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Microstructure of electrodeposited Fe–Pt binary alloy films
Materials Science and Engineering A 375–377 (2004) 1289–1293
Microstructure of electrodeposited Fe–Pt binary alloy films Feng Wang, Sayaka Doi, Kaori Hosoiri, Tohru Watanabe∗ Department of Applied Chemistry, Graduate School of Engineering, Tokyo Metropolitan University, Minami-Ohsawa, Hachioji-Shi, Tokyo 192-0397, Japan
Abstract Fe–Pt alloy films were electrodeposited in a novel bath containing ammonia solution and ammonium tartrate as complexing agent. The compositions of the Fe–Pt deposited films were controlled by bath composition and current density. The crystallographic structures of deposited films with compositions ranging from 34.0 at.% Pt to 81.8 at.% Pt were analysed by means of XRD, HRTEM and ESCA. The crystallographic structure of all the deposited films, which were obtained in the present study, consisted of a single solid solution of a face-centred cubic structure. © 2003 Elsevier B.V. All rights reserved. Keywords: Iron–platinum; Film; Electrodeposition; Crystallographic structure
1. Introduction Fe–Pt alloys have attracted significant attention as possible high-density recording media [1] because of their uniaxial magnetocrystalline anisotropy (Ku ∼ = 7 × 106 J/m3 ) [2], high coercive force, and good corrosion resistance as well as wear resistance. These characteristics have prompted numerous studies of the properties of Fe–Pt, both in bulk and in thin film form [3,4]. In the Fe–Pt alloy system, as shown in its thermal equilibrium phase diagram (Fig. 1) [5], three superlattices, Fe3 Pt, FePt, FePt3 , form at ∼800 ◦ C, ∼1300◦ and ∼650 ◦ C, respectively. Among these, it has long been known that the FePt has an ordered tetragonal, L10 type of structure, and this has the effect of greatly increasing the coercivity over that of the disordered alloy. In general, the face-centred cubic (fcc) phase, which exists in the as-synthesised Fe–Pt alloys with about 50 at.% Pt content, can be transformed into the face-centred tetragonal (fct) phase, namely, L10 ordered phase by heat treatment. Many research works have proved that this L10 ordered phase shows high magnetocrystalline anisotropy along the c axis, which is brought about by the large spin–orbit coupling of Pt atoms and the strong hybridisation of Pt d bands with highly polarised Fe d bands [6]. Numerous physical and chemical methods have been employed to produce Fe–Pt alloys including sputtering, metal ∗
Corresponding author. E-mail address: [email protected] (T. Watanabe).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.211
evaporation, grinding, and liquid phase metal salt reduction. However, there is no published data on Fe–Pt alloys obtained by using electrodeposition method since the preparation of stable and effective electrodeposition electrolyte is very difficult. The intent of the present study is to design the novel bath composition and electrodeposition condition in order to deposit the Fe–Pt alloy film with a wide alloy composition range. The chemical states of iron and platinum in the deposited film are determined by using electron spectroscopy of chemical analysis, and the crystallographic structures of the deposited films with various compositions are studied in detail.
2. Experimental The solution and operating condition for Fe–Pt electrodeposition are listed in Table 1. The bath contained H2 PtCl6 (0.05 − x mol/l), FeSO4 ·7H2 O (x mol/l) (where x was controlled as 0.045, 0.035 mol/l, respectively). In addition, NH3 ·H2 O (0.14 mol/l) and (NH4 )2 C4 H4 O6 (0.030 mol/l) were added as the complex-forming additives. All solutions were prepared using distilled water and reagent grade chemicals. The plating cell was a 300 ml beaker with agitation. The copper foil with the thickness of 35 m was used as substrate and the platinum film coated on the titanium mesh was used as anode material. The copper foils with the dimensions of 2 cm × 2 cm were electrochemically polished
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F. Wang et al. / Materials Science and Engineering A 375–377 (2004) 1289–1293
Fig. 1. Thermal equilibrium phase diagram of Fe–Pt binary alloy system [5].
