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Structure, magnetic properties and magnetostriction of Fe81Ga19 thin films
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 320 (2008) 769–773 www.elsevier.com/locate/jmmm
Structure, magnetic properties and magnetostriction of Fe81Ga19 thin films B.W. Wanga,b,, S.Y. Lia, Y. Zhoua, W.M. Huanga, S.Y. Caoa a
Research Center of Magnetic Technique and Magnetic Materials, Hebei University of Technology, Tianjin 300130, China b International Center for Materials Physics, Academia Sinica, Shenyang 110015, China Received 23 June 2007; received in revised form 1 August 2007 Available online 24 August 2007
Abstract The structure, magnetic properties and magnetostriction of Fe81Ga19 thin films have been investigated by using X-ray diffraction analysis, scanning electron microscope (SEM), vibrating sample magnetometer and capacitive cantilever method. It was found that the grain size of as-deposited Fe81Ga19 thin films is 50–60 nm and the grain size increases with increase in the annealing temperature. The remanence ratio (Mr/Ms) of the thin films slowly decreases with increase in the annealing temperature. However, the coercivity of the thin films goes the opposite way with increase in the annealing temperature. A preferential orientation of the Fe81Ga19 thin film fabricated under an applied magnetic field exists along /1 0 0S direction due to the function of magnetic field during sputtering. An in-plane-induced anisotropy of the thin film is well formed by the applied magnetic field during the sputtering and the formation of in-plane-induced anisotropy results in 901 rotations of the magnetic domains during magnetization and in the increase of magnetostriction for the thin film. r 2007 Elsevier B.V. All rights reserved. PACS: 75.8.+q Keywords: Fe–Ga thin film; Magnetic property; Magnetostriction; Structure
1. Introduction Magnetostrictive materials exhibit large magnetostriction preferably at low fields and are of interest in actuator and sensor applications. The magnetostriction and magnetization process of Dy0.7Tb0.3Fe2 alloy have been investigated [1,2]. Recently, it is found that single crystal /1 0 0S-oriented Fe81Ga19 alloy exhibits a giant magnetostriction approaching 400 ppm with low saturating magnetic fields of several hundred Oersteds and also displays a limited temperature dependence over a 20 to 80 1C range [3–5]. The investigation has found that magnetostrictive constant reaches 270 ppm in directional solidified polycrystalline Fe–Ga alloys [6] and that the Corresponding author. School of Electrical Engineering, Hebei University of Technology, Guangrong Road No. 8, Tianjin 300130, PR China. Tel.: +86 22 6020 4600; fax: +86 22 6020 4409. E-mail address: [email protected] (B.W. Wang).
0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.08.022
structural transformation of Fe–Ga alloys is very complex during cooling process [7–9]. Clearly, magnetoelastic alloys in the thin film form are of current interest as materials for integrated magnetostrictive actuators, which require the availability of thin film materials with low saturation field and low coercivity. Weston et al. [10] have investigated the characterization of (1 1 0)-oriented epitaxial Fe–Ga thin films and found that they are easily saturated in all in-plane orientations in fields under 500 Oe and their coercivity is 30–60 Oe. Butera et al. [11] have found that there is an in-plane angular variation of the resonance field. In fact, it is more interesting to prepare the thin films, which show an uniaxial in-plane anisotropy in order to meet the requirement of real application. This anisotropy should be oriented perpendicular to the external magnetic field, which results in 901 rotations of the magnetic domains during magnetization and thus in maximum magnetostriction. It has been found that the anisotropy of magnetic thin films can be improved by thermal annealing or by
ARTICLE IN PRESS B.W. Wang et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 769–773
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sputtering under an applied magnetic field [12,13]. Therefore, it is worthy to investigate the anisotropy change of Fe81Ga19 thin films caused by thermal annealing and field sputtering. In this paper, we report the results of the investigation of the structure, anisotropy and magnetostriction of Fe81Ga19 thin films. 2. Experimental procedures Fe81Ga19 thin films were prepared by DC magnetron sputtering on Si (1 0 0) substrate of 0.2 mm thickness and an alloy target consisting of Fe81Ga19 was used. The base pressure of the sputtering chamber was better than 2 108 Torr and an argon pressure during sputtering was 1.2 Pa. The sputtering power is 45 W and the sputtering time is 30 min. The thickness of thin films fabricated under no applied magnetic field, is nearly 660 nm and the deposition rate is 0.36 nm/s. Annealing was carried out for 1 h in a high vacuum of 5 105 Torr at various temperatures ranging from 100 to 500 1C. A substrate holder which permits to apply magnetic field to the samples during sputtering was used to form induced anisotropy. The magnetic field was generated by a couple Si (100) (110)
Intensity (a.u.)
