Electroless Ni Co P Ternary Alloy Deposits Preparation and

Surface and Coatings Technology 172(2003) 298–307

0257-8972/03/$ - see front matter� 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0257-8972(03)00315-3

Electroless Ni–Co–P ternary alloy deposits: preparation andcharacteristics

T.S.N. Sankara Narayanan *, S. Selvakumar , A. Stephena, b b

National Metallurgical Laboratory, Madras Centre, CSIR Complex, Taramani, Chennai 600 113, Indiaa

Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai 600 025, Indiab

Received 2 December 2002; accepted in revised form 31 January 2003

Abstract

Electroless Ni–Co–P ternary alloy deposits were prepared by varying the metallic ratio and were characterized using X-raydiffraction (XRD), differential scanning calorimetry(DSC), thermogravimetric analysis(TGA) and vibration sample magnetom-eter. The plating rate of electroless Ni–Co–P deposits is a function of concentration of sodium hypophosphite, pH of the platingbath, plating time and the metallic ratio. With increase in metallic ratio, the cobalt content of the deposits increases with asimultaneous decrease in the nickel content, while the phosphorus content decreases slightly. The electroless Ni–Co–P depositsof the present study are amorphous in their as-deposited condition. The DSC trace exhibits three distinct exothermic peaks,corresponding to the relaxation of lattice strain during the phase separation, the phase transformation of amorphous phase tonickel and nickel phosphide phases and the transformation of metastable phases to stable nickel phosphide phase. The XRDpattern of electroless Ni–Co–P deposits confirms the formation of Ni, Ni P , Ni P and Ni P phases during annealing at 3005 2 12 5 3

and 4008C for 1 h. Thermomagnetic study exhibits the Curie transition of nickel and non-stoichiometric Ni Co based alloys.3

Being amorphous in nature, the electroless Ni–Co–P deposits exhibit soft magnetic characteristics. The saturation magnetization,remanence and coercivity increase with cobalt content of the deposit.� 2003 Elsevier Science B.V. All rights reserved.

Keywords: Electroless deposition; Ni–Co–P alloy; Phase transition; X-ray diffraction

1. Introduction

Electroless deposition process experienced numerousmodifications to meet the challenging needs of a varietyof industrial applications since Brenner and Riddellinvented the process in 1946w1x. Development ofelectroless nickel polyalloy deposits is considered as themost effective method to alter the chemical and physicalproperties of binary Ni–P and Ni–B alloy depositsw2–4x. The choice of the additional element is made basedon the chemicalyphysical property to be imparted in thedeposit. Cobalt is considered to be the most commonadditional element for imparting magnetic properties inthe depositsw5,6x. Although a variety of techniques areavailable for preparing magnetic films, electroless plat-

*Corresponding author. Tel.:q91-44-254-2077; fax:q91-44-254-1027.

E-mail address: [email protected](T.S.N. Sankara Narayanan).

ing is found to be the most suitable method because ofits ability to provide a uniform surface and the cost-effectiveness of the processw7x. Electroless Co–Ni–Palloy films were studied for their use as a thin filmmagnetic recording mediaw6–11x. However, the mag-netic properties of the electroless Co–Ni–P films strong-ly depend on their thickness and microstructurew8,12,13x. The microstructure of electroless Co–Ni–Pfilms is, in turn, dependent on the chemical speciespresent in the plating bath and the operating conditionsemployedw8,14–16x. The dependence of magnetic prop-erties on the microstructure enables the possibility todeposit electroless Ni–Co–P films with different mag-netic properties from the same solution by changing theconditions of deposition. The use of programmed vari-ation of rotational speed of the substrate and simulta-neous electrolysis during electroless plating weresuccessfully adopted to manipulate the magnetic prop-erties of electroless Ni–Co–P filmsw17–20x. In this

299T.S.N. Sankara Narayanan et al. / Surface and Coatings Technology 172 (2003) 298–307

Table 1Bath composition and operating conditions employed for preparingelectroless Ni–Co–P ternary alloy deposits

