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Fabrication of L10 ordered FePt thin films with a canted easy magnetization axis on MgO (1 1 0) substrate
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) e557–e559
Fabrication of L10 ordered FePt thin films with a canted easy magnetization axis on MgO (1 1 0) substrate T. Shimaa,*, T. Sekia, K. Takanashia, Y.K. Takahashib, K. Honob a
Institute for Materials Research, Institute fir Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan b National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan
Abstract We have investigated the magnetic properties of highly coercive FePt films prepared on both MgO (0 0 1) and MgO (1 1 0) substrates with various film thicknesses (tN ). FePt (0 0 1) films with perpendicular magnetization and FePt (1 0 1) films with a canted easy magnetization axis (c-axis) were prepared on MgO (0 0 1) and MgO (1 1 0) substrates, respectively. At the critical thickness, where the film morphology changes from a particulate to a continuous state, a drastic decrease in the coercivity Hc has been observed for the FePt (0 0 1) film, while no jump of Hc has been observed for the FePt (1 0 1) films. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Vv; 68.55.Jk; 75.60.d Keywords: FePt; Thin film; Sputtering; Microstructure; Magnetic domain structure
Magnetization processes in nanometer-sized magnets are of great technological and scientific interest. They are important for understanding the high coercivity mechanism of micromagnets with potential applications in various future magnetic devices such as ultra highdensity recording media and bias magnets in monolithic microwave integrated circuits. An assembly of nanomagnets with large uniaxial anisotropy is required for these applications. The magnetization process, and therefore the coercivity should depend strongly on the characteristic size and the morphology of materials. However, there have been only a few experimental investigations on the correlation between the measured magnetization behavior and the actual microstructure of ultrathin films. L10 ordered FePt alloy with a large uniaxial magnetic anisotropy has attracted much attention and a lot of studies have been focused on the fabrication of L10 ordered FePt thin films [1–4]. In most cases, however, the coercivity realized in highly ordered *Corresponding author. Tel.: +81-22-215-2097; fax: +1-22215-2096. E-mail address: [email protected] (T. Shima).
FePt films was less than 20 kOe. In a previous paper [1], we demonstrated that a huge coercivity as high as 40 kOe was obtained for epitaxially grown FePt (0 0 1) films with small-island structure, and simultaneously, a drastic change in the coercivity by one order of magnitude was found at the critical thickness (tN ¼ 45 nm), where the film morphology change from a particulate to a continuous state that was observed by transmission electron microscopy (TEM). The study of the films with a different direction of the [0 0 1] axis (caxis), which coincides with the easy magnetization axis, may provide further information for understanding the relationship between magnetization process and film morphology. In this paper, L10 ordered FePt thin films with a canted easy magnetization axis were prepared on MgO (1 1 0) substrates and the comparison between the films grown on MgO (0 0 1) and MgO (1 1 0) substrates is reported. Samples were prepared in a high vacuum system (base pressure B5 1010 Torr) using multiple dc-sputtering with co-deposition of Fe and Pt directly onto single crystalline MgO (0 0 1) or MgO (1 1 0) substrates. The targets of 99.99 at% Fe and 99.9 at% Pt were used.