3. Results and discussion 3.1. Chemical content of deposited films In order to bring the potentials of iron and platinum close enough so that co-deposition can be carried out, the ammonia solution and ammonium tartrate were used to form complexes with metallic ions in aqueous solution. The electrodeposition was carried out under galvanostatic conditions. Fig. 2 illustrates the influence of the cathodic current density on the Pt content in the deposited film. The data indicates that the Pt content in the deposited film gradually increases
with increasing current density. Moreover, it increases with increasing Pt concentration in the bath. Based on the above result, it can be concluded that the Fe–Pt alloy films with the alloy composition ranging from 100
Pt Content in deposits, C/at%
to a mirror finish surface. All samples were rinsed with distilled water, and dried by nitrogen gas before deposition. The range of current density was from 100 to 1000 A/m2 . The bath temperature was maintained at 60 ± 0.1 ◦ C, which was regulated through a heating resistance whose power was controlled to keep the temperature constant. The deposited film composition was determined by energy dispersive X-ray analysis (EDS) (JED-2001, JEOL). The chemical states of Fe and Pt in the deposits were estimated by electron spectroscopy for chemical analysis (ESCA) (ESCA-3400, Shimadsu), and the structure of the deposited film was determined by X-ray diffraction (XRD) (MX Labo2, MAC Science) and a high-resolution transmission electron microscope (HRTEM) (JEM-2000FE, JEOL).
(a)
80
(b)
60
40
20
0 0
20
40
60
Current Density, i/A
80
100
m-2
Fig. 2. Effect of current density on Pt content in the Fe–Pt deposited films from various bath with different Pt concentration—(a) 0.045 mol/l, (b) 0.035 mol/l.
F. Wang et al. / Materials Science and Engineering A 375–377 (2004) 1289–1293
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Table 1 Solution composition and electrodeposition conditions of Fe–Pt deposited films Solution composition
Deposition conditions
Component
Concentration
Parameter
Value
FeSO4 ·7H2 O H2 PtCl6 (NH4 )2 C4 H4 O6 NH3 ·H2 O
0.05–x (mol/l) x (mol/l) 2 (0.05–x) (mol/l) Moderation
Anode Cathode Temperature Current density Agitation
Pt coating on Ti mesh Cu foil 60 ◦ C 100–1000 (A/m2 ) Yes
Fe0
Pt0
20 min
Intensity / CPS
Intensity / CPS
Fe3+
10 min
1 min
730
10 min 1 min
0 min
0 min
725
720
715
710
705
700
85
80
75
70
Binding energy / eV
Binding energy / eV
Fig. 3. Variations of Fe 2p, Pt 4f ESCA spectra of Fe–Pt deposited film with 57.2 at.% Pt under different etching time.
Cu substrate (g-Fe, Pt) (111)
81.8 at%Pt
Intensity (arb. unit)
735
20 min
75.5 at%Pt 60.4 at%Pt 57.2 at%Pt 51.1 at%Pt 34.0 at%Pt 40
50
60
70
80
90
Diffraction angle 2q (deg) Fig. 4. X-ray diffraction patterns of the as-deposited Fe–Pt films with various Pt content.
65
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F. Wang et al. / Materials Science and Engineering A 375–377 (2004) 1289–1293
34.0 at.% Pt to 81.8 at.% Pt have been electrodeposited from the bath using the NH3 ·H2 O and (NH4 )2 C4 H4 O6 as complex-forming additives.
