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High coercivity and perpendicular anisotropy in Co–Cu granular films
Physica B 327 (2003) 400–403
High coercivity and perpendicular anisotropy in Co–Cu granular films Nguyen Anh Tuana, Nguyen Hoang Luongb,*, Nguyen Chaub, Vuong Van Hiepb, Nguyen Minh Hab b
a International Training Institute for Materials Science (ITIMS), 1 Dai Co Viet, Hanoi, Viet Nam Center for Materials Science, Faculty of Physics, Viet Nam National University, 334 Nguyen Trai, Hanoi, Viet Nam
Abstract High coercivity was obtained in Co–Cu granular films, RF sputtered on Si(1 0 0) substrates, when the Co content is less than about 40 at% and annealed at high temperatures. Perpendicular anisotropy was observed in the Co-rich films, where the Co content is not less than about 40 at% atom. The reasons for the high coercivity and the perpendicular anisotropy in these Co–Cu films are discussed. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Granular films; Superparamagnetic; High coercivity; Perpendicular anisotropy
Magnetic granular films have been known about a decade ago for their giant magneto-resistance (GMR) [1]. Recently, these materials have again got considerable attention, not only from a viewpoint of fundamental physics [2,3] but also because of their potential application as high-density magnetic recording media [4]. These media require small magnetic particles having a high coercivity and perpendicular anisotropy. In this work we report some results of observations of the high coercivity and perpendicular anisotropy in Co–Cu granular films prepared by RF sputtering. These magnetic properties were investigated as a function of the Co fraction as well as of the annealing temperature, because these factors affect sensitively the structure characteristics and the magnetic properties.
*Corresponding author. Tel./Fax: +84-4-8589496. E-mail address: [email protected] (N.H. Luong).
The CoxCu1x films (x=0.12, 0.16, 0.20, 0.26, 0.34, 0.42, 0.59 and 0.77) were deposited on Si(1 0 0) substrates at room temperature by RF sputtering using Ar gas. The composition target was prepared from a Co target on which Cu pieces were attached. Sputtering power was 400 W, the basic pressure was 106 mbar, and the Ar pressure for discharging was 103 mbar. The thickness of ( measured by an the samples was fixed at 5000 A, Alpha Step apparatus. The Co fraction is determined by energy dispersive X-ray spectroscopy (EDS), and the structure of the films was characterized by X-ray diffraction (XRD) measurements using the radiation of CuKa : The magnetic properties were measured in a vibrating sample magnetometer (VSM). The thermal treatment of the samples was carried out in vacuum (B105 mbar) for 30 min at 1001C, 2001C, 3001C, 4001C and 5001C. Analysis of the XRD measurements for the samples showed that the Cu matrix has the FCC
0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 7 5 7 - X
N.A. Tuan et al. / Physica B 327 (2003) 400–403
structure and a so-called metastable phase of Co–Cu alloy is formed at low Co fraction. The XRD patterns for some selected samples are presented in Fig. 1. The Cu(1 1 1) and (2 0 0) peaks are very close to standard lines of bulk Cu (2yCuð1 1 1Þ E43:41) for Co-poor films (see, for example, diagram a for x ¼ 0:12 in Fig. 1), and these peaks are shifted to the larger 2y angle, closed to standard lines of bulk Co (2yCoð1 1 1Þ E44:21) for Co-richer films (see, for example, diagram d for x ¼ 0:59 in Fig. 1). Simultaneously, the intensity of these peaks decreases with increasing x. These results suggest that Co atoms form small clusters or fine particles at low Co fractions, and that these particles become larger when the Co fraction increases [5].
Counts (a.u.) →
Cu(111)
Cu(200)
Co(111)
a b c d 40
45
50 2θ (degree) →
401
55
Fig. 1. XRD diagrams for the CoxCu1x films. (a) x=0.12, (b) x=0.26, (c) x=0.42, (d) x=0.59.
1.0 Co 42 Cu 58
M/M10 kOe
Co 12 Cu 88 0.5 0.0 -0.5 -1.0 1.0 Co 20 Cu 80
Co 59 Cu 41
Co 34 Cu 66
Co 77 Cu 23
M/M10 kOe
0.5 0.0 -0.5
M/M10 kOe
-1.0 1.0 0.5 0.0 -0.5 -1.0 -10
-5
0
H (kOe)
5
10
-10
-5
0
5
H (kOe)
Fig. 2. Hysteresis loops measured in-plane for the CoxCu1x films.
