Electrodeposited Co-Ni Films: Electrolyte pH-Property Relationships

DOI 10.1007/s10948-012-1774-z O R I G I N A L PA P E R

Electrodeposited Co–Ni Films:

Electrolyte pH—Property Relationships

Received: 28 June 2012 / Accepted: 24 September 2012 / Published online: 5 October 2012 © Springer Science+Business Media, LLC 2012

Abstract The effect of electrolyte pH on structural, mag-netic, and magnetoresistive properties of Co–Ni films was studied. The films were deposited on a titanium substrate from the electrolytes with 4.10± 0.05, 3.14 ± 0.05, and 2.14± 0.05 pH values. The Co–Ni system exhibited anoma-lous codeposition. Structural analysis indicated that the films had (220) preferential oriented face-centered cubic structure and their surface became smoother as the elec-trolyte pH decreased. The compositional and magnetic anal-ysis revealed that an increase of the Co content in the films resulted in an increase in saturation magnetization and coercivity. Magnetoresistance curves indicated that the films show anisotropic magnetoresistance. Longitudinal and transversal magnetoresistances were found to be the highest values of 8 % and 7 %, respectively, for the film deposited at a low electrolyte pH. The variation of the Co:Ni ratio in deposits caused by the change of the electrolyte pH has a considerable effect on the properties of the films.

Keywords Anisotropic magnetoresistance·

Electrochemical synthesis· Magnetic properties · Ni–Co alloys

A. Karpuz (



Physics Department, Science & Literature Faculty, Balikesir University, Balikesir, Turkey

e-mail:alikarpuz@bau.edu.tr

Present address: A. Karpuz

Physics Department, Kamil Ozdag Science Faculty, Karamanoglu Mehmetbey University, Karaman, Turkey

e-mail:alikarpuz@kmu.edu.tr

M. Alper

Physics Department, Science & Literature Faculty, Uludag University, Bursa, Turkey

1 Introduction

Longitudinal (LMR) and transversal (TMR) magnetoresis-tances called as anisotropic magnetoresistance (AMR) have been one of the most interesting characteristics observed in magnetic films for a long time. They have been compre-hensively studied by the changes observed in the electri-cal resistance with an external magnetic field [1–3]. Co–Ni and its ternary or quaternary alloys are considered as sig-nificant materials because of their specific magnetic prop-erties [4,5]. In codeposition of Co and Ni, the interaction of hydrogen with the metal surface is a topic of consider-able experimental, theoretical, and industrial interest. Elec-trodeposition is one of the most useful tools for the pro-duction of Co–Ni and other iron group metals that have potential applications in computer read/write heads, micro-electromechanical systems, and data storage [5–7]. There are also a lot of advantages when the coatings are produced with electrodeposition, i.e., simple, vacuumless, and at room temperature deposition [8, 9]. The important parameter is experimental conditions for electrodeposition [10,11]. The reports [2,12–15] also expose that the electrolyte pH has a significant effect on the properties of films. Therefore, the purpose of the present study is to investigate the elec-trodeposited Co–Ni films and report their reaction to dif-ferent electrolyte pHs. The x-ray diffraction (XRD) results disclosed that the samples have a face centered cubic (fcc) structure in the region of 40–57 at.% Co. The region of 50–70 at.% Co was blank and unexplained in the previous study [16]. The values of LMR and TMR obtained by us-ing the van der Pauw (VDP) method increased up to 8 % and 7 %, respectively, were higher than the values found with other techniques in study [17]. The results have shown that the properties of Co–Ni films show strong dependence on the alloy composition, which can be controlled by the electrolyte pH.

Table 1 The data obtained from compositional, structural magnetic, and MR measurements pH (±0.05) EDX XRD SEM VSM VDP Co (at.%) Ni (at.%) Lattice parameter (nm) Surface appearance Ms (emu/cm3) Hc (Oe) LMR (%) TMR (%)

4.10 40 60 0.35269± 0.00078 Rough and bumpy 775 40 6 5

3.14 57 43 0.35493± 0.00079 Pebbly 855 47 6 6

2.14 53 47 0.35306± 0.00150 Smooth and straight 855 47 8 7

2 Experimental

The electrolyte prepared for deposition of Co–Ni films was composed of 0.4 M Ni(SO3·NH2)2·4H2O, 0.2 M CoSO4·7H2O, and 0.2 M H3BO3. Three different elec-trolyte pH levels (4.10± 0.05, 3.14 ± 0.05, and 2.14 ± 0.05) were selected for investigation. An electrodeposition sys-tem consisted of a potentiostat (EGG model 362) with three electrodes, a computer, and an electrochemical cell served for the film production. A polished titanium substrate and a platinum foil were used as a cathode and anode, respec-tively, while the saturated calomel electrode (SCE) was used as a reference electrode. The film thickness was calculated according to Faraday’s law as done in [18] and fixed at 3 µm. A cathode potential of−1.9 V vs. SCE was applied for all depositions in a continuous waveform.

