Growth of Co/Cu multilayered thin films by electro-deposition

phys. stat. sol. (a) 201, No. 8, 1658–1661 (2004) / DOI 10.1002/pssa.200304643

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

Growth of Co/Cu multilayered thin films by electro-deposition

Y. Hayashi *, 1, C.-G. Lee1, B.-H. Koo1, T. Sato2, M. Arita2, and M. Masuda2 1 Department of Metallurgy and Materials Science, Changwon National University, Sarim-Dong,

Changwon, 641-773, Korea 2 Department of Materials Science and Engineerin, Kyushu University, Fukuoka 812-8581, Japan

Received 12 November 2003, revised 2 March 2004, accepted 25 March 2004 Published online 8 June 2004

PACS 75.47.De, 75.70.Cn, 81.05.Bx

Electrochemical method has been applied for preparing Co/Cu multilayered thin films. The layers were electrodeposited after injecting Co and Cu containing solutions in a flow exchanged supporting electrolyte solution. In X-ray diffraction at low angle range a long period peak could be observed, and in the middle angle region a single broad peak showing an average lattice spacing of the film was observed. The electri-cal resistance showed the properties of Giant Magnetoresistance (GMR). The GMR ratio showed a varia-tion against Cu layer thickness as expected for anti-ferromagnetically coupled films. The maximum GMR ratio obtained in this study was 6.2% at room temperature for the film of [Co 2.0 nm/Cu 1.0 nm]15. Though we need further studies for choosing suitable deposition conditions, the electrodeposition method is a promising method for producing multilayered thin films.


1 Introduction Since the discovery of the giant magnetoresistance in anti-ferromagnetically coupled Fe/Cr [1], and the oscillation of the values of GMR against the thickness of nonmagnetic layer [2], preparation of metallic multilayered films have attracted much interest of various researchers. The multi-layers were prepared under well controlled conditions to obtain high quality layered structure in atomic scale. Therefore, physical vapour depositions, such as MBE and sputtering, have been recognized as suitable method for the preparation of well characterized multilayered films. On the other hand, for the convenience of facilities and easiness in operation, electrodeposition has been studied as a promising and competitive candidate for the multilayers fabrication. Remarkable applications of the electrochemical method to the growth of magnetic multilayers have been done for preparing multilayered nano-wires in cylindrical nano-pores of track etched polycarbonate membranes [3], and for multilayers observing GMR effect [4]. Some other electrochemical studies fol-lowed using a cyclic potential change method in one electrolytic cell containing both metal ions [5–9]. The multilayered films of Co/Cu grown by the cyclic potential change method showed a relatively large GMR ratio and also its oscillation property against the change of Cu layer thickness [9]. This cyclic potential change method can produce compositionally modulated multilayers easily, but, in principle, the layers are not of pure metals. The more noble metal, i.e. Cu, can be deposited as pure metal, but the less noble metal, i.e. Co, deposits as an alloy, Co–Cu alloy. In some features electrochemical method has advantages against the one using vacuum system. To prepare multilayered films of pure metals by elec-trochemical method with simple equipment may be useful in relation to the various applications of the multilayered films. 2 Experimental In this study pure Co and Cu layers have been prepared by injecting metal ion con-taining electrolytes in the flowing supporting electrolyte alternately for 15 cycles. The supporting elec- * Corresponding author: e-mail: [email protected], Tel. +82 55 279 7535, Fax. +82 55 261 7017

phys. stat. sol. (a) 201, No. 8 (2004) / www.pss-a.com 1659


trolyte was 0.1 ~ 0.3 mol/l boric acid aqueous solution, and the metal ion containing electrolytes were 1 ~ 2 × 10–3 mol/l copper sulphate and 5 × 10–3 mol/l cobalt sulphate aqueous solutions. The suitable concentrations of the solutions were decided after various experiments. The deposition potentials for Co and Cu were –1.1 V and –0.9 V vs. Ag/AgCl reference electrode, respectively. The substrates used were Si wafer plated with Au by vacuum evaporation to take electrical contact. The plated area was 10mm × 10mm. The layer structure was examined by X-ray diffraction, and the surface morphology was observed by AFM. The magnetization curve was taken by VSM, and the magneto resistance was meas-ured by Van der Pauw method with current supplying in plane with applied magnetic field parallel to the plane. The estimation of the thickness of the layers was as follows: Total amount of Co deposited was esti-mated from the saturation magnetization of the films. The long range period was determined from the peak in low angle X-ray diffraction. Knowing the number of repeated depositions, the thickness of the layers was estimated. The amount of electrical charge for metal deposition was also taken into consideration. 3 Results and discussion When the electrolytic solution is flowing during metal deposition, thickness control was very difficult, especially for very thin layers. Furthermore, the surface showed irregular streaks along the flow direction. Therefore, the flow of the liquid, 2.1 ml/s, was intermitted when the metals were deposited. To control the thickness of the layer, deposition time was changed. The cathodic current during deposition was recorded, and used for the estimation of the thickness of the layers. In stagnant condition surface of the film was smooth, and the control of thin layer deposition became easier than in flowing condition. The average root mean square surface roughness of the films after 15 times repeated depositions was 2 nm; the roughness of the Au buffer layer was 1 nm. The results obtained for the films prepared on Si(111) substrate with varying Cu thickness are shown. In Fig. 1 X-ray diffraction profiles of multilayered films with varying Cu layer thickness in the low and middle angle regions are

