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Thin Solid Films 475

Characterization of nano-oxide layer in specular spin valve multilayer

D.H. Lee*, S.Y. Yoon, J.H. Kim, S.J. Suh

Advanced Materials and Process Research Center for IT, Sungkyunkwan University, 300 Chunchun-dong, Jangan-gu, Suwon, Gyunggi-do, 440-746, Korea

Available online 25 August 2004

Abstract

To characterize the nano-oxide layer (NOL) in specular spin valve multilayer, naturally oxidized CoFe layer was inserted between pinned

CoFe layers. The under-, optimum, and over-oxidized samples were obtained by control of the oxidation time and the oxygen flow rate. The

NOL was analyzed to be Fe oxide mainly by X-ray photoelectron spectroscopy (XPS) data. As oxidation increased, the content of Fe oxide

and the thickness of the NOL increased and too much thicker NOL was obtained in case of over-oxidation. We concluded that the lower

magnetoresistance (MR) ratio and the more slanted magnetization curve than the optimum could be attributed to the Fe oxide as a non-

ferromagnetic defect or magnetic discontinuity in case of under-oxidation and the too thick NOL in case of over-oxidation.

D 2004 Elsevier B.V. All rights reserved.

Keywords: NOL; MR ratio; XPS; Fe oxide

1. Introduction

A multilayer system of specular spin valve giant

magnetoresistance (GMR) is one of the promising systems

for magnetic read head, because it is compatible with high

density recording media due to its high sensitivity and very

thin sensing layer.

The key layer in specular spin valve multilayer is known

to be the nano-oxide layer (NOL) between ferromagnetic

pinned layers, which reflects conduction electrons, thus

increases the magnetoresistance ratio (MR ratio) [1]. There

are several ways to form this NOL; plasma oxidation [2,3],

reactive sputtering, and natural oxidation [4,5]. Also, these

oxidation processes can change the property of NOL and, as

a result, the speculation of conduction electrons and

magnetic coupling between two ferromagnetic pinned

layers.

The NOL can be characterized by its microstructure and

oxidation state. However, the oxidation state and fine

structure of the NOL have not been studied thoroughly

and are still unclear. In this study, we tried the natural

oxidation process varying the oxidation condition, such as

0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2004.07.028

* Corresponding author. Tel.: +82 31 290 7373; fax: +82 31 290 7377.

E-mail address: duhyun@skku.edu (D.H. Lee).

flow rate and oxidation time. We found the optimum

oxidation condition to get the maximum MR ratio and

Hex. Also, the optimum oxidized sample was compared with

the under-oxidized and the over-oxidized samples, espe-

cially focused on the chemical property of the NOL. Finally,

we tried to relate this result to the magnetic properties of

specular spin valve multilayer with oxidation condition.

2. Experiment

Specular spin valves of Si substrate/Ta 5/NiFe 3.5/IrMn

6/CoFe 2(P1)/NOL/CoFe 2(P2)/Cu 2.5/CoFe 4(F)/Ta 2.5

multilayer were deposited by magnetron sputtering system.

The numerical values show the thickness in nm of each

layer. The Co–Fe layer in contact with the Mn–Ir layer is

called pinned layer and the Co–Fe layer situated above Cu

layer-free layer. The seed layer Ta/NiFe was used to

promote the (111) texture of antiferromagnetic Mn–Ir layer.

The base pressure was less than 3�10�8 Torr. Mn81–Ir19at.% was deposited from a Mn target with Ir chips attached

to it. A Ni81–Fe19 wt% and a Co90–Fe10 at.% alloy target

were used for the corresponding layers. The samples were

deposited under 5 mTorr of Ar at room temperature. In order

to induce unidirectional anisotropy, we applied a magnetic

field of 100 Oe to the samples during the process.

(2005) 251–255

D.H. Lee et al. / Thin Solid Films 475 (2005) 251–255252

The NOL was formed in the load lock chamber by

exposing to pure oxygen gas. Varying the oxidation time

from 15 to 60 s and the oxygen flow rate from 10 to 50

sccm, optimum, under-oxidized, and over-oxidized NOLs

was obtained. Post-annealing of 200 8C for 30 min under

magnetic field of 3 kOe induced the magnetic anisotropy

and stabilized the NOL.

The MR ratio is defined as: MR ratio=[R(maxi-

mum)�R(minimum)]/R(minimum)�100. Hex was meas-

ured from the shift of the M–H loop away from the zero-

field axis. These are obtained from four-point probe

method and vibrating sample magnetometer (VSM),

respectively.

The chemical property was analyzed with the X-ray

photoelectron spectroscopy (XPS-ESCA2000 from VG

Microthech) X-ray of 13 kV was extracted from Mg anode.

Regarding the analyzer, the pass energy was 20 eV and the

energy resolution was 0.2 eV. The binding energy was

calibrated with the surface carbon. During the XPS analysis,

the sample was etched with the rate of 0.2 nm/min and depth

profiles were obtained. The peaks of Co 2p and Fe 2p were

used to analyze the NOL.