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.11.322
ARTICLE IN PRESS T. Shima et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e557–e559
The sputtering pressure is kept at 1.4 mTorr and the substrates were heated to 700 C during the deposition. The nominal thickness (tN ) of FePt film was varied in the range between 15 and 100 nm. The composition of the films was determined to be Fe52Pt48 (at%) by electron probe X-ray microanalysis (EPMA). Magnetic properties were measured by a superconducting quantum interference device (SQUID) magnetometer in the field up to 55 kOe at room temperature. The X-ray diffraction measurements have revealed that the c-axis of L10 structure is aligned in the direction perpendicular to the film plane for the films grown on MgO (0 0 1) substrates (denoted as FePt (0 0 1) film, hereafter), while the [1 0 1] axis is mostly perpendicular to the film plane, and the c-axis is canted from the perpendicular to direction for the films grown on MgO (1 1 0) substrates (denoted as FePt (1 0 1) films, hereafter). It is noted that the direction of the c-axis for the films grown on MgO (1 1 0) substrates is different from previous results on the films with Pt buffer layer [5,6]. It is considered to arise from the strain stored between FePt film and MgO substrate. Magnetization curves for FePt (0 0 1) films and FePt (1 0 1) films with tN ¼ 15 and 50 nm are shown in Figs. 1(a)–(d). The easy magnetization axis lies in the perpendicular direction to the film plane for FePt (0 0 1) films. On the other hand, it exists between the perpendicular direction to the film plane and in-plane MgO [1 1% 0] direction for FePt (1 0 1) films, i.e., the easy magnetization axis is canted from the normal direction to the film plane. TEM observation and electrical resistance measurements have reveled that
Fig. 1. Magnetization curves for FePt (0 0 1) films and FePt (1 0 1) films with tN ¼ 15 ((a), (c)) and 50 nm ((b), (d)) deposited on MgO (0 0 1) and MgO (1 1 0) substrate, respectively. The crystal axes shown in (c) and (d) are these for the MgO (1 1 0) substrate.
100 Coercivity, HC (kOe)
e558
6 4
FePt (101)
2
10 6 4 2
Particulate
1
Continuous FePt (001)
6 4 2
0
20
40 60 80 FePt film thickness, tN (nm)
100
Fig. 2. The change of coercivity for FePt (0 0 1) films and FePt (1 0 1) films as a function of tN :
the film with tN ¼ 15 nm has discontinuous morphology, while the film with tN ¼ 50 nm has continuous one with interconnected islands for both FePt (0 0 1) and FePt (1 0 1) films. For FePt (0 0 1) films, Hc for tN ¼ 50 nm is much smaller than that for tN ¼ 15 nm. For FePt (1 0 1) films, however, the difference in Hc between tN ¼ 15 and 50 nm is not so remarkable. Hc as a function of tN for FePt (0 0 1) and FePt (1 0 1) films are shown in Fig. 2. With increasing tN ; Hc decreases slowly, but still keeps a quite large value of about 25 kOe for FePt (0 0 1) films with tN ¼ 45 nm. However, a drastic change by one order of magnitude is observed between tN ¼ 45 and 50 nm. This critical region corresponds to the change in the morphology of the films from particulate to continuous state [1]. In contrast to the behavior in the FePt (0 0 1) films, a gradual change of Hc has been observed and also no jump is seen at the critical thickness for FePt (1 0 1) films. The former indicates low resistance to the movement of the nucleated domain walls for the FePt (0 0 1) films, while the latter indicates the presence of pinning for the domain wall movement for FePt (1 0 1) films. The pinning is thought to due to planar defects like twins, which were induced in a course of the growing process and they play an effective role as pinning sites for the domain wall movement. In summary, different magnetization behavior has been observed between FePt (0 0 1) films and FePt (1 0 1) films when the film morphology changes from a particulate to continuous state. The further microstructural investigation of FePt (1 0 1) films is in progress. This work is partly supported by ‘‘Nanohetero Metallic Materials’’ Project from MEXT. The authors would like to acknowledge Mr. Y. Murakami for technical assistance.
ARTICLE IN PRESS T. Shima et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e557–e559
References [1] T. Shima, K. Takanashi, Y.K. Takahashi, K. Hono, Appl. Phys. Lett. 81 (2002) 1050. [2] B.M. Lairson, M.R. Viosokay, R. Sinclair, B.M. Clemens, Appl. Phys. Lett. 62 (1993) 639. [3] T. Shima, T. Moriguchi, S. Mitani, K. Takanashi, Appl. Phys. Lett. 80 (2002) 288.
e559
[4] M. Watanabe, M. Homma, Jpn. J. Appl. Phys. 35 (1996) L1264. [5] R.F.C. Farrow, D. Weller, R.F. Marks, M.F. Toney, J. Appl. Phys. 84 (1998) 934. [6] T. Seki, T. Shima, K. Takanashi, Proceedings of ICM 2003, Rome, 2003.