Type of deposit Electroless Ni–Co–P

Sample designation NCP3 NCP5 NCP7Metallic ratio 0.3 0.5 0.7

Bath compositionNiSO Ø6H O4 2 0.035 M 0.025 M 0.015 MCoSOØ7H O4 2 0.015 M 0.025 M 0.035 MNa C H O Ø2H O3 6 5 7 2 0.15 M 0.15 M 0.15 MNa H POØH O2 2 2 2 0.30 M 0.30 M 0.30 MNH Cl4 0.50 M 0.50 M 0.50 M

Operating conditionspH 9.0 9.0 9.0Temperature 80"1 8C 80"1 8C 80"1 8CTime 60 min 60 min 60 min

Deposit compositionNickel (wt.%) 60.00% 51.40% 39.70%Cobalt(wt.%) 31.00% 40.00% 52.20%Phosphorus(wt.%) 9.00% 8.60% 8.10%

Fig. 1. Effect of sodium hypophosphite concentration on the plating rate of electroless Ni–Co–P ternary alloy deposit(CoSOØ7H Os0.025 M;4 2

NiSO Ø6H Os0.025 M; Na C H Os0.15 M; NH Cls0.50 M; pH 9.0; Temperatures80 8C; Times60 min).4 2 3 6 5 7 4

perspective, the present work aims to prepare electrolessNi–Co–P ternary alloy deposits, and to evaluate theircharacteristic properties and magnetic behaviour.

2. Experimental details

Nickel sulphate hexahydrate and cobalt sulphate hep-tahydrate were used as the source of nickel and cobalt,respectively. Sodium hypophosphite was used as thereducing agent, which also forms the source of phos-phorus in the deposit. Trisodium citrate was used as the

complexing agent to control the rate of release of freemetal ion for the reduction reaction. In addition to otherconstituents, ammonium chloride was added to impartbuffering action. During plating, the bath was main-tained at a temperature of 80"1 8C by a constanttemperature bath. Besides, the pH of the bath was alsomaintained constant during the plating process with theaddition of NaOH. Electroless Ni–Co–P ternary alloydeposits having different amounts of nickel, cobalt andphosphorus were prepared by varying metallic ratio(CoSOyCoSOqNiSO ), viz., 0.3, 0.5 and 0.7. The4 4 4

phosphorus content of the deposit was determined usingatomic absorption spectrophotometer. The cobalt contentof the deposit was determined spectrophotometricallyusing nitroso-R-salt as the reagent whereas the nickelcontent of the deposit was determined after precipitatingnickel as Ni–dimethyl glyoxime complex. The bathcomposition, operating conditions employed for prepar-ing electroless Ni–Co–P ternary alloy deposits and thedeposit composition are given in Table 1. The electrolessNi–Co–P ternary alloy deposits prepared using differentmetallic ratios, viz., 0.3, 0.5 and 0.7 were designated as‘NCP3’, ‘NCP5’ and ‘NCP7’, respectively. These depos-its were characterized by X-ray diffraction(XRD),differential scanning calorimetry(DSC), thermomagnet-ic measurement and vibrating sample magnetometer(VSM) to determine their crystal structure, phase trans-formation behaviour, Curie transition and magnetic prop-erties, respectively. XRD patterns were obtained using acopper target(ls1.540598 A) in Rich Seifert(model˚3000) XRD. The DSC traces were recorded using aPerkin-Elmer DSC-7 in the temperature range of 220–

300 T.S.N. Sankara Narayanan et al. / Surface and Coatings Technology 172 (2003) 298–307

Fig. 2. Effect of pH of the plating bath on the plating rate of electroless Ni–Co–P ternary alloy deposit(CoSOØ7H Os0.025 M; NiSOØ6H Os4 2 4 2

