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Structural, morphological and magnetic characterization of electrodeposited Co–Fe–W alloys
Journal of Alloys and Compounds 611 (2014) 243–248
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Structural, morphological and magnetic characterization of electrodeposited Co–Fe–W alloys R. Della Noce a,⇑, A.V. Benedetti a, M. Magnani a, E.C. Passamani b, H. Kumar c, D.R. Cornejo c, C.A. Ospina d a
Instituto de Química, Universidade Estadual Paulista, UNESP, 14800-900 Araraquara, SP, Brazil Departamento de Física, Universidade Federal do Espírito Santo, 29075-910 Vitória, ES, Brazil c Instituto de Física, Universidade de São Paulo, USP, 05508-090 São Paulo, SP, Brazil d Electron Microscopy Laboratory, Brazilian Nanotechnology National Laboratory, 13083-970 Campinas, SP, Brazil b
a r t i c l e
i n f o
Article history: Received 12 February 2014 Received in revised form 22 May 2014 Accepted 24 May 2014 Available online 2 June 2014 Keywords: Co–Fe–W alloys Electrodeposition Mössbauer spectroscopy X-ray diffractometry
a b s t r a c t Structural, morphological and magnetic characterization of electrodeposited Co–Fe–W alloys, containing small amounts of W (up to 9 at.%), were performed using X-ray diffractometry, scanning (SEM) and transmission (TEM) electron microscopy, Mössbauer spectroscopy and magnetization measurements. Electrodeposited (Co100 xFex)100 yWy films (x = 63–72 at.% Fe, y = 4–9 at.% W) were successfully produced varying the applied cathodic current density (ic) between 0.5 and 10 mA cm 2. X-ray diffraction results revealed a bcc-like structure for all studied compositions with average crystallite size ranging from 16 to 35 nm, as also confirmed by TEM results. SEM images indicated that needle-type morphology is dominant for the deposits containing lower W content (up to 4.5 at.%.), while a cauliflower-type behavior is observed for higher W content deposits. Room temperature Mössbauer spectra indicate the presence of two magnetic species for all samples; one component associated with an ordered Co–Fe–W fraction (crystalline grain core) and a magnetic disordered Co–Fe–W contribution, which can be attributed to the grain boundaries/grain surfaces. Magnetization was observed to be in the film plane along the film direction, except the sample prepared at ic = 10 mA cm 2 that is slightly canted from in- to out-of-plane geometry. Magnetic measurements show high saturation magnetization values accompanied by low coercivity ones for the electrodeposited Co–Fe–W alloys, making these alloys potential materials for applications in magnetic devices such as read/write heads and hard disks. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction In the last decades, much attention has been paid to the Co–Fe system, in particular to the Co35Fe65 alloy, due to its excellent magnetic and mechanical properties [1–3]. Such properties include high saturation magnetization (>2.4 Tesla) and Curie temperature (>1000 °C), high electric permeability and good thermal stability. Moreover, the addition of refractory elements (W, Mo or V) to the Co–Fe binary system results in improvement on hardness and corrosion resistance, enhancement of durability, and increase of thermal resistance, among others. In this context, Co–Fe–M (M = W, Mo, V or Re) alloys have been lately produced by different techniques including sputtering, molecular beam epitaxy (MBE), physical vapor deposition (PVD), and electrodeposition etc. [4–7]. Among these synthesis techniques, electrodeposition has stood out due to the following ⇑ Corresponding author. Tel.: +55 16 33019782; fax: +55 16 33019637. E-mail address: [email protected] (R.D. Noce). http://dx.doi.org/10.1016/j.jallcom.2014.05.157 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
important features: cost-effectiveness, high quality deposits, production at room temperature (no need of high vacuum), and easy operating conditions. Therefore, various Co–Fe–M ternary alloys have been fabricated by electrodeposition with different purposes and applications [8–10]. The Co–Fe–M alloys, in particular the Co–Fe–W alloys, are also interesting from theoretical viewpoint because this system shows simultaneously the phenomena of anomalous [11–13] and induced co-deposition in electrodeposition processes [14–17]. Specifically, the above two effects have been described in detail by Esteves et al. [18] for electrodeposited Co–Ni–Mo thin films, and recently by our group for electrodeposited Co–Ni–W alloys from a glycine-containing bath [19]. Consequently, the present work is aimed to study the structure, morphology, atomic arrangements and magnetic properties of electrodeposited Co–Fe–W alloys, with composition close to Co35Fe65 and addition of low W contents (up to 9 at.%). Several works have been reported on the electrodeposition of Co–Fe binary alloys. However, as far as we know there are only three published papers concerning electrodeposited Co–Fe–W alloys hitherto
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[10,20,21]. In general, these works were focused on high W concentration, while the present work is aimed to add small W amounts (up to 9 at.%) to the Co–Fe system. Capel et al. [10] proposed the electrodeposition of Co–W and Co–Fe–W alloys onto stainless steel as an alternative to electrodeposited hard chrome. The authors studied the hardness and wear properties, and found that the Co–W alloy containing 27 at.% W exhibits the highest hardness after heat treatment (1185 kgf mm 2), a value even higher than electrodeposited hard chromium (848 kgf mm 2). For the Co43Fe30W27 alloy, on the other hand, the measured hardness value was 838 kgf mm 2. The others two Co–Fe–W papers were published by the same group in different journals [20,21]. Using Mössbauer spectroscopy (MS), they have been able to discussed the formation of amorphous state in Co–W–Fe alloys with W concentration higher than 14 at.% [21]. Therefore, in the present work, we focus on the structural, morphological and magnetic characterization of electrodeposited Co–Fe–W alloys for W concentration lower than 10 at.%. X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), Mössbauer spectroscopy and magnetic measurements have been used to analyze the structural and magnetic properties of electrodeposited Co–Fe–W films. 2. Experimental details Co–Fe–W alloys were galvanostatically (constant current application) prepared by using an EG & G PAR M283 potentiostat/galvanostat. A Tait-type cell was used for all electrodeposition experiments. The (1 1) cm2 copper plates (purity > 99.99%) were used as substrate. The alloys were electrodeposited from a citrate-containing unstirred bath composed of 0.04 M Fe2(SO4)3, 0.05 M CoSO4, 0.01 M Na2WO4, 0.2 M H3BO3 and 0.2 M citrate (Na3C6H5O7). By using this particular bath formulation, Fe-rich Co–Fe alloys with composition close to Co35Fe65 are expected to form due to the anomalous codeposition effect [11–13], where Fe is preferentially deposited in relation to Co (more noble metal). The bath pH was adjusted to 6 by utilizing diluted NaOH. The applied cathodic current density (ic) value was varied from 0.5 to 10 mA cm 2. Low ic-values were used to avoid the formation of undesired Fe-species such as oxides and hydroxides. A maximum of three or four depositions using the same bath were performed to avoid changes in the electrolyte composition. In all experiments, the charge passed to obtain the electrodeposits was equal to 35 C. The film thickness and the current efficiency were estimated from the mass of the deposits and their compositions by using the Faraday’s law. To perform these calculations, the number of electrons involved in the reduction processes of Fe, Co and W metallic were considered as 3, 2 and 6, respectively. The alloy composition was estimated by Energy Dispersive X-ray Spectrometry (EDS) coupled to SEM. The EDS analyses were performed by applying an accelerating voltage of 20 keV for all samples. X-ray diffraction (XRD) was employed to physically characterize the crystal structure of deposits. The XRD measurements were performed using Cu Ka radiation (1.5418 Å) in a Siemens D5000 instrument at scan rate of 0.002°/sec. Cross section TEM specimens, from the Cu (100 lm)/Co–Fe–W (5.2 lm) sample, were prepared by sandwiching them between Si substrates to ensure a good centering of the substrate and alloy interface for the subsequent manual, dimpling and ion polishing processes. To characterize the final TEM specimens, a JEM-2100 LaB6 microscope, operating in conventional mode at 200 kV, was used at the Electron Microscopy Laboratory, of the Brazilian Nanotechnology National Laboratory. Room temperature (RT) Conversion Electron Mössbauer Spectroscopy (CEMS) spectra were recorded with a 50 mCi 57Co(Rh) source in a conventional CEMS chamber. These CEMS spectra were fitted by a hyperfine field distribution using a histogram method plus one sextet. To reproduce the small asymmetry of the spectra, a linear correlation between hyperfine field (Bhf) and isomer shift (d) was assumed [22]. d-values are given relative to a-Fe at RT. Magnetic measurements were conducted at RT using a vibrating sample magnetometer (VSM). The probe magnetic field was applied in- and out-of-film-plane and varied from 20 kOe to 20 kOe.
