Journal of Magnetism and Magnetic Materials 234 (2001) 294–298

Giant magnetoresistance in Cu–Co films electrodepositedon n-Si

Gyana R. Pattanaik, Dinesh K. Pandya*, Subhash C. Kashyap

Indian Institute of Technology Delhi, Thin Film Laboratory, Department of Physics, Hauz Khas, New Delhi 110 016, India

Received 13 March 2001

Abstract

High quality Cu–Co alloy films with excellent metallic luster have been electrolytically deposited directly onto n-Si(1 0 0) substrate, thereby eliminating the need of a conducting seed layer, which is otherwise required when the filmswere grown on insulating substrates (Al2O3). The as-deposited Cu–Co films exhibit relatively higher magnetoresistance

(MR) in comparison with the as-deposited films on Al2O3 under identical conditions. The observed increase in MRcould be attributed to the reduced substrate current shunting. The MR further improves to 2.67% (at H ¼ 10 kOe) withvacuum annealing (at 4251C for 30min) of the films on Si. This has been ascribed to the separation of Cu and Co phases

resulting in a magnetic granular nanostructure. This value of MR of annealed films on Si is, however, lower incomparison with the value obtained for annealed films deposited on Al2O3. Glancing angle X-ray diffraction (GAXRD)has revealed the formation of copper silicide in these samples, which is responsible for the lower value of MR. Thus we

have observed good MR with a copper silicide host matrix. r 2001 Elsevier Science B.V. All rights reserved.

PACS: 75.70.Pa

Keywords: Giant magnetoresistance; Electrodeposition; Cu–Co; n-Si

Giant negative magnetoresistance (GMR) wasfirst discovered in antiferromagnetically coupledultrathin multilayers of ferromagnetic and non-magnetic metals [1]. Subsequently, granular me-tallic systems with magnetic particles embedded innon-magnetic metallic host were found to showthis effect [2,3]. Granular thin film systems havethe advantage of simplicity of fabrication over themultilayers. Of the various methods available for

the preparation of thin films exhibiting GMR,electrodeposition [4] is the simplest and leastexpensive. Electrodeposition becomes more attrac-tive due to its ability to deposit in geometrieswhere conventional deposition processes wouldfail. Recently, it has been used by some groups todeposit nanowires into the pores of nuclear tracketched polycarbonate membranes or anodic alu-minum oxide membranes [5], which are suitablefor current perpendicular plane (CPP) GMRinvestigations. The requirement of a conductingsubstrate for electrodeposition could be a draw-back of the process since during MR measurementit leads to current shunting in the current-in-plane

*Corresponding author. Tel.: +91-11-6591347; fax: +91-11-

6581114.

E-mail addresses: dkpandya@netearth.iitd.ac.in,

kant4p@hotmail.com (D.K. Pandya).

0304-8853/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 4 1 3 - 9

(CIP) geometry which reduces the signal inelectrodeposited thin films. The use of a thinconducting layer of a non-magnetic metal on aninsulating substrate has removed this drawback[6,7]. But this introduces an additional processingstep and does not overcome the problem of currentshunting completely. Electrodeposition of GMRthin films directly onto semiconducting substrates,such as suitably doped Si, is a better alternative asthe conductivity of these substrates could be goodenough to allow the electrodeposition and wouldnot short circuit during the transport measure-ments due to the relatively higher resistivity of Si.Moreover, electrodeposition of GMR thin filmsdirectly onto Si could lead to the easy integrationof GMR sensors with Si-based electronics. Therehave been a few reports of electrodeposition ofmultilayer GMR thin films and spin valvestructures directly onto n-Si [8] and n-GaAs [9],respectively. In this letter we present our study ofmagnetoresistance in Cu–Co alloy films electro-deposited directly onto n-Si (1 0 0) substrateseliminating the need of a conducting underlayer.

Single side polished n-Si (1 0 0) wafers were usedas substrates. The substrates were subjected toroutine cleaning followed by a 30 sec dip in 10%HF solution to remove the oxide layer. The back(unpolished) side of the substrate was painted withconducting silver paste and dried with a hot airblower for electrical contact. Acid resistant ad-hesive tape was used to mask off all the substrateexcept for the deposition area. Galvanostaticelectrodeposition was carried out from a citrateelectrolyte containing 0.072 M each of CoSO4,7H2O and CuSO4, 5H2O. Analytical grade re-agents from Merck were used for the preparationof the electrolytic solution in filtered deionizedwater. The pH of the solution was increased to 6.0with NH4OH. A two-electrode configuration wasused: n-Si (1 0 0) as cathode and a Pt plate asanode. A current density of 3mA/cm2 wasmaintained between the two electrodes. Thedeposition was carried out at room temperature(B201C). The deposited films were vacuumannealed at 5� 10�6 Torr at different temperaturesin the range 200–4501C. The thickness of the filmswas measured using a Talystep (Taylor Hobson,UK). All samples reported in this study were

