Journal of Magnetism and Magnetic Materials 240 (2002) 189–191

Interactive effect of impurities on giant magnetoresistance ofCo–Fe/Cu multilayers

Masakiyo Tsunoda*, Hideo Arai, Daisuke Takahashi, Satoshi Miura,Migaku Takahashi

Department of Electronic Engineering, Graduate School of Engineering, Tohoku University, Aobayana 05, Sendai 980-8579, Japan

Abstract

Giant magnetoresistance of Co–Fe–B/Cu multilayers fabricated in the sputtering atmosphere, where the amount of

oxygen impurity is varied, is discussed in connection with their interfacial roughness. The magnetoresistance (MR) ratioof Co–Fe/Cu multilayers is enhanced by up to 33% when the oxygen content is varied between 10 and 100 ppm ofprocessing Ar gas. The enhancement of the MR ratio was due to the flattening effect of impurity oxygen on themultilayer interfaces: the root mean square roughness of the multilayer was decreased from 7.5 to 5 (A. With increasing

boron content in Co–Fe layers, however, the enhancing effect of MR ratio by oxygen diminished and nearly vanishedfor 12 at%-B–(Co–Fe) case. The strong affinity of boron for oxygen is suggested as a probable mechanism. r 2002Elsevier Science B.V. All rights reserved.

Keywords: Giant magnetoresistance; Multilayer; Impurities; Interface roughness; Thin films

1. Introduction

The giant magnetoresistance (GMR) effect in

Co90Fe10/Cu multilayers [1] has been actively investi-gated to be applied to magnetic sensors. Key technicalfactors to obtain sensitive magnetoresistance (MR)

response are (1) flat interfaces of multilayers to retainlarge MR ratio, suppressing the so-called ‘‘orange-peel’’coupling [2] which prevents the antiparallel alignment of

the magnetizations of neighboring Co–Fe layers, and (2)soft magnetic properties of Co–Fe layers. These twofactors can be realized simultaneously by controllingimpurities in fabrication process and additives in multi-

layers. Oxygen, introduced into the sputtering atmo-sphere during film deposition, is an effective impurity toimprove the interfacial flatness, which has been demon-

strated for spin valves [3] and Co/Cu multilayers [4].Boron is a well-known additive to achieve magneticsoftening of Co–Fe films [5], because of its character-

istics as an amorphous former. However, the combined

effect of these elements on the multilayer properties isnot yet clarified. In the present study, we thus fabricatedCo–Fe–B/Cu multilayers in the sputtering atmosphere,

where the amount of oxygen impurity was varied, andinvestigated the GMR in connection with their micro-structure, especially the interfacial roughness.

2. Experimental procedure

The multilayers were deposited on the thermally

oxidized Si wafer with the structure of sub./((Co90-Fe10)1�xBx 10 (A/Cu dCu)30, by using a DC magnetronsputtering machine which was capable of pumpingresidual gases down to 4� 10�11 Torr. The Cu layer

thickness was optimized to make the MR ratiomaximum in the so-called ‘‘second peak’’ of the GMRoscillation, which ranged from 19 to 23 (A, under the

respective fabrication conditions. The boron contentwas changed from 0 to 12 at%. After being pumpeddown to the ultimate pressure, oxygen was introduced

through a variable leak valve to vary the partial pressureof the chamber from 10�8 to 10�6 Torr. Ultra-clean

*Corresponding author. Fax: +81-22-263-9416.

E-mail address: tsunoda@ecei.tohoku.ac.jp (M. Tsunoda).

0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 7 5 3 - 3

(9N) Ar gas was then introduced to make the totalchamber pressure reach 1mTorr. The interfacial flatness

of the multilayer was analyzed by X-ray reflectivity witha Cu-Ka radiation source. The magnetoresistance wasmeasured by a DC four-point probe method in a

magnetic field up to 13 kOe at room temperature. TheMR ratio was defined as Dr=rs � ðr0 � rsÞ=rs; where r0is the maximum resistivity at around zero field and rs isthe saturation resistivity.

3. Results and discussion

Prior to the multilayer experiment, we investigated theeffect of boron addition to Co–Fe layers on theirmagnetic and magnetoresistive properties. The coerciv-ity of 200- (A-thick Co–Fe–B single-layered films and the

magnetoresistance, Dr; of Co–Fe–B/Cu/Co–Fe–B sand-wich films are plotted as a function of boron content inFig. 1. A 70- (A-thick Mn–Ir layer deposited on the

sandwiches resulted in a well-separated magnetizationprocess of both Co–Fe–B layers and enabled us todetermine Dr from r2H curves as illustrated in the

inset. In other words, we artificially realized theantiparallel alignment of magnetization of neighboringCo–Fe–B layers, which is hard to be obtained in the

multilayers having ferromagnetic interlayer coupling asdiscussed below. The boron addition drastically di-minishes the coercivity of Co–Fe films, while themagnetoresistance gradually decreases with increasing

boron content. Dr decreases about one-third in the caseof 12 at% boron content, compared to that of the Co–Fecase. From these results, one can expect the softening of

the Co–Fe layer without serious diminishing of the spin-dependent scattering in the multilayers.