3. Results and discussion 3.1. Compositional analysis
-0.7
E / V vs. Hg/HgSO4/K2SO4 Sat
244
-0.9 -1.0 -1.1 -1.2 -1.3 -1.4 -1.5 -1.6 -1.7 0
200
400
600
800
1000
1200
1400
time / minutes Fig. 1. E–t profiles for the electrodeposition of Co–Fe–W alloys by using different applied cathodic current densities (ic).
decreases to more negative values, as would be expected. It was also possible to evaluate the thicknesses of the films and the cathodic current efficiency (CCE) taking into account the deposited mass. These data are summarized in Table 1 together to the compositional analysis of the alloys that was obtained by EDS. It can be seen that the thickness and the CCE augment as the ic-values are increased. It should first be mentioned that, in general, the increase of the ic-quantity results in a concomitant reduction of thickness and CCE for electrodeposited alloys due to the increase of hydrogen evolution rate [19,23,24]. Nevertheless, in our case, this behavior seems to not occur because we have used a complexing agent (citrate), which results in higher W content in the deposits when higher ic-values were used. In our previous work [19] on electrodeposition of Co–Ni–W alloys using a glycine-based bath, it was noticed that the W content, thickness and CCE diminish with the increase of the ic-quantity. It is known that citrate is a stronger complexing agent than glycine, thus higher ic-values are needed to break the metal (W)-citrate bond in citrate-based bath, which in turn leads to a higher W content as observed in Table 1. It can be seen that higher ic leads to high W concentration in the deposits, which increases the deposit thickness and CCE due to the higher atomic weight of W compared to both Fe and Co. A similar behavior was observed by Mizushima et al. [25] in electrodeposition of Ni–W alloys on comparing different complexing agents including citrate and glycine. They reported that an increase in ic from 10 to 100 mA cm 2 leads to: (i) increase in W concentration from 17 to 25 at.% for citrate-based bath in agreement with our findings; (ii) decrease in W concentration from 10 to 2 at.% for glycine-based bath. It is important to note that all deposits are Fe-rich due to the preferential Fe deposition in relation to the more noble metal (Co) according to the above-mentioned anomalous co-deposition effect [11–13]. In addition, the increase of ic-values leads to deposits with high Fe content; consequently the Co concentration is reduced. These results confirm that the alloys have compositions close to Co35Fe65 plus W addition as initially predicted. It is worth mentioning that within this thickness range (lm), the alloys magnetic
Table 1 Composition, thickness and cathodic current efficiency (CCE) of electrodeposited Co–Fe–W films as a function of applied cathodic current density (ic). ic/mA cm
Fig. 1 shows the potential vs. time (E t) profiles used to obtain the Co–Fe–W alloys at different ic-values. Considering the fact that the passed charge was kept constant (35 C), the total time for each experiment is decreased with the increase of ic. In addition, it can be noted that when the ic-quantity is increased, the potential (E)
-2
0.5 mA cm -2 1 mA cm -2 2 mA cm -2 5 mA cm -2 10 mA cm
-0.8
0.5 1 2 5 10
2
Co at.%
Fe at.%
W at.%
Thickness/lm
CCE/%
32.1 29.6 27.9 23.4 19.2
63.2 66.3 67.6 69.7 71.9
4.7 4.1 4.5 6.9 8.9
2.9 3.1 3.8 4.6 5.2
30.8 31.3 33.2 47.7 55.2
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properties, which are discussed below, are expected to be due to the bulk (low dimension effect can be neglected). 3.2. Structural and morphological characterization Fig. 2 depicts the SEM images of the electrodeposited Co–Fe–W alloys prepared at different ic-values. SEM images exhibit compact, uniform and homogenous deposits for all studied conditions. For the deposits with low W contents (up to 4.5 at.% obtained for ic = 0.5, 1 and 2 mA cm 2), a needle-like morphology is clearly seen as shown in Fig 2a. Specifically, the observed small holes in Fig. 2b are attributed to the surface region where hydrogen evolution occurred during the electrodeposition process, which impeded the alloy deposition. The needle-like morphology has also been observed by Kim et al. [26] in Co–Fe alloys prepared by electrodeposition and Zhou et al. [27] in Fe–Pt electrodeposits. When ic is increased up to 10 mA cm 2, an abrupt morphology change is detected, i.e. from needle-like (Fig. 2c) to cauliflower-type morphology (Fig. 2d). In the latter case, the alloy with highest W-content (implies in a large film thickness) presents micro-cracks as observed in SEM images (not shown); an effect associated with internal stress caused by the interfacial mechanical coupling between the film and substrate, which is enhanced with increasing the film thickness [28]. Fig. 3 displays the XRD patterns of electrodeposited Co–Fe–W alloys obtained at different ic-values. It is clear noticed that all deposits have a bcc Fe-like phase because Fe is the major alloy component. This result suggests that both Co and W are totally dissolved into the Fe matrix. As a consequence, it can be said that there is the formation of a substitutional solid solution in these conditions with no presence of intermetallic compounds and/or segregated Fe, or Co pure phases. This observation is in agreement
with the equilibrium phase diagrams for each binary alloy (Co–W or Fe–W) [29], which show that intermetallic compounds may be formed only for much higher W concentrations (>20 at.%). Furthermore, another evidence of the formation of substitutional solid solution can be inferred whether the most intense crystalline peak (1 1 0) is analyzed. In the case of pure Fe, the (1 1 0) peak is located at 44.67°; however the XRD patterns of the electrodeposited Co– Fe–W alloys show that this peak is shifted to smaller 2h values, e.g. 44.55° (ic = 0.5 mA cm 2). This is an indication that the alloy lattice parameter increases by incorporation of W atoms in the bcc Fe-like matrix. In brief, the alloys have a bcc Fe-like structure with expanded lattice parameter due to the inclusion of W atoms because Co having similar atomic radius to Fe, in these contents, would not change the XRD bcc-like peaks angular position. Another important parameter that can be obtained from the XRD data is the average crystallite size. By using the Scherrer formula [30] and considering the (1 1 0) peak as the most intense, it was calculated the average crystallite size (td) for the electrodeposited Co–Fe–W alloys in the studied conditions, which is listed in Table 2 in conjunction with the magnetic parameters. The results indicate that the td-parameter ranges from 16 to 35 nm, depending on the ic-values. The lowest td-value (16 nm) was obtained for the alloy electrodeposited at the highest ic-value (10 mA cm 2). The observed grain size refinement as a function of the ic-quantity can be found in electrodeposited materials [19,31,32], and it is directly affected by the hydrogen evolution rate and also by the amount of nuclei formed during the electrodeposition process. Moreover, the observed broadening of the peaks in the XRD patterns as ic-quantity increases reveals that the crystalline grain sizes would be reduced up to a complete transformation into a nanocrystalline/amorphous state whether the ic would be increased to values higher than 10 mA cm 2. This structural transition was
Fig. 2. SEM images of electrodeposited Co–Fe–W alloys obtained at different ic-values: (a) 0.5, (b) 2, (c) 5 and (d) 10 mA cm
2
.
