DSpace at VNU: Effects of mixed gases on characteristics of specular spin-valves containing oxide layers

The OXLs were formed by natural oxidation in different ambient atmos-pheres of pure oxygen, of oxygen/nitrogen, and of oxygen/argon gas mixtures.. 3 Results and discussion 3.1 Optimal c

Effects of mixed gases on characteristics

of specular spin-valves containing oxide layers

H D Quang**, 1

, N T Hien2

, S K Oh1

, and S C Yu **, 1 1

Department of Physics, Chungbuk National University, Cheongju 361-763, Korea

2

Cryogenic Laboratory, Department of Physics, College of Natural Science,

Vietnam National University, 334-Nguyen Trai Road, Hanoi, Vietnam

Received 7 May 2004, revised 6 September 2004, accepted 8 September 2004

Published online 11 November 2004

PACS 75.47.De, 75.50 Ss, 75.70.Ak

Specular spin valves containing oxide layers (OXLs), structured as substrate/seed/AF/P1/OXL/P2/Cu/ F/OXL, have been fabricated The OXLs were formed by natural oxidation in different ambient atmos-pheres of pure oxygen, of oxygen/nitrogen, and of oxygen/argon gas mixtures The fabrication conditions were optimized to enhance the magnetoresistance (MR) ratio, to suppress the interlayer coupling fields

(Hf) between the free and pinned layers, to suppress the high interface density of the OXL, to ease the control of the OXL thickness and to form a smooth OXL/P2 interface for promoting specular electron scattering The characteristics of our specular spin valves are as follows: the MR ratio of 14.1%, the

ex-change bias field of 44 – 45 mT and Hf weaker than 1.0 mT The optimal conditions for oxidation time, to-tal oxidation pressure and the annealing temperature were found to be of 300 s, 0.13 Pa (oxygen/ argon = 8/2), and 250 °C, respectively The results suggest that specular spin-valves containing OXLs sat-isfy the requirements of read head performances in high density (100 Gbits/in 2

) recording devices

© 2004 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim

1 Introduction

In recent studies [1 – 7] oxide layers (OXLs), formed within the MMn-based (M = Fe, Pt, Ir, etc.) spin valves (SVs) by annealing the latter in an ambient atmosphere of pure oxygen atmosphere, have been considered as a mean to enhance the degree of specular scattering at the outer surfaces of the spin valve structure, and the dependence of the enhanced magnetoresistance (MR) ratio on the annealing tempera-ture was investigated Furthermore, it was found that the bottom-pinned SV with the OXLs has a signifi-cant MR enhancement and a higher thermal stability than the top-pinned SV

A generic configuration of such bottom-pinned specular SV is given by substrate/seed/AF/P1/OXL/P2/ Cu/F/OXL Here P1 and P2 denote ferromagnetic (FM) layers pinned by an antiferromagnetic (AF) layer and F is a free ferromagnetic layer The OXL is formed by the oxidation of the P1 layer The F layer, e.g

in form of a CoFe layer, is oxidized to form the OXL capping layer aiming at enhancing the scattering at this same capping layer [2] To achieve an optimal operation of the SV, OXL/P2 interface should be smooth for the specular reflection of electrons, and there must be a strong ferromagnetic coupling be-tween the two ferromagnetic layers P1 and P2 through the OXL, which is exchange biased by the AF layer Therefore, one of the key processes in the fabrication of a SV containing OXLs is the uniform formation of OXLs with the above-mentioned features Many oxidation methods for OXL formation, such as plasma oxidation, ion beam oxidation and natural oxidation, have been developed for controlling the interface quality and the MR improvement are made mainly using a pure oxygen atmosphere In the