(a)
(211)
(b)
(c)
30
40
50
60 2θ (Deg.)
70
80
90
Fig. 1. X-ray diffraction patterns of Fe81Ga19 thin: (a) as-deposited, (b) annealed at 300 1C, and (c) annealed at 500 1C.
of NdFeB magnets and a magnetic field of 500–600 Oe was measured to be applied to the samples. The structure was observed by X-ray diffraction with Cu Ka radiation. The composition of samples was determined by energy dispersion spectroscopy (EDS) and the surface image was examined by scanning electron microscope (SEM). Magnetic properties were measured by using a vibrating sample magnetometer with a maximum magnetic field of 400 kA/m and sample dimensions are 5 5 mm2. The magnetostriction of thin films was measured by the capacitive cantilever method under in-plane magnetic fields up to 60 kA/m at room temperature [14–16] and sample dimensions are 5 20 mm2. 3. Results and discussion Fig. 1 shows the X-ray diffraction patterns of the asdeposited and annealing Fe81Ga19 thin films. The diffraction patterns of a-Fe(Ga) (1 1 0), (2 1 1) and Si (1 0 0) peaks occur for the as-deposited Fe81Ga19 thin film and the lattice parameter of the a-Fe(Ga) phase is 0.288 nm, which is less than that (0.29 nm) of bulk sample [6,17]. The EDS analysis shows that the composition of the as-deposited Fe81Ga19 thin films is nearly the target composition Fe81Ga19. After annealing at temperatures ranging in 100–300 1C, the diffraction patterns of the Fe81Ga19 thin films have changed a little (Fig. 1(b)). With further increase in annealing temperature, the diffraction peak breadth of a-Fe(Ga) (1 1 0) and (2 1 1) obviously becomes narrow (Fig. 1(c)) and it means that the grain size increases with increase in annealing temperature. Fig. 2 shows the SEM photographs of the as-deposited Fe81Ga19 thin film. We can find that the grain size of the thin film is 50–60 nm and the grain shape is round (Fig. 2(a)). The transverse surface image of the as-deposited Fe81Ga19 thin film has confirmed that the thin films grow in a manner of columnar grain growth (Fig. 2(b)). After annealing, the grain size of the thin film increases with increase in the annealing temperature and is 100–120 nm at 500 1C for 1 h. Fig. 3 shows the annealing temperature dependence of coercivity and the in-plane remanence ratio (Mr/Ms) to saturation magnetization for Fe81Ga19 thin films. The inplane remanence ratio (Mr/Ms) slowly decreases with
Fig. 2. SEM photograph of as-deposited Fe81Ga19 thin films: (a) surface image and (b) transverse surface image.
