystal orientation and interface rough-

ness of the antimagnets affect the Hex in the NiO/CoFebilayers.

2. Experimental

The NiO/CoFe bilayers are grown by radio frequency

magnetron sputtering. High purity Ar gas at a base

pressure B/1.0�/10�5 Pa is used for sputtering. TheNiO layers are sputtered onto corning #7059 glass, Si

(100), Si (110), and Si (111) single crystal substrates at a

distance of about 95 mm from the target at different Ar

pressures such as 0.13, 0.47 and 0.67 Pa. A pressure of

0.47 Pa is performed while sputtering the CoFe films.

The target compositions are Co90Fe10 and sintered NiO

target without the introduction of oxygen gas. The

temperature of the substrate (Ts) holder is controlled atroom temperature by refrigerated coolant. A 700 Oe

uniaxial deposition field is applied in the film plane

during sputtering. The sputtering rates are about 0.1 nm

* Corresponding author. Tel.: �/86-27-875-42993; fax: �/86-27-875-

42886.

E-mail address: chongyan@public.wh.hb.cn (C. Yan).

Materials Science and Engineering B99 (2003) 421�/424

www.elsevier.com/locate/mseb

0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0921-5107(02)00451-8

s�1 for CoFe layers and about 0.2 nm s�1 for NiO

layers.

The thickness of the each layer is controlled by the

deposition rate and time. The Hex defined by theasymmetric shift of the ferromagnet hystersis loop

from zero applied field is determined after many cycling

of measurements to ensure that no noticeable aftereffect

is present. A vibrating sample magnetometer (VSM) is

used for magnetization hysteresis curves. The crystal

texture and surface morphologies of NiO layers are

characterized by using X-ray diffraction (XRD) and

atomic force microscopy (AFM), respectively.

3. Results and discussion

In order to research the effects of the texture and

interface roughness of NiO/CoFe bilayers on the

exchange coupling, the NiO sputtering Ar pressures

are changed. Table 1 shows the Hex of NiO (70 nm)/

CoFe (6 nm) bilayers with glass substrates as a functionof NiO sputtering pressure of Ar. Fig. 1 shows the XRD

patterns of NiO at different pressures. There are three

diffraction peaks for all the samples sputtered at

different pressures. A strongest NiO (111) diffraction

peak is found at 0.67 Pa sputtering pressure sample and

a weakest NiO (111) diffraction peak is appeared at 0.13

Pa sputtering pressure sample among these three

samples. The AFM measurements of NiO thin films

prepared at the same time as the above films are showed

in Fig. 2. The roughness of the NiO surface increases

with increasing NiO sputtering pressure. The root-

mean-square roughness (RMS) determined by AFM

over an area of 1 mm2 varied from 0.331 to 0.559 nm

when the NiO sputtering Ar pressure increased from

0.13 to 0.67 Pa, respectively. These data show that the

increase of Hex with decreasing NiO sputtering pressure

could be due to the decreasing RMS of the NiO/CoFe

interface (Fig. 2) and not due to the increase of the NiO

(111) texture which is the spin uncompensated plane.

Table 1

Exchange coupling field Hex, rms roughness RMS of the NiO (70 nm)/

CoFe (6 nm) bilayers deposited on glass substrates at different Ar

pressure

PAr (Pa) 0.13 0.47 0.67

Hex (Oe) 68.3 54 49.7

RMS (nm) 0.311 0.496 0.559

Fig. 1. The XRD patterns of NiO grown on glass substrates at

different sputtering Ar pressure.

Fig. 2. The AFM images of the NiO (70 nm) deposited on glass

substrates at different Ar pressure.

C. Yan et al. / Materials Science and Engineering B99 (2003) 421�/424422

To investigate the effects of the texture and roughness

effect on the Hex farther, the NiO (70 nm)/CoFe (6 nm)

bilayers are deposited on different substrates under the

same condition. Fig. 3 shows the XRD patterns of NiO

sputtered at 0.47 Pa Ar pressure on Si (100), Si (110),

and Si (111) substrates. The #7059 glass samples XRD

patterns are shown in Fig. 1. It is found that NiO films

are not grown epitaxially on Si single crystal substrates

because of the unmatched crystal lattice. But it is still

found that a strongest NiO (111) diffraction peak is

appeared in Si (111) substrate sample. Fig. 4 plots the

AFM images of the same NiO films grown on Si (100),

Si (110), and Si (111) substrates and Table 2 shows the

relationship between the substrates and the exchange-

coupling field. We can also draw the same conclusion as

above that the increase of Hex is due to the decreasing

RMS and not due to the increase NiO (111) spin

uncompensated plane.

The random-field model proposed by Malozemoff

can be applied to explain the experimental results.

According to the random-field model [9], the Hex arises

from an energy difference, per unit areas of the

antiferromagnetic/ferromagnetic interface. At the inter-

face, the antiferromagnetic films break up into small

domains due to the interface roughness, and the domain

walls are perpendicular to interface. In this case, a

completely spin uncompensated plane at the interface is not necessary and the interface roughness is very

important.

4. Conclusion

In summary, it is found that the exchange couplingfield of NiO/CoFe bilayers strongly depend on the

interface roughness in our experimental results. Spin

complete uncompensated configuration at the interface

Fig. 3. The XRD patterns of NiO grown on Si (100), Si (110) and Si

(111) at 0.47 Pa sputtering Ar pressure.

Fig. 4. The AFM images of the NiO (70 nm) deposited on Si (100), Si

(110) and Si (111) at 0.47 Pa sputtering Ar pressure.

Table 2

Exchange coupling field Hex, rms roughness RMS of the NiO (70 nm)/

CoFe (6 nm) bilayers deposited on Si (100), Si (110) and Si (111) at

0.47 Pa sputtering Ar pressure

Substrates Si (100) Si (111) Si (110)

Hex (Oe) 64.5 53.8 51.6

RMS (nm) 0.327 0.524 0.547

C. Yan et al. / Materials Science and Engineering B99 (2003) 421�/424 423

is not necessary for increasing the exchange anisotropy.

The experimental results are explained by the random-

field model.

Acknowledgements

It is pleasure to thank Professor Lin Gengqi and

Professor Li Zhuoyi for preparing examined thin films

and Ph.D. Zheng Yuankai for AFM measurements.

This work was supported by the National Natural

Science Foundation of China (NSFC) under contract

number 69801003.

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