Thin ®lms of cobalt and granular copper±cobalt alloysby nebulized spray pyrolysis1

Sachin Parashar, A.R. Raju, C.N.R. Rao*

Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scienti®c Research, Jakkur Bangalore 560 064, India

Abstract

Thin ®lms of cobalt have been prepared by nebulized spray pyrolysis using Co(acac)2 precursor in hydrogen atmosphere. The ®lms show

excellent magnetic and metallic properties. Granular Cu±Co alloy ®lms of the composition Cu82Co18 and Cu50Co50 have been obtained by

the nebulized spray pyrolysis of mixtures of Co(acac)2 and Cu(acac)2 in hydrogen atmosphere. These ®lms show good magnetoresistance at

room temperature. # 1999 Elsevier Science S.A. All rights reserved.

Keywords: Cobalt ®lms; Cobalt±copper alloy ®lms; Granular ®lms; Magnetism; Magnetoresistance

1. Introduction

Thin ®lms of cobalt are known to be useful in memory

elements and in microelectronics because of their low

resistivity and high oxidation resistance. Although cobalt

®lms have been obtained by thermal evaporation and sput-

tering techniques, their study has been rather limited.

Recently, Li et al. [1] have reported the study of cobalt

®lms deposited by thermal evaporation and chemical vapor

deposition (CVD) of b-diketonate compounds. These work-

ers were concerned with the magnetic properties of Co ®lms

with respect to their surface roughness. Maruyama [2]

deposited Co ®lms of varying thickness by the CVD tech-

nique and studied their electrical transport properties with

varying thickness. We considered it most useful to obtain

good quality cobalt ®lms by the simple, low-cost technique

of nebulized spray pyrolysis [3,4]. We report the successful

deposition of cobalt ®lms obtained by this means in this

article.

Copper±cobalt granular ®lms are being investigated

extensively in recent years because of the giant magnetore-

sistance (GMR) exhibited by them. These ®lms have been

prepared by DC sputtering [5], magnetron sputtering [6,7]

and melt spinning [8]. In this communication, we report the

preparation of Cu±Co granular alloy ®lms by nebulized

spray pyrolysis, along with their magnetoresistance proper-

ties.

2. Experimental

Cobalt thin ®lms were deposited using a home-built

nebulized spray pyrolysis apparatus described elsewhere

[3,4]. A solution containing the cobalt acetylacetonate pre-

cursor in methanol solvent (�1 mM) was nebulized making

use of a PZT transducer (1.72 MHz). Cobalt(II) acetylace-

tonate {Co(acac)2 } was used as precursor in the study of Co

®lms. Similarly, a solution containing a mixture of cop-

per(II) acetylacetonate and cobalt(II) acetylacetonate of the

required composition was used to deposit the Cu±Co gran-

ular ®lms. The compositions of CuxCo1ÿx ®lms were x�0.5±

0.9. All the precursors were synthesized in the laboratory

using commercial acetylacetone and chloride salts of copper

and cobalt.

The ®lms were deposited on Si(1 0 0) as well as on

borosilicate glass substrates at substrate temperatures

598±673 K for 7±9 h. Zero grade hydrogen gas was used

as the carrier gas to transport the nebulized precursor with a

typical gas ¯ow rate of 1±1.5 l/min. In order to avoid the

formation of cobalt oxide, the growth chamber was thor-

oughly purged with hydrogen gas before and after the

deposition run.

The deposited ®lms were examined by X-ray diffraction

using a Seifert (XRD, XDL, TT, Cu target) instrument.

Conventional q±q scans were collected with a Bragg±Bren-

tano goniometer and a high resolution, 169 eV Si(Li) solid

state detector with 1 mm/2 mm slits. The surface morphol-

ogy of the ®lms was examined by scanning electron micro-

scopy with a Leica S-440I scanning electron microscope

(SEM). The ®lm thickness was obtained by cross-sectional

Materials Chemistry and Physics 61 (1999) 46±49

*Corresponding author. Fax: +91-80-331-1310

E-mail address: cnrrao@jncasr.ac.in (C.N.R. Rao)1Dedicated to Prof. S. Somiya.

0254-0584/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.