10
N.A. Tuan et al. / Physica B 327 (2003) 400–403
402 300
500
Co 12 Cu 88 Co 16 Cu 84 Co 20 Cu 80 Co 26 Cu 74 Co 42 Cu 58 Co 59 Cu 41
250 400
300
HC (Oe)
HC(Oe)
200 150 100 50
200
100
0 0
20
40
x (at %)
60
80
100
Fig. 3. Coercivity as a function of Co fraction for the CoxCu1x films.
0 0
100
200
300
400
500
600
Ta (oC) Fig. 4. Coercivity as a function of annealing temperature for the CoxCu1x films.
Fig. 2 shows the hysteresis loops of the selected CoxCu1x films. As can be seen in this figure, superparamagnetism or property of the fine particles systems is dominant for Co-poor samples (xo0:40). For the samples with xo0:20; the magnetization process is rather similar to that of a paramagnet, as can be seen for the Co12Cu88 sample. This property is less prominent when the Co content increases, and ferromagnetism dominates at the Co-richer samples (x > 0:40). Such behavior has been reported by some other authors [6,7]. Fig. 3 presents the dependence of the coercivity, HC ; on the Co content. As one can see in this figure, the coercivity first increases with increasing x; reaches a maximum value of about 250 Oe at x ¼ 0:34; and then decreases with further increasing x: The enhancement of the coercivity in granular films is known to be due to an increase in size of the magnetic particles [8]. The occurrence of such an increase was proved by the XRD measurements shown above (see Fig. 1). The hysteresis for fine particles system has been attributed to blocking of particles whose size exceeds the critical size for superparamagnetism [9]. However, the hysteresis of granular systems can also be explained by an interaction of magnetic particles rather than of blocked particles [10]. Another cause is surface anisotropy [3,11,12]. For the CoxCu1x films with xE0:2020:40; it could be suggested that the Co particles embedded
in the Cu matrix are single-domain fine particles, whereas for x > 0:40 they may be multi-domain [9]. It could be said that the Co-content threshold at x ¼ xp ; where xp lies between 0.34 and 0.40, is the magnetic percolation threshold [8]. The high coercivity for the low Co content films was also observed after annealing. Fig. 4 shows the dependence of the coercivity on annealing temperature, Ta ; for selected CoxCu1x films. As can be seen in this figure, HC increases with increasing Ta for the Co-poor samples (xo0:40), and decreases for Corich samples. Considerable increase of HC for Copoor samples with increasing Ta from 3001C can also be explained by the growth of the Co particles [12]. This has been confirmed by XRD measurements for samples as-deposited and annealed (see Fig. 5 for Co26Cu74 film, as an example); it is indicated by the shift to higher 2y angle of Cu(1 1 1) and Cu(2 0 0) peaks with increasing annealing temperature. Another important phenomenon observed for Co-rich films (x > 0:40) is that the shape of the hysteresis loops manifests a partly perpendicular magnetic anisotropy, as seen from the graphics on the right-hand side of Fig. 2. This has been observed in Co-rich granular films of Co–Ag systems, and has been suggested to be due to a preferential orientation of Co phase or preferential arrangement of the Co particles in the direction
N.A. Tuan et al. / Physica B 327 (2003) 400–403
403
Counts (a.u.) →
Cu(111)
C
Co(111) →
Ta = 500 oC Ta = 400 oC Ta = 300 oC As-deposited
40
42
44
46
48
2θ θ (degree) ( )
50
→
52
54
Fig. 5. XRD diagrams for Co26Cu74 film as-deposited and annealed.
perpendicular to the film plane [13]. However, the perpendicular anisotropy may originate from the surface magnetic anisotropy at the interfaces between Co particles and the Cu matrix [3,14].
Acknowledgements This work is supported by the State Program of Science & Technology of Viet Nam, Project KC02-13, and State Program of Fundamental Research, Project 420101.