*

Corresponding author: e-mail: scyu@chungbuk.ac.kr

**

e-mail: ducquang78vn@yahoo.com

2 Experimental

The specular spin valves structures as Ta(48 Å)/NiFe(23 Å)/IrMn(68 Å)/CoFe(23 Å)/OXL/CoFe(13 Å)/ Cu(24 Å)/CoFe(13 Å)/NiFe(43 Å)/Ta(48 Å) (sample 1) and Ta(48 Å)/NiFe(23 Å)/IrMn(68 Å)/CoFe (18 Å)/OXL/CoFe(18 Å)/Cu(20 Å)/CoFe(28 Å)/Cu(10 Å)/Ta(5 Å)/OXL/Ta(23 Å) (sample 2), which in-cludes a capping OXL and a buried OXL, were prepared at room temperature by dc magnetron sputter-ing onto Si(100)/SiO2(1300 Å) substrates Here we used targets of Ta, Cu, CoFe, NiFe, IrMn, where CoFe, NiFe, and InMn stand for Co90Fe10, Ni81Fe19, and Ir21Mn79, respectively The base pressure of the main chamber was below 2.8 × 10–6

Pa, while the working pressure was 0.13 ~ 0.28 Pa of argon and deposition rates were about 0.2 – 0.4 nm/min The OXL was formed in the pinned layer through natural oxidation to form the structure: CoFe/oxide/CoFe The oxidation was carried out in pure oxygen, oxy-gen/nitrogen and oxygen/argon ambient atmospheres, respectively After completing the deposition pro-cedure, the samples were annealed for 30 minutes in a high vacuum furnace (6.6 × 10–4

Pa) at tempera-tures ranging from 250 to 400 °C, followed by furnace cooling at room temperature in a magnetic field

of 100 mT, which was applied along the spin valve easy axis Ramp-up and cool down times were about

40 minutes and 120 minutes, respectively This procedure leads to an increase in the (111) orientation of the IrMn grains, resulting in a well-defined exchange field The MR ratios for the spin valves were

measured by dc four-point probe technique by applying magnetic fields ranging from 50 mT to 500 mT

in the film plane at both low and high temperatures The exchange field was calculated from the shift of the hysteresis loop, measured on a vibrating sample magnetometer (VSM) The profiles of sample sur-faces were observed and examined by atomic force microscopy (AFM) in order to clarify the interface properties of the spin valves

3 Results and discussion

3.1 Optimal conditions

Figure 1 shows the variation of MR ratio as a function of total pressure at different annealing ambient atmospheres, i.e., of pure oxygen gas and of mixed gases, respectively In the case of the mixed gas am-bient atmospheres, the mixing ratio of partial pressures was 1 : 1, for both oxygen/nitrogen and oxy-gen/argon gas mixtures For the OXL formation in sample 1, the oxidation time and annealing tempera-ture were fixed at 10 minutes and at 250 °C, respectively As shown in the figure, the MR ratio is in-creased from 8.12 to 10.75% when the total pressure increase is 0.13 Pa With pure oxygen gas, a de-crease in the MR ratio occurs at pressures above 0.54 Pa, while in the case of mixed gases, this dede-crease

is negligible for pressures up to 0.66 Pa Thus, for SVs with OXL formed in mixed gas ambient atmos-pheres, high MR ratio has been obtained in a more extended range of oxidation pressures The inset

of Fig 1 shows the MR curves, which were measured for the SVs with the OXL formed by annealing

in pure oxygen and mixed gas atmospheres, respectively, at the fixed pressure of 0.66 Pa (the optimum

Fig 1 MR ratio as a function of total partial pressure in pure oxygen and mixed gas oxidation

atmos-phere (oxygen/nitrogen = 1/1 and oxygen/argon = 1/1) in sample 1 The oxidation time and annealing temperature were fixed at 600 s and at 250 °C, respectively MR ratios denote the value obtained through the OXL formation by annealing in pure oxygen (solid square:
), in nitrogen/oxygen gas mixture (open circle: ϒ), and in argon/oxygen gas mixture (solid circle:  ) at 0.66 Pa, respectively Lines are guides to the eye The inset shows the MR curves

pressure suggested by Fig 1) For the SVs treated by mixed gas oxidation, exchange bias fields of 40 to