0.4 fcc Pt (0.3930 nm)
Fig. 3 shows the changes of ESCA spectra in the Fe 2p and Pt 4f regions for Fe–Pt deposited film with 57.2 at.%Pt. The as-deposited film was sputtered by using argon ionic etching for various durations to eliminate atmospherically oxidised and contaminated surface layers on the deposited film. The figure provides evidence that iron is not in the free state in the surface of the as-deposited film and the layer sputtered by 1 min argon ionic etching is almost in the oxidised state. This can be attributed to the passive film formed on the surface or presence of unreduced iron. However, the metallic iron peaks appears and the oxides peaks disappear after 10 and 20 min argon ionic etching. Moreover, the platinum also is not in the free state in the surface of the as-deposited film. This can be attributed to the existence of unreduced platinum. However, after 1, 10 and 20 min argon ionic etching, the metallic platinum peaks appear clearly in Fig. 3. Based on the above results, it can be concluded that in the Fe–Pt deposited films both iron and platinum exist in the metallic state. 3.3. Characterisation of Fe–Pt deposited films X-ray diffraction studies were carried out to analyse various Fe–Pt deposited films with different platinum content. According to the diffraction results shown in Fig. 4, only
Lattice constant L / nm
0.39
3.2. Chemical state of Fe–Pt deposited film
0.38
rg Ve
ul e ’s r d r a
0.37 fcc g-Fe (0.3630 nm)
0.36
0.35 0
20
40
60
80
100
Pt content in deposits C / at% Fig. 5. Relationship between Pt content in the deposited film and the lattice constant.
one peak around 2θ of 40.5◦ appears in all the diffraction patterns. This peak is similar to the standard diffraction peak of Pt or ␥-Fe which is the allotropic transformation of pure iron at the high temperature state, but gradually shifts to the lower degree with increasing Pt content in the deposited films, therefore the peak can be assigned to the (␥-Fe, Pt) solid solution of a face-centred cubic structure.
Fig. 6. TEM images and electron diffraction patterns of Fe–Pt deposited film with various Pt content—(a) 34.0 at.%, (b) 57.2 at.%, and (c) 75.5 at.%.
F. Wang et al. / Materials Science and Engineering A 375–377 (2004) 1289–1293
From the XRD results (Fig. 4), the variations of the lattice constant of the deposited films (Fig. 5) were calculated from the lattice spacing of these diffraction patterns. It was found that the lattice constant increases linearly with increasing Pt content in the deposited film, being in good agreement with Vegard’s rule. The deposited films form a single fcc solid solution with a wide alloy composition ranging form from 34.0 at.% Pt to 81.8 at.% Pt, which were obtained in the present study. However, the Fe–Pt alloys are predicted to form FePt, FePt3 and their mixtures at lower temperature according to the thermal equilibrium phase diagram (Fig. 1). These results indicated that the Fe–Pt deposited films form a metastable phase different to the stable phase predicted by the thermal equilibrium phase diagram. This is because the deposit is the quenched solid from high temperature state, which has been demonstrated by Girin and Haruyama [7,8]. Lattice images and the electron diffraction patterns of the Fe–Pt deposited films were investigated in order to examine the crystallographic structure of the deposited films with various Pt content (Fig. 6). The micrograph reveals that the crystal size of the deposited films changes with changing the Pt content in the deposited film. The crystal size of the deposited film with 57.2 at.% Pt is estimated to be around 5.0 ± 0.5 nm, showing a minute crystal structure. All of the electron diffraction patterns reveal that the deposited films form a single (␥-Fe, Pt) solid solution of a face-centred cubic structure.
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4. Conclusions Electrodeposited Fe–Pt binary alloy films with a wide range of compositions were prepared by changing the bath composition and electrodeposition condition. Their crystallographic structures were analysed by using XRD, HRTEM and ESCA. The results indicate that both iron and platinum exist as metallic states in the deposited film, and the crystallographic structure of deposited films with wide compositions ranging from 34.0 at.% Pt to 81.8 at.% Pt consists of a single solid solution of a face-centred cubic structure.
References [1] R.A. Ristau, K. Barmak, L.H. Lewis, K.R. Coffey, J.K. Howard, J. Appl. Phys. 86 (1999) 4527. [2] K. Inomata, T. Sawa, S. Hashimoto, J. Appl. Phys. 64 (1988) 2537. [3] S.H. Sun, C.B. Murry, D. Weller, L. Folks, A. Moser, Science 287 (2000) 989. [4] C.P. Luo, D.J. Sellmyer, Appl. Phys. Lett. 75 (1999) 3162. [5] T.B. Massalski, Binary Alloy Phase Diagram, The material Information Society, 1990, p. 1755. [6] A. Sakuma, J. Phys. Soc. Jpn. 63 (1994) 3053. [7] O.B. Girin, J. Electron. Mater. 24 (1995) 947. [8] Haruyama, DENKIKAGAKU, 31 (1963) 478.
