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Morphology, magnetic, magnetoresistance and optical properties of Co–Ni–Mo alloys thin films
Applied Surface Science 257 (2011) 10903–10909
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Morphology, magnetic, magnetoresistance and optical properties of Co–Ni–Mo alloys thin films S.I. Tanase a,b,∗ , D. Pinzaru Tanase a , M. Dobromir a , V. Georgescu a a b
Faculty of Physics, “Alexandru Ioan Cuza” University, Iasi, 700506, Romania Department of Physics, “Alexandru Cel Bun” College, Gura Humorului, 725300, Romania
a r t i c l e
i n f o
Article history: Received 8 March 2011 Received in revised form 30 July 2011 Accepted 31 July 2011 Available online 6 August 2011 Keywords: Co–Ni alloy Thin films Magnetic properties Magnetoresistance
a b s t r a c t We present in this paper several results concerning the preparation by means of electrolysis and characterization of Co–Ni–Mo thin films. Co–Ni–Mo thin films with different molybdenum content in the range 0–25 at% Mo were prepared from a complex solution containing ions of Co, Ni and Mo, using galvanostatic control, on aluminum substrates. The effects of applied current density on the morphology, magnetic, magnetoresistance, and optical properties of the electrodeposited Co–Ni–Mo films were investigated. The applied current density significantly influenced the film composition and their magnetic properties. The increase of molybdenum content in Co–Ni films (up to 25 at% Mo) enhances the resistivity, but it reduces the magnetoresistance effect. We report the first observation of magnetoresistance as high as 8% in Co–Ni–Mo thin films. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Magnetic thin films based on iron group elements have received much attention due to their importance in the electronic industries. Many studies were carried out to develop ternary alloys based on Co–Ni system with additions, because of their remarkable magnetic properties and potential applications in micro-electromechanical (MEMS) systems [1,2] or in the electronic industries for storage and high-density recording [3–6]. Cobalt alloys with soft-magnetic behavior have been studied with a view to microelectronic devices [7–9]. Thin films of Co, Ni, Fe-base alloys with Mo, N, Mg and P as impurity have been studied for a long time. Gomez et al. [10] reported that introduction of 5–11 wt.% molybdenum into cobalt electrodeposits leads to obtain materials with low-coercitivity. It was demonstrated that the presence of molybdenum in cobalt coatings led to a soft-magnetic response [11], so they could be used in magnetic devices such as sensors and actuators. Films containing Co–Mo [6], Ni–Mo [12], and electrodeposited Co–Ni–N [13], Co–Ni–P [14], Co–Ni–Mg [15], Co–Ni–Mg–N [16] were also studied. Mandreci [17] reported that thin Co–Ni–Mo films with good magnetic properties could be produced by electrodeposition. The properties of the Co–Ni alloy could be modified by the introduction of a third element in the alloy. In this line, the introduction of molybdenum in the Co–Ni films might improve their magnetic and magnetoresistance properties.
∗ Corresponding author. Tel.: +40 232 201050; fax: +40 232 201150. E-mail address: tanase [email protected] (S.I. Tanase). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.07.139
We are interested in the electrodeposition of Co–Ni–Mo films because they are used in a variety of applications. In addition, their electrodeposition behavior is unique, since molybdenum cannot be deposited from an aqueous electrolyte [6] – this type of electrodeposition is known as “induced co-deposition” [18] – but it can be co-deposited in the presence of iron group metallic ions. In our previous paper [13], we have reported on tunneling magnetoresistance (TMR) of Co–Ni alloys thin films with nitrogen as impurity, electrodeposited on aluminum substrate. In order to obtain the TMR effect for Co–Ni–Mo films, it was essential to use aluminum as substrate that oxidizes very easy. The purposes of this study are to obtain Co–Ni–Mo thin films by electrodeposition and to investigate the influence of the applied current density on the morphology, magnetic, magnetoresistance, and optical properties of thin electrodeposited Co–Ni–Mo films, correlated with the deposition parameters; this study is of interest for technological applications. The films were obtained as a particular case of the electrodeposition of Co–Ni–N thin films [13]. We did not find any study in the literature referring to the magnetic and magneto-transport properties of the electrodeposited Co–Ni–Mo thin films. Consequently, as far as we know, the magnetoresistance property of electrodeposited Co–Ni–Mo thin films has never been studied until now. Magnetoresistance in such thin films is a very promising research direction. 2. Experimental All the films studied in this paper were prepared by electrodeposition on aluminum substrates shaped as disks (geometric area = 3.14 cm2 ). The bath composition and the electrodeposi-
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Table 1 Electrolyte composition (g l−1 in double distilled water) for Mo, Co–Ni and Co–Ni–Mo films electrodeposited, labeled M, CN and CNM, respectively. Electrolytes
CoSO4 ·7H2 O
NiSO4 ·7H2 O
Na2 MoO4 ·2H2 O
NiCl2 ·6H2 O
Na2 SO4 ·10H2 O
Na3 C6 H5 O7 ·5H2 O
H3 BO3
NaCl
M CN CNM
– 30 30
– 50 50
0.6 – 0.6
– 10 10
50 50 50
– – 0.6
30 30 30
10 10 10
tion conditions were set accordingly to our previous publications [13]. The electrochemical experiments were performed galvanostatic in a conventional three-electrode cell by using a nickel anode and a working electrode of aluminum disk; quasi-reference electrode was a platinum wire that was embedded in a glass tube. By depositing some thick (∼450 nm) films of Co–Ni, Mo, and Co–Ni–Mo alloys in well-established electrolytic conditions, we determined the electrodeposition rate for these alloys. Taking into account these experiments, the thickness of the samples was controlled by the electric charge passing through the electrolyte during the plating process. In this paper we performed the studies of the morphology of the samples using a Vega II LSH scanning electron microscope (SEM) coupled with a detector EDX, model Quantax QX2. This detector is used for the quantitative and qualitative microanalysis of the thin films using Mn K␣ radiation. The X-ray photoelectron spectroscopy (XPS) experiments were carried out using a PHI 5000 Versa Probe ( ULVAC-PHI, Inc.) apparatus. A monochromatic Al K␣ Xray (1486.7 eV) was used as excitation source. The sputter-etching process was done using an Ar ion gun. Magnetic measurements were carried out at room temperature using an induction type device with data acquisition system and ac magnetic field applied in the film plane at a frequency of 50 Hz, in 60 kA m−1 maximum field. The resistance (R) measurements of the samples were performed at room temperature by using four-terminal contacts applied on the film surface, and an HM 8112-2 programmable multimeter working in regime of a constant current flowing in the plane of the films (current-in-plane geometry). The correction for the current passing the sample (7 A) is compensated automatically, by construction. Reflection spectra corrected for the effect of aluminum substrates were acquired in the wavelength range 400–1000 nm using a computer controlled STEAG-ETA Optic Spectrometer.