Fig. 2 shows the MR ratio and rs of multilayers as afunction of the partial pressure of oxygen, Po2 : In thecase of Co–Fe multilayer (circle), the MR ratio is

remarkably enhanced by introducing oxygen in the10�8�10�7 Torr range of partial pressure, while the rsremains constant in that range. The MR ratio reaches33% at Po2=4� 10�8 Torr. With further increase in thepartial pressure of oxygen, rs steeply increases. Thisincrease corresponds to the oxidization of the multilayer,resulting in the decrease of the MR ratio. These changes

of the MR ratio and rs show a good correspondencewith the changes observed in the Co/Cu multilayers [4].When the boron content is 4.4 at% (triangle), the MR

ratio is enhanced with introducing by oxygen similar tothe Co–Fe case, except for a shift of the onset pressure ofoxygen to a higher range, where the MR ratio starts to

increase. The maximum MR ratio of 30% is obtained atPo2=1� 10�7 Torr. On the other hand, when the boroncontent is 8.5 at% (reversed triangle), the enhancementof the MR ratio by introducing oxygen becomes very

small and the maximum MR ratio only reaches 5%.When the boron content is 12 at% (square marks), wecannot find the increase of the MR ratio any more, but

find a remarkable increase of rs starting from 10�8 Torrof Po2 : Judging from the gradual decrease of themagnetoresistance of the sandwich films (Fig. 1), one

should consider other reasons for the vanishing of theeffect of impurity oxygen on the GMR with increasingboron content in Co–Fe layer, rather than the decreaseof the spin-dependent scattering.

As mentioned earlier, the mechanism of enhancing theMR ratio by introducing oxygen is the flattening of

Fig. 1. Changes of coercivity of single-layered 200- (A-thick Co–

Fe–B film (solid circle) and magnetoresistance of Co–Fe–B/Cu/

Co–Fe–B sandwich films (open circle) as a function of the

boron content. The sandwich film comprises Ta 50 (A/Ni80Fe2020 (A/(Co90Fe10)–B 40 (A/Cu 25 (A/(Co90Fe10)–B 40 (A/Mn74Ir2670 (A/Ta 20 (A. The oxygen impurity was not introduced in the

processing chamber.

Fig. 2. Changes of MR ratio (solid marks) and rs (open marks)of multilayers, ((Co90Fe10)–B 10 (A/Cu 21 (A)30, as a function of

the partial pressure of oxygen introduced into the sputtering

chamber. Boron content in Co–Fe layer is 0 (circle), 4.4 at%

(triangle), 8.5 at% (reversed triangle), and 12 at% (square),

respectively.

M. Tsunoda et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 189–191190

interfaces of the multilayers, which suppresses the

obstructive ‘‘orange-peel’’ coupling [4]. In order toinvestigate the interfacial roughness, we measured theX-ray reflectivity of the multilayers. Fig. 3 shows the

changes of the reflectivity profile of the multilayers with(a) Co–Fe layers and (b) (Co–Fe)88B12 layers. As aguiding scale for comparing the profiles, 104 counts/s of

reflective intensity are shown as a horizontal line on eachprofile. In the case of Co–Fe multilayers, the diffractionpeaks, due to the artificial period, and finite-size peaks,which appear as high-frequency oscillations on the

profiles, become larger and clearer with an increase inPo2 ; indicating that the interfacial flatness in the multi-layers improves with increase in Po2 : It fairly follows theresults of Co/Cu multilayers [4]. On the other hand, inthe case of (Co–Fe)88B12 multilayers, the reflectivityprofile changes very little with an increase in Po2 : Toexamine quantitatively the interfacial roughness, theroot mean square (rms) roughness was calculated fromthe ratio of the observed reflectivity to the one calculatedin the ideal multilayer with no roughness [6]. Fig. 4

shows the rms roughness of Co–Fe–B multilayers withvarious boron contents, as a function of the partialpressure of oxygen, Po2 : One can confirm the flattening

interfaces with increasing Po2 only in the cases of theCo–Fe multilayer (circle) and the (Co–Fe)95.6B4.4 multi-layer (triangle). From these results, we conclude that the

effect of impurity oxygen to flatten the interface

diminishes with increase in the boron content ofmagnetic layers. This is due to the strong affinity ofboron for oxygen compared with that of Co–Fe. The

mechanism of rapid increase of rs in the case of the (Co–Fe)88B12 multilayer for the lower Po2 range (Fig. 2) canbe naturally understood by assuming the formation of

boron oxide in the multilayers. The increase of rs is dueto higher density of diffusive scattering centers ofconduction electrons: both the boron oxide itself and

the grain boundaries which result from a prevention ofgrain growth of multilayers by boron oxides.In conclusion, we have examined the availability of

the combined addition of oxygen and boron to Co–Fe/

Cu multi-layers to attain highly sensitive GMR effect.However, the interactive effect between oxygen andboron has been found to cause the loss of the effects of

oxygen on the micro-structure, that is the flattening ofthe interfaces of multilayers, which is indispensable toretain a large MR ratio.

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Fig. 3. Changes of X-ray reflectivity profile of the: (a) Co–Fe

multilayer and (b) (Co–Fe)88B12 multilayer, (Co–Fe(–B) 10 (A/

Cu 21 (A)30, as a function of the partial pressure of oxygen

introduced into the sputtering chamber. Horizontal lines are the

guiding scale for comparison of the profiles. A vertical dashed

line is the expected position of the diffraction peak due to the

artificial period.

Fig. 4. Changes of root mean square (rms) roughness of the

multilayers, (Co–Fe(–B) 10 (A/Cu 21 (A)30, as a function of the

partial pressure of oxygen introduced into the sputtering

chamber. Boron content in Co–Fe layer is 0 (circle), 4.4 at%

(triangle), 8.5 at% (reversed triangle), and 12 at% (square),

respectively.

M. Tsunoda et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 189–191 191