PII: S 0 2 5 4 - 0 5 8 4 ( 9 9 ) 0 0 1 1 2 - 1

scanning electron microscopy. The chemical composition of

granular ®lms was determined by using Links EDX analy-

zer. The electrical resistivity of the deposited ®lms was

measured by using four probe method. Magnetic hysteresis

loop measurements were carried out using Lake Shore

vibrating sample magnetometer model 7300. Magnetore-

sistance measurements were carried out at room tempera-

ture (300 K) with a magnetic ®eld of 2 kOe.

3. Results and discussion

3.1. Co films

X-ray diffraction (XRD) patterns of the Co ®lms depos-

ited from the Co(acac)2, precursor on Si(1 0 0) substrates at

598 and 673 K are shown in Fig. 1. The XRD patterns

indicate that the ®lms are polycrystalline but there is no

indication of cobalt oxide impurity. The Co ®lms deposited

on Si(1 0 0) at 598 K show the presence of a mixture of f.c.c.

and h.c.p. phases. At a higher deposition temperature of

673 K, however, only the f.c.c.phase is observed. On the

borosilicate glass substrate both f.c.c.and h.c.p. phases of Co

are observed at both the deposition temperatures. The

proportion of the f.c.c.phase increases with increasing tem-

perature.

SEM micrographs of the Co ®lms on Si(1 0 0) and glass

substrates are shown in Fig. 2(a)±(d). The thickness of the

®lms was in the range of 1.2±1.4 mm. The SEM images

reveal clear variation in the surface morphology of the ®lms

with the deposition temperature in all the ®lms. In the ®lms

deposited on the Si(1 0 0) substrate, a mixed morphology of

Fig. 1. The X-ray diffraction patterns of Co films deposited on Si(1 0 0)

substrates at (a) 598 K, (b) 673 K and on glass substrate at (c) 598 K and

(d) 673 K. *denotes Si(2 0 0) substrate peak.

Fig. 2. Scanning electron micrographs of Co films on Si(1 0 0) substrate at temperature (a) 598 K and (b) 673 K and on glass substrate at temperature (c)

598 K and (d) 673 K.

S. Parashar et al. / Materials Chemistry and Physics 61 (1999) 46±49 47

needle shaped ¯akes and granular cobalt is obtained at

598 K. Only granular cobalt morphology is observed at

673 K. The ®lms on the glass substrate exhibit granular

morphology at 598 K and a needle-shaped ¯aky-type mor-

phology at 673 K.

Electrical resistivity data of the Co ®lms on the Si(1 0 0)

and glass substrates deposited at 673 K are shown in

Fig. 3(a) and (b), respectively. The data clearly reveal the

expected metallic behavior. The values of resistivity of the

Co ®lms on the Si(1 0 0) substrate are 1.646 and

0.258 m cm respectively at 300 and 15 K. The resistivity

values of the ®lms on the glass substrate are 2.347 and

0.327 m cm at 300 and 15 K, respectively. The resistivity

of the ®lms on the glass substrate is somewhat higher

probably because of the presence of mixed phases of Co.

We recall that only the f.c.c. phase is found on the Si(1 0 0)

substrates.

Magnetic hysteresis loop measurements of the Co ®lms

on Si(1 0 0) and glass substrates were recorded with the

magnetic ®eld parallel and perpendicular to the ®lm. The

results are shown in Fig. 4. The ®lms deposited at 598 K

give good square loops when the ®eld is parallel to the ®lm

surface. The loops are not as good in the perpendicular

direction as generally expected in ®lms. The saturation

magnetization values for the ®lms deposited on Si(1 0 0)

at 598 K are 30 and 21 emu/g in the parallel and perpendi-

cular directions, respectively. For the ®lms deposited on

glass substrates, the saturation magnetization values are 65

and 53 emu/g respectively in the parallel and perpendicular

directions.

3.2. Cu±Co granular films

We have prepared Cu±Co granular ®lms of the composi-

tion Cu82Co18 and Cu50Co50 on Si(1 0 0) substrates. The

composition of the ®lms was established by EDX analysis.

