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Magnetic anisotropy and its thickness dependence for NiFe alloy films electrodeposited on polycrystalline Cu substrates
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 304 (2006) e736–e738 www.elsevier.com/locate/jmmm
Magnetic anisotropy and its thickness dependence for NiFe alloy films electrodeposited on polycrystalline Cu substrates Hakan Kockara,, Mursel Alperb, Hilal Kurua, Turgut Meydanc a
Physics Department, Science & Literature Faculty, Balikesir University, 10100 Balikesir, Turkey Physics Department, Science & Literature Faculty, Uludag University, Go¨ru¨kle, 16059 Bursa, Turkey c Wolfson Center for Magnetic Technology, School of Engineering, Cardiff University, Cardiff, UK
b
Available online 20 March 2006
Abstract The thickness dependence of magnetic properties of NiFe alloys electrodeposited on polycrystalline copper substrates has been investigated. In order to see how the film thickness affects their properties, the films with various thicknesses were deposited by keeping the cathode potential at 1.5 V vs. the saturated calomel reference electrode (SCE). Magnetic measurements show that the magnetic properties are very sensitive to the film thicknesses and, the easy axis of all films is in the film plane. The results showed that the 1 and 2 mm thick NiFe films are anisotropic and the degree of their anisotropy depends on film thickness whereas those deposited at the thickness of 3 mm show an isotropic magnetic behaviour. It is also found that the increase of the nickel content when increasing their thickness results in an increase in the coercivity values. r 2006 Elsevier B.V. All rights reserved. PACS: 75.30.Gw; 75.50.Bb; 81.15.Pq Keywords: Electrodeposition; NiFe films; Magnetic anisotropy, Coercivity; Fe and its alloys; Electroplating
1. Introduction Electrodeposited nickel–iron alloys, with in-plane magnetization, are used in a variety of industrial applications as, for example, in magnetic recording [1,2]. The magnetic properties of NiFe films vary with composition and thickness [3]. The purpose of this study was to deposit NiFe films and then to characterise their magnetic properties.
NiSO4 7H2 O, 0:01 M FeSO4 7H2 O and 0:1 M H3 BO3 . The electrolyte pH was 2:5 0:1. Before starting the deposition process, the substrates were mechanically and electrochemically polished, and then washed in 10% H2 SO4 and distilled water, respectively. The films were deposited at room temperature using the cathode potentials of 1.5 V vs. saturated calomel electrode (SCE) in continuous waveforms. 3. Results and discussion
2. Experimental NiFe alloy films were grown on Cu substrates ðGoodfellowtÞ from a single electrolyte containing Niþ2 and Feþ2 ions under the potentiostatic conditions. Electrodeposition was performed in an electrochemical cell with three electrodes using a potentiostat/Galvanostat (EGG Model 362). The electrolyte was composed of 0.05 M Corresponding author. Tel.: +90 266 6121278; fax: +90 266 6121215.
E-mail address: [email protected] (H. Kockar). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.02.207
In-plane hysteresis loops of all films were measured at various angles in the film plane and perpendicular to the film plane using a vibrating sample magnetometer (VSM, NOVO Moltspin Ltd.). A circular film of 6 mm diameter was used to minimise demagnetising effects in the film plane. The structural data in our previous work [4] showed that the NiFe film crystallizes in FCC structure. Fig. 1 shows the hysteresis loops of 1 mm NiFe films measured at 0 and 90 in the film plane. When the field is applied at 0 in the film plane, the loop is almost square
ARTICLE IN PRESS H. Kockar et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e736–e738
with a high coercivity, H c , 0:29 kA=m. When the field is applied along 90 , the loop exhibits a smaller coercivity, H c , 0:21 kA=m than the anisotropy field, H a , 4.20 kA/m, but not equal to zero. The coercivities obtained from loops of all samples in the film plane and perpendicular to the film plane are summarised in Table 1. The coercivities of the film decrease when the measurement angle is rotated from 0 to 90 . The coercivity field, H c , is different from the anisotropy field, H a , which means that when the field is applied along the easy axis, the magnetisation does not rotate uniformly but there is a nucleation and propagation of domain walls [5–7]. When the field is applied perpendicular to the film plane, the coercivity reaches a value of 0.82 kA/m, indicating the film has a planar magnetic anisotropy. The easy axis was at 0 and its reason may probably be due to thickness of the film. Therefore, further NiFe films with 2 and 3 mm thicknesses were produced. Fig. 2 shows the hysteresis loops of 2 mm NiFe measured at 0 and 90 in the film plane and also perpendicular to the film plane. Although the feature characteristics of the in-plane anisotropy were observed, the loops corresponding to the angles are different from the loops in Fig. 1. When the field is applied at 0 , the loop is not as square as in Fig. 2. The higher thickness makes the uniaxial magnetic anisotropy less well defined. Hysteresis loops of 3 mm NiFe indicate that sample does not have a uniaxial magnetic anisotropy. As seen in Table 1 3 mm NiFe is isotropic. Normalised remanent magnetisation to saturation magnetisation ðM r =M s Þ of films as a function of the angles between the applied field and magnetization easy axis is
Fig. 1. An example of anisotropic hysteresis loops of 1 mm NiFe film.