ARTICLE IN PRESS B.W. Wang et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 769–773
increase in the annealing temperature and it means that the soft magnetic property of the thin films becomes weak with increase in the annealing temperature. According to the results in Figs. 1 and 2, the grain size of the as-deposited thin film is in nanometers (50–60 nm) and increases with increase in annealing temperature. As we know, the magnetocrystalline anisotropy can be reduced by using nanometer or amorphous materials [18]; and therefore, the soft magnetic property of the as-deposited thin film is better than that of the annealed thin films. From Fig. 3, the coercivity of the as-deposited thin film is 27.8 Oe, which is less than that reported in Ref. [10]. The coercivity of thin films remains almost unchanged when the annealing temperature is below 200 1C and linearly increases with increase in the annealing temperature. It seems that the increase of grain size leads to the increase of coercivity for the thin films. 120
100
0.8 Mr/Ms
Mr/Ms
Hc (Oe)
0.6 80
60 0.4 40 Hc
20 100
200 300 400 Annealed temperature (°C)
0.2 500
Fig. 3. Annealing temperature dependence of the coercivity and remanence ratio (Mr/Ms) for Fe81Ga19 thin films.
0.10
The hysteresis loops for the as-deposited Fe81Ga19 thin film fabricated under no applied magnetic field are shown in Fig. 4(a). The in-plane saturation magnetization of the Fe81Ga19 thin film is 0.082 emu/cm2. We suppose that the density of the Fe81Ga19 thin film is nearly 7.8 g/cm3. Then, the magnetization of the thin film is 159 emu/g, which is consistent with that of bulk sample (Ms ¼ 152 emu/g for Fe80Ga20 alloy [6]). In the case of the as-deposited Fe81Ga19 thin film, the shapes of in-plane hysteresis loops are identical in different directions. However, a clear difference in the shapes of hysteresis loops exists for the Fe81Ga19 thin film, fabricated under an applied magnetic field, as shown in Fig. 4(b). The in-plane hysteresis loops measured in the easy direction is nearly squared with the remanence ratio (Mr/Ms) of 0.854, while the loops in the hard direction are inclined with the remanence ratio (Mr/Ms) of 0.662. This means that an in-plane induced anisotropy is well formed by the applied magnetic field during the sputtering. The formation of in-plane induced anisotropy will result in 901 rotations of the magnetic domains during magnetization, which increase the magnetostriction of the thin film. The X-ray diffraction patterns of the as-deposited Fe81Ga19 thin films, fabricated under no applied magnetic field and under an applied magnetic field, are shown in Fig. 5. Compared with the diffraction pattern of the no magnetic field sputtering thin film, the diffraction peak of (1 1 0) surface for the magnetic field sputtering thin film clearly becomes weak and the (2 0 0) peak occurs in the thin film. This means that there is a preferential orientation of the field sputtering Fe81Ga19 thin film along /1 0 0S direction due to the function of magnetic field during the sputtering. This preferential orientation of the field sputtering Fe81Ga19 thin film result in an induced anisotropy and have an effect on the magnetic properties of the thin film, as shown in Fig. 4(b).
0.10
in-plane-parallel in-plane-perpendicular out-of-plane
in-plane-easy in-plane-hard out-of-plane
0.05 M (emu/cm2)
M (emu/cm2)
0.05
0.00
-0.05
-0.10 -600
771
0.00
-0.05
-400
-200
0 H(Oe)
200
400
600
-0.10 -600
-400
-200
0 H (Oe)
200
400
600
Fig. 4. Magnetic field dependence of magnetization of as-deposited Fe81Ga19 thin films: (a) under magnetic field sputtering and (b) under no magnetic field sputtering.
ARTICLE IN PRESS B.W. Wang et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 769–773
772
80 Si (100)
a (110) (211)
λ x10-6
Intensity (a.u.)
a
60
b 40
20
b
(200)
0 30
40
50
60 2θ (deg.)
70
80
90
Fig. 5. X-ray diffraction patterns of as-deposited Fe81Ga19 thin films: (a) under no magnetic field sputtering and (b) under magnetic field sputtering.
80
60 As-deposited
λ x 10-6
300°C 400°C
40 500°C
20
0 0
10
20
30
40 50 H(kA/m)
60
70
80
Fig. 6. Magnetic field dependence of magnetostriction for Fe81Ga19 thin films, annealed at different temperatures.