Fig. 1. The variation of MR ratio as a function of oxygen flow rate and

oxidation time. Samples were annealed after deposition at the temperature

of 200 8C. The under-, optimum, and over-oxidation conditions are

indicated.

3. Results and discussion

Natural oxidation was carried out after the deposition of

P1 layer by exposing this layer to the low pressure oxygen

gas. To find out the optimum oxidation condition, at which

maximum MR ratio and Hex were obtained, oxidation time

and oxygen flow rate were changed. According to Fig. 1,

generally, it was good to be said that as the oxidation

increased, MR ratio and Hex increased to their maximum

values and then decreased. These results showed that the

NOL affect the MR and Hex in three different ways as the

oxidation goes on. We could get maximum MR ratio of

10.3% and Hex of 320 Oe when it was oxidized for 30 s with

the oxygen flow rate of 30 sccm. We assigned it as optimum

condition. The condition of 15 s with 10 sccm and 60 s with

50 sccm were assigned as under-oxidation and over-

oxidation conditions, respectively. Their MR ratio and Hex

are listed in Table 1. These results indicated that the

condition of the NOL can be related to magnetic properties

of specular spin valve because the change of the oxygen

flow rate and oxidation time can vary the micro and

chemical structure of NOL.

Fig. 2 shows the MR curves of optimum, under- and

over-, oxidation conditions. For the optimum sample, a

higher Hex, MR ratio, and a plateau in the field region of 0–

300 Oe were obtained. However, for the under oxidized

sample, a lower Hex, MR ratio, and a narrow plateau region

were obtained. Also, for the over-oxidized sample, there is

no plateau region. These results suggested that a shape of

the R–H curve was also related to the oxidation condition of

the NOL. As increasing the applied field, the MR jumps up

to the maximum by magnetization rotation of F layer. The

steep increase of MR indicates the coherent and soft rotation

of F layer. Further increase of the applied field rotates the

pinned layer and it shows slanted hysteresis curve. As

shown in Fig. 2, the under- and over-oxidized samples have

more slated curves than the optimum one. Fig. 3 shows the

M–H curve obtained form VSM, where a hysteresis loop

placed at the lower left is obtained from a free layer and a

hysteresis at the upper right is from a pinned layer. The

pinned layer of the over-oxidized sample (Fig. 3(b)) shows

an asymmetric hysteresis, whereas the pinned layer of the

under-oxidized sample (Fig. 3(b)) shows a symmetric

hysteresis. That is to say, there can be two reasons for

slanted curve of pinned layer. If the hysteresis loop of

pinned layer is symmetric, it can be said that the layer is

composed of magnetically single layer and the layer

converts its magnetization coherently. However, in case of

asymmetric hysteresis loop, the layer can be composed of

magnetically different layers, for example, different

exchange field or coercivity. The over-oxidized sample

had asymmetric hysteresis loop. It can be said that the

pinned layer is composed of magnetically different two

layers but they are separated ambiguously by NOL. The

optimum one showed similar asymmetry of loop. While on

the other hand, the under-oxidized sample had symmetric

hysteresis loop of pinned layer. It means that the layer acts

Fig. 3. Magnetization curves for the oxidation condition of NOL indicated

in Fig. 1.

Table 1

MR ratio and exchange biased field (Hex) for the oxidation condition of

NOL indicated in Fig. 1

Oxidation condition Under Optimum Over

Hex (Oe) 200 320 147

MR (%) 9.4 10.3 8.4

D.H. Lee et al. / Thin Solid Films 475 (2005) 251–255 253

as a single layer and the NOL does not play a role as a

separating layer.

For these samples, XPS analysis was carried out. By

etching the sample, Co 2p, Fe 2p, and O 1 s peaks were

surveyed. The point where the highest oxygen peak was

observed during the pinned layer survey was designated to

the center of NOL. Firstly, not shown in here, we analyzed

the Co XPS result. As oxidation progressed, a distinct

shoulder on the high binding energy side of Co 2p peak

indicates the presence of some Co oxide in the NOL

region. However, we cannot calculate or acquire the exact

quantity of Co oxide due to low oxidation of Co.

However, as oxidation goes, the more Co oxide may be

generated. Also, after oxidation, the amount of magnetic

moment loss of pinned layer supports the presence of Co

oxide in the NOL region. In case of Fe, however, Fe oxide

was dominant. The peaks of Fe 2p in the NOL region are

shown in Fig. 4. It is obvious that most of Fe in the NOL

region is oxidized in contrast to the Co. The depth profile

of Fe 2p2/3 peaks was obtained from manual peak fitting.