0.025 M; Na C H Os0.15 M; NH Cls0.50 M; NaH POs0.30 M; Temperatures80 8C; Times60 min).3 6 5 7 4 2 2

Fig. 3. Effect of plating time on the plating rate of electroless Ni–Co–P ternary alloy deposit(CoSOØ7H Os0.025 M; NiSOØ6H Os0.025 M;4 2 4 2

Na C H Os0.15 M; NH Cls0.50 M; NaH POs0.30 M; pH 9.0; Temperatures80 8C).3 6 5 7 4 2 2

520 8C at a heating rate of 108Cymin in argonatmosphere. The thermomagnetic study was performedusing a Perkin Elmer TGA-7 in the temperature rangeof 150–8008C at a heating rate of 108Cymin in argonatmosphere in the presence of a horseshoe magnet. Thenative weight of the electroless Ni–Co–P alloy depositsis first made as 0 and the weight due to the magneticfield is taken as the weight of the deposit, which isequated to 100% weight so that the 100% weight isentirely due to magnetic attraction. The magnetic prop-

erties, viz., saturation magnetic moment(s ), remanences

(M ) and coercivity(H ), were determined using EG&Gr c

Princeton Applied Research VSM(model 4500).

3. Results and discussion

The plating rate of electroless Ni–Co–P alloy depositas a function of concentration of sodium hypophosphite(0.1–0.3 M), pH of the plating bath(8.0–11.0), platingtime (10–60 min) and the metallic ratio(CoSOy4


Fig. 4. Effect of metallic ratio(CoSOy(CoSOqNiSO )) on the plating rate of electroless Ni–Co–P ternary alloy deposit(Na C H Os0.154 4 4 3 6 5 7

M; NH Cls0.50 M; NaH POs0.30 M; pH 9.0; Temperatures80 8C; Times60 min).4 2 2

Fig. 5. Effect of metallic ratio(CoSOy(CoSOqNiSO )) on the chemical composition of the electroless Ni–Co–P ternary alloy deposit.4 4 4

CoSOqNiSO ) is depicted in Figs. 1–4, respectively.4 4

The plating rate increases with increase in sodiumhypophosphite concentration in the range from 0.1 to0.3 M (Fig. 1). However, the extent of increase is notlinear throughout. This is due to the fact that eventhough the available electrons for metal ion reductionincreases with increase in concentration of sodium hypo-phosphite, the rate at which the availability of the metalion is limited by the amount of complexing agent presentin the bath.

The plating rate is found to increase with increase inpH from 8.0 to 10.0, beyond which, the bath becomesdestabilized and approximately pH 11.0, decompositionof the bath is quite evident(Fig. 2). Hence, it is saferto use the bath approximately pH 9.0, where there isgood plating rate as well as the stability of the bath.The plating rate increases with increase in processing

time throughout the 60 min of plating(Fig. 3). This canbe ascribed to the autocatalytic nature of the electrolessplating process. However, the extent of increase in


Fig. 6. XRD pattern of electroless Ni–Co–P ternary alloy deposits intheir as-plated condition(a) NCP3;(b) NCP5; and(c) NCP7.

plating rate observed for every 10 min period is notlinear. This is due to the build-up of orthophosphite ionsfollowing oxidation of sodium hypophosphite.The plating rate of electroless Ni–Co–P deposit

decreases with increase in metallic ratio(CoSOy4CoSOqNiSO ) (Fig. 4). This behaviour is attributed4 4

to the fact that nickel possesses a higher catalytic activitythan cobalt. Matsubara and Yamadaw18x and Kim et al.w7x have also reported a decrease in plating rate withincrease in cobalt content in the plating bath. Theseresults indicate that it is possible to vary the compositionof the alloy deposit by varying the metallic ratio. Basedon these results Ni–Co–P ternary alloy deposits withdifferent metallic ratios are produced and characterized.The nickel, cobalt and phosphorus content of the

electroless plated Ni–Co–P ternary alloy deposit as afunction of metallic ratio(CoSOyCoSOqNiSO ) is4 4 4

given in Fig. 5. With increase in metallic ratio, the

cobalt content of the deposit increases with a simulta-neous decrease in the nickel content. Younan et al.w21xalso reports a similar trend. Tarozaite and Jusysw9x havealso observed the dependence of composition of theelectroless Ni–Co–P deposit withwCo x:wNi x ratio.2q 2q