0
10
20
30 H (kA/m)
40
50
60
Fig. 7. Magnetic field dependence of magnetostriction for as-deposited Fe81Ga19 thin films: (a) under magnetic field sputtering and (b) under no magnetic field sputtering.
under an applied magnetic field. Lim et al. [13] have investigated the magnetostriction of Sm–Fe thin film and they argued that the small magnetostriction in the low fields observed in the field-sputtered thin films is relative to a large coercivity value. From the result in Fig. 4, the coercivity of the thin film fabricated under no applied magnetic field is 27.8 Oe, and that of fabricated under an applied magnetic field is 42.7 Oe. The low coercivity of the thin film fabricated under no applied magnetic field makes the magnetostriction increase in low magnetic fields. However, the magnetostriction of the thin films fabricated under an applied magnetic field is higher than that fabricated under no applied magnetic field when the magnetic field 48 kA/m. As we known from Figs. 4(b) and 5, an in-plane induced anisotropy of the thin film is well formed by the applied magnetic field during the sputtering and the in-plane induced anisotropy leads to the increase of magnetostriction. 4. Conclusions
Fig. 6 shows the magnetic field dependence of the magnetostriction for Fe81Ga19 thin films, fabricated under no applied magnetic field, at different annealing temperatures. The magnetostriction of the as-deposited thin film reaches 42 ppm at the field of 5 kA/m and it decreases with increasing annealing temperature. The decrease of the magnetostriction with increasing temperature may be due to the increase of grain size as well as the precipitation of a DO3 phase during the annealing process [7]. The magnetic field dependences of the magnetostriction for the asdeposited Fe81Ga19 thin film fabricated under no applied magnetic field and under an applied magnetic field are shown in Fig. 7. In low magnetic fields (0–8 kA/m), the magnetostriction of the thin film fabricated under no applied magnetic field is a little higher than that fabricated
The structure, magnetic properties and magnetostriction for Fe81Ga19 thin films have been investigated and it is found that the grain size of as-deposited Fe81Ga19 thin films is 50–60 nm and the grain size increases with increase in annealing temperature. The remanence ratio (Mr/Ms) and the coercivity of the as-deposited thin films are 0.68 and 27.8 Oe, respectively. An in-plane-induced anisotropy of the thin film is well formed by the applied magnetic field during the sputtering and the formation of in-planeinduced anisotropy results in the increase of magnetostriction for the thin film. The magnetostriction of the thin film fabricated under an applied magnetic field reaches 45 ppm at the field of 10 kA/m and it can be used as the material of magnetostrictive transducer.
ARTICLE IN PRESS B.W. Wang et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 769–773
Acknowledgment This work is supported by the National Natural Science Foundation of China (50571034). References [1] B.W. Wang, S.C. Busbridge, Y.X. Li, G.H. Wu, A.R. Piercy, J. Magn. Magn. Mater. 218 (2000) 198. [2] B.W. Wang, S.C. Busbridge, Z.J. Guo, Z.D. Zhang, J. Appl. Phys. 93 (10) (2003) 8489. [3] J.R. Cullen, A.E. Clark, M. Wun-fogle, J.B. Restorff, T.A. Lograsso, J. Magn. Magn. Mater. 226 (2001) 948. [4] R.A. Kellogg, A.B. Flatau, A.E. Clark, M. Wun-fogle, T.A. Lograsso, J. Appl. Phys. 91 (2002) 7821. [5] A.E. Clark, M. Wun-Fogle, J.B. Restorff, T.A. Lograsso, J.R. Cullen, IEEE Trans. Magn. 37 (2001) 2678. [6] N. Srisukhumbowomchai, S. Guruswamy, J. Appl. Phys. 90 (2001) 5680. [7] O. Ikeda, R. Kainuma, I. Ohnuma, K. Fukamichi, K. Ishida, J. Alloys Compd. 347 (1-2) (2002) 198.