e737
shown in Fig. 3. The total angular range of measurement in the film plane was 360 , with the 0 direction chosen arbitrarily. The remanence ratios for 1 mm NiFe and 2 mm NiFe decreases from 0 to 90 . This is followed by an increase from 90 to 180 . The cycles are repeated in the region between 180 and 360 . This indicates that the magnetic anisotropy with an easy axis is at 0 and 180 . Although 1 mm NiFe and 2 mm NiFe have the similar curves, the degree of the variation in the remanence ratios is different, 42% for 1 mm NiFe and 26% for 2 mm NiFe. In the case of 3 mm NiFe, the remanence ratio in the range of 0 to 360 varies around 5% as in Fig. 3. This is most likely the increase of thickness causes a uniform magnetic material production on the film plane, therefore the uniaxial in-plane anisotropy changes into isotropy. This result is in a good agreement with the findings
Fig. 2. In-plane magnetic anisotropy exhibited in a 2 mm NiFe film.
Fig. 3. Normalised remanence ratio ðM r =M s Þ as a function of angles between applied field and magnetisation easy axis.
Table 1 Composition analysis and coercivity measurement results for of nickel–iron films deposited at 1.5 V at different thickness NiFe film thickness ðmmÞ
1 2 3
Compositions
Coercivity, H c ðkA=mÞ
%Ni
%Fe
0
30
60
90
Perpendicular to the film plane
In-plane magnetic anisotropy
Perpendicular magnetic anisotropy
64.92 69.16 73.74
35.08 30.84 26.26
0.29 0.40 0.58
0.26 0.39 0.57
0.23 0.38 0.58
0.21 0.37 0.58
0.82 1.29 3.10
Yes Yes (slight) No
No No No
ARTICLE IN PRESS e738
H. Kockar et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e736–e738
obtained in Ref. [8], which indicates that the magnetic properties of NiFe films vary with thickness. The average variation of nickel and iron contents was measured in the films by using scanning electron microscope (SEM, Hitachi S-570) and presented in Table 1. The value of coercivity is known to be dependent on many parameters such as film thickness and composition [7]. The influence of the composition and thickness on coercivity can be observed from the data in Table 1. Other researchers [8,9] observed that the coercivity of the nickel alloys such as permalloy is as low as 0.40 A/m, while that of bulk nickel and iron are approximately 0.80 and 0.08 kA/m, respectively. The decrease of the iron content when increasing their thickness results in an increase (from 0.25 to 0.58 kA/m) of the average coercivity. 4. Conclusions Nickel–iron films with several thicknesses were deposited onto copper substrates and their magnetic properties were investigated. Anisotropic and/or isotropic magnetic materials could be deposited by choosing the proper thicknesses. This work provides a good platform for the possible production of NiFe films onto the metal-plated flexible polyimide ðKaptontÞ substrate as magneto-elastic sensors.
Acknowledgements This work is supported by the Scientific and Technical Research Council (TUBITAK) of Turkey under Grant no. TBAG-1771, and Balikesir University, Turkey under Grant no. 2001/02. The authors are grateful to Dr. P.I. Williams for his help to use the facilities in Wolfson Centre, Cardiff University, UK, and to Balikesir University, for the use of Research Centre of Applied Sciences (BURCAS), Turkey. References [1] S. Asaki, M. Sano, S. Li, Y. Tsuchi, O. Redon, T. Sasaki, N. Ito, K. Terunuma, H. Morita, M. Matsuzaki, J. Appl. Phys. 87 (2000) 5377. [2] K. O’Gready, H. Laidler, J. Magn. Magn. Mater. 20 (1999) 616. [3] D. Gangasingh, J.B. Talbot, J. Electrochem. Soc. 138 (12) (1991) 3605. [4] H. Kockar, M. Alper, H. Topcu, Eur. Phys. J. B 42 (4) (2004) 497. [5] H. Kockar, T. Meydan J. Magn. Magn. Mater. 242–245P1 (2002) 187. [6] M. Pruton, Thin Ferromagnetic Films, Butterworths, London, 1974. [7] H. Kockar, J. Superconductivity 17 (4) (2004) 531. [8] D. Jiles, Introduction to Magnetism and Magnetic Materials, Chapman & Hall, London, 1991. [9] J. Jakubovics, Magnetism and Magnetic Materials, Institute of Metals, London, 1987.