References [1] A.E. Berkowitz, J.R. Mitchell, M.J. Carey, A.P. Young, S. Zhang, F.E. Spada, F.T. Parker, A. Hutten, G. Thomas, Phys. Rev. Lett. 68 (1992) 3745; C.L. Chien, J.Q. Xiao, J.S. Jiang, J. Appl. Phys. 73 (1993) 5309. [2] S.L. Woods, J.R. Kirtley, S. Sun, R.H. Koch, Phys. Rev. Lett. 87 (2001) 137205.
[3] N.H. Duc, N.A. Tuan, A. Fnidiki, C. Dorien, J. Ben Youssef, H. Le Gall, J. Phys.: Condens. Matter 14 (2002) 1. [4] K. Ichihara, A. Kikitsu, K. Yusu, F. Nakamura, H. Ogiwara, IEEE Trans. Magn. 34 (1998) 1603. [5] J.H. Du, Q. Li, L.C. Wang, H. Sang, S.Y. Zang, Y.W. Du, D. Feng, J. Phys.: Condens. Matter 7 (1995) 9425. [6] S. Honda, M. Nawate, M. Tanaka, T. Okada, J. Appl. Phys. 82 (1997) 764. [7] H. Hamakake, K. Ishii, IEEE Trans. Magn. 35 (1999) 3457. [8] J.N. Zhou, A. Bureta, H. Jiang, J.A. Barnard, J. Appl. Phys. 84 (1998) 5693. [9] B.D. Cullity, Introduction to Magnetic Materials, Addison-Wesley publishing company, Reading, MA, 1972. [10] P. Allia, M. Coisson, P. Tiberto, F. Vinai, M. Knobel, J. Appl. Phys. 85 (1999) 4343. [11] X. Zianni, K.N. Trohidou, J.A. Blackman, J. Appl. Phys. 81 (1997) 4739. [12] C. Chen, O. Kitakami, Y. Shimada, J. Appl. Phys. 84 (1998) 2184. [13] J.Q. Xiao, C.L. Chien, A. Gavrin, J. Appl. Phys. 79 (1996) 5309. [14] X.J. Chen, T. Suzuki, J. Appl. Phys. 85 (1999) 5048.
High coercivity and perpendicular anisotropy in Co–Cu granular films Nguyen Anh Tuana, Nguyen Hoang Luongb,*, Nguyen Chaub, Vuong Van Hiepb, Nguyen Minh Hab b
a International Training Institute for Materials Science (ITIMS), 1 Dai Co Viet, Hanoi, Viet Nam Center for Materials Science, Faculty of Physics, Viet Nam National University, 334 Nguyen Trai, Hanoi, Viet Nam
Abstract High coercivity was obtained in Co–Cu granular films, RF sputtered on Si(1 0 0) substrates, when the Co content is less than about 40 at% and annealed at high temperatures. Perpendicular anisotropy was observed in the Co-rich films, where the Co content is not less than about 40 at% atom. The reasons for the high coercivity and the perpendicular anisotropy in these Co–Cu films are discussed. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Granular films; Superparamagnetic; High coercivity; Perpendicular anisotropy
Magnetic granular films have been known about a decade ago for their giant magneto-resistance (GMR) [1]. Recently, these materials have again got considerable attention, not only from a viewpoint of fundamental physics [2,3] but also because of their potential application as high-density magnetic recording media [4]. These media require small magnetic particles having a high coercivity and perpendicular anisotropy. In this work we report some results of observations of the high coercivity and perpendicular anisotropy in Co–Cu granular films prepared by RF sputtering. These magnetic properties were investigated as a function of the Co fraction as well as of the annealing temperature, because these factors affect sensitively the structure characteristics and the magnetic properties.
*Corresponding author. Tel./Fax: +84-4-8589496. E-mail address: [email protected] (N.H. Luong).