There is only a slight change in phosphorus content ofthe electroless Ni–Co–P deposit with increase in metal-lic ratio. The phosphorus content of electroless Co–Ni–P deposit increases with an increase in nickel sulphateconcentration of the plating bath and a correspondingincrease in nickel content of the deposit. This hasresulted due to the comparatively higher phosphoruscodeposition with nickel than with cobaltw9x.XRD patterns of electroless Ni–Co–P ternary alloy

deposits, prepared using a metallic ratio(CoSOy4CoSOqNiSO ) of 0.3, 0.5 and 0.7(NCP3, NCP5 and4 4

NCP7, respectively) in their as-deposited condition areshown in Fig. 6. A broad peak centered at approximately458 2u is evident in all the three samples studied, whichindicates the amorphous nature of the deposits. Inelectroless deposition process, the extent of segregationof metalloid alloy(phosphorus or boron) in the coatingdetermines its crystallinity. Since the required amountof phosphorus segregation(8.1–9.0 wt.% for samplesNCP3, NCP5 and NCP7) is relatively large, nucleationof nickelycobalt phase is prevented, and this has resultedin amorphous structure. The broad peak could eitherdue to hcp-Co(0 0 2) or the fcc-Ni(1 1 1).The phase transformation behaviour of electroless Ni–

Co–P ternary alloy deposits was studied using DSC.The DSC trace obtained for electroless Ni–Co–P depos-its (NCP3, NCP5 and NCP7) at a heating rate of 108Cymin in argon atmosphere is depicted in Fig. 7. TheDSC trace exhibits three distinct exothermic peaks inthe temperature range of 260–4008C.It has been established that the peak temperature is

very sensitive to the phosphorus contentw22–24x. Sur-prisingly, for electroless Ni–Co–P ternary alloy depositsof the present study there is not much change in thephosphorus content. However, it is interesting to notethat with increase in cobaltydecrease in nickel contentof the deposit, there observed to be a small change inpeak temperature for Peak I whereas a decrease in peaktemperature is noted for Peaks II and III. Based on thisobservation, first exothermic peak(Peak I for NCP3,NCP5 and NCP7) could be either due to structuralrelaxation or due to the separation of fcc nickel fromthe amorphous matrix, whereas, the second exothermicpeak with peak temperature at approximately 329–3378C (Peak II for NCP3, NCP5 and NCP7) might be dueto the phase transformation of amorphous phase tocrystalline nickel and nickel phosphide phases, and, thethird exothermic peak with peak temperature at approx-imately 347–3838C (Peak III for NCP3, NCP5 andNCP7) may possibly be due to the transformation ofmetastable to stable phase.


Fig. 7. DSC trace of electroless Ni–Co–P ternary alloy deposits(a) NCP3;(b) NCP5; and(c) NCP7.

To ascertain the phases formed at the respective peaktemperatures, electroless Ni–Co–P deposits wereannealed at 300 and 4008C for 1 h and the correspond-ing XRD patterns are shown in Figs. 8 and 9, respec-tively. It is evident from Fig. 8 that the amorphousnature of the deposits still remains, even after annealingat 300 8C for 1 h. Hence, the first exothermic peak(Peak I for NCP3, NCP5 and NCP7) is due to therelaxation of lattice strain during the phase separation.The XRD pattern of electroless Ni–Co–P depositsannealed at 4008C for 1 h reveals the presence of fccnickel and bct nickel phosphide(Ni P) phases. Based3

on this, the exothermic peak with peak temperature atapproximately 329–3378C (Peak II for NCP3, NCP5and NCP7) could be attributed to the phase transfor-mation of amorphous phase to nickel and nickel phos-phide phases. The exothermic peak with peaktemperature approximately 347–3838C (Peak III forNCP3, NCP5 and NCP7) could not be accounted forrecrystallization and growth of Ni P phase as such an3

occurrence is expected only approximately 4008C w20x,and hence, is due to the transformation of metastablephases to stable nickel phosphide phase. XRD patternof NCP3 annealed at 3008C for 1 h reveals the presenceof hexagonal Ni P phase(Fig. 8). The presence of5 2

tetragonal Ni P phase is evident even after annealing12 5

at 400 8C for 1 h (NCP3, NCP5 and NCP7) (Fig. 9).

