Preparation and characterization of electrodeposited metallic multilayers

ELSEVIER

Physica B 239 (1997)35-40

Preparation and characterization of electrodeposited metallic multilayers Y. H a y a s h i * , S. K a s h i w a b a r a , Y. J y o k o Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, Hakozaki, Fukuoka, 812 Japan

Abstract

Multilayered films of Co/Pt were prepared by alternating electrodeposition of Co and Pt using two electrolytic baths. The structure of the electrodeposited multilayers was analyzed by reflection electron microscopy (REM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The magnetic anisotropy of the films was examined by changing the thickness of Co layers. Though the perpendicular magnetization characteristics are not good enough, electrodeposition may be feasible for multilayer deposition. Keywords: Perpendicular magnetic anisotropy; Electrodeposition; Co/Pt multilayers; Overpotential

1. Introduction

Metallic multilayers have attracted various interest, and extensive studies have been done for their preparation and characterization [1]. Among others, magnetic properties of metallic multilayers have been extensively studied [2, 3]. Most of the multilayered films were prepared by physical vapor deposition, but electrodeposition has also possibility to produce nanoscopically controlled multilayers [4-5]. Though the temperature is limited in a narrow region, there are various ways of choosing electrolytic bath composition and deposition potential in the electrodeposition technique. Preparation of metallic multilayers by electrodeposition is interesting also from the viewpoint of controlled crystal growth by electrolysis. Multilayers of Co/Pt are interesting for their application to the recording media using magnetization perpendicular to the film surface [6, 7]. In this study, Co/Pt multilayers were prepared by

*Corresponding author.

dual-bath electrodeposition, and their structure and magnetic properties were analyzed.

2. Experimental

Layers of Co and Pt were deposited from each electrolytic bath on a Cu substrate. Electrolytes, 0.5 kmol/m 3 H2SO4 containing 0.1 kmol/m 3 COSO4 or 1 × 10 -3 kmol/m 3 H2PtC16, were prepared for Co or Pt deposition. The layers of Pt were deposited at constant electrode potential, -0.65V versus saturated Ag/AgC1 electrode, and Co layers were deposited at two different electrode potentials, -0.85 V (low overpotential) and -0.15 V (high overpotential). The deposition rate of Pt was 0.3 nm/s, and for Co 0.02 and 0.2 nm/s at low and high overpotentials, respectively. For the REM observations the substrate was (111) plane of a Pt sphere. Specimens for the cross-section TEM observations were sliced from layered films with an ultramicrotome. Magnetic properties were measured with a vibrating-sample magnetometer. The effective perpendicular magnetic anisotropy energy

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Y. Hayashi et al. / Physica B 239 (1997) 35-40

was calculated from the magnetization curves measured with applied field perpendicular and parallel to the film surface. The thickness of the layers was estimated by the electrical charge passed during deposition, and examined by low angle peak of XRD, saturation magnetization and cross-section observation.

3. Results and discussion 3.1. Deposits and structure

The early stage of deposition was observed by REM. Surfaces of 5 monolayers Co deposited on

P t ( l l l ) at different overpotentials are shown in Fig. 1. The deposits show simultaneous nucleation layered growth. Thin Co grows pseudomorphic to the substrate surface. Then 14 monolayers of Pt were deposited on the surfaces shown in Fig. 1, and observed again by REM which are shown in Fig. 2. The deposited Pt show rather irregular forms and no clear terraces were observed. Platinum may deposit as small particles, and the appearance changes with the substrate Co deposits. Layered films were prepared with various designed periods, and the cross-section was observed. Fig. 3 shows the examples of the films with rather thick layers sequence. Dark part is Pt and light part is Co. Platinum deposits in fine particles and Co

Fig. 1. Typical REM-RHEED pictures obtained from Pt(lll) surfaces covered with 5.0 monolayer Co electrodeposited under (a) a relativelylow overpotential and (b) a very high overpotential.

Y. Hayashi et al. / Physica B 239 (1997) 35-40

37

Fig. 2. REM-RHEEDpictures ofPt (14monolayers)/Co (5.0 monolayers)/Pt(lll)sandwich specimens. Pt wereelectrodeposited on the same surfaces and at the same conditions shown in Fig. 1.

Fig. 3. Cross-sectional TEM image of electrodeposited (a) [Co (40 nm)/Pt (40 nm)] multilayer and (b) [Co (2 nm)/Pt (8 nm)] multilayer.

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Y. Hayashi et al. / Physica B 239 (1997) 35-40

[Co(2nm)/Pt(4nm)]6o Low overpotential '

"

I

.

.

.

.