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Relative Intensity
3.3. Mössbauer spectroscopy and magnetic measurements
S
S
S -2 (110) -10 mA cm -2 -5 mA cm -2 -2 mA cm
(211)
(200)
-2 -1 mA cm -2 -0.5 mA cm
30
40
50
60 70 2 θ / degrees
80
(a) S
Relative Intensity
-2 -10 mA cm
pure Fe (44.67°)
(110)
-2 -5 mA cm -2 -2 mA cm
-2 -1 mA cm -2 -0.5 mA cm
40
45
50
2 θ / degrees
(b) Fig. 3. XRD patterns of electrodeposited Co–Fe–W alloys obtained at different ic-values (a). Expanded region concerning the (1 1 0) peak considered for the calculation of average crystallite size (b). The full black line corresponds to the Fe Bragg peak position. S denotes the copper substrate peaks.
Table 2 Crystalline grain size (td), coercive field (HC), saturation magnetization (MS), remanence (MR) and squareness (MR/MS) of Co–Fe–W deposits as a function of ic. ic/mA cm 0.5 1 2 5 10
2
td/nm
Hc/Oe
l0MS/T
l0MR/T
MR/MS
35 30 22 18 16
23 18 8 7 23
2.04 2.14 2.03 1.97 1.95
1.19 0.78 0.26 0.46 0.34
0.58 0.36 0.13 0.23 0.17
reported recently by our group [19] in the case of electrodeposited Co–Ni–W alloys, where it was observed that ic-values higher than 100 mA cm 2 lead to amorphous-like alloys. In order to confirm the crystallite size calculated by XRD, TEM characterizations were carried out in one of our samples. Almost all the precipitates in the sample prepared at 10 mA cm2 have a size between 10 and 20 nm as observed in Fig. 4a. This result is in a good agreement with the average crystalline grain size value obtained by XRD (see Table 2). Selected area electron diffraction (SAED) images (Fig. 4b) were also taken to calculate the interplanar spacings, which values are in the range 0.0755–0.2004 nm. In addition, it is noticed from this diffraction pattern that the sample exhibits polycrystalline character, as would be expected.
Fig. 5 exhibits the room temperature CEMS spectra for Co–Fe– W alloys prepared under different ic-values. It should first be said that: (i) the six emission lines of these spectra suggest a nuclear Zeeman interaction around 57Fe probes, i.e., 57Fe atoms are magnetically ordered due to non-zero magnetic hyperfine field (Bhf); (ii) except for the sample prepared at ic = 10 mA cm 2 (9 at.% W), the relative intensity ratio of 2 and 3 lines (I23 or I54 – 5 and 4) is larger than that of the ratio of lines 1 and 3 (I13), suggesting that the sample magnetizations are laying in the film plane (in our CEMS experiments, the gamma ray direction is perpendicular to the sample plane, consequently I23 near 4 indicates in-plane magnetization [28]). In case of the sample prepared at ic = 10 mA cm 2, the I23 value is smaller than 2 (I23 = 0.9), suggesting that the sample magnetization is canted to out-of-plane geometry; an effect may be due to the high internal stress above discussed; finally (iii) the emission spectra are relatively complexes, since they seem to be composed by a superposition of sharp and broad emission lines; the latter (broadening effect) can be attributed to the disordered character of the electrodeposited Co–Fe–W alloys. Therefore, the CEMS spectra suggest that some of the Fe atoms are placed in regions with a relatively high ordered feature (crystalline phase), while a substantial number of Fe atoms is located in more disordered regions. Considering the fact that the increase of the ic-value provoked a reduction of the crystalline grain size and morphology changes, one would expect that the disordered region could be enhanced in samples with high ic-values due to the surface/interface grain contribution. Therefore, to have a better understanding of the ic-quantity influence on the deposits, these CEMS spectra were fitted assuming two magnetic subspectra. A sextet component that is related to the ordered Co–Fe–W alloys (blue lines), may be associated with the core of the grains plus a magnetic hyperfine field distribution component (green lines), which takes into account the disorder features of the Co–Fe–W alloys (surface/interface fraction). The isomer shift (d = 0.00 (2) mm/s) values of these two components are similar, but the Bhf values are slightly smaller for the distribution component (Bhf 33.5 T) when compared to that of the sextet (Bhf 35.5 T); an effect observed in nanograined system [22]. In addition, the relative area of the sextet (attributed to the ordered Co–Fe–W alloy fraction) seems to reduce while the distribution component increases; an effect that is directly correlated to the observed grain size reduction and morphology modification caused by the increase of the ic-value (W content increase as well). Finally, our results seem to agree with those reported in literature suggesting that for alloys containing a higher W content, Co–Fe–W alloys would be found in amorphous-like state [10,33]. Fig. 6a presents the in-plane magnetic hysteresis loops recorded at room temperature, i.e. M(H) curves, for the Co–Fe–W alloys obtained at different ic-values. In the inset, it is shown the low-field region. It is worth mentioning that for out-of-plane measurements (not shown), the magnetization did not reach saturation regime (it increases nearly linear with the applied magnetic field) for the magnetic field range of 20 kOe. Consequently, as also shown by Mössbauer data, the films exhibit in-plane magnetic anisotropy; an effect mainly attributed to the dominant shape anisotropy contribution. The magnetic parameters extracted from the M(H) loops [coercive field (HC), saturation magnetization (MS) and remanence (MR)] are depicted in Table 2. The behavior of the MS and HC quantities as a function of the cobalt content is shown in Fig. 6b and Fig. 6c, respectively. As expected, the composition affects the alloys saturation magnetization (Fig. 6b). The maximum saturation is obtained for the alloy with approximately 30 at.% of cobalt. The minimum of HC field (Fig. 6c), on the other hand, is achieved in the alloy with 23 at.% of cobalt. Similar behavior was found by
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Fig. 4. (a) TEM and (b) SAED images of the Co–Fe–W sample prepared at 10 mA cm2.
