DSpace at VNU: High-coercivity FePt sputtered films

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 290–291 (2005) 559–561 www.elsevier.com/locate/jmmm High-coercivity FePt sputtered films N.H Luonga,Ã, V.V Hiepa, D.M Honga, N Chaua, N.D Linha, M Kurisub, D.T.K Anhb, G Nakamotob a Center for Materials Science, University of Science, Vietnam National University, Hanoi, 334 Nguyen Trai, Hanoi, Vietnam School of Materials Science, Japan Advanced Institute of Science and Technology, Tatsunokuchi, Ishikawa 923-1292, Japan b Available online 21 December 2004 Abstract Fe56Pt44 thin films have been prepared by RF magnetron sputtering on Si substrates The substrate temperature was kept at 350 1C The X-ray diffraction patterns of as-deposited FePt films exhibited a disordered structure Annealing of the films at 650–685 1C for h yielded an ordered L10 phase with FCT structure The high value for coercivity HC of 17 kOe was obtained at room temperature for the 68 nm thick film annealed at 685 1C The hard magnetic properties as well as grain structure of the films strongly depend on the annealing conditions r 2004 Elsevier B.V All rights reserved PACS: 75.70.Ak Keywords: Thin films; FePt; L10 ordered structure; Coercivity Recently, FePt thin films have drawn much attention for their potential application as high-density magnetic recording materials [1,2] The equiatomic FePt L10ordered alloy is one of the most promising candidates for magnetic recording because its magnetocrystalline anisotropy constant Ku of 5–8 Â 107 erg/cm3 [3,4] is an order of magnitude higher than that of the currently used Co–Cr-based alloys [5] Several studies on granular films or isolated particle arrays of FePt L10 have been reported so far [6–9] However, because of its high anisotropy field Hk ($100 kOe), it is difficult to realize the saturation recording Heat-assisted magnetic recording may be an effective method to avoid this problem [10] although there remain a lot of technological problems for practical applications [11,12] Wang et al [13] have obtained ÃCorresponding author Tel.: +84 5582216; +84 8589496 E-mail address: luongnh@vnu.edu.vn (N.H Luong) fax: H c ¼ 8:6 kOe; M r ¼ 0:87 T and (BH)max ¼ 16.5 MGOe for Fe56Pt44 thin film annealed at 600 1C for 10 Yung et al [14] investigated the magnetic hardening of singlelayer FePt thin films and obtained a maximum coercivity of 13 kOe Liu et al [15] have studied the magnetic hardening in Fe/Pt multilayer prepared by sputtering onto high-temperature glass or Si substrates They obtained the coercivity exceeding 20 kOe A huge coercivity exceeding 40 kOe has been achieved by Shima et al [16] in Fe52Pt48 film with a thickness of 10 nm prepared by sputtering However, during the deposition, their substrates were heated to 700 1C which is high for practical application Previous studies reveal that the magnetic properties of Fe–Pt films strongly depend on the preparation conditions, composition and annealing conditions In this work, we present the results of study on structure and magnetic properties of FePt thin films prepared by RF magnetron sputtering We show that hard magnetic properties as well as grain structure of the films strongly depend on the annealing conditions 0304-8853/$ - see front matter r 2004 Elsevier B.V All rights reserved doi:10.1016/j.jmmm.2004.11.534 ARTICLE IN PRESS 560 N.H Luong et al / Journal of Magnetism and Magnetic Materials 290–291 (2005) 559–561 FePt thin films with different thicknesses were directly deposited on thermally oxidized Si (1 0) substrates by RF magnetron sputtering system Leybold UNIVEX 450 The target for the FePt thin film fabrication consisted of a high-purity (99.99%) ferrous disk with several high-purity (99.99%) Pt segments on the ferrous target to facilitate co-sputtering FePt films The RF power was 50 W After the high-purity Ar gas was introduced, sputter pressure was fixed at 10 mTorr The substrate temperature was kept at 350 1C The composition analysis performed by energy dispersion spectrometer (EDS) included in the scanning electron microscope 5410 LV, Jeol, revealed that the films are of Fe56Pt44 composition The structure of as-deposited and annealed films was examined by an X-ray diffractometer D5005, Bruker, using CuKa radiation The films were annealed in the temperature range from 550 to 685 1C for h in high vacuum The hysteresis loops of the films at room temperature were measured by using a vibrating sample magnetometer and a SQUID magnetometer in magnetic field up to 70 kOe The thicknesses of the prepared Fe56Pt44 films, as revealed by small-angle X-ray measurements, are 33, 40, 68, 90 and 92 nm Fig 1a shows the X-ray diffraction pattern of as-deposited Fe56Pt44 film The disordered structure in this sample is seen with a 2y1 of about 40.81 The X-ray diffraction patterns of annealed Fe56Pt44 films of 68 nm in thickness are shown in Fig 1b It is observed that the film annealed