Electrodeposited Ni–Co films from electrolytes with different Co contents

Applied Surface Science 258 (2012) 4005–4010

Contents lists available at SciVerse ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Electrodeposited Ni–Co films from electrolytes with different Co contents Ali Karpuz a,∗ , Hakan Kockar a , Mursel Alper b , Oznur Karaagac a , Murside Haciismailoglu b a b

Physics Department, Science and Literature Faculty, Balikesir University, Balikesir, Turkey Physics Department, Science and Literature Faculty, Uludag University, Bursa, Turkey

a r t i c l e

i n f o

Article history: Received 5 October 2011 Received in revised form 16 December 2011 Accepted 17 December 2011 Available online 29 December 2011 Keywords: Cobalt alloys Magnetic analysis Magnetoresistance Structural analysis

a b s t r a c t The properties of electrodeposited Ni–Co films produced from electrolyte consisted of nickel sulfamate, cobalt sulfate and boric acid were investigated as a function of Co content in the films. The compositional analysis performed by an energy dispersive X-ray spectroscopy demonstrated that the Co content of the films increases as the cobalt sulfate concentration in the electrolyte increases. The anomalous codeposition behavior was observed for all concentrations. The crystal structure was analyzed using an X-ray diffraction technique. The face centered cubic (fcc) structure was observed in the films containing from 0 at.% Co to 58 at.% Co. For the higher atomic Co contents (64 at.% and 80 at.%), a mixed phase of dominantly fcc and hexagonal closed packed (hcp) structure was observed although the (10.0) and (10.1) hcp peaks had minor intensities in the patterns. Surface micrographs obtained from a scanning electron microscope revealed that the film surface has a rougher appearance as the Co content increases. Magnetic measurements showed that the saturation magnetization gradually increased with increasing Co content of the films. The coercivity, Hc can be controlled by the structural parameters such as average grain size and crystal structure. The results also indicated that the optimum film composition was 28–40 at.% Co since the lower Hc and higher magnetoresistance (MR) values with very smooth or slightly granular surfaces were achieved at this Co content. It is revealed that Co content has an important effect on structural, magnetic and MR properties of the Ni–Co films. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The interest in magnetic alloys has increased in the recent years due to their specific features ranging from recording head materials to decorative purposes [1,2]. To produce Ni–Co films, there are many growth processes such as molecular beam epitaxy and sputtering that require high vacuum condition. Besides, the films can also be prepared by electrodeposition technique [3,4], which does not need any vacuum system [5]. It also has rapid production, low cost and easy control of deposition parameters [6]. Electrodeposition of films is strongly dependent on deposition parameters such as electrolyte composition, electrolyte pH, deposition potential, etc. Thus, changes in the experimental parameters may result in important changes in film properties [7,8]. Many studies were achieved about Ni–Co alloys in recent the years. Whereas study [9] has aimed to investigate the tribological properties of Ni–Co alloys with different compositions, the other studies have dealt with influences of electroplating parameters in Ni–Co films produced from an acid chloride–sulfate bath [10], effect

of glycine [11], effect of pulse frequency and current density [12], and influence of different anion types [13] on properties of Ni–Co films. Also, these studies used several additives in electrochemical bath in addition to boric acid to improve electrolyte or deposit conditions and generate proper film structures. Besides, to our knowledge there are very limited studies based on the research of structural and magnetic properties of Ni–Co films deposited using a mixture of sulfamate and sulfate bath in the literature. In this study, structural, magnetic and magnetoresistance (MR) properties of the electrodeposited Ni–Co films were investigated as a function of Co concentration in sulfamate–sulfate electrolyte. Except for boric acid, no additive was used to get coherent and quality Ni–Co structures. The results showed that the characterized properties were strongly affected by different Co contents in the film, which can be controlled by the Co concentration in the electrolyte. Also, optimum film composition for experimental conditions applied in this study was determined by considering obtained film properties.