The CoxCu1x films (x=0.12, 0.16, 0.20, 0.26, 0.34, 0.42, 0.59 and 0.77) were deposited on Si(1 0 0) substrates at room temperature by RF sputtering using Ar gas. The composition target was prepared from a Co target on which Cu pieces were attached. Sputtering power was 400 W, the basic pressure was 106 mbar, and the Ar pressure for discharging was 103 mbar. The thickness of ( measured by an the samples was fixed at 5000 A, Alpha Step apparatus. The Co fraction is determined by energy dispersive X-ray spectroscopy (EDS), and the structure of the films was characterized by X-ray diffraction (XRD) measurements using the radiation of CuKa : The magnetic properties were measured in a vibrating sample magnetometer (VSM). The thermal treatment of the samples was carried out in vacuum (B105 mbar) for 30 min at 1001C, 2001C, 3001C, 4001C and 5001C. Analysis of the XRD measurements for the samples showed that the Cu matrix has the FCC
0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 7 5 7 - X
N.A. Tuan et al. / Physica B 327 (2003) 400–403
structure and a so-called metastable phase of Co–Cu alloy is formed at low Co fraction. The XRD patterns for some selected samples are presented in Fig. 1. The Cu(1 1 1) and (2 0 0) peaks are very close to standard lines of bulk Cu (2yCuð1 1 1Þ E43:41) for Co-poor films (see, for example, diagram a for x ¼ 0:12 in Fig. 1), and these peaks are shifted to the larger 2y angle, closed to standard lines of bulk Co (2yCoð1 1 1Þ E44:21) for Co-richer films (see, for example, diagram d for x ¼ 0:59 in Fig. 1). Simultaneously, the intensity of these peaks decreases with increasing x. These results suggest that Co atoms form small clusters or fine particles at low Co fractions, and that these particles become larger when the Co fraction increases [5].
Counts (a.u.) →
Cu(111)
Cu(200)
Co(111)
a b c d 40
45
50 2θ (degree) →
401
55
Fig. 1. XRD diagrams for the CoxCu1x films. (a) x=0.12, (b) x=0.26, (c) x=0.42, (d) x=0.59.
1.0 Co 42 Cu 58
M/M10 kOe
Co 12 Cu 88 0.5 0.0 -0.5 -1.0 1.0 Co 20 Cu 80
Co 59 Cu 41
Co 34 Cu 66
Co 77 Cu 23
M/M10 kOe
0.5 0.0 -0.5
M/M10 kOe
-1.0 1.0 0.5 0.0 -0.5 -1.0 -10
-5
0
H (kOe)
5
10
-10
-5
0
5
H (kOe)
Fig. 2. Hysteresis loops measured in-plane for the CoxCu1x films.
10
N.A. Tuan et al. / Physica B 327 (2003) 400–403
402 300
500
Co 12 Cu 88 Co 16 Cu 84 Co 20 Cu 80 Co 26 Cu 74 Co 42 Cu 58 Co 59 Cu 41
250 400
300
HC (Oe)
HC(Oe)
200 150 100 50
200
100
0 0
20
40
x (at %)
60
80
100
Fig. 3. Coercivity as a function of Co fraction for the CoxCu1x films.
0 0
100
200
300
400
500
600
Ta (oC) Fig. 4. Coercivity as a function of annealing temperature for the CoxCu1x films.
Fig. 2 shows the hysteresis loops of the selected CoxCu1x films. As can be seen in this figure, superparamagnetism or property of the fine particles systems is dominant for Co-poor samples (xo0:40). For the samples with xo0:20; the magnetization process is rather similar to that of a paramagnet, as can be seen for the Co12Cu88 sample. This property is less prominent when the Co content increases, and ferromagnetism dominates at the Co-richer samples (x > 0:40). Such behavior has been reported by some other authors [6,7]. Fig. 3 presents the dependence of the coercivity, HC ; on the Co content. As one can see in this figure, the coercivity first increases with increasing x; reaches a maximum value of about 250 Oe at x ¼ 0:34; and then decreases with further increasing x: The enhancement of the coercivity in granular films is known to be due to an increase in size of the magnetic particles [8]. The occurrence of such an increase was proved by the XRD measurements shown above (see Fig. 1). The hysteresis for fine particles system has been attributed to blocking of particles whose size exceeds the critical size for superparamagnetism [9]. However, the hysteresis of granular systems can also be explained by an interaction of magnetic particles rather than of blocked particles [10]. Another cause is surface anisotropy [3,11,12]. For the CoxCu1x films with xE0:2020:40; it could be suggested that the Co particles embedded