at Ta ¼ 550 1C exhibits a disordered g phase with FCC structure However, when Fig X-ray diffraction patterns of the 68 nm thick Fe56Pt44 films (a) as-deposited film and (b) film after annealing at different temperatures the annealing temperature increases, the FCT phase begins to form The (0 1) peak begins to appear and its intensity is more pronounced when the annealing temperature is higher The evidence of chemical ordering for the annealed films was signified by appearance of the tetragonal (0 1), (1 0), and (0 2) peaks [13] The ratio of the (0 1) peak to the (0 2) peak is an indication of the degree of chemical ordering in FePt [13] The same behavior of X-ray diffraction patterns is observed for the films of other thicknesses Fig shows the SEM images of Fe56Pt44 films annealed at different temperatures As mentioned above, X-ray diffraction analysis showed that as-deposited film (Fig 1a) has disordered structure At Ta ¼ 550 1C the gFCC crystallites started forming (Fig 1b, Ta1 and Fig 2a), with increasing annealing temperature transformed into the FCT grains (Fig 1b, Ta2 and Fig 2b) and developed with an average size of 124 nm (Fig 1b, Ta3 and Fig 2c) to 230 nm (Fig 1b, Ta4 and Fig 2d) The hysteresis loops of as-deposited and annealed Fe56Pt44 films of different thicknesses have been measured in the direction parallel to the film plane The as-deposited films behave like soft magnetic materials without significant coercivity due to the presence of disordered structure High coercivity is developed after the films are annealed at Ta ¼ 650 1C It is generally observed that coercivity increases with increasing annealing temperature Fig presents the hysteresis loop of a 68 nm thick Fe56Pt44 film annealed at Ta ¼ 685 1C Magnetic property measurements show that coercivity also depends on thickness of the films, as shown in Fig Thus, among our investigated samples the 68 nm thick Fe56Pt44 film has the highest value for coercivity of 17 kOe As grain size is larger than the single-domain size, coercivity of the film rapidly Fig Plan view SEM images of 68 nm thick Fe56Pt44 films annealed at 550 1C (a), 650 1C (b), 670 1C (c), 685 1C (d) ARTICLE IN PRESS N.H Luong et al / Journal of Magnetism and Magnetic Materials 290–291 (2005) 559–561 561 to the formation of the high-anisotropy ordered tetragonal FePt phase as now generally accepted (see, for instance, Refs [9,10]) The high in-plane coercivity of 17 kOe is clearly related with the forming of FCT L10 phase in the film due to annealing as well as related to enhancement in the chemical ordering This correlation between coercivity and chemical ordering is similar to that observed before in FePt [19] The authors acknowledge financial support by National Science and Technology Program (Project KC 02-13 01) and National Research Program (Project 811204) Fig Hysteresis loop at room temperature of 68 nm thick Fe56Pt44 film annealed at 685 1C for h Fig Dependence of coercivity of Fe56Pt44 films on thickness, at different annealing temperatures decreases [17] We suppose that in our films, the critical single-domain size occurs in the 68 nm thick film It is well known that coercivity is affected by anisotropy as well as inhomogeneity (grain size, various types of defects, etc.) Defects in magnetic materials, such as grain boundaries and phase boundaries, can form pinning sites that impede the movement of magnetic domain walls, leading to high coercivity [18] Although the rotation of magnetization including an incoherent process such as buckling is a dominating mechanism in small single-domain particles, the nucleation of domain becomes significant when the particles size increases with film thickness, resulting in the decrease of coercivity [16] Magnetic hardening of the annealed films is attributed References [1] C.P Luo, D.J Sellmyer, IEEE Trans Magn 31 (1995) 2764 [2] C.P Luo, Z.S Shan, P.J Sellmyer, J Appl Phys 79 (1996) 4899 [3] H Kanazawa, G Lanhoff, T Suzuki, J Appl Phys 87 (2000) 6143 [4] S Okamoto, N Kikuchi, O Kitakami, T Miyazaki, Y Shimada, K Fukamichi, Phys Rev B 66 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(2002) 1050 [17] C.M Kuo, P.C Kuo, H.C Wu, J Appl Phys 85 (1999) 2264 [18] J.D Livingston, J Appl Phys 52 (1981) 2544 [19] J.-C Shih, H.-H Hsiao, J.-L Tsai, T.-S Chin, IEEE Trans Magn 37 (2001) 1280  ... by National Science and Technology Program (Project KC 02-13 01) and National Research Program (Project 811204) Fig Hysteresis loop at room temperature of 68 nm thick Fe56Pt44 film annealed at. .. W After the high-purity Ar gas was introduced, sputter pressure was fixed at 10 mTorr The substrate temperature was kept at 350 1C The composition analysis performed by energy dispersion spectrometer... films were annealed in the temperature range from 550 to 685 1C for h in high vacuum The hysteresis loops of the films at room temperature were measured by using a vibrating sample magnetometer and