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[8] J.M. Borrego, J.S. Blazquez, C.F. Conde, S. Roth, Intermetallics 15 (2007) 193. [9] M.C. Zhang, X.X. Gao, H.L. Jiang, Y. Qiao, S.Z. Zhou, J. Alloys Compd. 431 (2007) 42. [10] J.L. Weston, A. Butera, T. Lograsso, M. Shamsuzzoha, I. Zana, G. Zangari, J. Barnard, IEEE Trans. Magn. 38 (2002) 2832. [11] A. Butera, J.L. Weston, J.A. Barnard, J. Magn. Magn. Mater. 284 (2004) 17. [12] S.M. Na, S.J. Suh, H.J. Kim, S.H. Lim, J. Magn. Magn. Mater. 239 (2002) 570. [13] S.H. Lim, S.H. Han, H.J. Kim, S.H. Song, D. Lee, J. Appl. Phys. 87 (2000) 5801. [14] E. Klokholm, IEEE Trans. Magn. MAG 12 (1976) 819. [15] E. du Tremolet de Lacheisserie, J.C. Peuzin, J. Magn. Magn. Mater. 136 (1994) 189. [16] H. Chiriac, M. Pletea, E. Hristoforou, Sens. Actuators A 68 (1998) 414. [17] H. Okamoto, L.E. Tanner, Phase diagrams of binary iron alloys, monograph series on alloy phase diagrams no. 9, in: H. Okamoto (Ed.), ASM International, Materials Parks, OH, 1993, pp. 49–61. [18] H. Fujimori, J.Y. Kim, S. Suzuki, H. Morita, N. Kataoka, J. Magn. Magn. Mater. 124 (1993) 115.
Journal of Magnetism and Magnetic Materials 320 (2008) 769–773 www.elsevier.com/locate/jmmm
Structure, magnetic properties and magnetostriction of Fe81Ga19 thin films B.W. Wanga,b,, S.Y. Lia, Y. Zhoua, W.M. Huanga, S.Y. Caoa a
Research Center of Magnetic Technique and Magnetic Materials, Hebei University of Technology, Tianjin 300130, China b International Center for Materials Physics, Academia Sinica, Shenyang 110015, China Received 23 June 2007; received in revised form 1 August 2007 Available online 24 August 2007
Abstract The structure, magnetic properties and magnetostriction of Fe81Ga19 thin films have been investigated by using X-ray diffraction analysis, scanning electron microscope (SEM), vibrating sample magnetometer and capacitive cantilever method. It was found that the grain size of as-deposited Fe81Ga19 thin films is 50–60 nm and the grain size increases with increase in the annealing temperature. The remanence ratio (Mr/Ms) of the thin films slowly decreases with increase in the annealing temperature. However, the coercivity of the thin films goes the opposite way with increase in the annealing temperature. A preferential orientation of the Fe81Ga19 thin film fabricated under an applied magnetic field exists along /1 0 0S direction due to the function of magnetic field during sputtering. An in-plane-induced anisotropy of the thin film is well formed by the applied magnetic field during the sputtering and the formation of in-plane-induced anisotropy results in 901 rotations of the magnetic domains during magnetization and in the increase of magnetostriction for the thin film. r 2007 Elsevier B.V. All rights reserved. PACS: 75.8.+q Keywords: Fe–Ga thin film; Magnetic property; Magnetostriction; Structure
1. Introduction Magnetostrictive materials exhibit large magnetostriction preferably at low fields and are of interest in actuator and sensor applications. The magnetostriction and magnetization process of Dy0.7Tb0.3Fe2 alloy have been investigated [1,2]. Recently, it is found that single crystal /1 0 0S-oriented Fe81Ga19 alloy exhibits a giant magnetostriction approaching 400 ppm with low saturating magnetic fields of several hundred Oersteds and also displays a limited temperature dependence over a 20 to 80 1C range [3–5]. The investigation has found that magnetostrictive constant reaches 270 ppm in directional solidified polycrystalline Fe–Ga alloys [6] and that the Corresponding author. School of Electrical Engineering, Hebei University of Technology, Guangrong Road No. 8, Tianjin 300130, PR China. Tel.: +86 22 6020 4600; fax: +86 22 6020 4409. E-mail address: [email protected] (B.W. Wang).