Journal of Magnetism and Magnetic Materials 304 (2006) e736–e738 www.elsevier.com/locate/jmmm
Magnetic anisotropy and its thickness dependence for NiFe alloy films electrodeposited on polycrystalline Cu substrates Hakan Kockara,, Mursel Alperb, Hilal Kurua, Turgut Meydanc a
Physics Department, Science & Literature Faculty, Balikesir University, 10100 Balikesir, Turkey Physics Department, Science & Literature Faculty, Uludag University, Go¨ru¨kle, 16059 Bursa, Turkey c Wolfson Center for Magnetic Technology, School of Engineering, Cardiff University, Cardiff, UK
b
Available online 20 March 2006
Abstract The thickness dependence of magnetic properties of NiFe alloys electrodeposited on polycrystalline copper substrates has been investigated. In order to see how the film thickness affects their properties, the films with various thicknesses were deposited by keeping the cathode potential at 1.5 V vs. the saturated calomel reference electrode (SCE). Magnetic measurements show that the magnetic properties are very sensitive to the film thicknesses and, the easy axis of all films is in the film plane. The results showed that the 1 and 2 mm thick NiFe films are anisotropic and the degree of their anisotropy depends on film thickness whereas those deposited at the thickness of 3 mm show an isotropic magnetic behaviour. It is also found that the increase of the nickel content when increasing their thickness results in an increase in the coercivity values. r 2006 Elsevier B.V. All rights reserved. PACS: 75.30.Gw; 75.50.Bb; 81.15.Pq Keywords: Electrodeposition; NiFe films; Magnetic anisotropy, Coercivity; Fe and its alloys; Electroplating
1. Introduction Electrodeposited nickel–iron alloys, with in-plane magnetization, are used in a variety of industrial applications as, for example, in magnetic recording [1,2]. The magnetic properties of NiFe films vary with composition and thickness [3]. The purpose of this study was to deposit NiFe films and then to characterise their magnetic properties.
NiSO4 7H2 O, 0:01 M FeSO4 7H2 O and 0:1 M H3 BO3 . The electrolyte pH was 2:5 0:1. Before starting the deposition process, the substrates were mechanically and electrochemically polished, and then washed in 10% H2 SO4 and distilled water, respectively. The films were deposited at room temperature using the cathode potentials of 1.5 V vs. saturated calomel electrode (SCE) in continuous waveforms. 3. Results and discussion
2. Experimental NiFe alloy films were grown on Cu substrates ðGoodfellowtÞ from a single electrolyte containing Niþ2 and Feþ2 ions under the potentiostatic conditions. Electrodeposition was performed in an electrochemical cell with three electrodes using a potentiostat/Galvanostat (EGG Model 362). The electrolyte was composed of 0.05 M Corresponding author. Tel.: +90 266 6121278; fax: +90 266 6121215.
E-mail address: [email protected] (H. Kockar). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.02.207
In-plane hysteresis loops of all films were measured at various angles in the film plane and perpendicular to the film plane using a vibrating sample magnetometer (VSM, NOVO Moltspin Ltd.). A circular film of 6 mm diameter was used to minimise demagnetising effects in the film plane. The structural data in our previous work [4] showed that the NiFe film crystallizes in FCC structure. Fig. 1 shows the hysteresis loops of 1 mm NiFe films measured at 0 and 90 in the film plane. When the field is applied at 0 in the film plane, the loop is almost square
ARTICLE IN PRESS H. Kockar et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e736–e738
with a high coercivity, H c , 0:29 kA=m. When the field is applied along 90 , the loop exhibits a smaller coercivity, H c , 0:21 kA=m than the anisotropy field, H a , 4.20 kA/m, but not equal to zero. The coercivities obtained from loops of all samples in the film plane and perpendicular to the film plane are summarised in Table 1. The coercivities of the film decrease when the measurement angle is rotated from 0 to 90 . The coercivity field, H c , is different from the anisotropy field, H a , which means that when the field is applied along the easy axis, the magnetisation does not rotate uniformly but there is a nucleation and propagation of domain walls [5–7]. When the field is applied perpendicular to the film plane, the coercivity reaches a value of 0.82 kA/m, indicating the film has a planar magnetic anisotropy. The easy axis was at 0 and its reason may probably be due to thickness of the film. Therefore, further NiFe films with 2 and 3 mm thicknesses were produced. Fig. 2 shows the hysteresis loops of 2 mm NiFe measured at 0 and 90 in the film plane and also perpendicular to the film plane. Although the feature characteristics of the in-plane anisotropy were observed, the loops corresponding to the angles are different from the loops in Fig. 1. When the field is applied at 0 , the loop is not as square as in Fig. 2. The higher thickness makes the uniaxial magnetic anisotropy less well defined. Hysteresis loops of 3 mm NiFe indicate that sample does not have a uniaxial magnetic anisotropy. As seen in Table 1 3 mm NiFe is isotropic. Normalised remanent magnetisation to saturation magnetisation ðM r =M s Þ of films as a function of the angles between the applied field and magnetization easy axis is
Fig. 1. An example of anisotropic hysteresis loops of 1 mm NiFe film.
