ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 310 (2007) 2524–2526 www.elsevier.com/locate/jmmm High coercivity and giant magnetoresistance of CoAg, CoCu granular films V.V Hiep, N Chau, D.M Hong, N.H Luongà Center for Materials Science, University of Science, Vietnam National University, Hanoi-334 Nguyen Trai Road, Hanoi, Vietnam Available online December 2006 Abstract We report our study on the structure, magnetic and giant magnetoresistance properties of two systems: CoxAg1Àx (x ¼ 33, 48, 49, 52 at%) and CoyCu1Ày (y ¼ 11, 13, 15, 17 at%) granular films prepared by RF sputtering The thermal transition measured by the SDT 2960 apparatus revealed exothermal peaks at 400 1C, corresponding to the crystallization of FCC-Co crystallites The studied films were annealed in a temperature range of 300–450 1C for h The structure and particle size were determined from the X-ray diffraction data Superparamagnetic state was shown in as-deposited films After appropriate heat treatment, coercivity increased up to 1100 Oe in the Co52Ag48 film annealed at 350 1C, and 690 Oe in the Co13Cu87 film annealed at 400 1C Maximum magnetoresistance up to 4.25% in the Co48Ag52 film, and 5.4% in the Co15Cu85 film annealed at 400 1C was obtained Our values for magnetoresistance are quite high compared with recent studies r 2006 Elsevier B.V All rights reserved PACS: 75.75.+a; 75.70.Ak; 75.50.Bb; 75.50.Tt Keywords: Giant magnetoresistance; Thin film; Superparamagnetism; Metal nanoparticle; Nanogranular alloy Magnetic granular films have been known since the beginning of 1990s due to the giant magnetoresistance (GMR) property [1] Recently, their high coercivity and GMR properties, which make them to have potential application as high-density recording media and various other devices, have also been studied widely [2–4] Basically, these properties depend on particle size, anisotropy, interaction between particles, spin-dependence scattering on particles etc In this paper, we report our study on structure, magnetic and GMR properties of two systems: CoxAg1Àx (x ¼ 33, 48, 49, 52 at%) and CoyCu1Ày (y ¼ 11, 13, 15, 17 at%) granular films Thin films were prepared by RF sputtering from Co50Ag50 and Co15Cu85 targets The thermal transition was measured by an SDT 2960 apparatus The structure and particle size were determined from the X-ray diffraction data The magnetic and GMR properties of granular ÃCorresponding author Tel.: +84 5582216; fax: +84 8589496 E-mail address: luongnh@vnu.edu.vn (N.H Luong) 0304-8853/$ - see front matter r 2006 Elsevier B.V All rights reserved doi:10.1016/j.jmmm.2006.11.136 films were investigated by VSM and the four-probes method The DSC curve for the as-deposited Co49Ag51 film of 310-nm thickness revealed exothermal peaks at Tp ¼ 400 1C, corresponding to the crystallization of FCCCo crystallites (Fig 1) The studied films were annealed in a temperature range from Ta ¼ 300 to 450 1C (around Tp) for h Fig shows the X-ray diffraction patterns of the CoxAg1Àx films There are two very close arranged peaks, one for the Co(1 1) FCC phase and another one for Ag(2 0) and Cu(2 0) (inset) (Fig 2) Particle size was determined from the X-ray diffraction data, and seemed to increase with annealing temprature from to 18 nm (Table 1) Superparamagnetic state was shown in the as-deposited films and is in good agreement with Langevin fitting (Fig 3) After appropriate heat treatment, coercivity increased up to 1100 Oe in the 350 1C-annealed Co52Ag48 film, and 690 Oe in the 400 1C-annealed Co13Cu87 film (Tables and 2) The maximum coercive field appears for ferromagnetic particles, which are single domain In this ARTICLE IN PRESS V.V Hiep et al / Journal of Magnetism and Magnetic Materials 310 (2007) 2524–2526 