Table 2 Notations for the series of Co–Ni and Co–Ni–Mo thin films deposited at different current density (j) from the solutions labeled M, CN and CNM. Sample
j (mA cm2 )
at.% Mo
Solution
TF1 TF2 TF3 TF4 TF5 TF6 TF7 TF8
0.95 1.60 2.54 2.10 5.75 7.15 11.34 15.92
69.24 0 0 14.58 16.88 20.42 21.71 25.36
M CN CN CNM CNM CNM CNM CNM
In the present work we deposited the series of thin films labeled for brevity TF1–TF8, as it is shown in Table 2, using the solutions from Table 1. The samples were prepared using different values of current density (j), in the range of 1–16 mA cm2 , but maintaining the others working parameters of the electrodeposition identical. From the SEM analysis we found that the molybdenum content (cMo ) of the deposits increases as the current density is increased. For example, the molybdenum content increases from 14.58 (sample TF4) to 25.36 at% Mo (sample TF8), when the current density increases from 2.10 to 15.92 mA cm−2 . The effect of the current density on the molybdenum content (cMo ) of the deposited films is shown in Fig. 1. Fig. 1 shows the dependence of the molybdenum content (cMo ) in the films on the current density (j) applied during electrodeposition, for the studied samples (line is only a guide for the eyes). We found that the current density exhibits a strong influence on the molybdenum content in films, determined from the SEM experiments. The Co–Ni–Mo films deposited at low current density contain a large amount of oxygen as in Ref. [19]. In order to interpret the changes observed due to addition of the Mo to the Co–Ni alloy films, morphological analysis of the deposits was performed by SEM. We have found that the surface morphology of the electrodeposited films changes as a result of increasing
3. Results and discussion We have started the research by experiments on electrodeposition performed separately of Co–Ni alloys and Mo element, in order to optimize the solution composition and deposition parameters. After that, the experiments were carried out for the Co–Ni and Co–Ni–Mo thin films deposition from the sulfate baths of the composition given in Table 1. For deposition of Mo films, the solution (labeled M) contained mainly Na2 MoO4 ·10H2 O and some additives were used. The deposition of Co–Ni films was performed from the solution labeled CN, containing CoSO4 ·7H2 O and NiSO4 ·7H2 O as the base elements. The solutions labeled CNM for electrodeposition of Co–Ni–Mo films contain compounds of the three metals and a similar content of additives (NiCl2 ·6H2 O, Na2 SO4 ·10H2 O, Na3 C6 H5 O7 ·5H2 O, H3 BO3 and NaCl,) for all baths. Before each experiment, the aluminum substrates were mechanically polished, degreased with acetone, etched for a short time in dilute HNO3 , and washed in distilled water. In all experiments, the temperature was kept at 25 ◦ C and the electrolyte pH was adjusted to 4.0 by adding H2 SO4 , without stirring the solution. The film thickness for all samples was controlled by electric charge passing through the electrolyte.
Fig. 1. The effect of applied current density (j) on the molybdenum content (cMo ) in films.
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Fig. 2. Scanning electron micrographs for the sample TF1 (a), electrodeposited at 0.95 mA cm−2 from the solution labeled M and for the samples TF4 (b) and TF6 (c) electrodeposited from the solution labeled CNM at 2.10 mA cm−2 , and 7.15 mA cm−2 , respectively.
current density (j). Fig. 2 shows the scanning electron micrograph for the samples TF1 (2a), electrodeposited at 0.95 mA cm−2 from the solution labeled M and for the samples TF4 (2b) and TF6 (2c) electrodeposited from the solution labeled CNM at 2.10 mA cm−2 , and 7.15 mA cm−2 , respectively. Fig. 2 shows that the films contain polygonal grains with size of 2–70 m composed of molybdenum oxides crystallites (as we will show by XPS experiments). The Co–Ni grains are grown from different nucleation sites onto the aluminum substrate and they tend to form conglomerates, especially in the case of the samples TF4 and TF6 (Fig. 2b and c), obtained at 2.10 mA cm−2 and 7.15 mA cm−2 , respectively. The morphology of the samples is similar with those obtained by Gomez [20] in the case of Co–Ni–Mo films electrodeposited onto glass/ITO support, where Co–Ni–Mo alloy islands were grown over the initial molybdenum oxide film. From the preliminary SEM experiments, we have found that the film composition is influenced by the applied current density. For example, the films labeled TF4 deposited at 2.10 mA cm−2 contain 14.58 at% Mo, and those deposited at 7.15 mA cm−2 contain 20.42 at% Mo (sample TF6). From the SEM/EDAX analyses, we can conclude that the Co–Ni–Mo films consist in polygonal grains with various diameters which nucleate in different sites on the substrate. The surface morphology is similar for all sam-
ples and the SEM images also illustrate that the remaining surface between the polygonal grains contains Al and O. The uncovered sites on the substrate oxidize very easy and the conditions for electric transport are changed, favoring tunneling magnetoresistance effect due to Al2 O3 and molybdenum oxides low conductive layers. The XPS analysis permits the determination of the oxidation state of the metals present in the alloys deposits. The molybdenum and aluminum oxidation state was estimated by deconvolution of the peaks in the Mo3d and Al2p region. We investigated the studied samples and we gave as example in Fig. 3a the spectrum of the sample TF1 (67.99 at.% Mo, 16.95 at.% Al, 15.06 at.% O) deposited from the solution labeled M (Table 2) with core levels Mo3d (insets). The Mo3d spectrum typically consisted of a Mo3d3/2 –Mo3d5/2 doublet due to the spin orbit coupling. Taking into account the fact that Mo exists in a number of oxidation state as oxides and hydroxides [21], these doublets correspond to the different Mo species in the deposits. After peak deconvolution (Fig. 3b) we have investigated a mixture of polyvalent Mo oxides (mainly MoO2 at 229.9 eV and Mo2 O5 at 231.93 eV). Although it is not possible to determine the relative amount of the oxides and of metallic Mo because it is a mixture, it can be concluded that in all case we had a deposit basically constituted of oxides doped with a small amount Mo metallic.
