Cu multilayer film produced by electrodeposition method

Cu multilayer film produced by electrodeposition method

~ D Journal of Magnetismand Magnetic Materials 156 (1996) 350-352 4~ Journal ofsm am:Inetl magnetic N ELSEVIER ,i~ materials Magnetoresistance e...

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Journal of Magnetismand Magnetic Materials 156 (1996) 350-352

4~ Journal ofsm am:Inetl magnetic

N ELSEVIER

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materials

Magnetoresistance effect of Co/Cu multilayer film produced by electrodeposition method Y. Ueda *, N. Hataya, H. Zaman Dept. of Electrical and Electronic Engineering, Muroran Institute of Technology, 27-1 Mizumoto-cho, Muroran 050, Japan

Abstract Electrodeposition conditions for the preparation of Co/Cu multilayer films and the relationship between the magnetization and the magnetoresistance (MR) of the prepared films have been investigated. The MR for the Co/Cu multilayers depends on the composition of the Co-rich ferromagnetic layer and the Cu-rich nonmagnetic interlayer. The MR ratio decreases remarkably with increasing content of Co into the Cu interlayer.

Much attention has been paid to giant magnetoresistance (GMR), since the first observation of large resistance changes has been found in the F e / C r multilayer film with antiferromagnetic coupling between adjacent Fe layers. The phenomenon has been observed in several other antiferromagnetically coupled multilayers such as Co/Cu and Co/Ag [1,2]; furthermore, it has been found in Co-Cu granular alloy films, which are immiscible combinations produced by the sputter and electrodeposition methods [3-5]. It is not yet clear why two separated Co and Cu phases of the alloy films as well as the multilayer produce GMR. That is, it is unclear how the size, the distribution of the magnetic particles and the magnetic orientation of the particles affect the antiferromagnetic coupling. Electrodeposition is the method of electrochemical precipitation through the reducing reaction of metal ions at low temperature from an electrolyte solution. Although it is difficult to produce nonequilibrium alloys by ordinary melting, which usually results in the formation of an equilibrium alloy, it is possible to produce the alloy by controlling the electrodeposition conditions [5]. Furthermore, it is possible to produce a multilayer film or a compositionally modulated film of a suitable alloy composition and thickness by controlling the electrode potential (square potential wave) and quantity of electricity (pulse width) [6]. In this respect the electrodeposition technique has merit over other deposition techniques. Ultimately, it offers the possibility of precipitation of the nonequilibrium alloy and multilayers with a period of single atomic layers.

We investigated the electrodeposition conditions required for the preparation of multilayers with compositional modulation, and also the relationship between the magnetism and the magnetoresistance of the films. The chemical composition of the electroplating bath (Co95Cu 5) was 53.4 g of CoSO4 • 7H20, 2.45 g of CuSO4 • 5H20, 76.5 g of Na3C6HsO 7 and 2 g of NaCI in 1 liter of solution. The substrates for electrodeposition were copper thin films vapor-deposited on glass plates. The crystalline orientation of the Cu substrate was not observed. Electrodeposition was carried out with a square wave of 0.3-20 m A / c m 2 in a plating solution maintained at a pH value of 5.0. The number of CoCu/CuCo bilayers in the multilayers was 50. The concentrations of Co and Cu in the deposited films were determined by X-ray fluorescence spectroscopy. We supposed that the relation between the composition and the current density in the film of 1000 ,~ is also applicable to Co thin films of 15 ,~.

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0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSD1 0304-8853(95)00894-2

Y. Ueda et al. / Journal of Magnetism and Magnetic Materials 156 (1996) 350-352

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Fig. 1 shows the composition of the electrodeposited film as a function of current density. The precipitation of Cu only is observed in the region of current densities less than 0.4 m A / c m 2. The film of Co88Cu12 was deposited at 20 m A / c m 2. The MR ratio for the films with various Co compositions in the ferromagnetic layer is described as a function of the layer thickness of Cu in Fig. 2. The MR ratio giVeSoa 1st peak in the Cu interlayer thickness range of 13-18 A, and shows a 2nd peak at about 35/k. The MR ratio has a maximum value of 15% (Cu = 14 A, room temperature) at the Co composition of 86 at%. The maximum of the MR ratio shifts to the thicker side of the Cu film thickness with increasing Co concentration of the Co-rich layer. The tendency does not agree with the results of sputtered films [7]. It is not clear whether the reason is due to the film structure or the film composition depending on the electrodeposition conditions. Fig. 3 shows A R / R as a function of the magnetic field for Co~Cu~o0_dCu ( x = 7 1 , 83, 86 and 88) films. A R / R is the measured value at the first peak in Fig. 2, and does not saturate at 15 kOe. Fig. 4 shows the comparison between the magnetic field dependence of A R / R and the magnetization curve. A R / R does not saturate in the field range up to 4 kOe for the film with a copper layer thickness of 14 A. The magnetization curve, however, shows near saturation at small magnetic fields. The magnetic field dependence of A R / R is not in agreement with that of the magnetization curve, but that of the Cu layer thickness of 35 A is fairly in agreement with it. The magnetic field dependence of A R / R has a different tendency on changing the direction of the applied field. The effect of Co addition into the Cu layer on the MR ratio is shown in Fig. 5. The MR ratio decreases remarkably with increasing Co concentration into the layer. The MR is affected significantly by the purity of the Cu interlayer.

References [1] S.S.P. Parkin, R. Bhadra and K.P. Roche, Phys. Rev. Lett. 66 (1991) 2152. [2] W.P. Pratt Jr., S.F. Lee, J.M. Slaughter, R. Loloee, P.A. Schroeder and J. Bass, Phys. Rev. Lett. 66 (1991)3060. [3] J.Q. Xiao, J.S. Jiang and C.L Chien, Phys. Rev. Lett. 68 (1992) 3749.

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Y. Ueda et al. / Journal of Magnetism and Magnetic Materials 156 (1996) 350-352

[4] A.E. Berkowitz, J.R. Mitchell, M.J. Carey, A.P. Young, S. Zhang, F.E. Spada, F.T. Parker, A. Hutten and G. Thomas, Phys. Rev. Lett. 68 (1992) 3745. [5] Y. Ueda and M. Ito, Jpn. J. Appl. Phys. 33 (1994) L1403. [6] M. Alper, K. Attenborough, R. Hart, S.J. Lane, D.S. Lash-

more, C. Younes and W. Schwarzacher, Appl. Phys. Lett. 63 (1993) 2144. [7] D.H. Mosca, F. Petroff, A. Fert, P.A. Schroeder, W.P. Pratt Jr. and R. Laloee, J. Magn. Magn. Mater. 94 (1991) L1.