2. Experimental

∗ Corresponding author at: Balıkesir Üniversitesi Fen Edebiyat Fakültesi C¸a˘gıs¸ Yerles¸kesi 10145 Balıkesir, Türkiye. Tel.: +90 266 6121278; fax: +90 266 6121215. E-mail address: [email protected] (A. Karpuz). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.12.088

Electrodeposition was carried out at room temperature (24 ± 2 ◦ C) and in a three electrode cell using EGG-362 model potentiostat/galvanostat under potential control conditions. While fixing the nickel sulfamate, Ni(SO3 NH2 )2 ·4H2 O and boric acid, H3 BO3

4006

A. Karpuz et al. / Applied Surface Science 258 (2012) 4005–4010

Table 1 The data obtained from compositional, structural, magnetic and MR measurements. XRD

Ni (at.%)

Co (at.%)

Lattice parameter (nm)

100 85 72 60 42 36 20

0 15 28 40 58 64 80

0.35196 0.35232 0.35306 0.35232 0.35381 0.35306 0.35381

± ± ± ± ± ± ±

0.00079 0.00079 0.00080 0.00079 0.00151 0.00151 0.00079

SEM

VSM

Grain size (nm)

Surface appearance

Ms (emu/cm3 )

Hc (Oe)

LMR (%)

TMR (%)

16 15 14 14 16 16 18

Very smooth Very smooth Very smooth Slightly granular Pebbly Acicular Homogeneous spherical granular

358 509 611 704 833 939 1000

61 42 36 36 45 78 107

3.5 8.5 11.0 10.0 5.0 6.0 4.8

3.0 6.0 4.0 8.0 6.0 5.1 6.1

concentrations at 0.40 M and 0.20 M, respectively, cobalt sulfate, CoSO4 ·7H2 O concentration was changed from 0.00 M to 0.35 M and shown in Table 1. A platinum sheet with an area of 6 cm2 was used as counter electrode and titanium sheet was served as substrate. The reference electrode was a saturated calomel electrode (SCE) and all potentials are pronounced with respect to the SCE. Electrolyte pH was chosen as 2.80 ± 0.20 and film thickness was fixed to 3 ␮m by using Faraday law [7,8]. The deposition potential was determined by using a cyclic voltammetry (CV) method. An energy dispersive X-ray spectroscopy, EDX (GENESIS APEX 4 – EDAX, AMETEK) combined with a scanning electron microscope, SEM (FEITM , NOVA NANOSEM 430) was employed to find film contents at the same time with investigation of surface morphology. The structural measurements were realized with an X-ray diffraction technique, XRD (PANalytical) in 2 = 40◦ –100◦ by using Cu-K␣ radiation ( = 0.15406 nm). A commercial vibrating sample magnetometer, VSM (ADE technologies DMS-EV9) was used to investigate the magnetic properties of the films. The MR measurements were performed with a van der Pauw (VDP) method with four-point probes and the magnetic field applied in the square shaped film plane was ±10 kOe. The longitudinal magnetoresistance (LMR) measurements were realized by applying current density parallel to the field, and in the transverse magnetoresistance (TMR) measurements, the current density was perpendicular to the field. The percentage changes in the VDP-MR were calculated with relation given in Refs. [14,15]. The films were peeled off from their substrates before all characterizations. 3. Results and discussion 3.1. Characterization of the electrolyte and composition of the deposits The CV curves of the electrolytes used for determination of deposition potential were indicated in Fig. 1. The scans were carried out with potential scanning rate of 20 mV/s and in the cathodic direction from +1.0 V to −2.0 V. According to CV curve of the electrolyte containing Ni(SO3 ·NH2 )2 ·4H2 O and H3 BO3 (void of Co), the current is almost zero in the region between +1 V and −1.0 V. After the value of −1.0 V, a cathodic current occurred with the increase of the potential due to the deposition of Ni ions. The CV curve of the second electrolyte containing CoSO4 ·7H2 O and H3 BO3 (void of Ni) showed that Co deposition started at around −0.8 V since the Co ions deposited to the cathode surface at this potential value. When the scan was reversed (anodic direction), a broad peak formed in a region between around −0.4 V and +1.0 V. This peak probably represents the dissolution of Co. The present CV results are more detailed and explanatory than those of [16] since Ni and Co were investigated individually for electrolyte characterization in our study. The study [16] mentioned that consecutive dissolution of Ni and Co from deposits led to two separated oxidation peaks without certain detection of oxidation and reduction potentials of Ni and Co. In the present study, satisfactory results were found out about these