45 mT have been obtained, and there appears a pronounced plateau for the MR ratios in the field interval between 0 mT to 40 mT However, for the SVs treated in pure oxygen atmosphere, an exchange bias field of 36 mT is observed, and there appears no noticeable plateau for the MR ratio The MR ratio pro-gressively decreases and vanishes at fields above 60 mT This suggests that a denser OXL structure was formed when the sample was annealed in pure oxygen Thereby, we expect a possible over-oxidation at the MnIr/P1 interface, which might induce a weak direct exchange coupling between the P1 and P2 layer

as observed (or inferred from the results) by the authors in Refs [2] and [5] Nevertheless, we think that oxidation using mixed gases is more useful for the suppression of a closed dense OXL structure and for the improvement in the OXL depth control

3.2 Interlayer exchange coupling and microstructure

In order to understand the ferromagnetic coupling between the two ferromagnetic layers though the OXL, a simple Stoner – Wohlfarth type energy model for a bottom-pinned spin valve was developed Here, a single domain for each magnetic layer was assumed The total energy contains the following terms representing the dominant energy contributions:

(1) Zeeman energy terms (EZ):

EZ = –MP1tP1H cos θP1 – MP2tP2H cos θP2 – MFtFH cos θF ,

where M i is the saturation magnetization of ith layer, t i is its thickness and θi is the angle between its

magnetization and the externally applied field H

(2) Interlayer coupling terms (EC):

EC = –JCu cos (θP2 – θF) –JOXL cos (θP2 – θP1) ,

for which the coupling through the OXL is strongly ferromagnetic (JOXL > 0) while the coupling through

the Cu layer (JCu) is weakly ferromagnetic or anti-ferromagnetic For our samples, as shown in the inset

of Fig 1, JCu is the ferromagnetic coupling (0.008 ~ 0.011 mJ/m2)

(3) Exchange bias term (Eex):

Eex = –Jex cos (θP1)

inset of Fig 1 for spin valves with OXLs formed in pure oxygen atmosphere Finally, for JOXL smaller

than about half of Jex, the two layers will have almost independent switching behaviour In the latter regime, the switching field of unpinned P2 is within the range between 10 mT and 20 mT Therefore,

with increasing JOXL, the right hand slope of the MR loop quickly decreases and the plateau region with antiparallel alignment between P1 and F becomes wider From the MR curves in the inset of Fig 1, it is deduced that JOXL-nitrogen/oxygen (> 0.3 mJ/m2) > JOXL-argon/oxygen > Jex (0.22 ~ 0.23 mJ/m2) > JOXL-pure oxygen,

where the Jex value for the spin valve without OXLs could be calculated by using the expression Jex =

pinned layer as well as the enhancement of the GMR effect in the specular spin valves are very important parameters for determining the bias-point of a spin valve sensor So, the effect of the oxidation time on

these parameters (MR ratio and the interlayer coupling field Hf) was investigated and the results are

shown in Fig 2 Here, the interlayer coupling field Hf as a function of oxidation time is presented for spin valves with OXLs, obtained by annealing at 250 °C and at 0.13 Pa total pressure (the optimum pres-sure as inferred from Fig 1) As can be seen from the figure, when the oxidation time was increased from 300 to 1200 s, the MR ratio does not vary significantly from 10.65% in all oxidation atmospheres, and a degradation of the MR curve shape is not observed Interestingly, while the MR ratio increases rap-idly in the annealing time interval between 0 and 60 s for oxidation in pure oxygen and oxygen/argon am-bient atmospheres, it only slowly increases in a much wider oxidation time interval between 0 and 300 s in the oxygen/nitrogen atmosphere (see Fig 2) This difference may be related to the oxidation of the P1 layer

in such a way that nitrogen may suppress the formation of a smooth OXL surface formed by oxygen and,

