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Structural characterization and magnetic properties of electrodeposited CoPt alloys
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) e883–e884
Structural characterization and magnetic properties of electrodeposited CoPt alloys Fernando M.F. Rhen*, J.M.D. Coey Physics Department, Trinity College, Dublin 2, Ireland
Abstract Shiny and smooth Co1xPtx (0:05oxo0:65) films were electrodeposited from a single bath on copper substrates with thickness up to 2.0 mm. X-ray diffraction of the samples indicates the presence of Co3Pt as the major phase depending on the potential. Films with composition Co78Pt22 showed coercivities of 0.15 T when magnetized perpendicular to the film plane. r 2004 Published by Elsevier B.V. PACS: 82.45.Q; 81.15.P; 75.50.W Keywords: Magnetization; L10 CoPt; Electrodeposition; Film
1. Introduction Hard magnetic films processed at low temperatures (o300 C) are required for applications compatible with silicon-based microelectronic technology. Sputtered L10 FePt and CoPt show high coercivities (>1 T), however, the major disadvantage of this method is the requirement for high-temperature post-annealing (500–700 C) [1–3]. Electrodeposited materials may be the most suitable candidates for these applications. Many studies have been carried out to develop electrodeposited Co–Pt films, most of them involving Co–Pt–P alloys [4,5]. Here we report on the preparation and magnetic and structural characterization of CoPt electrodeposited from a new single bath. CoPt films were potentiostatically electrodeposited on polycrystalline Cu substrates from a single bath containing 1 mmol/l H2 PtCl6 ; 0.1 mol/l Na2 SO4 and 0.1 mol/l CoSO4 : The solution pH was adjusted to 3 by adding a small amount hydrochloric acid (HCl). A graphite rod of 6 mm diameter and 50 mm length was used as a counter electrode and Ag/AgCl saturated *Corresponding author. Tel.: 0353-01-608-2171; fax: 0035301-671-1759. E-mail address: [email protected] (F.M.F. Rhen). 0304-8853/$ - see front matter r 2004 Published by Elsevier B.V. doi:10.1016/j.jmmm.2003.12.207
with KCl (factory calibration of 20575 mV) as the reference electrode. All potentials are quoted with respect to this reference electrode. Solutions were prepared from 100 ml deionized water. Cu substrates were used as working electrodes and cut out from 99.9% pure copper foil 0.5 mm thick into 5 mm 5 mm squares. These substrates were coated on one side with varnish and connected to a 1 mm copper wire. Just before electrodeposition, the substrates were dipped into a 10% H2 SO4 solution to remove any oxide layers, and then dipped into deionized water. Deposition was carried out from fresh solutions either vigorously stirred or unstirred in open atmosphere at room temperature and the deposition occurred immediately after placing the substrate into the bath because Pt deposition occurs even in the absence of an applied potential. The deposition was controlled using an EG&G model 263A potentiostat. Composition of the samples was determined by energy dispersive X-ray spectroscope (EDX) in a scanning electron microscopy (SEM). The crystalline structure was investigated by X-ray diffraction (XRD) with CuKa radiation and a 0.02 step. Magnetic properties were measured in a 5 T superconducting quantum interference device (SQUID) magnetometer and the thickness was evaluated in an atomic force microscope (AFM) in the contact mode.
ARTICLE IN PRESS F.M.F. Rhen, J.M.D. Coey / Journal of Magnetism and Magnetic Materials 272–276 (2004) e883–e884
65 60 55 50 45 40 35 30 25 20 15 10 5
perpendicular parallel
68.0
stirred unstirred
34.0 σ (J/TKg)
Pt (atm%)
e884
0.0
-34.0 -68.0 -3
-0.90
-0.85 -0.80 -0.75 -0.70 -0.65 Potential quoted to Ag/AgCl (V)
-0.60
Fig. 1. Platinum content as a function of the stirring conditions and overpotential applied to the electrochemical cell.
Platinum is electrodeposited from a complex ion PtCl2 6 in two consecutive steps on the working electrode 2 PtCl2 6 þ 2e2PtCl4 þ 2Cl
ð1Þ
PtCl2 4 þ 2e2Pt þ 4Cl
ð2Þ
The standard reduction potentials for the reactions (1) and (2) are 0.52 and 0.54 V quoted relative to Ag/AgCl reference electrode [6]. The standard potential for Co+2 reduction is 0.50 V [6]. In this case, for the potentials applied to the electrochemical cell, the deposition of Pt is mainly governed by mass transport. By measuring the thickness of the film in an AFM (2.0 mm) and taking account of the integrated charge (7.5 C) we estimated the current efficiency. A value of about 15% at 0.9 V was found. Under strong stirring conditions, Pt content in the films was found to vary almost linearly between 0.6 and 0.9 V as shown in Fig. 1. Films electrodeposited under no stirring conditions are Co rich. All films were shiny and homogeneous and tiny amounts of Na and Cl were found in most of the films which are likely to come from the compounds Na2 SO4 and H2 PtCl6 in the solutions. Magnetic measurements are shown in Fig. 2. The sample was electrodeposited at 0.9 V and exhibited coercivity of 0.07 T in a field parallel to the plane of the films and 0.15 T in a perpendicular field. The remanence ratio sr/ss calculated for this sample is smaller than 0.3, suggesting a high or complete absence of intergrain exchange interaction among grains. It is likely that part
-2
-1
0 µ0 H(T)
1
2
3
Fig. 2. Room temperature hyteresis loop of CoPt electrodeposited at 0.9 V.