Journal of Magnetism and Magnetic Materials 272–276 (2004) e557–e559
Fabrication of L10 ordered FePt thin films with a canted easy magnetization axis on MgO (1 1 0) substrate T. Shimaa,*, T. Sekia, K. Takanashia, Y.K. Takahashib, K. Honob a
Institute for Materials Research, Institute fir Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan b National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan
Abstract We have investigated the magnetic properties of highly coercive FePt films prepared on both MgO (0 0 1) and MgO (1 1 0) substrates with various film thicknesses (tN ). FePt (0 0 1) films with perpendicular magnetization and FePt (1 0 1) films with a canted easy magnetization axis (c-axis) were prepared on MgO (0 0 1) and MgO (1 1 0) substrates, respectively. At the critical thickness, where the film morphology changes from a particulate to a continuous state, a drastic decrease in the coercivity Hc has been observed for the FePt (0 0 1) film, while no jump of Hc has been observed for the FePt (1 0 1) films. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Vv; 68.55.Jk; 75.60.d Keywords: FePt; Thin film; Sputtering; Microstructure; Magnetic domain structure
Magnetization processes in nanometer-sized magnets are of great technological and scientific interest. They are important for understanding the high coercivity mechanism of micromagnets with potential applications in various future magnetic devices such as ultra highdensity recording media and bias magnets in monolithic microwave integrated circuits. An assembly of nanomagnets with large uniaxial anisotropy is required for these applications. The magnetization process, and therefore the coercivity should depend strongly on the characteristic size and the morphology of materials. However, there have been only a few experimental investigations on the correlation between the measured magnetization behavior and the actual microstructure of ultrathin films. L10 ordered FePt alloy with a large uniaxial magnetic anisotropy has attracted much attention and a lot of studies have been focused on the fabrication of L10 ordered FePt thin films [1–4]. In most cases, however, the coercivity realized in highly ordered *Corresponding author. Tel.: +81-22-215-2097; fax: +1-22215-2096. E-mail address: [email protected] (T. Shima).
FePt films was less than 20 kOe. In a previous paper [1], we demonstrated that a huge coercivity as high as 40 kOe was obtained for epitaxially grown FePt (0 0 1) films with small-island structure, and simultaneously, a drastic change in the coercivity by one order of magnitude was found at the critical thickness (tN ¼ 45 nm), where the film morphology change from a particulate to a continuous state that was observed by transmission electron microscopy (TEM). The study of the films with a different direction of the [0 0 1] axis (caxis), which coincides with the easy magnetization axis, may provide further information for understanding the relationship between magnetization process and film morphology. In this paper, L10 ordered FePt thin films with a canted easy magnetization axis were prepared on MgO (1 1 0) substrates and the comparison between the films grown on MgO (0 0 1) and MgO (1 1 0) substrates is reported. Samples were prepared in a high vacuum system (base pressure B5 1010 Torr) using multiple dc-sputtering with co-deposition of Fe and Pt directly onto single crystalline MgO (0 0 1) or MgO (1 1 0) substrates. The targets of 99.99 at% Fe and 99.9 at% Pt were used.