e737
shown in Fig. 3. The total angular range of measurement in the film plane was 360 , with the 0 direction chosen arbitrarily. The remanence ratios for 1 mm NiFe and 2 mm NiFe decreases from 0 to 90 . This is followed by an increase from 90 to 180 . The cycles are repeated in the region between 180 and 360 . This indicates that the magnetic anisotropy with an easy axis is at 0 and 180 . Although 1 mm NiFe and 2 mm NiFe have the similar curves, the degree of the variation in the remanence ratios is different, 42% for 1 mm NiFe and 26% for 2 mm NiFe. In the case of 3 mm NiFe, the remanence ratio in the range of 0 to 360 varies around 5% as in Fig. 3. This is most likely the increase of thickness causes a uniform magnetic material production on the film plane, therefore the uniaxial in-plane anisotropy changes into isotropy. This result is in a good agreement with the findings
Fig. 2. In-plane magnetic anisotropy exhibited in a 2 mm NiFe film.
Fig. 3. Normalised remanence ratio ðM r =M s Þ as a function of angles between applied field and magnetisation easy axis.
Table 1 Composition analysis and coercivity measurement results for of nickel–iron films deposited at 1.5 V at different thickness NiFe film thickness ðmmÞ
1 2 3
Compositions
Coercivity, H c ðkA=mÞ
%Ni
%Fe
0
30
60
90
Perpendicular to the film plane
In-plane magnetic anisotropy
Perpendicular magnetic anisotropy
64.92 69.16 73.74
35.08 30.84 26.26
0.29 0.40 0.58
0.26 0.39 0.57
0.23 0.38 0.58
0.21 0.37 0.58
0.82 1.29 3.10
Yes Yes (slight) No
No No No
ARTICLE IN PRESS e738
H. Kockar et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e736–e738
obtained in Ref. [8], which indicates that the magnetic properties of NiFe films vary with thickness. The average variation of nickel and iron contents was measured in the films by using scanning electron microscope (SEM, Hitachi S-570) and presented in Table 1. The value of coercivity is known to be dependent on many parameters such as film thickness and composition [7]. The influence of the composition and thickness on coercivity can be observed from the data in Table 1. Other researchers [8,9] observed that the coercivity of the nickel alloys such as permalloy is as low as 0.40 A/m, while that of bulk nickel and iron are approximately 0.80 and 0.08 kA/m, respectively. The decrease of the iron content when increasing their thickness results in an increase (from 0.25 to 0.58 kA/m) of the average coercivity. 4. Conclusions Nickel–iron films with several thicknesses were deposited onto copper substrates and their magnetic properties were investigated. Anisotropic and/or isotropic magnetic materials could be deposited by choosing the proper thicknesses. This work provides a good platform for the possible production of NiFe films onto the metal-plated flexible polyimide ðKaptontÞ substrate as magneto-elastic sensors.
Acknowledgements This work is supported by the Scientific and Technical Research Council (TUBITAK) of Turkey under Grant no. TBAG-1771, and Balikesir University, Turkey under Grant no. 2001/02. The authors are grateful to Dr. P.I. Williams for his help to use the facilities in Wolfson Centre, Cardiff University, UK, and to Balikesir University, for the use of Research Centre of Applied Sciences (BURCAS), Turkey. References [1] S. Asaki, M. Sano, S. Li, Y. Tsuchi, O. Redon, T. Sasaki, N. Ito, K. Terunuma, H. Morita, M. Matsuzaki, J. Appl. Phys. 87 (2000) 5377. [2] K. O’Gready, H. Laidler, J. Magn. Magn. Mater. 20 (1999) 616. [3] D. Gangasingh, J.B. Talbot, J. Electrochem. Soc. 138 (12) (1991) 3605. [4] H. Kockar, M. Alper, H. Topcu, Eur. Phys. J. B 42 (4) (2004) 497. [5] H. Kockar, T. Meydan J. Magn. Magn. Mater. 242–245P1 (2002) 187. [6] M. Pruton, Thin Ferromagnetic Films, Butterworths, London, 1974. [7] H. Kockar, J. Superconductivity 17 (4) (2004) 531. [8] D. Jiles, Introduction to Magnetism and Magnetic Materials, Chapman & Hall, London, 1991. [9] J. Jakubovics, Magnetism and Magnetic Materials, Institute of Metals, London, 1987.