0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.08.022
structural transformation of Fe–Ga alloys is very complex during cooling process [7–9]. Clearly, magnetoelastic alloys in the thin film form are of current interest as materials for integrated magnetostrictive actuators, which require the availability of thin film materials with low saturation field and low coercivity. Weston et al. [10] have investigated the characterization of (1 1 0)-oriented epitaxial Fe–Ga thin films and found that they are easily saturated in all in-plane orientations in fields under 500 Oe and their coercivity is 30–60 Oe. Butera et al. [11] have found that there is an in-plane angular variation of the resonance field. In fact, it is more interesting to prepare the thin films, which show an uniaxial in-plane anisotropy in order to meet the requirement of real application. This anisotropy should be oriented perpendicular to the external magnetic field, which results in 901 rotations of the magnetic domains during magnetization and thus in maximum magnetostriction. It has been found that the anisotropy of magnetic thin films can be improved by thermal annealing or by
ARTICLE IN PRESS B.W. Wang et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 769–773
770
sputtering under an applied magnetic field [12,13]. Therefore, it is worthy to investigate the anisotropy change of Fe81Ga19 thin films caused by thermal annealing and field sputtering. In this paper, we report the results of the investigation of the structure, anisotropy and magnetostriction of Fe81Ga19 thin films. 2. Experimental procedures Fe81Ga19 thin films were prepared by DC magnetron sputtering on Si (1 0 0) substrate of 0.2 mm thickness and an alloy target consisting of Fe81Ga19 was used. The base pressure of the sputtering chamber was better than 2 108 Torr and an argon pressure during sputtering was 1.2 Pa. The sputtering power is 45 W and the sputtering time is 30 min. The thickness of thin films fabricated under no applied magnetic field, is nearly 660 nm and the deposition rate is 0.36 nm/s. Annealing was carried out for 1 h in a high vacuum of 5 105 Torr at various temperatures ranging from 100 to 500 1C. A substrate holder which permits to apply magnetic field to the samples during sputtering was used to form induced anisotropy. The magnetic field was generated by a couple Si (100) (110)
Intensity (a.u.)
(a)
(211)
(b)
(c)
30
40
50
60 2θ (Deg.)
70
80
90
Fig. 1. X-ray diffraction patterns of Fe81Ga19 thin: (a) as-deposited, (b) annealed at 300 1C, and (c) annealed at 500 1C.
of NdFeB magnets and a magnetic field of 500–600 Oe was measured to be applied to the samples. The structure was observed by X-ray diffraction with Cu Ka radiation. The composition of samples was determined by energy dispersion spectroscopy (EDS) and the surface image was examined by scanning electron microscope (SEM). Magnetic properties were measured by using a vibrating sample magnetometer with a maximum magnetic field of 400 kA/m and sample dimensions are 5 5 mm2. The magnetostriction of thin films was measured by the capacitive cantilever method under in-plane magnetic fields up to 60 kA/m at room temperature [14–16] and sample dimensions are 5 20 mm2. 3. Results and discussion Fig. 1 shows the X-ray diffraction patterns of the asdeposited and annealing Fe81Ga19 thin films. The diffraction patterns of a-Fe(Ga) (1 1 0), (2 1 1) and Si (1 0 0) peaks occur for the as-deposited Fe81Ga19 thin film and the lattice parameter of the a-Fe(Ga) phase is 0.288 nm, which is less than that (0.29 nm) of bulk sample [6,17]. The EDS analysis shows that the composition of the as-deposited Fe81Ga19 thin films is nearly the target composition Fe81Ga19. After annealing at temperatures ranging in 100–300 1C, the diffraction patterns of the Fe81Ga19 thin films have changed a little (Fig. 1(b)). With further increase in annealing temperature, the diffraction peak breadth of a-Fe(Ga) (1 1 0) and (2 1 1) obviously becomes narrow (Fig. 1(c)) and it means that the grain size increases with increase in annealing temperature. Fig. 2 shows the SEM photographs of the as-deposited Fe81Ga19 thin film. We can find that the grain size of the thin film is 50–60 nm and the grain shape is round (Fig. 2(a)). The transverse surface image of the as-deposited Fe81Ga19 thin film has confirmed that the thin films grow in a manner of columnar grain growth (Fig. 2(b)). After annealing, the grain size of the thin film increases with increase in the annealing temperature and is 100–120 nm at 500 1C for 1 h. Fig. 3 shows the annealing temperature dependence of coercivity and the in-plane remanence ratio (Mr/Ms) to saturation magnetization for Fe81Ga19 thin films. The inplane remanence ratio (Mr/Ms) slowly decreases with
Fig. 2. SEM photograph of as-deposited Fe81Ga19 thin films: (a) surface image and (b) transverse surface image.