Lee and Hur w25x and Hur et al. w26x reported theformation of Ni P phase upon annealing of electroless5 2

Ni–Co–P and Ni–Cu–P deposits at 3258C. Yo et al.w27x have also observed the formation of Ni P and5 2

Ni P phases during the phase transformation of elec-12 5

troless Ni–18.0Cu–9.3P deposits upon annealing. Theelectroless Ni–Co–P deposits of the present study havea high phosphorus content(Fig. 5), which enables ahigher phosphorus segregation in the grain boundaryregion. During annealing the high phosphorus region isfurther increased by the extraction of phosphorus atomsdissolved in nickel grains, which in turn gave rise toprecipitation of the Ni P and Ni P phases. These5 2 12 5

metastable phases get transformed into stable Ni P phase3

at higher temperatures, supporting the observations madeearlier by Lee and Hurw25x, Hur et al. w26x and Yu etal. w27x. The various stages of phase transformation ofthe electroless Ni–Co–P deposit are depicted in Fig.10.The thermogravimetric analysis(TGA) trace of the

electroless Ni–Co–P ternary alloy deposit(NCP7) uponannealing at a heating rate of 108Cymin in argonatmosphere in presence of a magnetic field is shown inFig. 11. The initial weight gain is not a real weight gainbut a magnetic flux gain following the removal ofoccluded hydrogen in the electroless Ni–Co–P ternaryalloy deposit. The first Curie transition noted at 326.69


Fig. 8. XRD pattern of electroless Ni–Co–P ternary alloy depositsheat-treated at 3008C for 1 h (a) NCP3;(b) NCP5; and(c) NCP7.

Fig. 9. XRD pattern of electroless Ni–Co–P ternary alloy depositsheat-treated at 4008C for 1 h (a) NCP3;(b) NCP5; and(c) NCP7.

Fig. 10. Schematic of the sequential transformation of phases of electroless Ni–Co–P deposits following heat treatment at 300, 400 and)4008C for 1 h.


Fig. 11. TGA trace of electroless Ni–Co–P ternary alloy deposit(NCP7) obtained in presence of magnetic field.

Table 2Curie transitions of electroless Ni–Co–P ternary alloy deposits

Sample First Curie transition(T )c1 Second Curie transition(T )c2designation (8C) (8C)

NCP3 315 625NCP5 317 607NCP7 327 529

8C is the Curie transition(T ) of nickel. T for standardc c

nickel is 354 8C and for electrodeposited nickel it isapproximately 3208C. The decrease in magnetic fluxafter the first Curie transition is due to the formation ofnon-magnetic phases, viz., Ni P , Ni P , Ni P, etc. The5 2 12 5 3

DSC and XRD studies confirm the formation suchphases. The second Curie transition is noted above 5208C and this Curie transition increases with increase incobalt content of the deposit(Table 2). This may bedue to the non-stoichiometric Ni Co and Ni–Co based3

alloys. After the second Curie transition, there observedto be a gain in the magnetic flux. This may be due toNi–Co alloy formation. Further studies are required toascertain the nature of alloy formed under suchconditions.The magnetic properties of electroless Ni–Co–P ter-

nary alloy deposits were studied in their as-depositedcondition. The hysteresis loop obtained for as-plated

electroless Ni–Co–P deposits(NCP3, NCP5 and NCP7)is shown in Fig. 12. The magnetic properties, viz.,saturation magnetic moment(s ), remanence(M ) ands r