Microstructure of electrodeposited Fe–Pt binary alloy films Feng Wang, Sayaka Doi, Kaori Hosoiri, Tohru Watanabe∗ Department of Applied Chemistry, Graduate School of Engineering, Tokyo Metropolitan University, Minami-Ohsawa, Hachioji-Shi, Tokyo 192-0397, Japan
Abstract Fe–Pt alloy films were electrodeposited in a novel bath containing ammonia solution and ammonium tartrate as complexing agent. The compositions of the Fe–Pt deposited films were controlled by bath composition and current density. The crystallographic structures of deposited films with compositions ranging from 34.0 at.% Pt to 81.8 at.% Pt were analysed by means of XRD, HRTEM and ESCA. The crystallographic structure of all the deposited films, which were obtained in the present study, consisted of a single solid solution of a face-centred cubic structure. © 2003 Elsevier B.V. All rights reserved. Keywords: Iron–platinum; Film; Electrodeposition; Crystallographic structure
1. Introduction Fe–Pt alloys have attracted significant attention as possible high-density recording media [1] because of their uniaxial magnetocrystalline anisotropy (Ku ∼ = 7 × 106 J/m3 ) [2], high coercive force, and good corrosion resistance as well as wear resistance. These characteristics have prompted numerous studies of the properties of Fe–Pt, both in bulk and in thin film form [3,4]. In the Fe–Pt alloy system, as shown in its thermal equilibrium phase diagram (Fig. 1) [5], three superlattices, Fe3 Pt, FePt, FePt3 , form at ∼800 ◦ C, ∼1300◦ and ∼650 ◦ C, respectively. Among these, it has long been known that the FePt has an ordered tetragonal, L10 type of structure, and this has the effect of greatly increasing the coercivity over that of the disordered alloy. In general, the face-centred cubic (fcc) phase, which exists in the as-synthesised Fe–Pt alloys with about 50 at.% Pt content, can be transformed into the face-centred tetragonal (fct) phase, namely, L10 ordered phase by heat treatment. Many research works have proved that this L10 ordered phase shows high magnetocrystalline anisotropy along the c axis, which is brought about by the large spin–orbit coupling of Pt atoms and the strong hybridisation of Pt d bands with highly polarised Fe d bands [6]. Numerous physical and chemical methods have been employed to produce Fe–Pt alloys including sputtering, metal ∗
Corresponding author. E-mail address: [email protected] (T. Watanabe).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.211
evaporation, grinding, and liquid phase metal salt reduction. However, there is no published data on Fe–Pt alloys obtained by using electrodeposition method since the preparation of stable and effective electrodeposition electrolyte is very difficult. The intent of the present study is to design the novel bath composition and electrodeposition condition in order to deposit the Fe–Pt alloy film with a wide alloy composition range. The chemical states of iron and platinum in the deposited film are determined by using electron spectroscopy of chemical analysis, and the crystallographic structures of the deposited films with various compositions are studied in detail.
2. Experimental The solution and operating condition for Fe–Pt electrodeposition are listed in Table 1. The bath contained H2 PtCl6 (0.05 − x mol/l), FeSO4 ·7H2 O (x mol/l) (where x was controlled as 0.045, 0.035 mol/l, respectively). In addition, NH3 ·H2 O (0.14 mol/l) and (NH4 )2 C4 H4 O6 (0.030 mol/l) were added as the complex-forming additives. All solutions were prepared using distilled water and reagent grade chemicals. The plating cell was a 300 ml beaker with agitation. The copper foil with the thickness of 35 m was used as substrate and the platinum film coated on the titanium mesh was used as anode material. The copper foils with the dimensions of 2 cm × 2 cm were electrochemically polished
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F. Wang et al. / Materials Science and Engineering A 375–377 (2004) 1289–1293
Fig. 1. Thermal equilibrium phase diagram of Fe–Pt binary alloy system [5].