Fig 2 Dependence of the MR ratio (left-hand side ordinate, dotted line) and the interlayer coupling field

H f (right-hand side ordinate, solid line) on the oxidation time, for spin valves prepared with different oxi-dation atmospheres: pure oxygen ( □), oxygen/nitrogen ( ▲), and oxygen/argon (●)

Fig 3 Representative AFM topographies and roughness data for the SV with OXLs (sample 1) that

were formed by annealing at 250 °C in different atmospheres: a) pure oxygen gas or oxygen/argon gas mixture (Rrms ~ 2.1 Å, Rave ~ 1.6 Å) and b) oxygen/nitrogen gas mixture (Rrms ~ 3.4 Å,Rave ~ 2.6 Å)

thus, induce a rougher interface As we can clearly see from the AFM topography shown in Fig 3, the

SV with OXLs formed in oxygen/nitrogen atmosphere shows higher roughness than the one annealed in

other conditions (almost two times for the following parameters: peak-to-valley-roughness, Rp–v;

root-mean-square roughness, Rrms; and average roughness, Rave) This effect of nitrogen can also explain the

stronger ferromagnetic coupling field Hf in SV with OXLs prepared in oxygen/nitrogen atmosphere with oxidation times between 0 and 300 s, compared to those of SVs prepared in pure-oxygen and oxy-gen/argon oxidation atmospheres (see the right-hand side of Fig 2) This lead us to the speculation that a

rougher interface can cause an increase in Hf owing to high Néel orange peel coupling [6, 17] Moreover,

Hf clearly decreased with increasing oxidation time under all oxidation conditions When we increase the

oxidation time from 0 to 300 s, Hf decreases rapidly from 2.3 mT to 1.5 mT, and then shows slower

de-crease for longer oxidation times This dede-crease in Hf could be ascribed to the formation of a smoother interface [6] with increasing oxidation time and a decrease in magnetization of the pinned layer with the OXL These observations are related to the original Néel orange peel coupling [6], which is generally expressed by

λ

λ

π

− π

P f

f

M 2

t

h

where the modified Néel orange peel coupling taking accounts of the finite magnetic film thickness of the bottom pinned spin valve [17] is expressed as

λ

π

f

f

M

2

P

P

h

where h and λ are the amplitude and wavelength of the roughness profile, tf, tP and tS are the thickness of the free-, pinned- and spacer- (Cu) layer, respectively, and MP is the magnetization of the pinned layer The left hand side of Fig 4 shows the dependence of saturation magnetization of the pinned layer with the OXL on the oxidation time in pure oxygen atmosphere The magnetization decreased from 1.50 × 106

J T–1

m–3

to 1.14 × 106

J T–1

m–3

when the oxidation time was increased from 0 to 1200 s By taking the MP value from Fig 4 and adopting λ ≈ 200 Å, which was estimated from AFM topography,

the values for h was calculated by making use of (1) and (2) The variation of h as a function of oxidation time is shown in Fig 4 (see the right-hand side) It is quite clear that both h and MP decrease with the

increasing oxidation time Consequently, for high MR ratio and low Hf, it is found that the oxidation time should be longer than 300 s

3.3 Thermal stability problems and remarks

The thermal stability of spin valves with OXLs, formed under the optimum oxidation conditions, has also been investigated Figure 5 presents the dependence of the MR ratio on the annealing temperature,

T a, where the total oxidation pressure was fixed at 0.13 Pa As is seen, the MR ratios of SVs with OXL (S1) rapidly decrease as the annealing temperature was raised over 300 °C For the SVs with the struc-ture of sample 2 (S2), OXL1 was formed in oxygen/argon atmosphere in the oxidation time of 300 s, while OXL2 was formed in oxygen/argon atmosphere in the the120 s oxidation time Sample 2 exhibits the MR ratio of 14.1%, which is higher than that of the spin valve with only one OXL This is due to the double specular scattering by both the capping OXL and the buried OXL Nevertheless, the MR ratio decreases from 14.1% to 4.60% when the annealing temperature was increased from 250 °C to 400 °C, while the slope of the MR ratio versus annealing temperature curves is similar for both types of spin valves This decrease in the MR ratio can be related to the thermal degradation of the multilayer struc-tures, which might be caused by Mn diffusion [9, 14, 15] This can be clarified by secondary-ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS) depth profile analysis that are under-way