350 nm thick. The chemical compositional analysisof the films was carried out by using an atomicabsorption spectrometer (ECIL AAS-4129). Theconstituent phases and the crystallographic struc-ture of the films were investigated by Glancingangle X-ray diffraction (GAXRD), using a dif-fractometer with Cu Ka radiation obtained from a4 KW (40 KV, 100 mA) rotating anode (modelGiegerflex-D/max-RB-RU200, Rigaku, Japan).Room temperature magnetoresistance measure-ment in four-terminal van der Pauw geometrywas carried out using a Keithley 224 program-mable current source and a Keithley 181 nano-voltmeter at magnetic field up to 10 kOe. Both thecurrent and the magnetic field were in the plane ofthe film and parallel to each other.

High quality thin films of composition Cu80Co20

with excellent metallic luster were obtained on n-Si(1 0 0) by electrodeposition. In order to obtain agranular magnetic structure the as-deposited filmwas vacuum annealed at 5� 10�6 Torr. Theannealing helps in the segregation of Co, dissolvedin the Cu. Fig. 1 shows the GAXRD pattern of anas-deposited film. The XRD pattern resembles thatof a polycrystalline single phasic FCC structuresimilar to Cu with preferential orientation along(1 1 1). The lattice parameter, calculated from thepeak search results, is found to be 3.601 (AA. Thelattice parameter of pure Cu (FCC) is 3.615 (AA andthat of pure Co (FCC) is 3.544 (AA. The equilibriumphase diagram of Cu–Co binary system showsvirtual immiscibility of Cu and Co in each other

Fig. 1. Glancing angle X-ray diffraction pattern of a Cu80Co20

film directly electrodeposited on n-Si (1 0 0).

G.R. Pattanaik et al. / Journal of Magnetism and Magnetic Materials 234 (2001) 294–298 295

[10,11]. The observation of single phasic FCCpattern with an in between lattice constant thusimplies that the as-deposited film consists of ametastable Cu–Co alloy. It has already beenestablished in our previous works that the electro-deposited Cu–Co film is a metastable alloy [7]exhibiting a solid solution-like behaviour (Ve-gard’s law) [6,12,13], although some phase separa-tion occurs in the as-deposited films as revealed byTEM and magnetization studies [7,12].

As the annealing temperature (TA) is increasedbeyond 2001C the initiation of a reaction is visiblein the film. The formation of copper silicide hasbeen reported to initiate in the annealing tempera-ture range of 200–2501C by rapid thermal anneal-ing [14]. However in the case of Co–Si system,CoSi forms at B4501C whereas other cobaltsilicide phases form at still higher temperatures[15]. Fig. 2 shows the sheet resistance of ourelectrodeposited Cu80Co20/n-Si(1 0 0) films an-nealed at temperatures in the range 200–4501Cfor 30 min each. The sheet resistance was measuredafter cooling the sample to room temperature. Thecurve has three regions with distinct change inslopes at B2001C and B4501C respectivelycorresponding to the formation temperatures ofCu–Si and Co–Si. Copper silicide formation seemsto be complete by 4251C and for TA > 4251C theincrease in sheet resistance could be due to the

formation of cobalt silicide. Fig. 3 shows thetemperature dependence of resistance of as-depos-ited and 4251C annealed films. The resistance ofthe film is observed to increase by a factor of 4upon annealing, which is ascribed to the higherresistivity of copper silicide. The slope of the RBTcurve is also increased by an order of magnitudeon annealing indicating the formation of an alloyphase.

Fig. 4 shows the GAXRD pattern of a filmannealed at 4251C for 30 min. The d-spacingsalongwith the relative intensities of the peaksobserved in XRD pattern are given in Table 1. Acollection of d-spacings observed for differentphases of copper silicides [17,18] are also presented

Fig. 2. Room temperature sheet resistance, measured at zero

field, for Cu80Co20/n-Si (1 0 0) films annealed at temperatures in

the range 200–4501C for 30min.

Fig. 3. Temperature dependence of resistance (at zero field) of

as-deposited and 4251C annealed Cu80Co20 films. Upon

annealing the resistance increases by a factor of 4 and the

slope of the RBT curve increases by an order of magnitude.

G.R. Pattanaik et al. / Journal of Magnetism and Magnetic Materials 234 (2001) 294–298296

in Table 1. A comparison of the observed d-spacings with those of the various copper silicidephases clearly shows the presence of Cu4Si andCu3Si as dominant phases. Annealing of Cu–Coalloy films leads to the segregation of Co, thus fora film annealed at 4251C for 30 min a granularstructure with Co particles dispersed in a coppersilicide matrix is a more appropriate model.