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Fig. 3. XPS spectrum of the sample TF1 (a) grown on aluminum substrates using electrochemical deposition, with core level Mo3d (insets); deconvoluted core level spectra of Mo3d (b).
In Fig. 4a we give as an example the XPS spectrum of the sample TF4 (14.58 at.% Mo, 12.19 at.% Co, 6.35 at.% Ni, 38.75 at.% Al, 28.13 at.% O) deposited from the solution labeled CNM (Table 2) with core levels Mo3d and Al2p (insets). After peak deconvolution (Fig. 4b) we have investigated a mixture of polyvalent Mo oxides as in the case of sample TF1; the aluminum oxidation state was estimated by deconvolution of the peak in the Al2p region (Fig. 4c). In the case of Co–Ni–Mo films, there are a lot of uncovered sites on Al substrate, as we have found from the SEM measurements, indicative of a good phase separation between the ferromagnetic granules and the molybdenum oxides and Al2 O3 substrate regions. Moreover, the intense O1s signal present in the XPS spectrum of sample TF4 (Fig. 4a) is a proof that the Al substrate is in fact oxidized. By these experiments we demonstrated that the aluminum is in fact oxidized and thus is enhances the films resistivity; on the other hand, we suppose that the shape of the magnetoresistance curves is influenced by the existence of the boundaries between grains (rich in molybdenum oxides and Al2 O3 ) which contributes to the increased resistance of the films. It should be emphasized that the large values of the magnetoresistance are apparently affected by both the presence of the molybdenum in the films and the existence of the aluminum oxide on the grains frontiers. By changing the current density, we found that both the surface morphology and the molybdenum content (cMo ) in the Co–Ni–Mo films are different and this leads to the modification of the magnetic properties of the films. Starting from these observations, in the fol-
Fig. 4. XPS spectrum of the sample TF4 (a) grown on aluminum substrates using electrochemical deposition, with core level Mo3d and Al2p (insets); deconvoluted core level spectra of Mo3d (b) and Al2p (c).
lowing part of this paper, we will compare the hysteresis loops of the four representative samples. Fig. 5a and b show the hysteresis loops recorded at room temperature for the following films: (i) TF2 (Co–Ni in Fig. 5a – 69.71 at.% Co, 16.95 at.% Ni, 4.96 at.% Al, 8.38 at.% O) and (ii) Co–Ni–Mo thin films (samples TF4, TF6 and TF8) electrodeposited at different current densities (Co–Ni–Mo in Fig. 5b). The shapes of M(H) loops are different, this being a function of the microstructure of the sample,
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Fig. 6. The dependence of the coercive field (Hc ) of electrodeposited Co–Ni–Mo thin films on the Mo content (at.%) in films.
Fig. 5. Comparison between hysteresis curves of the sample TF2 (from Co–Ni alloy) deposited at 2.54 mA cm−2 (a) and for the samples TF4, TF6, TF8 (from Co–Ni–Mo alloy) deposited at 2.10 mA cm−2 , 7.15 mA cm−2 and 15.92 mA cm−2 , respectively (b).
changed according to the applied current density. The changes of the shape of the hysteresis loops reflect the changes in the magnetization processes and in the switching properties of the films. The presence of Mo into Co–Ni plating baths modifies the morphology of the deposits because of their influence on the growth of the initial crystal nuclei. The hysteresis loops recorded for the studied samples reveal that the saturation magnetization decreases when the molybdenum content of the films increases from 14.58 at.% to 25.36 at.%. Simultaneously, a gradually increasing coercivity is observed in the range of 16.5–29 kA/m (Fig. 5a and b). For the film of ∼14.58 at.% Mo, a coercivity (Hc ) of about 17.8 kA/m was obtained, while for the film TF8 (∼25.36 at.% Mo) a coercivity of about 28 kA/m. Low but moderate contents of molybdenum should be more suitable to attain small coercivity values maintaining high saturation magnetization. Increase of the molybdenum content in the films results in changing the morphology of samples. From SEM experiments we have found that the morphology of the samples is different for the samples with different Mo content. This leads to changes in magnetic anisotropy of the films, due to changes in shape and size of crystallites of Co–Ni and thus result in changes of the magnetic field at which magnetization saturation occurs. The dependence of the coercive field (Hc ) was also studied as a function of the Mo content in the film. For the film of ∼14.58 at.% Mo, a coercivity (Hc ) of about 17.8 kA/m was obtained, while for the film TF8 (∼25.36 at.% Mo) a coercivity of about 28 kA/m. Low but moderate contents of molybdenum should be
more suitable to attain small coercivity values maintaining high saturation magnetization. As we have found by performing hysteresis loop measurements, the coercivity increased while the saturation magnetization decreased as the molybdenum content (cMo ) was increased, in the same manner as it was shown for some Co–Mo films in [22]. Fig. 6 shows the dependence of the coercivity (Hc ) on the molybdenum content (cMo ) in films (line is only a guide for the eyes). The coercive field varied between 16.5 and 29 kA m−1 depending on cMo . This behavior should be correlated with the magnetic domain structure (modified due to different morphology). The phenomenon of domain walls pinning on the Mo impurities is increased when the Mo content in films is increased, therefore coercive field increase in films with larger Mo content. The measurements of resistance versus applied dc magnetic field were performed at room temperature using four-probe method for applied magnetic fields varied between ±220 kA/m. In this study, the magnetoresistance was defined as R(H) − R(Hmax ) R (%) = × 100 R R(Hmax )
(1)
In Eq. (1), R(H) represents the film resistance measured in the field H and R(Hmax ) is the resistance in the maximum applied magnetic field. It was known that the addition of molybdenum to the Ni–Fe deposits enhances the resistivity, but reduces the MR effect [23]. In this paper, we study the effect of the Mo content to Co–Ni films. Fig. 7a–c shows the variation of resistance as a function of the applied magnetic field curves for the samples TF4 (Fig. 7a), TF6 (Fig. 7b) and TF8 (Fig. 7c). The magnetoresistance effect is decreased for higher Mo content. The magnetoresistance ratio (R/R%) varied between 1.4% in the case of the sample labeled TF8, and about 8.3% for the sample TF4, depending on the molybdenum content in film and having a similar kind of field dependence. From magnetoresistance measurements, we have found that the increase in molybdenum content in films (up to 25 at% Mo) enhances the resistance of the samples, but it reduces the MR effect. One possible explanation is on the one hand the existence of a combined effect of both composition and physical microstructure on the MR behavior. On the other hand, the behavior of the magnetoresistance can be explained similar with the results obtained in [13]. We suppose that the shape of the magnetoresistance curves is influenced by the existence of the boundaries between grains (rich in molybdenum oxides and Al2 O3 ) which contribute to the increased resistance of
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Fig. 8. Reflection spectra recorded for Co–Ni and Co–Ni–Mo thin films.