3. Results and discussion 3.1. Chemical content of deposited films In order to bring the potentials of iron and platinum close enough so that co-deposition can be carried out, the ammonia solution and ammonium tartrate were used to form complexes with metallic ions in aqueous solution. The electrodeposition was carried out under galvanostatic conditions. Fig. 2 illustrates the influence of the cathodic current density on the Pt content in the deposited film. The data indicates that the Pt content in the deposited film gradually increases
with increasing current density. Moreover, it increases with increasing Pt concentration in the bath. Based on the above result, it can be concluded that the Fe–Pt alloy films with the alloy composition ranging from 100
Pt Content in deposits, C/at%
to a mirror finish surface. All samples were rinsed with distilled water, and dried by nitrogen gas before deposition. The range of current density was from 100 to 1000 A/m2 . The bath temperature was maintained at 60 ± 0.1 ◦ C, which was regulated through a heating resistance whose power was controlled to keep the temperature constant. The deposited film composition was determined by energy dispersive X-ray analysis (EDS) (JED-2001, JEOL). The chemical states of Fe and Pt in the deposits were estimated by electron spectroscopy for chemical analysis (ESCA) (ESCA-3400, Shimadsu), and the structure of the deposited film was determined by X-ray diffraction (XRD) (MX Labo2, MAC Science) and a high-resolution transmission electron microscope (HRTEM) (JEM-2000FE, JEOL).
(a)
80
(b)
60
40
20
0 0
20
40
60
Current Density, i/A
80
100
m-2
Fig. 2. Effect of current density on Pt content in the Fe–Pt deposited films from various bath with different Pt concentration—(a) 0.045 mol/l, (b) 0.035 mol/l.
F. Wang et al. / Materials Science and Engineering A 375–377 (2004) 1289–1293
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Table 1 Solution composition and electrodeposition conditions of Fe–Pt deposited films Solution composition
Deposition conditions
Component
Concentration
Parameter
Value
FeSO4 ·7H2 O H2 PtCl6 (NH4 )2 C4 H4 O6 NH3 ·H2 O
0.05–x (mol/l) x (mol/l) 2 (0.05–x) (mol/l) Moderation
Anode Cathode Temperature Current density Agitation
Pt coating on Ti mesh Cu foil 60 ◦ C 100–1000 (A/m2 ) Yes
Fe0
Pt0
20 min
Intensity / CPS
Intensity / CPS
Fe3+
10 min
1 min
730
10 min 1 min
0 min
0 min
725
720
715
710
705
700
85
80
75
70
Binding energy / eV
Binding energy / eV
Fig. 3. Variations of Fe 2p, Pt 4f ESCA spectra of Fe–Pt deposited film with 57.2 at.% Pt under different etching time.
Cu substrate (g-Fe, Pt) (111)
81.8 at%Pt
Intensity (arb. unit)
735
20 min
75.5 at%Pt 60.4 at%Pt 57.2 at%Pt 51.1 at%Pt 34.0 at%Pt 40
50
60
70
80
90
Diffraction angle 2q (deg) Fig. 4. X-ray diffraction patterns of the as-deposited Fe–Pt films with various Pt content.
65
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F. Wang et al. / Materials Science and Engineering A 375–377 (2004) 1289–1293
34.0 at.% Pt to 81.8 at.% Pt have been electrodeposited from the bath using the NH3 ·H2 O and (NH4 )2 C4 H4 O6 as complex-forming additives.
0.4 fcc Pt (0.3930 nm)
Fig. 3 shows the changes of ESCA spectra in the Fe 2p and Pt 4f regions for Fe–Pt deposited film with 57.2 at.%Pt. The as-deposited film was sputtered by using argon ionic etching for various durations to eliminate atmospherically oxidised and contaminated surface layers on the deposited film. The figure provides evidence that iron is not in the free state in the surface of the as-deposited film and the layer sputtered by 1 min argon ionic etching is almost in the oxidised state. This can be attributed to the passive film formed on the surface or presence of unreduced iron. However, the metallic iron peaks appears and the oxides peaks disappear after 10 and 20 min argon ionic etching. Moreover, the platinum also is not in the free state in the surface of the as-deposited film. This can be attributed to the existence of unreduced platinum. However, after 1, 10 and 20 min argon ionic etching, the metallic platinum peaks appear clearly in Fig. 3. Based on the above results, it can be concluded that in the Fe–Pt deposited films both iron and platinum exist in the metallic state. 3.3. Characterisation of Fe–Pt deposited films X-ray diffraction studies were carried out to analyse various Fe–Pt deposited films with different platinum content. According to the diffraction results shown in Fig. 4, only
Lattice constant L / nm
0.39
3.2. Chemical state of Fe–Pt deposited film
0.38
rg Ve
ul e ’s r d r a
0.37 fcc g-Fe (0.3630 nm)
0.36
0.35 0
20
40
60
80
100
Pt content in deposits C / at% Fig. 5. Relationship between Pt content in the deposited film and the lattice constant.