2.0
fD = disordered fraction
34.5 T
-2
0.5 mA cm -2 1 mA cm -2 2 mA cm -2 5 mA cm -2 10 mA cm
1.5 1.0
(b)
μ0 M (Tesla)
f = 69 % D
f = 77 % D
0.5 0.0
2.0 1.5
-0.5
μ0M (Tesla)
(a)
Relative emission
(a)
-1.0
1.0 0.5 0.0
-0.5 -1.0
-1.5
-1.5
(c)
(PBHF)
-2.0
-2.0 f = 77 % D
-20
0
20
40
H (Oe)
-15
-10
-5
0 H (kOe)
5
10
15
24
2.2
(d)
-40
f = 80 % D
20
-8
-6
I3 I4
I2 -4
-2
0
I5 2
v (mm/s)
4
μ0MS (T)
(e)
I1
f = 87 % D
I6 6
Hc (Oe)
2.1 16
12
2.0
8
0 5 10 15 20 25 30 35 40
8
BHF(T)
(b)
Fig. 5. Room temperature CEMS spectra recorded for Co–Fe–W alloys obtained at different ic-values [ic = 0.5 (a), 1.0 (b), 2.0 (c), 5.0 (d) and (e) 10 mA cm 2]. The full black line is the least square fits resulting from the two subspectra: green sextet and blue magnetic hyperfine field distribution. In the right side is plotted the corresponding magnetic hyperfine field distribution curves of the blue component. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Kuhrt and Schultz [34], studying the magnetic properties of nanocrystalline mechanically alloyed Co–Fe alloys. It should be noted that the MS values, obtained in our work, are smaller than those reported in Ref. [34]; an effect that could be related to the addition of W (nonmagnetic element) to the Co–Fe binary system. It is known that the HC field of a certain material is related to the magnetic anisotropy; a property that depends on grain size and shapes/ morphologies, composition (magnetocrystalline contribution), and others [35]. In this sense, it is observed in Table 2 that the alloy HC field decreases with lowering grain sizes in the first four samples.
(c) 4
1.9 18 20 22 24 26 28 30 32 34
18 20 22 24 26 28 30 32 34
at% Co
at% Co
Fig. 6. Room temperature magnetic hysteresis loops obtained for Co–Fe–W alloys prepared under different ic-values (ic values are indicated inside the figure) (a): the inset shows the region corresponding to low magnetic fields; Plots of saturation magnetization (MS) (b) and coercivity (HC) (c) as a function of Co content in Co–Fe– W alloys.
Precisely, these four samples have similar morphology, characterized by needle-like particles as shown by SEM. The change in morphology observed in the sample prepared at 10 mA cm2 is accompanied by an increase in HC field value. Nalwa [36] described that the behavior of HC field for a thin film usually enhances with decreasing grain size; an effect associated with the transition from magnetic multi- to single-domains [35]. Another factor that may chance the HC-values is the W content in the deposits, however, a direct relationship between HC field and W content was not
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observed in our samples, probably because the W content was not very high. With respect to the remanence (MR) of the Fe–Co–W alloys, it can be observed a clear difference between the first sample and the others (Table 2). Indeed, the sample prepared with lower current density has a Mr close to 60% of the saturation value, which indicates a dominant magnetizing interaction between their magnetic grains [37]. Differently, all other samples have ratios MR/MS below 50%, with a minimum of 13% in the sample prepared with ic = 2 mA cm 2. Then, the control of the current density and the W content allow obtaining alloys of Co–Fe–W with interesting magnetic characteristics: low HC field (approximately between 10 to 20 Oe) and high MS value (higher or close to 2 T), which makes these materials a possible candidate for applications such as read/write heads and hard disks in magnetic recording. 4. Conclusions The structural, morphological and magnetic characterization of electrodeposited Co–Fe–W alloys, containing small contents of W (up to 9 at.%), were investigated by XRD, SEM, TEM, Mössbauer spectroscopy and magnetic measurements. By varying the applied cathodic current density (ic), electrodeposited (Co100 xFex)100 yWy films (x = 63–72 at.% Fe, y = 4–9 at.% W) were successfully prepared. The XRD results revealed a bcc Fe-like structure for all studied compositions with average crystallite size ranging from 16 to 35 nm which was confirmed by TEM. The SEM images indicated that a needle-like morphology is dominant for the deposits containing lower W content (up to 4.5 at.%), and a cauliflower-type one is observed in the case of higher W content deposits. Mössbauer spectroscopy has shown the contribution of two magnetic phases: one ordered associated with the core grain of the Co–Fe–W alloys and a disordered contribution that is related to the grain boundary/surface with Co–Fe–W features. Due to the needle-like morphology, the magnetization seems to be in the film-plane (orientated along the larger grain size), while in the cauliflower-type morphology is slightly canted from in- to outof-plane geometry. Magnetic measurements, performed in the VSM, confirmed a ferromagnetic character for all samples at room temperature. High saturation values accompanied by low coercivity ones were obtained for electrodeposited Co–Fe–W alloys. These results show that Co–Fe–W alloys are potential materials for applications in magnetic devices such as read/write heads and hard disks. Acknowledgments The authors gratefully acknowledge financial support and scholarships from the Brazilian funding agencies CNPq, Fapesp,
Prope/Unesp and Fapes. Authors also thank the Brazilian Nanotechnology National Laboratory by TEM analyses. References [1] C.H. Shang, R.C. Cammarata, T.P. Weihs, J. Mater. Res. 15 (2000) 835–837. [2] C.H. Shang, T.P. Weihs, R.C. Cammarata, J. Appl. Phys. 87 (2000) 6508–6510. [3] D. Zhou, M. Zhou, M. Zhu, X. Yang, M. Yue, J. Appl. Phys. 111 (2012). 07A31913. [4] K. Kumasaka, K. Ono, IEEE Trans. Magn. 18 (1982) 941–944. [5] N.D. Ha, M.-H. Phan, C.-O. Kim, J. Appl. Phys. 99 (2006) 08F105. [6] P. Galimberti, S. Lay, A. Antoni-Zdziobek, S. Coindeau, M. Véron, F. Bley, M. De Boissieu, Intermetallics 19 (2011) 556–562. [7] I. Tabakovic, S. Riemer, V. Vas’ko, M. Kief, Electrochim. Acta 53 (2008) 8008– 8014. [8] M. Schwartz, C. Arcos, K. Nobe, Plat. Surf. Fin. 90 (2003) 46–51. [9] M. Schwartz, B.Y. Yoo, K. Nobe, ECS Trans. 33 (2011) 19–31. [10] H. Capel, P.H. Shipway, S.J. Harris, Wear 255 (2003) 917–923. [11] D.W. Ernst, M.L.J. Holt, J. Electrochem. Soc. 105 (1958) 686–692. [12] E. Chassaing, K. Quang, R. Wiart, J. Appl. Electrochem. 19 (1989) 839–843. [13] E.J. Podlaha, M. Matlosz, D. Landolt, J. Electrochem. Soc. 140 (1993) L149–L151. [14] H. Dahms, I.M. Croll, J. Electrochem. Soc. 112 (1965) 771–775. [15] D. Gangasingh, J.B. Talbot, J. Electrochem. Soc. 138 (1991) 3605–3611. [16] M. Matlosz, J. Electrochem. Soc. 140 (1993) 2272–2279. [17] N. Zech, E.J. Podlaha, D. Landolt, J. Electrochem. Soc. 146 (1999) 2892–2900. [18] M.C. Esteves, P.T.A. Sumodjo, E.J. Podlaha, Electrochim. Acta 56 (2011) 9082– 9087. [19] A.M.P. Sakita, E.C. Passamani, H. Kumar, D.R. Cornejo, C.S. Fugivara, R.D. Noce, A.V. Benedetti, Mater. Chem. Phys. 141 (2013) 576–581. [20] V.M. Fedosyuk, J.M. Riveiro, J. Non-Cryst. Solids 204 (1996) 99–104. [21] E.A. Vasilev, T.M. Tkachenko, V.M. Fedosyuk, O.I. Kasyutich, E.A. Dmitrieva, Russ. Metall. 