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), 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) 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: luongnh@vnu.edu.vn (N.H Luong) The CoxCu1Àx 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) 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 10À6 mbar, and the Ar pressure for discharging was 10À3 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 (B10À5 mbar) for 30 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 - ( ) 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 The Cu(1 1) and (2 0) peaks are very close to standard lines of bulk Cu (2yCuð1 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 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 XRD diagrams for the CoxCu1Àx 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 H (kOe) 10 -10 -5 H (kOe) Fig Hysteresis loops measured in-plane for the CoxCu1Àx 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 20 40 x (at %) 60 80 100 Fig Coercivity as a function of Co fraction for the CoxCu1Àx films 0 100 200 300 400 500 600 Ta (oC) Fig Coercivity as a function of annealing temperature for the CoxCu1Àx films Fig shows the hysteresis loops of the selected CoxCu1Àx 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 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 CoxCu1Àx 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 shows the dependence of the coercivity on annealing temperature, Ta ; for selected CoxCu1Àx 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 for Co26Cu74 film, as an example); it is indicated by the shift to higher 2y angle of Cu(1 1) and Cu(2 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 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 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) [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 (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 ... 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... 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... 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