of the platinum in the samples goes to grain boundaries isolating the grains in the films. The samples are magnetically anisotropic. X-ray diffraction of the samples indicates the presence of Co3 Pt as the major phase. Optimized CoPt 2.0 mm thick films deposited from a single bath on Cu exhibit coercivity of 0.15 T and remanence of 20 J/Tkg in a field perpendicular to the plane of the films. These films may suit silicon technology requirements since no annealing is necessary to obtain coercivity. A copper seed layer may be deposited on Si-substrate for later deposition of CoPt. Addition of NaH2 PO2 to the bath may improve the coercivity of these films.
References [1] K.R. Coffey, M.A. Parker, J.K. Howard, IEEE Trans. Magn. 31 (1995) 2737. [2] C.H. Lee, R.F.C. Farrow, C.J. Lin, E.E. Marinero, Phys. Rev. B. 42 (7) (1990) 11384. [3] P.F. Garcia, Z.G. Li, W.B. Zeper, J. Magn. Magn. Mater. 121 (1993) 452. [4] G. Zangari, P. Bucher, N. Lecis, P.L. Cavallotti, L. Callegaro, E. Puppin, J. Magn. Magn. Mater. 157 (1996) 256. [5] P.L. Cavalloti, N. Lecis, H. Fauser, A. Zielonka, J.P. Celis, G. Wooters, J. Machado da Silva, J.M. Brochado Oliveira, M.A. Sa, Surf, Coat. Technol. 105 (1998) 232. [6] Robert C.Weast, Handbook of Chemistry and Physics, 53rd edition. The Chemical Rubber Co., Cleveland, 1972, p. D-112.
Journal of Magnetism and Magnetic Materials 272–276 (2004) e883–e884
Structural characterization and magnetic properties of electrodeposited CoPt alloys Fernando M.F. Rhen*, J.M.D. Coey Physics Department, Trinity College, Dublin 2, Ireland
Abstract Shiny and smooth Co1xPtx (0:05oxo0:65) films were electrodeposited from a single bath on copper substrates with thickness up to 2.0 mm. X-ray diffraction of the samples indicates the presence of Co3Pt as the major phase depending on the potential. Films with composition Co78Pt22 showed coercivities of 0.15 T when magnetized perpendicular to the film plane. r 2004 Published by Elsevier B.V. PACS: 82.45.Q; 81.15.P; 75.50.W Keywords: Magnetization; L10 CoPt; Electrodeposition; Film
1. Introduction Hard magnetic films processed at low temperatures (o300 C) are required for applications compatible with silicon-based microelectronic technology. Sputtered L10 FePt and CoPt show high coercivities (>1 T), however, the major disadvantage of this method is the requirement for high-temperature post-annealing (500–700 C) [1–3]. Electrodeposited materials may be the most suitable candidates for these applications. Many studies have been carried out to develop electrodeposited Co–Pt films, most of them involving Co–Pt–P alloys [4,5]. Here we report on the preparation and magnetic and structural characterization of CoPt electrodeposited from a new single bath. CoPt films were potentiostatically electrodeposited on polycrystalline Cu substrates from a single bath containing 1 mmol/l H2 PtCl6 ; 0.1 mol/l Na2 SO4 and 0.1 mol/l CoSO4 : The solution pH was adjusted to 3 by adding a small amount hydrochloric acid (HCl). A graphite rod of 6 mm diameter and 50 mm length was used as a counter electrode and Ag/AgCl saturated *Corresponding author. Tel.: 0353-01-608-2171; fax: 0035301-671-1759. E-mail address: [email protected] (F.M.F. Rhen). 0304-8853/$ - see front matter r 2004 Published by Elsevier B.V. doi:10.1016/j.jmmm.2003.12.207
with KCl (factory calibration of 20575 mV) as the reference electrode. All potentials are quoted with respect to this reference electrode. Solutions were prepared from 100 ml deionized water. Cu substrates were used as working electrodes and cut out from 99.9% pure copper foil 0.5 mm thick into 5 mm 5 mm squares. These substrates were coated on one side with varnish and connected to a 1 mm copper wire. Just before electrodeposition, the substrates were dipped into a 10% H2 SO4 solution to remove any oxide layers, and then dipped into deionized water. Deposition was carried out from fresh solutions either vigorously stirred or unstirred in open atmosphere at room temperature and the deposition occurred immediately after placing the substrate into the bath because Pt deposition occurs even in the absence of an applied potential. The deposition was controlled using an EG&G model 263A potentiostat. Composition of the samples was determined by energy dispersive X-ray spectroscope (EDX) in a scanning electron microscopy (SEM). The crystalline structure was investigated by X-ray diffraction (XRD) with CuKa radiation and a 0.02 step. Magnetic properties were measured in a 5 T superconducting quantum interference device (SQUID) magnetometer and the thickness was evaluated in an atomic force microscope (AFM) in the contact mode.