The compositional analysis was executed by energy dispersive x-ray spectroscopy (EDX, GENESIS APEX 4-EDAX, AMETEK). The XRD measurements were per-formed with a diffractometer (PANalytical) by using Cu-Kα radiation, in the range of 40◦<2θ < 100◦, where θ

is the Bragg’s angle. The surface morphology was investi-gated with the scanning electron microscope (SEM, FEITM, NOVA NANOSEM 430). A commercial vibrating sample magnetometer (VSM, The DMS Model EV9, ADE Tech-nologies) calibrated with Ni foil was used to determine the magnetic properties in the range of±10 kOe and all deposi-tions and measurements were done at the room temperature. Magnetoresistance (MR) curves were obtained by using the VDP method with four point contacts at ±10 kOe in the film plane. The films were square and their surface sizes were 1 cm2. The AMR measurements were carried out by applying current density parallel (LMR) and perpendicular (TMR) to the magnetic field [2].

3 Results and Discussion

The results of compositional analysis of the deposits are pre-sented in Table1. As indicated in the table, the Co content was 40 at.% for the film deposited at the high pH (4.10) and increased to 57 at.% as the electrolyte pH decreased to 3.14. The analysis obtained from different electrolytes

Fig. 1 The XRD patterns of the films deposited at the different elec-trolyte pHs

showed that the Co content remains in the range of 50– 57 at.% even if the electrolyte pH decreased to the lowest pH (2.14). According to the results, anomalous codeposi-tion occurred at all deposits obtained from the present elec-trolyte since the Co content (40–57 at.%) in the deposits is higher than the relative ion concentration of cobalt in the electrolyte. The anomalous codeposition of Co–Ni alloys was also reported at different electrodeposition conditions [5,17]. On the contrary, the anomalous codeposition of Co and Ni was not observed due to insufficient thickness of the deposits in [19].

The effect of electrolyte pH on the crystalline structure of the films was detected by using XRD. It was clearly ob-served that all films have a fcc structure. The XRD patterns of the films are shown in Fig. 1. As seen from the pat-terns, the reflections from the characteristic crystal planes of (111), (200), (220), and (311) are observed at 2θ ∼= 44◦, 51◦, 76◦, and 92◦, respectively as found in [5,16]. The fcc phase can be seen in the Co–Ni films since they were grown from an acidic electrolyte caused by the low electrolyte pH as in [4,20]. According to study [16], the phase structure for Co–Ni alloys changes gradually from fcc for Ni rich

de-posits (up to 50 wt.% Co) to mixed fcc+ hcp and complete hcp structure for deposits containing 70–100 wt.% Co. The present study exposed that the fcc structure is observed for the deposits containing the Co content between 50–57 at.%, which were undisclosed in [16]. To assess the texture forma-tion of the films, the relative peak intensities of the observed reflections were considered. It was seen that all films have (220) preferential orientation. The average grain sizes of the crystallites were calculated by using Scherrer’s method [21] and found that they were around 15 nm. The work [17] re-ported that the average grain sizes of Co–Ni were around 10 nm for all electrolyte concentration range. The grain sizes calculated using the Scherrer formula for various Co–Ni al-loys in [16] showed that they altered between 18 nm and 23 nm for entire content range. The study [16] also reported that the grain size for 50 % Co–Ni deposits was 11.5 nm. The grain sizes in the present study are compatible with the studies [16,17]. The small differences between the present work and [17] may be attributed to different substrate (such as Si/Cr/Cu) and electrolyte chemicals (such as Na2SO4, H2NSO3) that were used during the deposition of films. The larger sizes than that of present study were reported in study [22] that investigated the effect of high electrolyte tempera-ture on grain sizes. It can be said that the room temperatempera-ture deposits have lower grain size, as in our study. The lattice parameters were also found to be 0.35269± 0.00078 nm, 0.35493± 0.00079 nm, and 0.35306 ± 0.00150 nm for the films deposited at 4.10, 3.14, and 2.14 electrolyte pH, spectively, by using the least squares technique [21]. The re-sults are in accordance with JCPDS 004-0850 and 15-0806 XRD data.