2 (1994) 88–91. [22] J.A. Gómez, S.K. Xia, E.C. Passamani, B. Giordanengo, E.M. Baggio-Saitovitch, J. Magn. Magn. Mater. 223 (2001) 112–118. [23] M. Schwartz, N.V. Myung, K. Nobe, J. Electrochem. Soc. 151 (2004) C468–C477. [24] M. Bucˇko, J. Rogan, S.I. Stevanovic´, S. Stankovic´, J.B. Bajat, Surf. Coat. Technol. 228 (2013) 221–228. [25] I. Mizushima, P.T. Tang, H.N. Hansen, M.A.J. Somers, Electrochim. Acta 51 (2005) 888–896. [26] D. Kim, D.-Y. Park, B.Y. Yoo, P.T.A. Sumodjo, N.V. Myung, Electrochim. Acta 48 (2003) 819–830. [27] H.-R. Zhou, Y.-D. Yu, G.-Y. Wei, H.-L. Ge, Int. J. Electrochem. Sci. 7 (2012) 5544– 5556. [28] E.C. Passamani, C. Larica, P.S. Moscon, P. Mendoza-Zélis, H.H. Sanchez, J. Appl. Phys. 110 (2011). 0439061-7. [29] T.B. Massalski, Binary Alloy Phase Diagrams, second ed., vol. 2, ASM International, OH, 1992. pp. 1258 and 1792. [30] B.D. Cullity, Elements of X-ray Diffraction, second ed., Addison-Wesley Publishing Company Inc., Reading, MA, 1978. p. 91. [31] R.D. Noce, N. Barelli, R.F.C. Marques, P.T.A. Sumodjo, A.V. Benedetti, Surf. Coat. Technol. 202 (2007) 107–113. [32] A.M. Rashidi, A. Amadeh, Surf. Coat. Technol. 202 (2008) 3772–3776. [33] S.S. Grabchikov, O.I. Potuzhnaya, L.B. Sosnovskaya, M.U. Sheleg, Russ. Metall. (Metally) 2 (2009) 164–171. [34] G. Herzer, IEEE Trans. Magn. 26 (1990) 1397–1402. [35] R.C. O’Handley, Modern Magnetic Materials, J. Wiley & Sons Inc., New York, 2000. [36] H.S. Nalwa, Handbook of Thin Films Materials: Deposition and Processing of Thin Films, vol. 1, Academic Press, Sand Diego, CA, 2002. p. 509. [37] E.P. Wohlfarth, J. Appl. Phys. 29 (1958) 595–596.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Structural, morphological and magnetic characterization of electrodeposited Co–Fe–W alloys R. Della Noce a,⇑, A.V. Benedetti a, M. Magnani a, E.C. Passamani b, H. Kumar c, D.R. Cornejo c, C.A. Ospina d a
Instituto de Química, Universidade Estadual Paulista, UNESP, 14800-900 Araraquara, SP, Brazil Departamento de Física, Universidade Federal do Espírito Santo, 29075-910 Vitória, ES, Brazil c Instituto de Física, Universidade de São Paulo, USP, 05508-090 São Paulo, SP, Brazil d Electron Microscopy Laboratory, Brazilian Nanotechnology National Laboratory, 13083-970 Campinas, SP, Brazil b
a r t i c l e
i n f o
Article history: Received 12 February 2014 Received in revised form 22 May 2014 Accepted 24 May 2014 Available online 2 June 2014 Keywords: Co–Fe–W alloys Electrodeposition Mössbauer spectroscopy X-ray diffractometry
a b s t r a c t Structural, morphological and magnetic characterization of electrodeposited Co–Fe–W alloys, containing small amounts of W (up to 9 at.%), were performed using X-ray diffractometry, scanning (SEM) and transmission (TEM) electron microscopy, Mössbauer spectroscopy and magnetization measurements. Electrodeposited (Co100 xFex)100 yWy films (x = 63–72 at.% Fe, y = 4–9 at.% W) were successfully produced varying the applied cathodic current density (ic) between 0.5 and 10 mA cm 2. X-ray diffraction results revealed a bcc-like structure for all studied compositions with average crystallite size ranging from 16 to 35 nm, as also confirmed by TEM results. SEM images indicated that needle-type morphology is dominant for the deposits containing lower W content (up to 4.5 at.%.), while a cauliflower-type behavior is observed for higher W content deposits. Room temperature Mössbauer spectra indicate the presence of two magnetic species for all samples; one component associated with an ordered Co–Fe–W fraction (crystalline grain core) and a magnetic disordered Co–Fe–W contribution, which can be attributed to the grain boundaries/grain surfaces. Magnetization was observed to be in the film plane along the film direction, except the sample prepared at ic = 10 mA cm 2 that is slightly canted from in- to out-of-plane geometry. Magnetic measurements show high saturation magnetization values accompanied by low coercivity ones for the electrodeposited Co–Fe–W alloys, making these alloys potential materials for applications in magnetic devices such as read/write heads and hard disks. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction In the last decades, much attention has been paid to the Co–Fe system, in particular to the Co35Fe65 alloy, due to its excellent magnetic and mechanical properties [1–3]. Such properties include high saturation magnetization (>2.4 Tesla) and Curie temperature (>1000 °C), high electric permeability and good thermal stability. Moreover, the addition of refractory elements (W, Mo or V) to the Co–Fe binary system results in improvement on hardness and corrosion resistance, enhancement of durability, and increase of thermal resistance, among others. In this context, Co–Fe–M (M = W, Mo, V or Re) alloys have been lately produced by different techniques including sputtering, molecular beam epitaxy (MBE), physical vapor deposition (PVD), and electrodeposition etc. [4–7]. Among these synthesis techniques, electrodeposition has stood out due to the following ⇑ Corresponding author. Tel.: +55 16 33019782; fax: +55 16 33019637. E-mail address: [email protected] (R.D. Noce). http://dx.doi.org/10.1016/j.jallcom.2014.05.157 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
important features: cost-effectiveness, high quality deposits, production at room temperature (no need of high vacuum), and easy operating conditions. Therefore, various Co–Fe–M ternary alloys have been fabricated by electrodeposition with different purposes and applications [8–10]. The Co–Fe–M alloys, in particular the Co–Fe–W alloys, are also interesting from theoretical viewpoint because this system shows simultaneously the phenomena of anomalous [11–13] and induced co-deposition in electrodeposition processes [14–17]. Specifically, the above two effects have been described in detail by Esteves et al. [18] for electrodeposited Co–Ni–Mo thin films, and recently by our group for electrodeposited Co–Ni–W alloys from a glycine-containing bath [19]. Consequently, the present work is aimed to study the structure, morphology, atomic arrangements and magnetic properties of electrodeposited Co–Fe–W alloys, with composition close to Co35Fe65 and addition of low W contents (up to 9 at.%). Several works have been reported on the electrodeposition of Co–Fe binary alloys. However, as far as we know there are only three published papers concerning electrodeposited Co–Fe–W alloys hitherto