ARTICLE IN PRESS F.M.F. Rhen, J.M.D. Coey / Journal of Magnetism and Magnetic Materials 272–276 (2004) e883–e884
65 60 55 50 45 40 35 30 25 20 15 10 5
perpendicular parallel
68.0
stirred unstirred
34.0 σ (J/TKg)
Pt (atm%)
e884
0.0
-34.0 -68.0 -3
-0.90
-0.85 -0.80 -0.75 -0.70 -0.65 Potential quoted to Ag/AgCl (V)
-0.60
Fig. 1. Platinum content as a function of the stirring conditions and overpotential applied to the electrochemical cell.
Platinum is electrodeposited from a complex ion PtCl2 6 in two consecutive steps on the working electrode 2 PtCl2 6 þ 2e2PtCl4 þ 2Cl
ð1Þ
PtCl2 4 þ 2e2Pt þ 4Cl
ð2Þ
The standard reduction potentials for the reactions (1) and (2) are 0.52 and 0.54 V quoted relative to Ag/AgCl reference electrode [6]. The standard potential for Co+2 reduction is 0.50 V [6]. In this case, for the potentials applied to the electrochemical cell, the deposition of Pt is mainly governed by mass transport. By measuring the thickness of the film in an AFM (2.0 mm) and taking account of the integrated charge (7.5 C) we estimated the current efficiency. A value of about 15% at 0.9 V was found. Under strong stirring conditions, Pt content in the films was found to vary almost linearly between 0.6 and 0.9 V as shown in Fig. 1. Films electrodeposited under no stirring conditions are Co rich. All films were shiny and homogeneous and tiny amounts of Na and Cl were found in most of the films which are likely to come from the compounds Na2 SO4 and H2 PtCl6 in the solutions. Magnetic measurements are shown in Fig. 2. The sample was electrodeposited at 0.9 V and exhibited coercivity of 0.07 T in a field parallel to the plane of the films and 0.15 T in a perpendicular field. The remanence ratio sr/ss calculated for this sample is smaller than 0.3, suggesting a high or complete absence of intergrain exchange interaction among grains. It is likely that part
-2
-1
0 µ0 H(T)
1
2
3
Fig. 2. Room temperature hyteresis loop of CoPt electrodeposited at 0.9 V.
of the platinum in the samples goes to grain boundaries isolating the grains in the films. The samples are magnetically anisotropic. X-ray diffraction of the samples indicates the presence of Co3 Pt as the major phase. Optimized CoPt 2.0 mm thick films deposited from a single bath on Cu exhibit coercivity of 0.15 T and remanence of 20 J/Tkg in a field perpendicular to the plane of the films. These films may suit silicon technology requirements since no annealing is necessary to obtain coercivity. A copper seed layer may be deposited on Si-substrate for later deposition of CoPt. Addition of NaH2 PO2 to the bath may improve the coercivity of these films.
References [1] K.R. Coffey, M.A. Parker, J.K. Howard, IEEE Trans. Magn. 31 (1995) 2737. [2] C.H. Lee, R.F.C. Farrow, C.J. Lin, E.E. Marinero, Phys. Rev. B. 42 (7) (1990) 11384. [3] P.F. Garcia, Z.G. Li, W.B. Zeper, J. Magn. Magn. Mater. 121 (1993) 452. [4] G. Zangari, P. Bucher, N. Lecis, P.L. Cavallotti, L. Callegaro, E. Puppin, J. Magn. Magn. Mater. 157 (1996) 256. [5] P.L. Cavalloti, N. Lecis, H. Fauser, A. Zielonka, J.P. Celis, G. Wooters, J. Machado da Silva, J.M. Brochado Oliveira, M.A. Sa, Surf, Coat. Technol. 105 (1998) 232. [6] Robert C.Weast, Handbook of Chemistry and Physics, 53rd edition. The Chemical Rubber Co., Cleveland, 1972, p. D-112.