see in Fig. 8, the reflection coefficient is different for the various samples and it changes especially in the wavelength domain where this factor has maximum value. The maximum values are at 950 nm for the samples TF4, TF6 and TF8, which explains the different colors of the samples. One can observe that up to the wavelength of 500 nm the reflectance coefficient is very low (for the samples TF2 and TF3), indicating a high light absorption in that respective spectral region. From optical measurements we have found an increase of reflection coefficient with the wavelength within spectral domain 450–950 nm, both for the sample TF6 and TF8. Moreover, one can remark the gradually increase of the reflectance up to about 90% (in the case of samples TF6 and TF8) at 950 nm wavelength. This indicates that, similar to the magnetic properties, the optical properties of the Co–Ni–Mo thin films are strongly dependent on the molybdenum content in the films. 4. Conclusions
Fig. 7. Resistance (R) vs magnetic field curves for the samples TF4 (a), TF6 (b), and TF8 (c).
the films. It should be emphasized that the large values of the magnetoresistance are apparently affected by both the presence of the molybdenum in the films and the existence of the aluminum oxide on the grains frontiers.By the visual examination of the samples, we observed that they have different colors depending on the current density at which they were electrodeposited. For this reason, in order to characterize the optical properties of the films, the reflection spectra were measured. The reflection spectra for the studied samples are presented in Fig. 6. The spectra were recorded using an aluminum sheet as reference. Fig. 8 shows the typical reflection spectra in the wavelength range 400–1000 nm for Co–Ni and Co–Ni–Mo thin films. As we can
Co–Ni and Co–Ni–Mo alloys thin films were electrodeposited on Al substrate, using a complex solution containing CoSO4 ·7H2 0, NiSO4 ·7H2 0, Na2 MoO4 ·10H2 O, Na3 C6 H5 O7 ·5H2 O and specific additives. We have found that the composition of deposits was controlled by the applied current density. Taking into account the values of the applied current density, three thin films obtained at 2.10 mA cm2 (TF4) of Co–Ni–Mo alloy, 7.15 mA cm2 (TF6) and 15.92 mA cm2 (TF8), respectively, were presented as examples in this paper. In the identical preparation conditions we have obtained thin films with different compositions, only by the variation of the applied current density. The current density was varied with the aim to compare the influence of these parameters on the morphology and physical properties of the films. As a consequence of the increase in current density, the Mo content in the samples increases and this has an important role on the magnetic, optical and magnetoresistance properties of the films. The values of the coercive field varied in the range Hc = (16.5–29) kA m−1 . The electrodeposited Co–Ni–Mo alloys thin films display a MR effect in the range 1.4–8.3%. From the magnetoresitance measurements we have found that the increase in molybdenum content in films enhances the resistance of the samples, but reduces the MR effect. Acknowledgements The authors are grateful to Prof. G.G. Rusu from Faculty of Physisc, “Alexandru Ioan Cuza” University for the optical measure-
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ments and to Mr. A.V. Sandu from the Faculty of Materials Science and Engineering, “Gheorghe Asachi” Technical University for the SEM measurements. S.I.T. wishes to thank Ms. C. Minculescu for her help during this work. This work was partially supported by the Ministry of Education, Research, Youth and Sport and by the Social European Funds provided through projects 5244 No. RO08POSDRU-6/1.5/S/25 and POSDRU 89/1.5/S/49944. References [1] T.S. Chin, J. Magn. Magn. Mater. 209 (2000) 75. [2] N.V. Myung, D.Y. Park, B.Y. Yoo, P.T.A. Sumodjo, J. Magn. Magn. Mater. 265 (2003) 189–198. [3] T. Homma, Y. Kita, T. Osaka, J. Electrochem. Soc. 147 (1) (2000) 160. [4] T. Osaka, Electrochim. Acta 45 (2000) 3311–3321. [5] E.I. Cooper, et al., IBM J. Res. Dev. 49 (2005) 103–126. [6] G.W. Qin, K. Oikawa, M. Sato, O. Kitakami, Y. Shimada, K. Fukamichi, K. Ishida, IEEE Trans. Mag. 41 (2005) 918–920. [7] H.-S. Nam, T. Yokoshima, T. Nakanishi, T. Osaka, Y. Yamazaki, D.N. Lee, Thin Solid Films 384 (2001) 288. [8] I. Tabakovic, V. Inturi, S. Riemer, J. Electrochem. Soc. 149 (2002) C18.