in the Cu matrix are single-domain fine particles, whereas for x > 0:40 they may be multi-domain [9]. It could be said that the Co-content threshold at x ¼ xp ; where xp lies between 0.34 and 0.40, is the magnetic percolation threshold [8]. The high coercivity for the low Co content films was also observed after annealing. Fig. 4 shows the dependence of the coercivity on annealing temperature, Ta ; for selected CoxCu1x films. As can be seen in this figure, HC increases with increasing Ta for the Co-poor samples (xo0:40), and decreases for Corich samples. Considerable increase of HC for Copoor samples with increasing Ta from 3001C can also be explained by the growth of the Co particles [12]. This has been confirmed by XRD measurements for samples as-deposited and annealed (see Fig. 5 for Co26Cu74 film, as an example); it is indicated by the shift to higher 2y angle of Cu(1 1 1) and Cu(2 0 0) peaks with increasing annealing temperature. Another important phenomenon observed for Co-rich films (x > 0:40) is that the shape of the hysteresis loops manifests a partly perpendicular magnetic anisotropy, as seen from the graphics on the right-hand side of Fig. 2. This has been observed in Co-rich granular films of Co–Ag systems, and has been suggested to be due to a preferential orientation of Co phase or preferential arrangement of the Co particles in the direction
N.A. Tuan et al. / Physica B 327 (2003) 400–403
403
Counts (a.u.) →
Cu(111)
C
Co(111) →
Ta = 500 oC Ta = 400 oC Ta = 300 oC As-deposited
40
42
44
46
48
2θ θ (degree) ( )
50
→
52
54
Fig. 5. XRD diagrams for Co26Cu74 film as-deposited and annealed.
perpendicular to the film plane [13]. However, the perpendicular anisotropy may originate from the surface magnetic anisotropy at the interfaces between Co particles and the Cu matrix [3,14].
Acknowledgements This work is supported by the State Program of Science & Technology of Viet Nam, Project KC02-13, and State Program of Fundamental Research, Project 420101.
References [1] A.E. Berkowitz, J.R. Mitchell, M.J. Carey, A.P. Young, S. Zhang, F.E. Spada, F.T. Parker, A. Hutten, G. Thomas, Phys. Rev. Lett. 68 (1992) 3745; C.L. Chien, J.Q. Xiao, J.S. Jiang, J. Appl. Phys. 73 (1993) 5309. [2] S.L. Woods, J.R. Kirtley, S. Sun, R.H. Koch, Phys. Rev. Lett. 87 (2001) 137205.
[3] N.H. Duc, N.A. Tuan, A. Fnidiki, C. Dorien, J. Ben Youssef, H. Le Gall, J. Phys.: Condens. Matter 14 (2002) 1. [4] K. Ichihara, A. Kikitsu, K. Yusu, F. Nakamura, H. Ogiwara, IEEE Trans. Magn. 34 (1998) 1603. [5] J.H. Du, Q. Li, L.C. Wang, H. Sang, S.Y. Zang, Y.W. Du, D. Feng, J. Phys.: Condens. Matter 7 (1995) 9425. [6] S. Honda, M. Nawate, M. Tanaka, T. Okada, J. Appl. Phys. 82 (1997) 764. [7] H. Hamakake, K. Ishii, IEEE Trans. Magn. 35 (1999) 3457. [8] J.N. Zhou, A. Bureta, H. Jiang, J.A. Barnard, J. Appl. Phys. 84 (1998) 5693. [9] B.D. Cullity, Introduction to Magnetic Materials, Addison-Wesley publishing company, Reading, MA, 1972. [10] P. Allia, M. Coisson, P. Tiberto, F. Vinai, M. Knobel, J. Appl. Phys. 85 (1999) 4343. [11] X. Zianni, K.N. Trohidou, J.A. Blackman, J. Appl. Phys. 81 (1997) 4739. [12] C. Chen, O. Kitakami, Y. Shimada, J. Appl. Phys. 84 (1998) 2184. [13] J.Q. Xiao, C.L. Chien, A. Gavrin, J. Appl. Phys. 79 (1996) 5309. [14] X.J. Chen, T. Suzuki, J. Appl. Phys. 85 (1999) 5048.