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.11.322
ARTICLE IN PRESS T. Shima et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e557–e559
The sputtering pressure is kept at 1.4 mTorr and the substrates were heated to 700 C during the deposition. The nominal thickness (tN ) of FePt film was varied in the range between 15 and 100 nm. The composition of the films was determined to be Fe52Pt48 (at%) by electron probe X-ray microanalysis (EPMA). Magnetic properties were measured by a superconducting quantum interference device (SQUID) magnetometer in the field up to 55 kOe at room temperature. The X-ray diffraction measurements have revealed that the c-axis of L10 structure is aligned in the direction perpendicular to the film plane for the films grown on MgO (0 0 1) substrates (denoted as FePt (0 0 1) film, hereafter), while the [1 0 1] axis is mostly perpendicular to the film plane, and the c-axis is canted from the perpendicular to direction for the films grown on MgO (1 1 0) substrates (denoted as FePt (1 0 1) films, hereafter). It is noted that the direction of the c-axis for the films grown on MgO (1 1 0) substrates is different from previous results on the films with Pt buffer layer [5,6]. It is considered to arise from the strain stored between FePt film and MgO substrate. Magnetization curves for FePt (0 0 1) films and FePt (1 0 1) films with tN ¼ 15 and 50 nm are shown in Figs. 1(a)–(d). The easy magnetization axis lies in the perpendicular direction to the film plane for FePt (0 0 1) films. On the other hand, it exists between the perpendicular direction to the film plane and in-plane MgO [1 1% 0] direction for FePt (1 0 1) films, i.e., the easy magnetization axis is canted from the normal direction to the film plane. TEM observation and electrical resistance measurements have reveled that
Fig. 1. Magnetization curves for FePt (0 0 1) films and FePt (1 0 1) films with tN ¼ 15 ((a), (c)) and 50 nm ((b), (d)) deposited on MgO (0 0 1) and MgO (1 1 0) substrate, respectively. The crystal axes shown in (c) and (d) are these for the MgO (1 1 0) substrate.
100 Coercivity, HC (kOe)
e558
6 4
FePt (101)
2
10 6 4 2
Particulate
1
Continuous FePt (001)
6 4 2
0
20
40 60 80 FePt film thickness, tN (nm)
100
Fig. 2. The change of coercivity for FePt (0 0 1) films and FePt (1 0 1) films as a function of tN :
the film with tN ¼ 15 nm has discontinuous morphology, while the film with tN ¼ 50 nm has continuous one with interconnected islands for both FePt (0 0 1) and FePt (1 0 1) films. For FePt (0 0 1) films, Hc for tN ¼ 50 nm is much smaller than that for tN ¼ 15 nm. For FePt (1 0 1) films, however, the difference in Hc between tN ¼ 15 and 50 nm is not so remarkable. Hc as a function of tN for FePt (0 0 1) and FePt (1 0 1) films are shown in Fig. 2. With increasing tN ; Hc decreases slowly, but still keeps a quite large value of about 25 kOe for FePt (0 0 1) films with tN ¼ 45 nm. However, a drastic change by one order of magnitude is observed between tN ¼ 45 and 50 nm. This critical region corresponds to the change in the morphology of the films from particulate to continuous state [1]. In contrast to the behavior in the FePt (0 0 1) films, a gradual change of Hc has been observed and also no jump is seen at the critical thickness for FePt (1 0 1) films. The former indicates low resistance to the movement of the nucleated domain walls for the FePt (0 0 1) films, while the latter indicates the presence of pinning for the domain wall movement for FePt (1 0 1) films. The pinning is thought to due to planar defects like twins, which were induced in a course of the growing process and they play an effective role as pinning sites for the domain wall movement. In summary, different magnetization behavior has been observed between FePt (0 0 1) films and FePt (1 0 1) films when the film morphology changes from a particulate to continuous state. The further microstructural investigation of FePt (1 0 1) films is in progress. This work is partly supported by ‘‘Nanohetero Metallic Materials’’ Project from MEXT. The authors would like to acknowledge Mr. Y. Murakami for technical assistance.
ARTICLE IN PRESS T. Shima et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e557–e559
References [1] T. Shima, K. Takanashi, Y.K. Takahashi, K. Hono, Appl. Phys. Lett. 81 (2002) 1050. [2] B.M. Lairson, M.R. Viosokay, R. Sinclair, B.M. Clemens, Appl. Phys. Lett. 62 (1993) 639. [3] T. Shima, T. Moriguchi, S. Mitani, K. Takanashi, Appl. Phys. Lett. 80 (2002) 288.
e559
[4] M. Watanabe, M. Homma, Jpn. J. Appl. Phys. 35 (1996) L1264. [5] R.F.C. Farrow, D. Weller, R.F. Marks, M.F. Toney, J. Appl. Phys. 84 (1998) 934. [6] T. Seki, T. Shima, K. Takanashi, Proceedings of ICM 2003, Rome, 2003.