We referenced the handbook of XPS to fit of Fe 2p2/3peaks [6]. It shows two kinds of Fe oxide–FeO (709.82

eV) and Fe2O3 (710.64 eV), and metallic Fe (707.14 eV)

as shown in Fig. 4(b). In addition, the Fe oxide is

composed of FeO and Fe2O3. This indicated that the

magnetic properties and shape of magnetization curve can

be affected by Fe oxide. Fig. 5 shows the depth profile of

Fe oxide around the NOL. The depth profiles have

Gaussian-like profile. Fitting by this function, we esti-

Fig. 4. XPS narrow scans of Fe 2p peaks for (a) the oxidation condition

indicated in the Fig. 1 and (b) the fitting result of optimum Fe XPS spectra.

Fig. 2. MR curves for the oxidation condition of NOL indicated in the

Fig. 1.

Fig. 5. XPS depth profile of Fe oxide for the oxidation condition of NOL

indicated in Fig. 1. The content of Fe oxide was deduced from the relative

peak area to that of the metallic Fe.

D.H. Lee et al. / Thin Solid Films 475 (2005) 251–255254

mated the thickness of the NOL by the width of the

Gaussian function. Our previous TEM result shows that, in

case of optimum oxidized sample, the NOL is formed

discontinuously and the thickness is about 0.5–2 nm and

some NOL regions are so thin that the upper CoFe layer is

directly connected with the lower CoFe layer [7]. We

estimated the NOL thickness from considering Fe oxide

distribution since the NOL mainly composed of Fe oxide.

The over-oxidized sample had the thickness about

1.8F0.176 nm, and the under-oxidized and the optimum

samples had the thickness about 1.5F0.054 and 1.4F0.129

nm, respectively. The estimated NOL thickness from XPS

well agreed with TEM result. However, there may be

difference between the estimated thickness of NOL from

XPS and that of actual thickness with oxidation condition

since we only consider the distribution of Fe oxide. The

content of Fe oxide in under-oxidized sample was lower

than the optimum and over-oxidized samples. This means

that only a part of Fe is oxidized in case of under-oxidized

sample due to the lack of oxygen. In case of over-oxidized

sample, the as-deposited NOL had excess oxygen and it

diffused out from NOL during the post-annealing. The

thicker NOL could be attributed to this.

The preferential oxidation of Fe against the Co could be

attributed to the concentrated Fe at the grain boundary and

the surface of the P1 [8]. So it could be said that Fe was

oxidized preferentially and the Co and the remnant Fe

present in the inner part were not oxidized. According to

Gillies et al. [8], Fe oxides are indispensable to the specular

reflection of conduction electrons. In our case, we can

suggest that the preferential oxidation of Fe made specular

oxide layer at the surface of P1.

Considering the above results, the magnetic properties

and structure of NOL depended strongly on the oxidation

condition. In case of the under-oxidation, the content of

Fe oxide was relatively small comparing with the

optimum and over-oxidation. However, its distribution

range was comparable to the optimum. So the density of

the oxide is low and it cannot make a continuous oxide

layer but a non-ferromagnetic defect or magnetic disconti-

nuity. Thus, the P1 and the P2 are not separated firmly.

The symmetrically slanted curve of the under-oxidized

sample can be attributed to this defect that obstructs the

coherent rotation of magnetization in the pinned layer.

That is to say, as oxidation goes on, firstly the Fe-oxide

islands will be developed at grain boundaries of CoFe

layer [8] and this small amount of Fe oxide may act as a

non-ferromagnetic defect or magnetic discontinuity in the

CoFe pinned layer although many direct contacting paths

will exist in under oxidized NOL. This Fe oxide in the

NOL may be reason for the slant R–H curve and lower

MR ratio of the under oxidized specular spin valve. The

symmetric slanted curve of the under-oxidized sample can

be attributed to this Fe oxide that obstructs the magnet-

ization rotation. In the case of the optimal oxidized NOL,

a mixture of island and layer of mainly Fe-oxide may

exist. Also direct contacting path between P1 and P2 will

exist and less disruption of the CoFe grain structure [7].

At the over-oxidation condition, the direct path will be

diminished and the continuous layer growth will be

predominant. The Fe oxide and Co oxide may invade

into grain of CoFe. This microstructure may lead to a

lower MR ratio and a slant R–H curve.

4. Conclusion

To characterize the NOL of specular spin valve multi-

layer, we had made differently oxidized samples and

analyzed them. As a result of XPS study, we could find

out that Fe was oxidized preferentially and the thickness of

NOL was around 1.4–1.8 nm depending on the oxidation

condition. The slanted MR curve and the lower MR ratio

can be attributed to the Fe oxide as a non-ferromagnetic

defect or magnetic discontinuity in case of the under-

oxidation and to the too thick NOL in case of over-

oxidation, respectively.

Acknowledgement

This work was supported by the Advanced Materials and

Process Research Center for IT at Sungkyunkwan Univer-

sity (Grant No. R12-2002-057-01001-0).

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