VDP

8 void of Ni void of Co

4

Current (mA)

0.00 (S) 0.05 (S1) 0.10 (S2) 0.15 (S3) 0.25 (S4) 0.30 (S5) 0.35 (S6)

EDX

0 -4 -8 -12 -2

-1,5

-1

-0,5

0

0,5

1

Voltage (V) Fig. 1. CV curves of the electrolytes containing Ni(SO3 ·NH2 )2 ·4H2 O and H3 BO3 (void of Co), and containing CoSO4 ·7H2 O and H3 BO3 (void of Ni). The arrows in the CV curves show the scan direction.

potentials for codeposition of Ni and Co ions. According to the measurements, it was decided that the potential value for codeposition of Ni and Co ions should be higher than −1.0 V. Depositing the films and then looking at the features of the films, it was approved that the proper films can be grown at −1.9 V. The compositional analysis results are presented in Table 1. As seen in the table, the Co content increased gradually from 0 at.% to 80 at.% when the Co concentration of the electrolyte was increased from 0.00 M to 0.35 M. Similar trend of Co content was also reported in Ref. [9]. Fig. 2 shows the dependence of Co content of the films on the concentration of Co ions in the electrolyte. The percentages of Co in the films were higher than those of Co ions in the electrolyte for all depositions. This refers to anomalous codeposition of Ni and 100

Co percentage of film

Co concentration (M)

47; 80

80

43; 64 60

38; 58

40

27; 40

11; 15

(0.3 M Co)

(0.25 M Co)

(0.15 M Co)

(0.1 M Co)

20; 28 20

(0.35 M Co)

(0.05 M Co)

0; 0 (0 M Co) 0 0

10

20

30

40

50

60

Co percentage of electrolyte Fig. 2. Dependence of Co content of the films on the concentration of Co ions in the electrolyte.

A. Karpuz et al. / Applied Surface Science 258 (2012) 4005–4010

0 M Co fcc(111) fcc(200)

15 at. % Co fcc(311) 28 at. % Co

fcc(220)

fcc(111) fcc(200)

fcc(311)

0.15 M Co

fcc(220)

fcc(111)

18

16 16 16

16 15 14

14

14

12

40 at. % Co

10

fcc(200)

0

fcc(311) 58 at. % Co

0.25 M Co

fcc(111)

fcc(311)

0.3 M Co

64 at. % Co

fcc(111)

fcc(220)

fcc(200)

fcc(311)

0.35 M Co

80 at. % Co

fcc(111) A B fcc(200)

fcc(220) fcc(311)

60

70

2 θ (degree)

80

90

15

30

45

60

75

90

Co content of the films (at.%) Fig. 4. Variation of average grain size as a function of Co content for the Ni–Co films deposited from the electrolytes with different Co concentrations.

fcc(220)

fcc(200)

50

Average grain size (nm)

fcc(220)

0.1 M Co Intensity (a.u.)

18

fcc(311)

fcc(111) fcc(200)

40

Seri 1 grain size Average

fcc(220)

0.05 M Co

A

20

0 a t. % Co

4007

100

A: hcp(10.0) B: hcp(10.1)

Fig. 3. XRD patterns of Ni–Co films deposited from the electrolytes containing different Co concentrations.

Co ions. That is, the less noble Co ions preferentially deposited to the cathode surface according to Ni ions. Several studies [4,9,12,17] based on electrodeposition of Ni–Co alloys support the anomalous type of Ni and Co codeposition as in our study. In the studies [9] and [12], the degree of anomalous codeposition was enhanced in comparison to our study, since the anomalous Co content in the films is around 20–25% for Co content of 4–5% in electrolyte. As seen in Fig. 2, the films with 20% Co can be deposited from the electrolyte contains around 15% Co content in our study. This may be because of different bath temperatures and types used for deposition of the films. Also, Co percentage in the electrolyte was plotted at larger range (0–47%) in Fig. 2 in comparison to [4] and [9] for more comprehensive investigation of anomalous codeposition behavior. 3.2. Structure of the deposits The XRD patterns of the films are shown in Fig. 3. The pattern of Ni film (0 at.% Co) labeled as S in Table 1 indicated the peaks of face centered cubic (fcc) phase with a cell parameter of 0.35196 ± 0.00079 nm. The fcc peaks were observed in all XRD patterns. However, the films S5 and S6 (64% Co and 80% Co, respectively, see Table 1) showed small peaks of hexagonal closed packed (hcp) structure at 2 ∼ = 41◦ and 48◦ . It was seen that intensities of the hcp peaks increased while the fcc peak intensities decreased with the increase of Co content in contrast to recent study [10] which detected increasing intensity of fcc peaks with increasing Co content. The results disclosed that fcc phase turned to a mixed crystalline structure of dominantly fcc and hcp. The changes detected