(c)

(b)

42 43 44 45 46 47

2 q (Cu-Ka )

Inte

nsity

(arb

unit)

0 5 10

2 q(Cu-Ka)

Inte

nsity

(arb

unit)

Co(2.0nm)/Cu(1.0nm)]



(a)

(b)

(a)

(c)

Fig. 1 Middle and low angle X-ray diffraction profiles of Co/Cu multilayers with varying Cu deposition time; (a) 10 s, (b) 15 s, and (c) 20 s, respectively.

1660 Y. Hayashi et al.: Growth of Co/Cu multilayered thin films by electro-deposition


0

5

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-5000 -2500 0 2500 5000

Applied magnetic field H/Oe

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/R (

%)

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-5000 -2500 0 2500 5000


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)

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Apped magnetic field H/Oe

-10000 -5000 0 5000 10000


-10000 -5000 0 5000 10000


(a) [Co(2.0nm)/Cu(1.0nm)]

(b) [Co(2.0nm)/Cu(1.4nm)]

(c) [Co(2.0nm)/Cu(2.0nm)]

Mag

netiz

atio

nM

/Ms

Mag

netiz

atio

nM

/Ms

Mag

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nM

/Ms

DD

Fig. 2 Room temperature magnetization curve and field dependence of magnetoresistance of Co/Cu multilayers with varing Cu layer thickness; (a) tCu = 1.0 nm, (b) tCu = 1.4 nm, and (c) tCu = 2.0 nm, respec-tively.

shown. At low angle range a long period peak could be observed, though the peak dwindled owing to the irregularity enhancement with the increase of Cu thickness. In the middle angle region a single broad peak showing an average fcc (111) lattice spacing of the film was obtained. The thickness shown in the figure is the estimated value. The results of the magnetization curves and magnetoresistance for the films (a), (b), (c) shown in Fig. 1 are shown in Fig. 2. The magnetization curve was measured applying the field parallel to the film surface, and magnetoresistance was measured with current supplying in plane

phys. stat. sol. (a) 201, No. 8 (2004) / www.pss-a.com 1661


and magnetic field parallel to the surface. The magnetization curves were different from single layer Co films, and showed similar characteristics of anti-ferromagnetic coupled multilayers. The electrical resis-tance showed the GMR characters. Though the magnetization curves did not show any distinct difference between the films with varying Cu thickness, the GMR ratio showed a variation against the thickness of Cu layers as expected for the anti-ferromagnetically coupled GMR films. The maximum GMR ratio obtained was 6.2% at room temperature for the specimen of [Co 2.0 nm/ Cu 1.0 nm]15. Though we need further studies for choosing a suitable deposition potential and improving the electro-lyte composition, the flow electrodeposition method is proved to be a promising method for producing magnetic layered thin films. 4 Conclusion By electrochemical deposition Co/Cu multilayered films were grown. An X-ray diffrac-tion peak corresponding to long period of the multilayers was observed. Though the layer structure was not complete, relatively large magnetoresistance was observed. The maximum GMR ratio obtained in this experiment was 6.2% at room temperature for the film of [Co 2.0 nm/Cu 1.0 nm]15, and showed variation against the thickness of the Cu layers. Although we need further studies for choosing a suitable deposition potential and improving the electrolyte composition, the flow electrolyte exchange electrode-position method is proved to be a promising method for producing multilayered thin films.

Acknowledgements One of the author, Hayashi, thanks to Korean Federation of Science and Technology Socie-ties and Korea Science and Engineering Foundation for their support to stay in Korea. The authors thank to Prof. A.S.M.A. Haseeb of Bangladesh University of Engineering and Technology for his valuable comments and discus-sion.

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Growth of Co/Cu multilayered thin films by electro-deposition