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[10,20,21]. In general, these works were focused on high W concentration, while the present work is aimed to add small W amounts (up to 9 at.%) to the Co–Fe system. Capel et al. [10] proposed the electrodeposition of Co–W and Co–Fe–W alloys onto stainless steel as an alternative to electrodeposited hard chrome. The authors studied the hardness and wear properties, and found that the Co–W alloy containing 27 at.% W exhibits the highest hardness after heat treatment (1185 kgf mm 2), a value even higher than electrodeposited hard chromium (848 kgf mm 2). For the Co43Fe30W27 alloy, on the other hand, the measured hardness value was 838 kgf mm 2. The others two Co–Fe–W papers were published by the same group in different journals [20,21]. Using Mössbauer spectroscopy (MS), they have been able to discussed the formation of amorphous state in Co–W–Fe alloys with W concentration higher than 14 at.% [21]. Therefore, in the present work, we focus on the structural, morphological and magnetic characterization of electrodeposited Co–Fe–W alloys for W concentration lower than 10 at.%. X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), Mössbauer spectroscopy and magnetic measurements have been used to analyze the structural and magnetic properties of electrodeposited Co–Fe–W films. 2. Experimental details Co–Fe–W alloys were galvanostatically (constant current application) prepared by using an EG & G PAR M283 potentiostat/galvanostat. A Tait-type cell was used for all electrodeposition experiments. The (1 1) cm2 copper plates (purity > 99.99%) were used as substrate. The alloys were electrodeposited from a citrate-containing unstirred bath composed of 0.04 M Fe2(SO4)3, 0.05 M CoSO4, 0.01 M Na2WO4, 0.2 M H3BO3 and 0.2 M citrate (Na3C6H5O7). By using this particular bath formulation, Fe-rich Co–Fe alloys with composition close to Co35Fe65 are expected to form due to the anomalous codeposition effect [11–13], where Fe is preferentially deposited in relation to Co (more noble metal). The bath pH was adjusted to 6 by utilizing diluted NaOH. The applied cathodic current density (ic) value was varied from 0.5 to 10 mA cm 2. Low ic-values were used to avoid the formation of undesired Fe-species such as oxides and hydroxides. A maximum of three or four depositions using the same bath were performed to avoid changes in the electrolyte composition. In all experiments, the charge passed to obtain the electrodeposits was equal to 35 C. The film thickness and the current efficiency were estimated from the mass of the deposits and their compositions by using the Faraday’s law. To perform these calculations, the number of electrons involved in the reduction processes of Fe, Co and W metallic were considered as 3, 2 and 6, respectively. The alloy composition was estimated by Energy Dispersive X-ray Spectrometry (EDS) coupled to SEM. The EDS analyses were performed by applying an accelerating voltage of 20 keV for all samples. X-ray diffraction (XRD) was employed to physically characterize the crystal structure of deposits. The XRD measurements were performed using Cu Ka radiation (1.5418 Å) in a Siemens D5000 instrument at scan rate of 0.002°/sec. Cross section TEM specimens, from the Cu (100 lm)/Co–Fe–W (5.2 lm) sample, were prepared by sandwiching them between Si substrates to ensure a good centering of the substrate and alloy interface for the subsequent manual, dimpling and ion polishing processes. To characterize the final TEM specimens, a JEM-2100 LaB6 microscope, operating in conventional mode at 200 kV, was used at the Electron Microscopy Laboratory, of the Brazilian Nanotechnology National Laboratory. Room temperature (RT) Conversion Electron Mössbauer Spectroscopy (CEMS) spectra were recorded with a 50 mCi 57Co(Rh) source in a conventional CEMS chamber. These CEMS spectra were fitted by a hyperfine field distribution using a histogram method plus one sextet. To reproduce the small asymmetry of the spectra, a linear correlation between hyperfine field (Bhf) and isomer shift (d) was assumed [22]. d-values are given relative to a-Fe at RT. Magnetic measurements were conducted at RT using a vibrating sample magnetometer (VSM). The probe magnetic field was applied in- and out-of-film-plane and varied from 20 kOe to 20 kOe.
3. Results and discussion 3.1. Compositional analysis
-0.7
E / V vs. Hg/HgSO4/K2SO4 Sat
244
-0.9 -1.0 -1.1 -1.2 -1.3 -1.4 -1.5 -1.6 -1.7 0
200
400
600
800
1000
1200
1400
time / minutes Fig. 1. E–t profiles for the electrodeposition of Co–Fe–W alloys by using different applied cathodic current densities (ic).
decreases to more negative values, as would be expected. It was also possible to evaluate the thicknesses of the films and the cathodic current efficiency (CCE) taking into account the deposited mass. These data are summarized in Table 1 together to the compositional analysis of the alloys that was obtained by EDS. It can be seen that the thickness and the CCE augment as the ic-values are increased. It should first be mentioned that, in general, the increase of the ic-quantity results in a concomitant reduction of thickness and CCE for electrodeposited alloys due to the increase of hydrogen evolution rate [19,23,24]. Nevertheless, in our case, this behavior seems to not occur because we have used a complexing agent (citrate), which results in higher W content in the deposits when higher ic-values were used. In our previous work [19] on electrodeposition of Co–Ni–W alloys using a glycine-based bath, it was noticed that the W content, thickness and CCE diminish with the increase of the ic-quantity. It is known that citrate is a stronger complexing agent than glycine, thus higher ic-values are needed to break the metal (W)-citrate bond in citrate-based bath, which in turn leads to a higher W content as observed in Table 1. It can be seen that higher ic leads to high W concentration in the deposits, which increases the deposit thickness and CCE due to the higher atomic weight of W compared to both Fe and Co. A similar behavior was observed by Mizushima et al. [25] in electrodeposition of Ni–W alloys on comparing different complexing agents including citrate and glycine. They reported that an increase in ic from 10 to 100 mA cm 2 leads to: (i) increase in W concentration from 17 to 25 at.% for citrate-based bath in agreement with our findings; (ii) decrease in W concentration from 10 to 2 at.% for glycine-based bath. It is important to note that all deposits are Fe-rich due to the preferential Fe deposition in relation to the more noble metal (Co) according to the above-mentioned anomalous co-deposition effect [11–13]. In addition, the increase of ic-values leads to deposits with high Fe content; consequently the Co concentration is reduced. These results confirm that the alloys have compositions close to Co35Fe65 plus W addition as initially predicted. It is worth mentioning that within this thickness range (lm), the alloys magnetic