in the crystalline structure were also observed in earlier studies [4,7,9,13]. In the study [4] which was employed a sulfamate electrolyte, fcc structure of films turned to fcc + hcp and only hcp structure for 70 wt.% Co and 100 wt.% Co, respectively. In study [18], the film consisted of 79% Co was investigated and found that the film has fcc crystalline structure since characteristic peaks of hcp phase are absent in its XRD pattern unlike the present study. A mixed crystalline structure of fcc + hcp was reported for 83% Co content in [18]. The differences observed in crystalline structure according to Co content may be because of different experimental conditions such as electrolyte pH and substrate. Another study [17] suggested that further electron microscopy studies would be necessary to obtain firm evidence about the presence of an hcp phase for the samples with Co contents above 65%. In this work, since the characteristic reflections of hcp were clearly observed in the XRD patterns of samples with Co contents of 64% and 80% (see Fig. 3), the presence of an hcp structure can be detected without further electron microscopy studies. The average grain sizes calculated using Scherrer formula [19] showed that they were 16, 15, 14, 14, 16, 16, 18 nm for the films with 0%, 15%, 28%, 40%, 58%, 64%, 80% Co content, respectively as shown in Fig. 4 and Table 1. The lattice parameters were also found between 0.35196 ± 0.00079 nm and 0.35381 ± 0.00151 nm as listed in Table 1 for Ni–Co films, by using the least squares technique. The results are compatible with JCPDS 15-0806 and 004-0850 XRD data. Preferential textures of the films were detected by using the relative peak intensities of the XRD patterns as done in Ref. [20] and it was found that all films have (2 2 0) preferential orientation. In study [9], the pure Ni film showed (2 0 0) preferential growth orientation since noticeable (2 0 0) peak occurred at 2 ∼ = 51◦ in XRD ◦ ◦ pattern achieved at 2 between 35 and 75 . In this study, the XRD patterns were obtained at 2 between 40◦ and 100◦ since the position of the main fcc peak (2 2 0) is at 2 ∼ = 76◦ . Different preferential orientations which were detected in the present study and [9] may be because of different range of 2 used in XRD measurements. 3.3. Surface morphology of Ni–Co films The Ni–Co films labeled as S, S1 and S2 in Table 1 (Co content up to 28%) exhibited very smooth surface as seen in Fig. 5a–c. The morphology became slightly granular (Fig. 5d) with increasing Co content to 40%. The pebbly and acicular surfaces were obtained as the Co content increased to 58% Co and 64% Co, respectively (see Fig. 5e and f). The surface with homogeneous distributed spherical granules were observed in the SEM image of S6 and showed in

4008

A. Karpuz et al. / Applied Surface Science 258 (2012) 4005–4010

Fig. 5. SEM images of Ni–Co films deposited from the electrolytes containing different Co concentrations, (a) S (0% Co), (b) S1 (15% Co), (c) S2 (28% Co), (d) S3 (40% Co), (e) S4 (58% Co), (f) S5 (64% Co), (g) S6 (80% Co).