ARTICLE IN PRESS B.W. Wang et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 769–773
increase in the annealing temperature and it means that the soft magnetic property of the thin films becomes weak with increase in the annealing temperature. According to the results in Figs. 1 and 2, the grain size of the as-deposited thin film is in nanometers (50–60 nm) and increases with increase in annealing temperature. As we know, the magnetocrystalline anisotropy can be reduced by using nanometer or amorphous materials [18]; and therefore, the soft magnetic property of the as-deposited thin film is better than that of the annealed thin films. From Fig. 3, the coercivity of the as-deposited thin film is 27.8 Oe, which is less than that reported in Ref. [10]. The coercivity of thin films remains almost unchanged when the annealing temperature is below 200 1C and linearly increases with increase in the annealing temperature. It seems that the increase of grain size leads to the increase of coercivity for the thin films. 120
100
0.8 Mr/Ms
Mr/Ms
Hc (Oe)
0.6 80
60 0.4 40 Hc
20 100
200 300 400 Annealed temperature (°C)
0.2 500
Fig. 3. Annealing temperature dependence of the coercivity and remanence ratio (Mr/Ms) for Fe81Ga19 thin films.
0.10
The hysteresis loops for the as-deposited Fe81Ga19 thin film fabricated under no applied magnetic field are shown in Fig. 4(a). The in-plane saturation magnetization of the Fe81Ga19 thin film is 0.082 emu/cm2. We suppose that the density of the Fe81Ga19 thin film is nearly 7.8 g/cm3. Then, the magnetization of the thin film is 159 emu/g, which is consistent with that of bulk sample (Ms ¼ 152 emu/g for Fe80Ga20 alloy [6]). In the case of the as-deposited Fe81Ga19 thin film, the shapes of in-plane hysteresis loops are identical in different directions. However, a clear difference in the shapes of hysteresis loops exists for the Fe81Ga19 thin film, fabricated under an applied magnetic field, as shown in Fig. 4(b). The in-plane hysteresis loops measured in the easy direction is nearly squared with the remanence ratio (Mr/Ms) of 0.854, while the loops in the hard direction are inclined with the remanence ratio (Mr/Ms) of 0.662. This means that an in-plane induced anisotropy is well formed by the applied magnetic field during the sputtering. The formation of in-plane induced anisotropy will result in 901 rotations of the magnetic domains during magnetization, which increase the magnetostriction of the thin film. The X-ray diffraction patterns of the as-deposited Fe81Ga19 thin films, fabricated under no applied magnetic field and under an applied magnetic field, are shown in Fig. 5. Compared with the diffraction pattern of the no magnetic field sputtering thin film, the diffraction peak of (1 1 0) surface for the magnetic field sputtering thin film clearly becomes weak and the (2 0 0) peak occurs in the thin film. This means that there is a preferential orientation of the field sputtering Fe81Ga19 thin film along /1 0 0S direction due to the function of magnetic field during the sputtering. This preferential orientation of the field sputtering Fe81Ga19 thin film result in an induced anisotropy and have an effect on the magnetic properties of the thin film, as shown in Fig. 4(b).