2525 Heat Flow (a.u) Table Particle size (D), in-plane (HCJ) and perpendicular coercivity (HC?) for 508 nm Co52Ag48 films annealed at Ta ¼ 300, 350, 400 and 450 1C for h 200 400 300 1C 350 1C 400 1C 450 1C D (nm) HcJ (Oe) Hc? (Oe) 240 225 14 1100 1020 16 1000 1080 18 625 645 Table In-plane coercivity (Oe) of 350 nm Co13Cu87, 506 nm Co15Cu85 and 590 nm Co17Cu83 films annealed in a temperature range of 360–420 1C/1 h 800 600 Ta T (°C) Co13Cu87 Co15Cu85 Co17Cu83 380 585 690 560 325 440 455 375 255 320 325 515 Fig DSC curve for 310 nm thick Co49Ag51 film 360 1C 380 1C 400 1C 420 1C Intensity (a.u) (111) (200) (111) 400°C Co15Cu85 450°C/1h 350°C 400°C/1h 300°C/1h 40 30 50 60 2θ (°) Fig XRD diagrams for 508 nm Co52Ag48 films; annealed at 300 1C, 350 1C, 400 1C and 450 1C/1 h Inset is the result for Co17Cu83 films GMR (%) 350°C/1h as-deposited 350°C/1h 450°C -15 300°C -10 -5 H (kOe) 10 15 Fig GMR curves for the Co15Cu85 films annealed at 300, 350, 400 and 450 1C/1 h Inset is GMR for other CoAg films M (memu) -1 -2 -15 -10 -5 10 15 H (kOe) Fig Hysteresis loops of the 508 nm as-deposited and annealed Co52Ag48 films case, the magnetization reversal occurs through a coherent rotation mechanism For larger dimensions, particles are multi-domain and domain wall motions are possible leading to reducing coercive fields GMR in granular materials originates from the spindependent scattering at interfaces between magnetic particles and the metallic nonmagnetic matrix GMR also depends on the magnetic state of the particles superparamagnetic, single domain or multi domain For small particles, the value of GMR is small and increases slowly with applied magnetic field In this case, GMR arises from non-aligned superparamagnetic or ferromagnetic single domain particles For larger but still single-domain particles, the GMR increases more rapidly with applied field When the particles size becomes larger, the material behaves progressively as a bulk ferromagnetic with multidomain particles and magnetic interactions between them and consequently the GMR decreases Maximum GMR up to 4.25% in the Co48Ag52 film (the inset) and 5.4% in the Co15Cu85 film annealed at 400 1C were obtained (Fig 4) It is noted that the MRs of the studied films are higher than that of recent studies [3,4] In conclusion, the two systems of granular films CoAg and, CoCu were prepared and studied Hc enhanced up to 1100 Oe in the Co52Ag48 film annealed at 350 1C, GMR ARTICLE IN PRESS 2526 V.V Hiep et al / Journal of Magnetism and Magnetic Materials 310 (2007) 2524–2526 reached up to 5.4% in the Co15Cu85 film annealed at 400 1C/1 h, have been presented We acknowledge the financial support from the Vietnam National Fundamental Research Program for this work References [1] C.L Chien, J.Q Xiao, J.S Jiang, J Appl Phys 73 (1993) 5309 [2] N.A Tuan, N.H Luong, N Chau, V.V Hiep, N.T.M Ha, Physica B 327 (2003) 400 [3] I Bergenti, A Deriu, E Bosco, M Baricco, E Angeli, D Bisero, A Da Re, F Ronconi, F Spizzo, P Vavassori, J Magn Magn Mater 272–276 (2004) 1554 [4] S Kenane, J Voiron, N Benbrahim, E Chainet, F Robaut, J Magn Magn Mater 297 (2006) 99 ... granular materials originates from the spindependent scattering at interfaces between magnetic particles and the metallic nonmagnetic matrix GMR also depends on the magnetic state of the particles... systems of granular films CoAg and, CoCu were prepared and studied Hc enhanced up to 1100 Oe in the Co52Ag48 film annealed at 350 1C, GMR ARTICLE IN PRESS 2526 V.V Hiep et al / Journal of Magnetism and. .. the Co48Ag52 film (the inset) and 5.4% in the Co15Cu85 film annealed at 400 1C were obtained (Fig 4) It is noted that the MRs of the studied films are higher than that of recent studies [3,4] In conclusion,