Fig. 5g. The change of surface morphologies is consistent with [7] which investigated effect of deposition potential on properties of Ni–Co films since the same changes with the change of Co content were observed in our study. Thus, it is seen that the increase of Co content in the films resulted in an alteration (from smooth to granular) in the surface of the films. Granules in the film surface became more evident and their amount increased with increasing

intensities of the hcp peaks. Hence, it turned out that the alteration in surface morphology is related to the change of crystal structure caused by increase of Co content in the films. However, some studies [4,9,12] revealed that the Ni–Co films with rougher and more granular surface can be obtained for low Co content (0–30%) unlike the present study. This may be due to different electrodeposition parameters that have significant effect on surface morphology

A. Karpuz et al. / Applied Surface Science 258 (2012) 4005–4010

4009

1200 1000 800

1000 939 833

107 100

704 611

600

120

61

80

78

509

60

400

42 358

200

45 36

36

30

45

Hc (Oe)

3

Ms (emu/cm )

140

Ms Hc

40 20 0

0 0

15

60

75

90

Co content of the films (at.%) Fig. 7. Variation of Ms and Hc values as a function of Co content for the Ni–Co films deposited from the electrolytes with different Co concentrations.

Fig. 6. Histeresis loops of the films deposited from the electrolytes containing different Co concentrations (//: parallel and ⊥: perpendicular).

such as electrolyte pH, temperature, or additives in bath. Furthermore, quality of the films was at high level and coherent structures were achieved for all Co contents as seen in Fig. 5. The smoother and finer surfaces could be obtained from sulfamate–sulfate bath in comparison to [10] which optimized deposition parameters and generated smooth and uniform fine grained surfaces from acid chloride–sulfate bath. Conversely, cracked deposits caused by hydrogen bubbles growing on the electrode surface were obtained in previous study [16]. The study [4] emphasized that low stressed coatings are obtained from sulfamate electrolyte. Therefore, the difference of deposit quality between present work and [16] may be due to different substrate or sulfamate bath used in this study. 3.4. Magnetic and MR properties of Ni–Co films Fig. 6 shows the hysteresis loops of the films. The in-plane saturation magnetization, Ms and in-plane coercivity, Hc values were presented in Table 1 and plotted as a function of Co content of the films in Fig. 7. The films exhibit high Ms values at high Co contents. As the Co content increased from 0% Co to 80% Co, Ms increased gradually from 358 emu/cm3 to 1000 emu/cm3 since Ms value of bulk Co (1420 emu/cm3 ) is higher than that of bulk Ni (480 emu/cm3 ) as indicated in Ref. [21]. In other words, the increase of magnetic moment per volume depending on Co content is caused by the increase of Co amount in the films, since Co has higher magnetic dipole moment than Ni. Therefore, it could be said that Ms depends firmly on Ni:Co ratios of the films. While decrease of Ms was due to increasing Ni content of the films in our study, incorporation of copper in Ni–Co films resulted in a reduction of the Ms values in study [18]. The minimum Hc value was found as 36 Oe for the films S2 and S3 (28% and 40% Co, respectively), see Table 1. It was understood that these films can be magnetized easier compared to the others. The same trends of Ms and Hc values were also detected in other study [13] investigated the effect of sulfate and chloride baths on magnetic properties of Ni–Co alloys. By considering the magnetic results obtained from this study and [13], it can be concluded that Hc has the same dependency according to Co content for the films produced from sulfate, chloride or mixture of sulfamate and sulfate electrolytes. The value of ∼20 Oe