Table 1 Composition, thickness and cathodic current efficiency (CCE) of electrodeposited Co–Fe–W films as a function of applied cathodic current density (ic). ic/mA cm
Fig. 1 shows the potential vs. time (E t) profiles used to obtain the Co–Fe–W alloys at different ic-values. Considering the fact that the passed charge was kept constant (35 C), the total time for each experiment is decreased with the increase of ic. In addition, it can be noted that when the ic-quantity is increased, the potential (E)
-2
0.5 mA cm -2 1 mA cm -2 2 mA cm -2 5 mA cm -2 10 mA cm
-0.8
0.5 1 2 5 10
2
Co at.%
Fe at.%
W at.%
Thickness/lm
CCE/%
32.1 29.6 27.9 23.4 19.2
63.2 66.3 67.6 69.7 71.9
4.7 4.1 4.5 6.9 8.9
2.9 3.1 3.8 4.6 5.2
30.8 31.3 33.2 47.7 55.2
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properties, which are discussed below, are expected to be due to the bulk (low dimension effect can be neglected). 3.2. Structural and morphological characterization Fig. 2 depicts the SEM images of the electrodeposited Co–Fe–W alloys prepared at different ic-values. SEM images exhibit compact, uniform and homogenous deposits for all studied conditions. For the deposits with low W contents (up to 4.5 at.% obtained for ic = 0.5, 1 and 2 mA cm 2), a needle-like morphology is clearly seen as shown in Fig 2a. Specifically, the observed small holes in Fig. 2b are attributed to the surface region where hydrogen evolution occurred during the electrodeposition process, which impeded the alloy deposition. The needle-like morphology has also been observed by Kim et al. [26] in Co–Fe alloys prepared by electrodeposition and Zhou et al. [27] in Fe–Pt electrodeposits. When ic is increased up to 10 mA cm 2, an abrupt morphology change is detected, i.e. from needle-like (Fig. 2c) to cauliflower-type morphology (Fig. 2d). In the latter case, the alloy with highest W-content (implies in a large film thickness) presents micro-cracks as observed in SEM images (not shown); an effect associated with internal stress caused by the interfacial mechanical coupling between the film and substrate, which is enhanced with increasing the film thickness [28]. Fig. 3 displays the XRD patterns of electrodeposited Co–Fe–W alloys obtained at different ic-values. It is clear noticed that all deposits have a bcc Fe-like phase because Fe is the major alloy component. This result suggests that both Co and W are totally dissolved into the Fe matrix. As a consequence, it can be said that there is the formation of a substitutional solid solution in these conditions with no presence of intermetallic compounds and/or segregated Fe, or Co pure phases. This observation is in agreement
with the equilibrium phase diagrams for each binary alloy (Co–W or Fe–W) [29], which show that intermetallic compounds may be formed only for much higher W concentrations (>20 at.%). Furthermore, another evidence of the formation of substitutional solid solution can be inferred whether the most intense crystalline peak (1 1 0) is analyzed. In the case of pure Fe, the (1 1 0) peak is located at 44.67°; however the XRD patterns of the electrodeposited Co– Fe–W alloys show that this peak is shifted to smaller 2h values, e.g. 44.55° (ic = 0.5 mA cm 2). This is an indication that the alloy lattice parameter increases by incorporation of W atoms in the bcc Fe-like matrix. In brief, the alloys have a bcc Fe-like structure with expanded lattice parameter due to the inclusion of W atoms because Co having similar atomic radius to Fe, in these contents, would not change the XRD bcc-like peaks angular position. Another important parameter that can be obtained from the XRD data is the average crystallite size. By using the Scherrer formula [30] and considering the (1 1 0) peak as the most intense, it was calculated the average crystallite size (td) for the electrodeposited Co–Fe–W alloys in the studied conditions, which is listed in Table 2 in conjunction with the magnetic parameters. The results indicate that the td-parameter ranges from 16 to 35 nm, depending on the ic-values. The lowest td-value (16 nm) was obtained for the alloy electrodeposited at the highest ic-value (10 mA cm 2). The observed grain size refinement as a function of the ic-quantity can be found in electrodeposited materials [19,31,32], and it is directly affected by the hydrogen evolution rate and also by the amount of nuclei formed during the electrodeposition process. Moreover, the observed broadening of the peaks in the XRD patterns as ic-quantity increases reveals that the crystalline grain sizes would be reduced up to a complete transformation into a nanocrystalline/amorphous state whether the ic would be increased to values higher than 10 mA cm 2. This structural transition was
Fig. 2. SEM images of electrodeposited Co–Fe–W alloys obtained at different ic-values: (a) 0.5, (b) 2, (c) 5 and (d) 10 mA cm
2
.
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Relative Intensity
3.3. Mössbauer spectroscopy and magnetic measurements
S
S
S -2 (110) -10 mA cm -2 -5 mA cm -2 -2 mA cm
(211)
(200)
-2 -1 mA cm -2 -0.5 mA cm
30
40
50
60 70 2 θ / degrees
80
(a) S
Relative Intensity
-2 -10 mA cm
pure Fe (44.67°)
(110)
-2 -5 mA cm -2 -2 mA cm
-2 -1 mA cm -2 -0.5 mA cm
40
45
50
2 θ / degrees
(b) Fig. 3. XRD patterns of electrodeposited Co–Fe–W alloys obtained at different ic-values (a). Expanded region concerning the (1 1 0) peak considered for the calculation of average crystallite size (b). The full black line corresponds to the Fe Bragg peak position. S denotes the copper substrate peaks.
Table 2 Crystalline grain size (td), coercive field (HC), saturation magnetization (MS), remanence (MR) and squareness (MR/MS) of Co–Fe–W deposits as a function of ic. ic/mA cm 0.5 1 2 5 10
2
td/nm
Hc/Oe
l0MS/T
l0MR/T
MR/MS
35 30 22 18 16
23 18 8 7 23
2.04 2.14 2.03 1.97 1.95
1.19 0.78 0.26 0.46 0.34
0.58 0.36 0.13 0.23 0.17
reported recently by our group [19] in the case of electrodeposited Co–Ni–W alloys, where it was observed that ic-values higher than 100 mA cm 2 lead to amorphous-like alloys. In order to confirm the crystallite size calculated by XRD, TEM characterizations were carried out in one of our samples. Almost all the precipitates in the sample prepared at 10 mA cm2 have a size between 10 and 20 nm as observed in Fig. 4a. This result is in a good agreement with the average crystalline grain size value obtained by XRD (see Table 2). Selected area electron diffraction (SAED) images (Fig. 4b) were also taken to calculate the interplanar spacings, which values are in the range 0.0755–0.2004 nm. In addition, it is noticed from this diffraction pattern that the sample exhibits polycrystalline character, as would be expected.