I

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i i i ~ , , , , 5*

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55*

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High overpotential

I

5"

10"

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20 (Co-Ka)

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50 °

55 °

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Fig. 4. Typical X-ray diffraction profiles of [Co (2 nm)/Pt (4 nm)] multilayers electrodeposited Co layer at (a) a relatively high overpotential and (b) a very high overpotential.

shows grown grains. Though the interface was not regular, modulated structure was observed even in a film with thin-layers sequence. Low- and high-angle X-ray diffraction profiles for [Co (2 nm)/Pt (4 nm)]6o films are shown in Fig. 4. Periodic modulation could be observed though the peaks were broad and higher-order peaks were not distinguished. The layered films were FCC structure and showed no preferred orientation. The X-ray diffraction showed no signifi-

cant difference in films prepared with different deposition potentials.

3.2. Magnetic properties Magnetization curves were measured applying magnetic field parallel and perpendicular to the film surface for specimens prepared with various layer designs. Examples of the M - H curves are

Y. Hayashi et al. / Physica B 239 (1997) 35-40

Co -0.85V(O.3nm)/Pt -0.65V(1.0nm)

39

2.0

=.

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Applied magnetic field, H/kOe

-2.0

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t

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,

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,

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t co/nm

Co -1.SV(O.2nm)lPt -0.65V(1.0nm) *~

e~

Fig. 6. Effective anisotropy energy tcoKeff, versus Co layer thickness tco for electrodeposited Co/Pt multilayers. Co layers electrodeposited under (O) a relatively low overpotential and (71) a very high overpotential,

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Applied magnetic field, H/kOe Fig. 5. Typical M - H curves of Co/Pt multilayers electrodeposited Co layer at (a) a relatively high overpotential and (b) a very high overpotential.

shown in Fig. 5. A film prepared at low overpotential for Co deposition showed a tendency of perpendicular magnetization. The effective magnetic anisotropy energy was calculated from the area of perpendicular and parallel M - H curves. The relation between the effective perpendicular anisotropy energy per unit volume of Co layer (Keff) and the thickness of Co layer (tco) is shown in Fig. 6. The plot of t c o K e f f v e r s u s tco shows a straight line. This relation can be expressed as [8] tcoKeff = Ks + K v t c o ,

where Ks is the surface anisotropy energy per unit area of the interface and Kv is the effective bulk

anisotropy energy per unit volume of Co layer. For comparison, the result obtained for the specimen prepared by MBE [9] is shown by a dashed line in the figure. Georgescu et al. [10] showed better perpendicular anisotropy with their electrodeposited multilayers than the films presented here, but the dependence on the Co thickness is different. The parameters Ks and Kv obtained here are different from those of MBE film. Irregularity of the interface may be the reason for the low value of Ks. Difference in elastic deformation of Co crystals near the interface may cause difference in the value of Kv. Some different ways of controlling the interface will be necessary to obtain a good perpendicular anisotoropic films by electrodeposition.

4. Conclusion Films of Co/Pt multilayers were prepared by electrodeposition using separate electrolytic baths for Co and Pt. Difference in deposition potential has an effect on the magnetic anisotropy of the Co/Pt layered films. Further effort is still needed for the control of geometry and crystalline coherence

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Y. Hayashi et al. / Physica B 239 (1997) 35-40

of the interface to prepare perpendicular magnetization films by electrodeposition.

References [1] For a recent review see, J.A.C. Bland, B. Heinrich (Eds.), Ultrathin Magnetic Structure, Springer, 1994, Berlin. [2] H. Takahashi, S. Tunashima, S. Iwata, S. Uchiyama, J. Magn. Magn. Mater. 126 (1993) 282. [3] G.H.O. Daalderop, P.J. Kelly, M.F.H. Schuurmans, Phys. Rev. B 42 (1992) 7270.

[4] Y. Jyoko, S. Kashiwabara, Y. Hayashi, Mater. Trans. JIM. 34 (1993) 946. [5] S. Kashiwabara Y. Jyoko, Y. Hayashi, Mater. Trans. JIM., 37 (1996) 289. [6] P.F. Carcia, A.P. Meinhaldd, A. Suna, Appl. Phys. Lett. 43 (1985) 178. [7] P.F. Carcia, J. Appl. Phys. 63 (1988) 5066. [8] F.J.A. den Broeder, W. Hoving, P.J.H. Bloemen, J. Magn. Magn. Mater. 93 (1991) 562. [9] C.J. Lin, G.I. Gorman, C.H. Lee, R.C. Farrow, E.E. Marinero, H.V. Do, H. Notarys, J. Magn. Magn. Mater. 93 (1991) 194. [-10] V. Georgescu, V. Mazur, O. Cheloglu, J. Magn. Magn. Mater. 156 (1996) 27.