The magnetoresistance of the films was calcu-lated as

MR% ¼ ½ðRH � RH¼0Þ=RH¼0��100:

The as-deposited Cu80Co20 films on n-Si(1 0 0)showed a maximum magnetoresistance of 1% in amagnetic field of 10 kOe (Fig. 5). The correspond-ing magnitude of magnetoresistance for a similarfilm deposited on Cu-coated alumina was 0.6%.The higher value of the magnetoresistance in thecase of the film deposited on n-Si substrate isattributed to the relatively higher resistivity of Si incomparison with the coated Cu layer on Al2O3.

Magnetoresistance ratios for various annealedsamples are also shown in Fig. 5. The observednegative magnetoresistance shows a behavioursimilar to that of the as-deposited samples. For afilm annealed at 3001C for 1 h the magnitude of theMR increases to 2.24%. Maximum magnetoresis-tance of 2.67% is observed for a film annealed at4251C for 30 min. These magnetoresistance valuesare lower than those for the films deposited on Cu-

coated alumina substrates (B4%). The magne-toresistance of magnetic granular films is directlyproportional to the electronic mean free path forthe matrix [16]. As the resistance of the coppersilicide is larger than that of pure copper [14], theelectronic mean free path for the copper silicidematrix is less than that for copper matrix. Hence areduced MR is observed for the annealed films onn-Si relative to those on Cu-coated alumina.Further increase in annealing temperature to4501C for 30 min led to the lowering of MR to2.53%. On increasing annealing temperature

Fig. 4. Glancing angle X-ray diffraction pattern of a Cu80Co20

film electrodeposited on n-Si (1 0 0) and vacuum-annealed at

4251C for 30min.

Table 1

Diffraction spacings and relative intensities of the annealed Cu–

Co/n-Si film. A collection of d-spacings reported for different

phases of copper silicides are also presented. All d-spacings are

in units of (AA

Dobs I=I0 Cu4Si [18] I=I0 Cu3Si [17] Cu15Si4 [18] I=I0

3.483 28 3.54 vw

3.167 88 3.16 3 3.19 vw 3.07 40

2.816 23 2.80 3

2.68

2.583 24 2.6 80

2.443 30 2.45w 2.43 40

2.233 29 2.24 3 2.34

2.122 40 2.1 100 2.13m

2.09

2.065 93 2.08 25 2.07 2.07 100

2.036 100 2.03 st

1.987 58 1.97 31 1.95 1.98 60

1.892 20 1.9 25 1.91 80

1.85 24 1.83 10

1.805 40

1.76 34 1.75 17 1.76w 1.77 20

1.66 3 1.72 40

1.67 40

1.59 1.58 60

1.559 16 1.5 40

1.532 13

1.43m 1.43 20

1.37 40

1.35 80

1.334 11 1.32

1.3 40

1.281 10 1.29 5 1.28 40

1.257 12 1.26 5

1.234 11 1.21 31 1.23w 1.23

1.189 14 1.19 3 1.18

1.165 27 1.17 3 1.17m 1.16

1.152 10 1.15 8 1.15

G.R. Pattanaik et al. / Journal of Magnetism and Magnetic Materials 234 (2001) 294–298 297

beyond 4501C or increasing annealing time at4501C a rapid decrease in MR is observed. Thereason for this MR lowering could be due to theformation of non-ferromagnetic cobalt silicide,which is known [15] to be formed at 4501C.

In conclusion, we have prepared high qualityCu–Co alloy films by direct electrodeposition ontothe semiconducting n-Si (1 0 0) substrates eliminat-ing the need of any conducting seed layer. Theas-deposited films show relatively larger magne-toresistance in comparison with the films depositedon Cu/Al2O3. This is ascribed to the minimizationof current shunting because of substrate conduc-tivity. MR improves with vacuum annealing of theas-deposited metastable Cu–Co alloy films becauseof phase separation. Despite the silicide formationon annealing, observed by GAXRD studies, thenegative magnetoresistive behavior in the annealed

films is maintained. Thus, ferromagnetic (Co)particles now dispersed in a copper silicide hostis observed to exhibit a good MR value of 2.67%though slightly lower in comparison with those onCu-coated alumina substrates. This is ascribed tothe lower conductivity of the copper silicidematrix.

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Fig. 5. Magnetoresistance ratios as a function of applied

magnetic field of Cu80Co20 films electrodeposited on (a) n-Si

(1 0 0): (i) as-deposited, (ii) annealed at 3501C for 1 h, (iii)

annealed at 4251C for 30min, (iv) annealed at 4501C for 15min,

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30min.

G.R. Pattanaik et al. / Journal of Magnetism and Magnetic Materials 234 (2001) 294–298298