In Fig. 5(a) we show the XRD pattern of the as-deposited

®lms of Cu82Co18 at 673 K. On annealing the ®lm in

hydrogen at 923 K, we notice phase separation in the

Cu82Co18 ®lm as can be seen from the XRD pattern in

Fig. 5(b). In the case of the Cu50Co50 composition, there is

phase separation in the as-deposited ®lm (at 673 K) as can

be seen in Fig. 5(c). Such phase separation on annealing

Cu82Co18 is known in the literature [5].

Magnetic hysteresis loop measurements were carried out

on the Cu82Co18 and Cu50Co50 ®lms. In Fig. 6, we show

typical hysteresis loops obtained with the Cu50Co50 ®lm

deposited on Si(1 0 0) at 673 K. The loops obtained with the

®eld parallel and perpendicular to the ®lm are both shown in

Fig. 3. Electrical resistivity of Co films deposited at 673 K on (a) Si(1 0 0)

and (b) glass.

Fig. 4. Magnetic hysteresis loops of Co films deposited at 598 K on

Si(1 0 0) with field (a) parallel and (c) perpendicular; on glass substrate

with field (b) parallel and (d) perpendicular to the film surface.

Fig. 5. X-ray diffraction patterns of Cu±Co granular films deposited at

673 K (a) as-deposited and (b) hydrogen annealed at 923 K for the films of

Cu82Co18 and (c) as-deposited film of Cu50Co50. *denotes Si(2 0 0)

substrate peak.

48 S. Parashar et al. / Materials Chemistry and Physics 61 (1999) 46±49

the ®gure. In the parallel orientation, good square loops are

obtained. The magnetization values for the Cu50Co50 ®lm

deposited at 673 K are 29 and 28.5 emu/g respectively with

the ®eld parallel and perpendicular to the ®lm surface.

Magnetoresistance (MR) measurements were carried out

on both the Cu50Co50 and Co18Cu82 ®lms. The results are

presented in Fig. 7. We have obtained MR of 0.28% and

0.14% respectively for the Cu50Co50 and Cu82Co18 ®lms at a

®eld of 1.2 kOe at 300 K. This is quite satisfactory, con-

sidering that other workers have obtained around 3% MR at

a ®eld of 5 tesla or higher [6,7].

4. Conclusions

1. The present study shows that nebulized spray pyrolysis

in hydrogen atmosphere provides satisfactory ®lms of

cobalt as well as Cu±Co granular alloys.

2. The cobalt films thus prepared show good characteristics

with respect to magnetic and electrical properties.

3. The Cu±Co granular alloy films prepared by nebulized

spray pyrolysis show satisfactory magnetoresistive beha-

vior.

References

[1] M. Li, G.C. Wang, H.G. Min, J. Appl. Phys. 83(10) (1998) 5313±

5320.

[2] T. Maruyama, Jpn. J. Appl. Phys. 36(6A) (1997) L705±707.

[3] A.R. Raju, H.N. Aiyer, C.N.R. Rao, Chem. Mater. 7 (1995) 225±

231.

[4] H.N. Aiyer, A.R. Raju, C.N.R. Rao, Chem. Mater. 9 (1997) 755±759.

[5] J.R. Childress, C.L. Chien, Phys. Rev. B 4(10) (1991) 8089±8092.

[6] A.E. Berkowitz, J.R. Mitchell, M.J. Carey, A.P. Young, S. Zang, F.E.

Spada, F.T. Parker, A. Hutten, G. Thomas, Phys. Rev. Lett. 68(25)

(1992) 3745±3748.

[7] A.E. Berkowitz, J.R. Mitchell, M.J. Carey, A.P. Rao, D. Rao, A.

Starr, S. Zang, F.E. Spada, F.T. Parker, A. Hutten, G. Thomas, J.

Appl. Phys. 73(10) (1994) 5320±5325.

[8] W. Wang, F. Zhu, J. Weng, J. Xiao, W. Lai, Appl. Phys. Lett. 72

(1998) 1118±1120.

Fig. 6. Magnetic hysteresis loops of Cu50Co50 film deposited at 673 K on

Si(1 0 0) with field (a) parallel and (b) perpendicular to the surface of the

film.

Fig. 7. Field dependence of the magnetoresistance of the Cu50Co50 and

Cu82Co18 films deposited at 673 K on Si(1 0 0).

S. Parashar et al. / Materials Chemistry and Physics 61 (1999) 46±49 49