0.10
in-plane-parallel in-plane-perpendicular out-of-plane
in-plane-easy in-plane-hard out-of-plane
0.05 M (emu/cm2)
M (emu/cm2)
0.05
0.00
-0.05
-0.10 -600
771
0.00
-0.05
-400
-200
0 H(Oe)
200
400
600
-0.10 -600
-400
-200
0 H (Oe)
200
400
600
Fig. 4. Magnetic field dependence of magnetization of as-deposited Fe81Ga19 thin films: (a) under magnetic field sputtering and (b) under no magnetic field sputtering.
ARTICLE IN PRESS B.W. Wang et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 769–773
772
80 Si (100)
a (110) (211)
λ x10-6
Intensity (a.u.)
a
60
b 40
20
b
(200)
0 30
40
50
60 2θ (deg.)
70
80
90
Fig. 5. X-ray diffraction patterns of as-deposited Fe81Ga19 thin films: (a) under no magnetic field sputtering and (b) under magnetic field sputtering.
80
60 As-deposited
λ x 10-6
300°C 400°C
40 500°C
20
0 0
10
20
30
40 50 H(kA/m)
60
70
80
Fig. 6. Magnetic field dependence of magnetostriction for Fe81Ga19 thin films, annealed at different temperatures.
0
10
20
30 H (kA/m)
40
50
60
Fig. 7. Magnetic field dependence of magnetostriction for as-deposited Fe81Ga19 thin films: (a) under magnetic field sputtering and (b) under no magnetic field sputtering.
under an applied magnetic field. Lim et al. [13] have investigated the magnetostriction of Sm–Fe thin film and they argued that the small magnetostriction in the low fields observed in the field-sputtered thin films is relative to a large coercivity value. From the result in Fig. 4, the coercivity of the thin film fabricated under no applied magnetic field is 27.8 Oe, and that of fabricated under an applied magnetic field is 42.7 Oe. The low coercivity of the thin film fabricated under no applied magnetic field makes the magnetostriction increase in low magnetic fields. However, the magnetostriction of the thin films fabricated under an applied magnetic field is higher than that fabricated under no applied magnetic field when the magnetic field 48 kA/m. As we known from Figs. 4(b) and 5, an in-plane induced anisotropy of the thin film is well formed by the applied magnetic field during the sputtering and the in-plane induced anisotropy leads to the increase of magnetostriction. 4. Conclusions
Fig. 6 shows the magnetic field dependence of the magnetostriction for Fe81Ga19 thin films, fabricated under no applied magnetic field, at different annealing temperatures. The magnetostriction of the as-deposited thin film reaches 42 ppm at the field of 5 kA/m and it decreases with increasing annealing temperature. The decrease of the magnetostriction with increasing temperature may be due to the increase of grain size as well as the precipitation of a DO3 phase during the annealing process [7]. The magnetic field dependences of the magnetostriction for the asdeposited Fe81Ga19 thin film fabricated under no applied magnetic field and under an applied magnetic field are shown in Fig. 7. In low magnetic fields (0–8 kA/m), the magnetostriction of the thin film fabricated under no applied magnetic field is a little higher than that fabricated
The structure, magnetic properties and magnetostriction for Fe81Ga19 thin films have been investigated and it is found that the grain size of as-deposited Fe81Ga19 thin films is 50–60 nm and the grain size increases with increase in annealing temperature. The remanence ratio (Mr/Ms) and the coercivity of the as-deposited thin films are 0.68 and 27.8 Oe, respectively. An in-plane-induced anisotropy of the thin film is well formed by the applied magnetic field during the sputtering and the formation of in-planeinduced anisotropy results in the increase of magnetostriction for the thin film. The magnetostriction of the thin film fabricated under an applied magnetic field reaches 45 ppm at the field of 10 kA/m and it can be used as the material of magnetostrictive transducer.
ARTICLE IN PRESS B.W. Wang et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 769–773
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