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Morphology, magnetic, magnetoresistance and optical properties of Co–Ni–Mo alloys thin films S.I. Tanase a,b,∗ , D. Pinzaru Tanase a , M. Dobromir a , V. Georgescu a a b
Faculty of Physics, “Alexandru Ioan Cuza” University, Iasi, 700506, Romania Department of Physics, “Alexandru Cel Bun” College, Gura Humorului, 725300, Romania
a r t i c l e
i n f o
Article history: Received 8 March 2011 Received in revised form 30 July 2011 Accepted 31 July 2011 Available online 6 August 2011 Keywords: Co–Ni alloy Thin films Magnetic properties Magnetoresistance
a b s t r a c t We present in this paper several results concerning the preparation by means of electrolysis and characterization of Co–Ni–Mo thin films. Co–Ni–Mo thin films with different molybdenum content in the range 0–25 at% Mo were prepared from a complex solution containing ions of Co, Ni and Mo, using galvanostatic control, on aluminum substrates. The effects of applied current density on the morphology, magnetic, magnetoresistance, and optical properties of the electrodeposited Co–Ni–Mo films were investigated. The applied current density significantly influenced the film composition and their magnetic properties. The increase of molybdenum content in Co–Ni films (up to 25 at% Mo) enhances the resistivity, but it reduces the magnetoresistance effect. We report the first observation of magnetoresistance as high as 8% in Co–Ni–Mo thin films. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Magnetic thin films based on iron group elements have received much attention due to their importance in the electronic industries. Many studies were carried out to develop ternary alloys based on Co–Ni system with additions, because of their remarkable magnetic properties and potential applications in micro-electromechanical (MEMS) systems [1,2] or in the electronic industries for storage and high-density recording [3–6]. Cobalt alloys with soft-magnetic behavior have been studied with a view to microelectronic devices [7–9]. Thin films of Co, Ni, Fe-base alloys with Mo, N, Mg and P as impurity have been studied for a long time. Gomez et al. [10] reported that introduction of 5–11 wt.% molybdenum into cobalt electrodeposits leads to obtain materials with low-coercitivity. It was demonstrated that the presence of molybdenum in cobalt coatings led to a soft-magnetic response [11], so they could be used in magnetic devices such as sensors and actuators. Films containing Co–Mo [6], Ni–Mo [12], and electrodeposited Co–Ni–N [13], Co–Ni–P [14], Co–Ni–Mg [15], Co–Ni–Mg–N [16] were also studied. Mandreci [17] reported that thin Co–Ni–Mo films with good magnetic properties could be produced by electrodeposition. The properties of the Co–Ni alloy could be modified by the introduction of a third element in the alloy. In this line, the introduction of molybdenum in the Co–Ni films might improve their magnetic and magnetoresistance properties.
∗ Corresponding author. Tel.: +40 232 201050; fax: +40 232 201150. E-mail address: tanase [email protected] (S.I. Tanase). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.07.139
We are interested in the electrodeposition of Co–Ni–Mo films because they are used in a variety of applications. In addition, their electrodeposition behavior is unique, since molybdenum cannot be deposited from an aqueous electrolyte [6] – this type of electrodeposition is known as “induced co-deposition” [18] – but it can be co-deposited in the presence of iron group metallic ions. In our previous paper [13], we have reported on tunneling magnetoresistance (TMR) of Co–Ni alloys thin films with nitrogen as impurity, electrodeposited on aluminum substrate. In order to obtain the TMR effect for Co–Ni–Mo films, it was essential to use aluminum as substrate that oxidizes very easy. The purposes of this study are to obtain Co–Ni–Mo thin films by electrodeposition and to investigate the influence of the applied current density on the morphology, magnetic, magnetoresistance, and optical properties of thin electrodeposited Co–Ni–Mo films, correlated with the deposition parameters; this study is of interest for technological applications. The films were obtained as a particular case of the electrodeposition of Co–Ni–N thin films [13]. We did not find any study in the literature referring to the magnetic and magneto-transport properties of the electrodeposited Co–Ni–Mo thin films. Consequently, as far as we know, the magnetoresistance property of electrodeposited Co–Ni–Mo thin films has never been studied until now. Magnetoresistance in such thin films is a very promising research direction. 2. Experimental All the films studied in this paper were prepared by electrodeposition on aluminum substrates shaped as disks (geometric area = 3.14 cm2 ). The bath composition and the electrodeposi-
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Table 1 Electrolyte composition (g l−1 in double distilled water) for Mo, Co–Ni and Co–Ni–Mo films electrodeposited, labeled M, CN and CNM, respectively. Electrolytes
CoSO4 ·7H2 O
NiSO4 ·7H2 O
Na2 MoO4 ·2H2 O
NiCl2 ·6H2 O
Na2 SO4 ·10H2 O
Na3 C6 H5 O7 ·5H2 O
H3 BO3
NaCl
M CN CNM
– 30 30
– 50 50
0.6 – 0.6
– 10 10
50 50 50
– – 0.6
30 30 30
10 10 10
tion conditions were set accordingly to our previous publications [13]. The electrochemical experiments were performed galvanostatic in a conventional three-electrode cell by using a nickel anode and a working electrode of aluminum disk; quasi-reference electrode was a platinum wire that was embedded in a glass tube. By depositing some thick (∼450 nm) films of Co–Ni, Mo, and Co–Ni–Mo alloys in well-established electrolytic conditions, we determined the electrodeposition rate for these alloys. Taking into account these experiments, the thickness of the samples was controlled by the electric charge passing through the electrolyte during the plating process. In this paper we performed the studies of the morphology of the samples using a Vega II LSH scanning electron microscope (SEM) coupled with a detector EDX, model Quantax QX2. This detector is used for the quantitative and qualitative microanalysis of the thin films using Mn K␣ radiation. The X-ray photoelectron spectroscopy (XPS) experiments were carried out using a PHI 5000 Versa Probe ( ULVAC-PHI, Inc.) apparatus. A monochromatic Al K␣ Xray (1486.7 eV) was used as excitation source. The sputter-etching process was done using an Ar ion gun. Magnetic measurements were carried out at room temperature using an induction type device with data acquisition system and ac magnetic field applied in the film plane at a frequency of 50 Hz, in 60 kA m−1 maximum field. The resistance (R) measurements of the samples were performed at room temperature by using four-terminal contacts applied on the film surface, and an HM 8112-2 programmable multimeter working in regime of a constant current flowing in the plane of the films (current-in-plane geometry). The correction for the current passing the sample (7 A) is compensated automatically, by construction. Reflection spectra corrected for the effect of aluminum substrates were acquired in the wavelength range 400–1000 nm using a computer controlled STEAG-ETA Optic Spectrometer.