Fig. 5 exhibits the room temperature CEMS spectra for Co–Fe– W alloys prepared under different ic-values. It should first be said that: (i) the six emission lines of these spectra suggest a nuclear Zeeman interaction around 57Fe probes, i.e., 57Fe atoms are magnetically ordered due to non-zero magnetic hyperfine field (Bhf); (ii) except for the sample prepared at ic = 10 mA cm 2 (9 at.% W), the relative intensity ratio of 2 and 3 lines (I23 or I54 – 5 and 4) is larger than that of the ratio of lines 1 and 3 (I13), suggesting that the sample magnetizations are laying in the film plane (in our CEMS experiments, the gamma ray direction is perpendicular to the sample plane, consequently I23 near 4 indicates in-plane magnetization [28]). In case of the sample prepared at ic = 10 mA cm 2, the I23 value is smaller than 2 (I23 = 0.9), suggesting that the sample magnetization is canted to out-of-plane geometry; an effect may be due to the high internal stress above discussed; finally (iii) the emission spectra are relatively complexes, since they seem to be composed by a superposition of sharp and broad emission lines; the latter (broadening effect) can be attributed to the disordered character of the electrodeposited Co–Fe–W alloys. Therefore, the CEMS spectra suggest that some of the Fe atoms are placed in regions with a relatively high ordered feature (crystalline phase), while a substantial number of Fe atoms is located in more disordered regions. Considering the fact that the increase of the ic-value provoked a reduction of the crystalline grain size and morphology changes, one would expect that the disordered region could be enhanced in samples with high ic-values due to the surface/interface grain contribution. Therefore, to have a better understanding of the ic-quantity influence on the deposits, these CEMS spectra were fitted assuming two magnetic subspectra. A sextet component that is related to the ordered Co–Fe–W alloys (blue lines), may be associated with the core of the grains plus a magnetic hyperfine field distribution component (green lines), which takes into account the disorder features of the Co–Fe–W alloys (surface/interface fraction). The isomer shift (d = 0.00 (2) mm/s) values of these two components are similar, but the Bhf values are slightly smaller for the distribution component (Bhf 33.5 T) when compared to that of the sextet (Bhf 35.5 T); an effect observed in nanograined system [22]. In addition, the relative area of the sextet (attributed to the ordered Co–Fe–W alloy fraction) seems to reduce while the distribution component increases; an effect that is directly correlated to the observed grain size reduction and morphology modification caused by the increase of the ic-value (W content increase as well). Finally, our results seem to agree with those reported in literature suggesting that for alloys containing a higher W content, Co–Fe–W alloys would be found in amorphous-like state [10,33]. Fig. 6a presents the in-plane magnetic hysteresis loops recorded at room temperature, i.e. M(H) curves, for the Co–Fe–W alloys obtained at different ic-values. In the inset, it is shown the low-field region. It is worth mentioning that for out-of-plane measurements (not shown), the magnetization did not reach saturation regime (it increases nearly linear with the applied magnetic field) for the magnetic field range of 20 kOe. Consequently, as also shown by Mössbauer data, the films exhibit in-plane magnetic anisotropy; an effect mainly attributed to the dominant shape anisotropy contribution. The magnetic parameters extracted from the M(H) loops [coercive field (HC), saturation magnetization (MS) and remanence (MR)] are depicted in Table 2. The behavior of the MS and HC quantities as a function of the cobalt content is shown in Fig. 6b and Fig. 6c, respectively. As expected, the composition affects the alloys saturation magnetization (Fig. 6b). The maximum saturation is obtained for the alloy with approximately 30 at.% of cobalt. The minimum of HC field (Fig. 6c), on the other hand, is achieved in the alloy with 23 at.% of cobalt. Similar behavior was found by
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Fig. 4. (a) TEM and (b) SAED images of the Co–Fe–W sample prepared at 10 mA cm2.
2.0
fD = disordered fraction
34.5 T
-2
0.5 mA cm -2 1 mA cm -2 2 mA cm -2 5 mA cm -2 10 mA cm
1.5 1.0
(b)
μ0 M (Tesla)
f = 69 % D
f = 77 % D
0.5 0.0
2.0 1.5
-0.5
μ0M (Tesla)
(a)
Relative emission
(a)
-1.0
1.0 0.5 0.0
-0.5 -1.0
-1.5
-1.5
(c)
(PBHF)
-2.0
-2.0 f = 77 % D
-20
0
20
40
H (Oe)
-15
-10
-5
0 H (kOe)
5
10
15
24
2.2
(d)
-40
f = 80 % D
20
-8
-6
I3 I4
I2 -4
-2
0
I5 2
v (mm/s)
4
μ0MS (T)
(e)
I1
f = 87 % D
I6 6
Hc (Oe)
2.1 16
12
2.0
8
0 5 10 15 20 25 30 35 40
8
BHF(T)
(b)
Fig. 5. Room temperature CEMS spectra recorded for Co–Fe–W alloys obtained at different ic-values [ic = 0.5 (a), 1.0 (b), 2.0 (c), 5.0 (d) and (e) 10 mA cm 2]. The full black line is the least square fits resulting from the two subspectra: green sextet and blue magnetic hyperfine field distribution. In the right side is plotted the corresponding magnetic hyperfine field distribution curves of the blue component. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Kuhrt and Schultz [34], studying the magnetic properties of nanocrystalline mechanically alloyed Co–Fe alloys. It should be noted that the MS values, obtained in our work, are smaller than those reported in Ref. [34]; an effect that could be related to the addition of W (nonmagnetic element) to the Co–Fe binary system. It is known that the HC field of a certain material is related to the magnetic anisotropy; a property that depends on grain size and shapes/ morphologies, composition (magnetocrystalline contribution), and others [35]. In this sense, it is observed in Table 2 that the alloy HC field decreases with lowering grain sizes in the first four samples.
(c) 4
1.9 18 20 22 24 26 28 30 32 34
18 20 22 24 26 28 30 32 34
at% Co
at% Co
Fig. 6. Room temperature magnetic hysteresis loops obtained for Co–Fe–W alloys prepared under different ic-values (ic values are indicated inside the figure) (a): the inset shows the region corresponding to low magnetic fields; Plots of saturation magnetization (MS) (b) and coercivity (HC) (c) as a function of Co content in Co–Fe– W alloys.
Precisely, these four samples have similar morphology, characterized by needle-like particles as shown by SEM. The change in morphology observed in the sample prepared at 10 mA cm2 is accompanied by an increase in HC field value. Nalwa [36] described that the behavior of HC field for a thin film usually enhances with decreasing grain size; an effect associated with the transition from magnetic multi- to single-domains [35]. Another factor that may chance the HC-values is the W content in the deposits, however, a direct relationship between HC field and W content was not
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observed in our samples, probably because the W content was not very high. With respect to the remanence (MR) of the Fe–Co–W alloys, it can be observed a clear difference between the first sample and the others (Table 2). Indeed, the sample prepared with lower current density has a Mr close to 60% of the saturation value, which indicates a dominant magnetizing interaction between their magnetic grains [37]. Differently, all other samples have ratios MR/MS below 50%, with a minimum of 13% in the sample prepared with ic = 2 mA cm 2. Then, the control of the current density and the W content allow obtaining alloys of Co–Fe–W with interesting magnetic characteristics: low HC field (approximately between 10 to 20 Oe) and high MS value (higher or close to 2 T), which makes these materials a possible candidate for applications such as read/write heads and hard disks in magnetic recording. 4. Conclusions The structural, morphological and magnetic characterization of electrodeposited Co–Fe–W alloys, containing small contents of W (up to 9 at.%), were investigated by XRD, SEM, TEM, Mössbauer spectroscopy and magnetic measurements. By varying the applied cathodic current density (ic), electrodeposited (Co100 xFex)100 yWy films (x = 63–72 at.% Fe, y = 4–9 at.% W) were successfully prepared. The XRD results revealed a bcc Fe-like structure for all studied compositions with average crystallite size ranging from 16 to 35 nm which was confirmed by TEM. The SEM images indicated that a needle-like morphology is dominant for the deposits containing lower W content (up to 4.5 at.%), and a cauliflower-type one is observed in the case of higher W content deposits. Mössbauer spectroscopy has shown the contribution of two magnetic phases: one ordered associated with the core grain of the Co–Fe–W alloys and a disordered contribution that is related to the grain boundary/surface with Co–Fe–W features. Due to the needle-like morphology, the magnetization seems to be in the film-plane (orientated along the larger grain size), while in the cauliflower-type morphology is slightly canted from in- to outof-plane geometry. Magnetic measurements, performed in the VSM, confirmed a ferromagnetic character for all samples at room temperature. High saturation values accompanied by low coercivity ones were obtained for electrodeposited Co–Fe–W alloys. These results show that Co–Fe–W alloys are potential materials for applications in magnetic devices such as read/write heads and hard disks. Acknowledgments The authors gratefully acknowledge financial support and scholarships from the Brazilian funding agencies CNPq, Fapesp,
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