was reported as a minimum value for the film has 50% Co in Ref. [13]. Although the similarities of Hc alteration came into prominence in the present study and [13], the differences were seen in minimum Hc values. This may be attributed to the differences in preferential film texture. In study [1], it was emphasized that preferential film texture affects the Hc of the films. Also, lower Hc values were obtained in our study compared to [1]. This can be attributed to higher grain sizes or different electrodeposition parameters in Ref. [1]. However, the recent study [11] has stated that acicular morphologies lead to magnetically harder deposits as increased Co content in films encourages acicular morphologies, whereas the Ni-richer ones show magnetically softer behavior in response to presence of round-shaped grains. It is possible to mention different morphological–magnetic relation in the present study in comparison to Ref. [11] because both very smooth surface and granular surface were observed for almost the same Hc values (S1 and S4 films) or softer deposit of S5 in comparison with S6 was obtained with acicular morphology (see Fig. 5 and Table 1). To determine easy-axis direction of magnetization, perpendicular hysteresis loops were also measured to compare with parallel ones. As an example, perpendicular loop of S6 was presented in Fig. 6. According to figure, all in-plane hysteresis loops have a lower Hc and higher remanent magnetization than the perpendicular loop. This refers that the easy-axis direction of magnetization is parallel to the film plane as found in Ref. [18]. The obtained results were compared in order to correlate the structural and magnetic properties of Ni–Co films. As shown in Fig. 4 and Fig. 7, the variation of average grain size is in accordance with Hc for all Co contents. The Hc was found at the lowest values for the lowest average grain size (14 nm) and it started to increase for higher grain sizes. The change of Hc values with different Co content can be related to alteration of average grain size. In addition, existence of (10.0) and (10.1) hcp peaks for S5 and S6 films contributed to enhancement of Hc values (78 Oe and 107 Oe) in comparison to the other films. It can be noted that the microstructural parameters such as average grain size and crystal structure have a firm influence on the magnetic parameters such as Hc . MR measurements revealed that all films exhibited anisotropic magnetoresistance (AMR) since LMR increased while TMR decreased as the magnetic field increased. The change in magnitude of LMR and TMR was listed in Table 1. As an example, the change in LMR and TMR magnitude for the film S5 (64% Co) was illustrated in Fig. 8. It is seen that LMR magnitudes varied between 3.5% and 11% and TMR magnitudes changed between 3.0% and 8.0% as Co content increased from 0% to 80%. Relatively high MR values

4010

A. Karpuz et al. / Applied Surface Science 258 (2012) 4005–4010

Magnetoresistance (%)

8 LMR TMR

7

MR system, by TUBITAK under Grant No. TBAG-1771 for electrodeposition system, and by State Planning Organization, Turkey under Grant No. 2005K120170 for VSM system. The authors are grateful to Dr. H. Guler for XRD measurement, Balikesir University, Turkey. Thanks also go to Bilkent University, Turkey – UNAM for EDX measurements and SEM micrographs.

Co Concentration 0.30 M

6 5 4

References

3 2 1 0 -10

-5

0

5

10

Applied magnetic field (kOe) Fig. 8. The LMR and TMR curves of the film deposited from the electrolyte containing 0.30 M Co concentration.

were detected for S2 and S3 films consisted of 28–40 at.% Co content as shown in Table 1. 4. Conclusions The Ni–Co films were grown with electrodeposition from the electrolytes containing different Co concentrations. The anomalous codeposition was detected in all films. The films with 64 at.% Co and 80 at.% Co have a mixture of dominantly fcc and hcp structure except for the rest of the films having fcc structure. The SEM images showed that qualified and coherent structures occurred for the films deposited from a mixture of sulfamate and sulfate electrolyte and roughness of the film surface increased with increasing Co content. The magnetic results disclosed that Ms increased with the increase of Co content and the highest Ms was found to be 1000 emu/cm3 for the film consisted of 80% Co. Furthermore, the easy axis direction of magnetization was found to be in the film plane for all films. The Hc of the films was strongly affected by changes in the structural properties caused by different film content. According to MR measurements, the AMR behavior and different magnitudes of LMR and TMR were detected as the Co content changed. The optimized film properties, i.e., lower Hc , higher MR values, very smooth or slightly granular surfaces were obtained at 28–40 at.% Co content range achieved by using 0.10–0.15 M CoSO4 ·7H2 O concentrations. The results indicate that the proper film structures and surfaces for applications could be obtained from electrolyte consisted of nickel sulfamate, cobalt sulfate and boric acid. The structural, magnetic and MR properties were significantly affected by the change of film content caused by the change of electrolyte concentration. It is anticipated that the investigation may supply a good contribution and platform for the potential applications of reading/writing heads and magnetic sensors. Acknowledgments This paper was financially supported by Balikesir University under Grant No. BAP 2010/34, under Grant No. BAP 2001/02 for