Table 2 Notations for the series of Co–Ni and Co–Ni–Mo thin films deposited at different current density (j) from the solutions labeled M, CN and CNM. Sample
j (mA cm2 )
at.% Mo
Solution
TF1 TF2 TF3 TF4 TF5 TF6 TF7 TF8
0.95 1.60 2.54 2.10 5.75 7.15 11.34 15.92
69.24 0 0 14.58 16.88 20.42 21.71 25.36
M CN CN CNM CNM CNM CNM CNM
In the present work we deposited the series of thin films labeled for brevity TF1–TF8, as it is shown in Table 2, using the solutions from Table 1. The samples were prepared using different values of current density (j), in the range of 1–16 mA cm2 , but maintaining the others working parameters of the electrodeposition identical. From the SEM analysis we found that the molybdenum content (cMo ) of the deposits increases as the current density is increased. For example, the molybdenum content increases from 14.58 (sample TF4) to 25.36 at% Mo (sample TF8), when the current density increases from 2.10 to 15.92 mA cm−2 . The effect of the current density on the molybdenum content (cMo ) of the deposited films is shown in Fig. 1. Fig. 1 shows the dependence of the molybdenum content (cMo ) in the films on the current density (j) applied during electrodeposition, for the studied samples (line is only a guide for the eyes). We found that the current density exhibits a strong influence on the molybdenum content in films, determined from the SEM experiments. The Co–Ni–Mo films deposited at low current density contain a large amount of oxygen as in Ref. [19]. In order to interpret the changes observed due to addition of the Mo to the Co–Ni alloy films, morphological analysis of the deposits was performed by SEM. We have found that the surface morphology of the electrodeposited films changes as a result of increasing
3. Results and discussion We have started the research by experiments on electrodeposition performed separately of Co–Ni alloys and Mo element, in order to optimize the solution composition and deposition parameters. After that, the experiments were carried out for the Co–Ni and Co–Ni–Mo thin films deposition from the sulfate baths of the composition given in Table 1. For deposition of Mo films, the solution (labeled M) contained mainly Na2 MoO4 ·10H2 O and some additives were used. The deposition of Co–Ni films was performed from the solution labeled CN, containing CoSO4 ·7H2 O and NiSO4 ·7H2 O as the base elements. The solutions labeled CNM for electrodeposition of Co–Ni–Mo films contain compounds of the three metals and a similar content of additives (NiCl2 ·6H2 O, Na2 SO4 ·10H2 O, Na3 C6 H5 O7 ·5H2 O, H3 BO3 and NaCl,) for all baths. Before each experiment, the aluminum substrates were mechanically polished, degreased with acetone, etched for a short time in dilute HNO3 , and washed in distilled water. In all experiments, the temperature was kept at 25 ◦ C and the electrolyte pH was adjusted to 4.0 by adding H2 SO4 , without stirring the solution. The film thickness for all samples was controlled by electric charge passing through the electrolyte.
Fig. 1. The effect of applied current density (j) on the molybdenum content (cMo ) in films.
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Fig. 2. Scanning electron micrographs for the sample TF1 (a), electrodeposited at 0.95 mA cm−2 from the solution labeled M and for the samples TF4 (b) and TF6 (c) electrodeposited from the solution labeled CNM at 2.10 mA cm−2 , and 7.15 mA cm−2 , respectively.
current density (j). Fig. 2 shows the scanning electron micrograph for the samples TF1 (2a), electrodeposited at 0.95 mA cm−2 from the solution labeled M and for the samples TF4 (2b) and TF6 (2c) electrodeposited from the solution labeled CNM at 2.10 mA cm−2 , and 7.15 mA cm−2 , respectively. Fig. 2 shows that the films contain polygonal grains with size of 2–70 m composed of molybdenum oxides crystallites (as we will show by XPS experiments). The Co–Ni grains are grown from different nucleation sites onto the aluminum substrate and they tend to form conglomerates, especially in the case of the samples TF4 and TF6 (Fig. 2b and c), obtained at 2.10 mA cm−2 and 7.15 mA cm−2 , respectively. The morphology of the samples is similar with those obtained by Gomez [20] in the case of Co–Ni–Mo films electrodeposited onto glass/ITO support, where Co–Ni–Mo alloy islands were grown over the initial molybdenum oxide film. From the preliminary SEM experiments, we have found that the film composition is influenced by the applied current density. For example, the films labeled TF4 deposited at 2.10 mA cm−2 contain 14.58 at% Mo, and those deposited at 7.15 mA cm−2 contain 20.42 at% Mo (sample TF6). From the SEM/EDAX analyses, we can conclude that the Co–Ni–Mo films consist in polygonal grains with various diameters which nucleate in different sites on the substrate. The surface morphology is similar for all sam-
ples and the SEM images also illustrate that the remaining surface between the polygonal grains contains Al and O. The uncovered sites on the substrate oxidize very easy and the conditions for electric transport are changed, favoring tunneling magnetoresistance effect due to Al2 O3 and molybdenum oxides low conductive layers. The XPS analysis permits the determination of the oxidation state of the metals present in the alloys deposits. The molybdenum and aluminum oxidation state was estimated by deconvolution of the peaks in the Mo3d and Al2p region. We investigated the studied samples and we gave as example in Fig. 3a the spectrum of the sample TF1 (67.99 at.% Mo, 16.95 at.% Al, 15.06 at.% O) deposited from the solution labeled M (Table 2) with core levels Mo3d (insets). The Mo3d spectrum typically consisted of a Mo3d3/2 –Mo3d5/2 doublet due to the spin orbit coupling. Taking into account the fact that Mo exists in a number of oxidation state as oxides and hydroxides [21], these doublets correspond to the different Mo species in the deposits. After peak deconvolution (Fig. 3b) we have investigated a mixture of polyvalent Mo oxides (mainly MoO2 at 229.9 eV and Mo2 O5 at 231.93 eV). Although it is not possible to determine the relative amount of the oxides and of metallic Mo because it is a mixture, it can be concluded that in all case we had a deposit basically constituted of oxides doped with a small amount Mo metallic.