Figures2(a)–(c) show the SEM images of the films de-posited at different electrolyte pHs. It is clearly seen that the film has a rough and bumpy surface at high pH (4.10) (see Fig.2(a)). The structure turned to a more homogeneous and pebbly surface when the electrolyte pH decreased to 3.14 (see Fig. 2(b)). The film surface became a smoother and straight appearance when the electrolyte pH was de-creased to 2.14 as shown in Fig. 2(c). It may be said that the smoother film surfaces formed from high acidic elec-trolytes caused by reduction of pH. This is also confirmed by another study [19]. SEM morphologies of Co–Ni alloys deposited at the 4.0 electrolyte pH value were also investi-gated in [23]. Although the Co content of the some alloys in [23] is almost the same with the film deposited at the 4.10 electrolyte pH in the present study (40 at.%), there are re-markable differences between SEM images of these films. The surface with homogeneous distributed spherical gran-ules was obtained in [23]. This may be attributed to the dif-ferent electrolyte type, steel substrate, or 45◦C of bath tem-perature used in [23].

Hysteresis loops were obtained to investigate the mag-netic properties and illustrated in Fig.3. The saturation

mag-Fig. 2 The SEM micrographs of the Co–Ni films deposited at the dif-ferent electrolyte pHs, (a) 4.10 pH, (b) 3.14 pH, (c) 2.14 pH

netization, Msof the film deposited at pH= 4.10 was found to be 775 emu/cm3. As the pH decreased to 3.14, the Ms

in-Fig. 3 The hysteresis loops of the Co–Ni films deposited at the differ-ent electrolyte pHs

creased to around 855 emu/cm3and remained almost at this value for the 2.14 electrolyte pH. Therefore, it can be said that the film grown at the low pH shows higher Ms, corre-sponding to the high Co content compared to the film grown at the high pH, since the Msof bulk Co (1420 emu/cm3) is higher than bulk Ni (480 emu/cm3) [24]. The similar trend was also seen for coercivity: Hc values that are 40 Oe for 4.10 pH and around 47 Oe for 3.14 and 2.14 pH. This refers that it is getting harder to magnetize the Co–Ni films as the electrolyte pH decreased. The results of the study [25] show similarities concerning the tendency of the Ms values pur-suant to the film composition.

The percentage changes in the VDP resistance, (MR)%, were calculated according to the following relation [2]; MR(%)=R(H )− Rmin



/Rmin



× 100 (1) where R(H ) is the value of the resistance at any magnetic field and Rminis the minimum resistance that was measured. MR measurements of the films were shown in Fig.4. It was clearly seen from figure that the films exhibited AMR since the LMR increases and the TMR decreases with the increase of magnetic field. The LMR and TMR magnitudes enlarged from 6 % to 8 % and from 5 % to 7 % as the electrolyte pH decreased, respectively (see Table 1). To the AMR re-sults, the AMR magnitudes increased as the electrolyte pH decreased. The AMR values are larger than the results ob-tained in the other study [17]. Since the percentage changes in the AMR measured with the VDP method are not nec-essarily equal to the percentage changes in true LMR and TMR [1,12,26], therefore, one must be careful when com-paring the MR ratios obtained using the VDP method to those obtained by other methods.

Fig. 4 The MR curves of the Co–Ni films deposited at the different electrolyte pHs

4 Conclusions

The Co–Ni films were produced and characterized to in-vestigate the effect of the electrolyte pH on the structural, magnetic, and magnetoresistive properties of the films. The results exposed that all Co–Ni films produced in the range of the 4.10± 0.05 and 2.14 ± 0.05 electrolyte pH bring about an anomalous codeposition. The fcc structure, 15 nm grain sizes, and the (220) preferential texture were obtained at all electrolyte pH range. More proper film structures and mirror-like appearances were generated at the low pH levels. The highest magnitude of the AMR was obtained at the low-est pH (2.14). It was noted that the structural, magnetic, and MR properties are influenced with the change of the Co:Ni ratio caused by the different electrolyte pHs. This can pro-vide a possible production of Co–Ni films used in the imple-mentation of sensor, actuators and recording media. Acknowledgements This paper was financially supported by Ba-likesir University under Grant no. BAP 2010/34, under Grant no. BAP

2001/02 for the MR system, by TUBITAK under Grant no. TBAG-1771 for the electrodeposition system, and by the State Planning Orga-nization, Turkey, under Grant no. 2005K120170 for the VSM system. The authors are grateful to Dr. Halil Guler for facilities provided in XRD measurements realized in Balikesir University, Turkey. The au-thors’ thanks also go to Bilkent University, Turkey–UNAM, Institute of Materials Science and Nanotechnology for SEM micrographs and EDX measurements.

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