coercivity (H ), derived from the hysteresis loop, arec

given in Table 3. The shape of the hysteresis loopsseems to be very similar to that exhibited by partiallyor totally amorphous materialsw28x. Fenineche et al.w29x have also observed a similar type of narrowhysteresis loop for Ni–Co–P deposits electrodepositedat 2 Aydm , which has an amorphous structure. It is2

well known that magnetic characteristics of amorphousmaterials are significantly smaller than those of crystal-line materials w30x. Being amorphous in nature, theelectroless Ni–Co–P deposits of the present study exhib-it soft magnetic characteristics. According to Tarozaiteet al. w19x the formation of certain crystallites shape,which determines high coercivity, takes place only atdefinite phosphorus content. The phosphorus content,


Fig. 12. Hysteresis loop obtained for electroless Ni–Co–P ternaryalloy deposits in their as-plated condition(a) NCP3; (b) NCP5; and(c) NCP7.

Table 3Magnetic properties of electroless Ni–Co–P ternary alloy deposits

Sample Saturation Remanence(M )r Coercivity (H )cdesignation magnetization(s )s (emuyg) (Oe)

(emuyg)

NCP3 11.54 3.16 67.62NCP5 28.10 7.74 64.65NCP7 59.41 20.03 63.70

necessary for the formation of grains boundaries, differsfor the films, deposited under different conditions,because of the grains with different size and shapeformation. According to them, electroless Ni–Co–Pdeposits exhibit a high coercivity when the phosphoruscontent of the deposit lies between 4 and 6 wt.%. Whenthe phosphorus content is lower than 4 wt.%, films ofcoarse crystallites are deposited, resulting in lower coer-civity. If phosphorus content exceeds 6 wt.%, filmsconsisting of very fine grains are deposited which alsoleads to a decrease in coercivity.A comparison of the magnetic characteristics of elec-

troless Ni–Co–P deposits of the present study revealsthat the saturation magnetization and remanence arefound to increase with cobalt content of the deposit(Table 3). Rivero et al.w31x report a linear increase inmagnetic moment with increase in cobalt content forelectrodeposited Ni–Co–P amorphous ribbons. Anincrease in saturation magnetization of electroless Ni–Co–P deposits with increase in cobalt content of thedeposit was also reported by Matsubara and Yamadaw18x.

4. Conclusions

The plating rate of electroless Ni–Co–P deposits is afunction of concentration of sodium hypophosphite, pHof the plating bath, plating time and the metallic ratio.With increase in metallic ratio, the cobalt content of thedeposits increases with a simultaneous decrease in thenickel content, while the phosphorus content decreasesslightly. Electroless Ni–Co–P deposits are amorphousin their as-deposited condition. The DSC trace exhibitsthree distinct exothermic peaks, corresponding to therelaxation of lattice strain during the phase separation,the phase transformation of amorphous phase to nickeland nickel phosphide phases and the transformation ofmetastable phases to stable nickel phosphide phase. TheXRD pattern of electroless Ni–Co–P deposits confirmsthe phases formed during annealing at 300 and 4008Cfor 1 h. Thermomagnetic study exhibits the Curietransition of nickel and non-stoichiometric Ni Co and3

Ni–Co based alloys. Being amorphous in nature, theelectroless Ni–Co–P deposits of the present study exhib-it soft magnetic characteristics. The saturation magneti-zation, remanence and coercivity are found to increasewith cobalt content of the deposit.

Acknowledgments

The authors express their sincere thanks to Prof. S.P.Mehrotra, Director, National Metallurgical Laboratory,Jamshedpur, Dr S. Srikanth, Scientist-in-Charge, NMLMadras Centre, CSIR Complex, Chennai and Prof. P.R.Subramanian, Professor and Head, Department of Nucle-ar Physics, University of Madras, Guindy Campus,Chennai, for their constant support and encouragementto carry out this work.

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Electroless Ni Co P Ternary Alloy Deposits Preparation and