In Fig 6, we show the MR ratio for the SVs with the structure of sample 1 vs partial pressure ratio for the mixed-gas oxidation atmospheres The oxidation time, the total oxidation pressure and the annealing temperature fixed at optimal values (as inferred from Fig 1 and Fig 4) are as follows: 300 s, 0.13 Pa and

Fig 5 Dependence of the MR ratio on the annealing temperature (T a) for the spin valve with the structure

of sample 1 (S1) in different ambient atmospheres (open square, open circle, solid square, solid circle) and for the spin valve with the structure of sample 2 (-S2: /Ta/NiFe/IrMn/CoFe/OXL1/CoFe/Cu/ CoFe/Cu/Ta1/OXL2/Ta2 (Ta2 includes a capping OXL and a buried OXL: solid triangle; OXL1 was formed

in 5 minutes oxidation in oxygen/argon, OXL2 was formed in 20 minutes oxidation in oxygen/argon) The lines are guides to the eye

250 °C The optimum values of the MR ratio (10.9%) were obtained in the oxygen/argon atmosphere at partial pressure mixing ratios from 0.1 to 0.2 By increasing the argon partial pressure, we get a slight decrease in the MR ratio On the other hand, the optimal values of the MR ratio from 10.6 to 10.7% was obtained in oxygen/nitrogen atmosphere at the partial pressure mixing ratios from 0.1 to 0.5, and by increasing the nitrogen partial pressure we get a stronger decrease in the MR ratio down to 9.4% This implies that a higher level of nitrogen in the mixture can lead to a rougher OXL/P2 interface and causes diffuse scattering, in a manner similar that mentioned previously These results suggest that the optimal partial pressure of oxygen to obtain the best MR ratio differs depending upon its mixture with nitrogen and argon, and the usable range of nitrogen partial pressure for smooth and uniform OXL formation is narrower than that of argon

Based on the experimental results discussed above, our specular spin valves have an MR ratio of 14.1%, an exchange bias field of 50 mT, and an interlayer coupling field weaker than 1.0 mT Oxidation time, total oxidation pressure and annealing temperature of 300 s, 0.13 Pa (oxygen/argon ratio of 4/1), and 250 °C, respectively, have been found as the optimal conditions for natural oxidation These spin valves with OXLs can satisfy the read head performance at 100 Gbits/in2 with structural thermal stability

up to 300 °C

4 Conclusions

In this work, natural oxidation conditions using mixed gases (oxygen/nitrogen and oxygen/argon) have been adopted to develop a uniform OXL in specular spin valves The optimal conditions, for the

en-hancement of the MR ratio and for the suppression of the interlayer coupling field Hf, have been found It has been shown that the oxidation in mixed gas ambient atmosphere is reliable for the suppression of a dense, closed OXL structure, for the ease of control of the OXL depth, and for the formation of a smoother OXL/P2 interface promoting specular electron scattering There is no significant difference in the thermal stability of structures formed in pure oxygen gas or mixed gas annealing atmospheres

Acknowledgements Research at Chungbuk National University was supported by the Korea Research Foundation

Grant (KRF-2003-005-C00018)

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Fig 6 Dependence of the MR ratio on the partial pressure

ratio for argon or nitrogen in mixed gas oxidation with the OXL formation in spin valves with the structure of sample

1 (S1): oxygen/nitrogen (solid circle) and oxygen/argon (open triangle) The solid lines are guides to the eye