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Fig. 3. XPS spectrum of the sample TF1 (a) grown on aluminum substrates using electrochemical deposition, with core level Mo3d (insets); deconvoluted core level spectra of Mo3d (b).
In Fig. 4a we give as an example the XPS spectrum of the sample TF4 (14.58 at.% Mo, 12.19 at.% Co, 6.35 at.% Ni, 38.75 at.% Al, 28.13 at.% O) deposited from the solution labeled CNM (Table 2) with core levels Mo3d and Al2p (insets). After peak deconvolution (Fig. 4b) we have investigated a mixture of polyvalent Mo oxides as in the case of sample TF1; the aluminum oxidation state was estimated by deconvolution of the peak in the Al2p region (Fig. 4c). In the case of Co–Ni–Mo films, there are a lot of uncovered sites on Al substrate, as we have found from the SEM measurements, indicative of a good phase separation between the ferromagnetic granules and the molybdenum oxides and Al2 O3 substrate regions. Moreover, the intense O1s signal present in the XPS spectrum of sample TF4 (Fig. 4a) is a proof that the Al substrate is in fact oxidized. By these experiments we demonstrated that the aluminum is in fact oxidized and thus is enhances the films resistivity; on the other hand, we suppose that the shape of the magnetoresistance curves is influenced by the existence of the boundaries between grains (rich in molybdenum oxides and Al2 O3 ) which contributes to the increased resistance of the films. It should be emphasized that the large values of the magnetoresistance are apparently affected by both the presence of the molybdenum in the films and the existence of the aluminum oxide on the grains frontiers. By changing the current density, we found that both the surface morphology and the molybdenum content (cMo ) in the Co–Ni–Mo films are different and this leads to the modification of the magnetic properties of the films. Starting from these observations, in the fol-
Fig. 4. XPS spectrum of the sample TF4 (a) grown on aluminum substrates using electrochemical deposition, with core level Mo3d and Al2p (insets); deconvoluted core level spectra of Mo3d (b) and Al2p (c).
lowing part of this paper, we will compare the hysteresis loops of the four representative samples. Fig. 5a and b show the hysteresis loops recorded at room temperature for the following films: (i) TF2 (Co–Ni in Fig. 5a – 69.71 at.% Co, 16.95 at.% Ni, 4.96 at.% Al, 8.38 at.% O) and (ii) Co–Ni–Mo thin films (samples TF4, TF6 and TF8) electrodeposited at different current densities (Co–Ni–Mo in Fig. 5b). The shapes of M(H) loops are different, this being a function of the microstructure of the sample,
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Fig. 6. The dependence of the coercive field (Hc ) of electrodeposited Co–Ni–Mo thin films on the Mo content (at.%) in films.
Fig. 5. Comparison between hysteresis curves of the sample TF2 (from Co–Ni alloy) deposited at 2.54 mA cm−2 (a) and for the samples TF4, TF6, TF8 (from Co–Ni–Mo alloy) deposited at 2.10 mA cm−2 , 7.15 mA cm−2 and 15.92 mA cm−2 , respectively (b).
changed according to the applied current density. The changes of the shape of the hysteresis loops reflect the changes in the magnetization processes and in the switching properties of the films. The presence of Mo into Co–Ni plating baths modifies the morphology of the deposits because of their influence on the growth of the initial crystal nuclei. The hysteresis loops recorded for the studied samples reveal that the saturation magnetization decreases when the molybdenum content of the films increases from 14.58 at.% to 25.36 at.%. Simultaneously, a gradually increasing coercivity is observed in the range of 16.5–29 kA/m (Fig. 5a and b). For the film of ∼14.58 at.% Mo, a coercivity (Hc ) of about 17.8 kA/m was obtained, while for the film TF8 (∼25.36 at.% Mo) a coercivity of about 28 kA/m. Low but moderate contents of molybdenum should be more suitable to attain small coercivity values maintaining high saturation magnetization. Increase of the molybdenum content in the films results in changing the morphology of samples. From SEM experiments we have found that the morphology of the samples is different for the samples with different Mo content. This leads to changes in magnetic anisotropy of the films, due to changes in shape and size of crystallites of Co–Ni and thus result in changes of the magnetic field at which magnetization saturation occurs. The dependence of the coercive field (Hc ) was also studied as a function of the Mo content in the film. For the film of ∼14.58 at.% Mo, a coercivity (Hc ) of about 17.8 kA/m was obtained, while for the film TF8 (∼25.36 at.% Mo) a coercivity of about 28 kA/m. Low but moderate contents of molybdenum should be
more suitable to attain small coercivity values maintaining high saturation magnetization. As we have found by performing hysteresis loop measurements, the coercivity increased while the saturation magnetization decreased as the molybdenum content (cMo ) was increased, in the same manner as it was shown for some Co–Mo films in [22]. Fig. 6 shows the dependence of the coercivity (Hc ) on the molybdenum content (cMo ) in films (line is only a guide for the eyes). The coercive field varied between 16.5 and 29 kA m−1 depending on cMo . This behavior should be correlated with the magnetic domain structure (modified due to different morphology). The phenomenon of domain walls pinning on the Mo impurities is increased when the Mo content in films is increased, therefore coercive field increase in films with larger Mo content. The measurements of resistance versus applied dc magnetic field were performed at room temperature using four-probe method for applied magnetic fields varied between ±220 kA/m. In this study, the magnetoresistance was defined as R(H) − R(Hmax ) R (%) = × 100 R R(Hmax )
(1)
In Eq. (1), R(H) represents the film resistance measured in the field H and R(Hmax ) is the resistance in the maximum applied magnetic field. It was known that the addition of molybdenum to the Ni–Fe deposits enhances the resistivity, but reduces the MR effect [23]. In this paper, we study the effect of the Mo content to Co–Ni films. Fig. 7a–c shows the variation of resistance as a function of the applied magnetic field curves for the samples TF4 (Fig. 7a), TF6 (Fig. 7b) and TF8 (Fig. 7c). The magnetoresistance effect is decreased for higher Mo content. The magnetoresistance ratio (R/R%) varied between 1.4% in the case of the sample labeled TF8, and about 8.3% for the sample TF4, depending on the molybdenum content in film and having a similar kind of field dependence. From magnetoresistance measurements, we have found that the increase in molybdenum content in films (up to 25 at% Mo) enhances the resistance of the samples, but it reduces the MR effect. One possible explanation is on the one hand the existence of a combined effect of both composition and physical microstructure on the MR behavior. On the other hand, the behavior of the magnetoresistance can be explained similar with the results obtained in [13]. We suppose that the shape of the magnetoresistance curves is influenced by the existence of the boundaries between grains (rich in molybdenum oxides and Al2 O3 ) which contribute to the increased resistance of
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Fig. 8. Reflection spectra recorded for Co–Ni and Co–Ni–Mo thin films.