[1] I.Z. Rahman, M.V. Khaddem-Mousavi, A.A. Gandhi, T.F. Lynch, M.A. Rahman, Growth of electrodeposited Ni–Co and Fe–Co magnetic films on Cu substrates, Journal of Physics: Conference Series 61 (2007) 523–528. [2] L. Burzynska, E. Rudnik, The influence of electrolysis parameters on the composition and morphology of Co–Ni alloys, Hydrometallurgy 54 (2000) 133–149. [3] C. Lupi, D. Pilone, Electrodeposition of nickel–cobalt alloys: The effect of process parameters on energy consumption, Minerals Engineering 14 (11) (2001) 1403–1410. [4] M. Srivastava, V.E. Selvi, V.K.W. Grips, K.S. Rajam, Corrosion resistance and microstructure of electrodeposited nickel–cobalt alloy coatings, Surface & Coatings Technology 201 (2006) 3051–3060. [5] A. Karpuz, M. Alper, H. Kockar, The role of Cu content on properties of electrodeposited Fe–Cu films, Sensor Letters 7 (2009) 1–4. [6] M. Safak Haciismailoglu, M. Alper, H. Kockar, Magnetoresistance of CoNiCu/Cu multilayers electrodeposited from electrolytes with different Ni ion concentrations, Journal of the Electrochemical Society 157 (10) (2010) D538–D545. [7] A. Karpuz, H. Kockar, M. Alper, The effect of different chemical compositions caused by the variation of deposition potential on properties of Ni–Co films, Applied Surface Science 257 (2011) 3632–3635. [8] H. Kockar, M. Alper, T. Sahin, O. Karaagac, Role of electrolyte pH on structural and magnetic properties of Co–Fe films, JMMM 322 (2010) 1095– 1097. [9] L. Wang, Y. Gao, Q. Xue, H. Liu, T. Xu, Microstructure and tribological properties of electrodeposited Ni–Co alloy deposits, Applied Surface Science 242 (2005) 326–332. [10] Y.-F. Yang, B. Deng, Z.-H. Wen, Preparation of Ni–Co alloy foils by electrodeposition, Advances in Chemical Engineering and Science 1 (2011) 27–32. [11] O. Ergeneman, K.M. Sivaraman, S. Pané, E. Pellicer, A. Teleki, A.M. Hirt, M.D. Baró, B.J. Nelson, Morphology, structure and magnetic properties of cobalt–nickel films obtained from acidic electrolytes containing glycine, Electrochimica Acta 56 (2011) 1399–1408. [12] C.K. Chung, W.T. Chang, Effect of pulse frequency and current density on anomalous composition and nanomechanical property of electrodeposited Ni–Co films, Thin Solid Films 517 (2009) 4800–4804. [13] D. Kim, D.-Y. Park, B.Y. Yoo, P.T.A. Sumodjo, N.V. Myung, Magnetic properties of nanocrystalline iron group thin film alloys electrodeposited from sulfate and chloride baths, Electrochimica Acta 48 (2003) 819–830. [14] M. Safak, M. Alper, H. Kockar, Parameters affecting microstructure and magnetoresistance of electrodeposited Co–Cu alloy films, Journal of Magnetism and Magnetic Materials 304 (2006) e784–e786. [15] M. Alper, H. Kockar, M. Safak, M.C. Baykul, Comparison of Ni–Cu alloy films electrodeposited at low and high pH levels, Journal of Alloys and Compounds 453 (2008) 15–19. ˇ ˇ A. Orinák, G. Vering, I. Talian, R.M. Smith, H.F. Arlinghaus, Influ[16] R. Orináková, ence of pH on the electrolytic deposition of Ni–Co films, Thin Solid Films 516 (2008) 3045–3050. [17] B.G. Toth, L. Peter, A. Revesz, J. Padar, I. Bakonyi, Temperature dependence of the electrical resistivity and the anisotropic magnetoresistance (AMR) of electrodeposited Ni–Co alloys, European Physical Journal B 75 (2010) 167–177. [18] E. Gómez, S. Pané, E. Vallés, Electrodeposition of Co–Ni and Co–Ni–Cu systems in sulphate–citrate medium, Electrochimica Acta 51 (2005) 146–153. [19] B.D. Cullity, Elements of X-Ray Diffraction, Addison-Wesley, USA, 1978. [20] H.J. Kim, K. Kim, S. Rae Lee, W.Y. Jeung, Thickness dependence of (0 0 1) texture evolution and magnetic properties of sputter-deposited FePt:MgO nanocomposite films, IEEE Transactions on Magnetics 44 (11) (2008) 3535– 3538. [21] D. Jiles, Introduction to Magnetism and Magnetic Materials, Chapman and Hall, London, 1996.