one peak around 2θ of 40.5◦ appears in all the diffraction patterns. This peak is similar to the standard diffraction peak of Pt or ␥-Fe which is the allotropic transformation of pure iron at the high temperature state, but gradually shifts to the lower degree with increasing Pt content in the deposited films, therefore the peak can be assigned to the (␥-Fe, Pt) solid solution of a face-centred cubic structure.
Fig. 6. TEM images and electron diffraction patterns of Fe–Pt deposited film with various Pt content—(a) 34.0 at.%, (b) 57.2 at.%, and (c) 75.5 at.%.
F. Wang et al. / Materials Science and Engineering A 375–377 (2004) 1289–1293
From the XRD results (Fig. 4), the variations of the lattice constant of the deposited films (Fig. 5) were calculated from the lattice spacing of these diffraction patterns. It was found that the lattice constant increases linearly with increasing Pt content in the deposited film, being in good agreement with Vegard’s rule. The deposited films form a single fcc solid solution with a wide alloy composition ranging form from 34.0 at.% Pt to 81.8 at.% Pt, which were obtained in the present study. However, the Fe–Pt alloys are predicted to form FePt, FePt3 and their mixtures at lower temperature according to the thermal equilibrium phase diagram (Fig. 1). These results indicated that the Fe–Pt deposited films form a metastable phase different to the stable phase predicted by the thermal equilibrium phase diagram. This is because the deposit is the quenched solid from high temperature state, which has been demonstrated by Girin and Haruyama [7,8]. Lattice images and the electron diffraction patterns of the Fe–Pt deposited films were investigated in order to examine the crystallographic structure of the deposited films with various Pt content (Fig. 6). The micrograph reveals that the crystal size of the deposited films changes with changing the Pt content in the deposited film. The crystal size of the deposited film with 57.2 at.% Pt is estimated to be around 5.0 ± 0.5 nm, showing a minute crystal structure. All of the electron diffraction patterns reveal that the deposited films form a single (␥-Fe, Pt) solid solution of a face-centred cubic structure.
1293
4. Conclusions Electrodeposited Fe–Pt binary alloy films with a wide range of compositions were prepared by changing the bath composition and electrodeposition condition. Their crystallographic structures were analysed by using XRD, HRTEM and ESCA. The results indicate that both iron and platinum exist as metallic states in the deposited film, and the crystallographic structure of deposited films with wide compositions ranging from 34.0 at.% Pt to 81.8 at.% Pt consists of a single solid solution of a face-centred cubic structure.
References [1] R.A. Ristau, K. Barmak, L.H. Lewis, K.R. Coffey, J.K. Howard, J. Appl. Phys. 86 (1999) 4527. [2] K. Inomata, T. Sawa, S. Hashimoto, J. Appl. Phys. 64 (1988) 2537. [3] S.H. Sun, C.B. Murry, D. Weller, L. Folks, A. Moser, Science 287 (2000) 989. [4] C.P. Luo, D.J. Sellmyer, Appl. Phys. Lett. 75 (1999) 3162. [5] T.B. Massalski, Binary Alloy Phase Diagram, The material Information Society, 1990, p. 1755. [6] A. Sakuma, J. Phys. Soc. Jpn. 63 (1994) 3053. [7] O.B. Girin, J. Electron. Mater. 24 (1995) 947. [8] Haruyama, DENKIKAGAKU, 31 (1963) 478.