see in Fig. 8, the reflection coefficient is different for the various samples and it changes especially in the wavelength domain where this factor has maximum value. The maximum values are at 950 nm for the samples TF4, TF6 and TF8, which explains the different colors of the samples. One can observe that up to the wavelength of 500 nm the reflectance coefficient is very low (for the samples TF2 and TF3), indicating a high light absorption in that respective spectral region. From optical measurements we have found an increase of reflection coefficient with the wavelength within spectral domain 450–950 nm, both for the sample TF6 and TF8. Moreover, one can remark the gradually increase of the reflectance up to about 90% (in the case of samples TF6 and TF8) at 950 nm wavelength. This indicates that, similar to the magnetic properties, the optical properties of the Co–Ni–Mo thin films are strongly dependent on the molybdenum content in the films. 4. Conclusions
Fig. 7. Resistance (R) vs magnetic field curves for the samples TF4 (a), TF6 (b), and TF8 (c).
the films. It should be emphasized that the large values of the magnetoresistance are apparently affected by both the presence of the molybdenum in the films and the existence of the aluminum oxide on the grains frontiers.By the visual examination of the samples, we observed that they have different colors depending on the current density at which they were electrodeposited. For this reason, in order to characterize the optical properties of the films, the reflection spectra were measured. The reflection spectra for the studied samples are presented in Fig. 6. The spectra were recorded using an aluminum sheet as reference. Fig. 8 shows the typical reflection spectra in the wavelength range 400–1000 nm for Co–Ni and Co–Ni–Mo thin films. As we can
Co–Ni and Co–Ni–Mo alloys thin films were electrodeposited on Al substrate, using a complex solution containing CoSO4 ·7H2 0, NiSO4 ·7H2 0, Na2 MoO4 ·10H2 O, Na3 C6 H5 O7 ·5H2 O and specific additives. We have found that the composition of deposits was controlled by the applied current density. Taking into account the values of the applied current density, three thin films obtained at 2.10 mA cm2 (TF4) of Co–Ni–Mo alloy, 7.15 mA cm2 (TF6) and 15.92 mA cm2 (TF8), respectively, were presented as examples in this paper. In the identical preparation conditions we have obtained thin films with different compositions, only by the variation of the applied current density. The current density was varied with the aim to compare the influence of these parameters on the morphology and physical properties of the films. As a consequence of the increase in current density, the Mo content in the samples increases and this has an important role on the magnetic, optical and magnetoresistance properties of the films. The values of the coercive field varied in the range Hc = (16.5–29) kA m−1 . The electrodeposited Co–Ni–Mo alloys thin films display a MR effect in the range 1.4–8.3%. From the magnetoresitance measurements we have found that the increase in molybdenum content in films enhances the resistance of the samples, but reduces the MR effect. Acknowledgements The authors are grateful to Prof. G.G. Rusu from Faculty of Physisc, “Alexandru Ioan Cuza” University for the optical measure-
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ments and to Mr. A.V. Sandu from the Faculty of Materials Science and Engineering, “Gheorghe Asachi” Technical University for the SEM measurements. S.I.T. wishes to thank Ms. C. Minculescu for her help during this work. This work was partially supported by the Ministry of Education, Research, Youth and Sport and by the Social European Funds provided through projects 5244 No. RO08POSDRU-6/1.5/S/25 and POSDRU 89/1.5/S/49944. References [1] T.S. Chin, J. Magn. Magn. Mater. 209 (2000) 75. [2] N.V. Myung, D.Y. Park, B.Y. Yoo, P.T.A. Sumodjo, J. Magn. Magn. Mater. 265 (2003) 189–198. [3] T. Homma, Y. Kita, T. Osaka, J. Electrochem. Soc. 147 (1) (2000) 160. [4] T. Osaka, Electrochim. Acta 45 (2000) 3311–3321. [5] E.I. Cooper, et al., IBM J. Res. Dev. 49 (2005) 103–126. [6] G.W. Qin, K. Oikawa, M. Sato, O. Kitakami, Y. Shimada, K. Fukamichi, K. Ishida, IEEE Trans. Mag. 41 (2005) 918–920. [7] H.-S. Nam, T. Yokoshima, T. Nakanishi, T. Osaka, Y. Yamazaki, D.N. Lee, Thin Solid Films 384 (2001) 288. [8] I. Tabakovic, V. Inturi, S. Riemer, J. Electrochem. Soc. 149 (2002) C18.
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