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Observation of Magnetization Reversal Process in Ni–Fe Nanowire Using Magnetic Field Sweeping-Magnetic Force Microscopy
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Japanese Journal of Applied Physics Vol. 46, No. 37, 2007, pp. L898–L900 #2007 The Japan Society of Applied Physics
Observation of Magnetization Reversal Process in Ni–Fe Nanowire Using Magnetic Field Sweeping-Magnetic Force Microscopy Yasushi E NDO1 , Yusuke M ATSUMURA1 , Hideki F UJIMOTO1 , Ryoichi NAKATANI1;2 , and Masahiko Y AMAMOTO1 1 2
Department of Materials Science and Engineering, Graduate School of Engineering, Osaka University, Yamadaoka, Suita, Osaka 565-0871, Japan Center for Atomic and Molecular Technologies, Graduate School of Engineering, Osaka University, Yamadaoka, Suita, Osaka 565-0871, Japan
(Received July 7, 2007; accepted August 16, 2007; published online September 21, 2007)
The details of the magnetization reversal process of Ni–Fe nanowires, which were 10, 30, and 50-nm thick, were successfully observed using our newly proposed magnetization measurement method, namely, magnetic field sweeping (MFS)-magnetic force microscopy (MFM). All the points within the nanowire show marked phase changes (stray fields change) as the magnetic field is varied. In particular, each nanowire edge displays a hysteresis loop, while the center shows a sharp jump or a plateau area. These results demonstrate that domain wall motion is dominant in the magnetization reversal process of a 10-nmthick Ni–Fe nanowire and that domain wall motion along with domain wall pinning play important roles in the magnetization reversal process in both 30- and 50-nm-thick Ni–Fe nanowires. [DOI: 10.1143/JJAP.46.L898] KEYWORDS: nanosized magnet, Ni–Fe nanowire, MFM, magnetization reversal process, domain wall
The shape and size of a nanosized magnet markedly influence its behavior.1,2) Because nanosized magnets are promising materials for practical applications, the magnetization reversal processes and domain wall configurations of nanosized magnets have been studied intensively, e.g., for nonvolatile memory devices,3,4) magnetic storage media,5) and magnetic logic gates.6,7) In particular, magnetic nanowires are attractive as nanosized magnets from both fundamental and applied points of view8–11) because inducing a magnetic field10) or current11) can control their domain wall motion. For these studies, a detailed understanding of the magnetic behavior of a single nanosized magnet is very important. However, conventional experimental techniques such as vibrating sample magnetometry (VSM) and superconducting quantum interference device (SQUID) magnetometry cannot be used to determine the magnetic behavior of a single nanosized magnet because these techniques are not sensitive enough to measure the magnetization of a single nanosized magnet. Recently, techniques, which include micro-SQUID12,13) and the use of a micro-Hall probe,14,15) have been used to determine the magnetic behavior of a single nanosized magnet. However, difficulties remain; micro-SQUID is limited by the operation temperature of the SQUID element and the shape of the specimen is restricted when using the micro-Hall probe owing to complications in the fabrication process. Hence, we have proposed a new magnetization measurement method, magnetic field sweeping (MFS)-magnetic force microscopy (MFM),16,17) which employs a MFM tip as a detector while sweeping the magnetic field. MFS-MFM has the following advantages: (1) The precise magnetic state in a local point of a single nanosized magnet can be directly observed at room temperature. (2) The shape of the specimen is not restricted. (3) The measuring time is less than 1 min. Consequently, we have demonstrated that the movement and annihilation of a vortex core in an isolated Co-Fe circular dot can be directly observed using MFS-MFM.16) We have also successfully observed domain wall trapping in a Ni constriction structure.17) In this paper, we report observation results of a detailed magnetization reversal process in a Ni–Fe nanowire using MFS-MFM.
E-mail address:
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Fig. 1. (a)–(c) SEM images of Ni–Fe nanowires (length ¼ 2100 nm, width ¼ 200 nm) with thicknesses of (a) 10, (b) 30, and (c) 50 nm. (d)–(f) MFM images in zero field of Ni–Fe nanowires with various thicknesses of (d) 10, (e) 30, and (f) 50 nm. Magnetic field is applied in the longitudinal direction of the nanowire plane. The arrows are visual guides to the domain walls in each nanowire.
Ni-20 at. %Fe (Ni–Fe) nanowires were fabricated by electron-beam lithography, DC magnetron sputtering, and a liftoff technique onto thermally oxidized Si(100) substrates. The length and the width of the nanowires were 2100 and 200 nm, respectively, while the thicknesses varied between 10 and 50 nm. The shape of the nanowires was observed by scanning electron microcopy (SEM). The magnetization of some ten thousands of nanowires was investigated by magneto-optical Kerr effect (MOKE) measurements, while the magnetic state of a nanowire was observed by our proposed measurement method, namely, MFS-MFM. In MFS-MFM, a MFM tip is used as a detector as the magnetic field is swept between þ3:0 and 3:0 kOe, so that the magnetic state of a local point in a nanowire can be directly observed by measuring the phase (the stray field). The MFM tip used in this method is a Si cantilever coated with a 30-nm-thick CoPtCr film, which has a low magnetic moment and is initially magnetized in the perpendicular direction. This tip is fixed 10 nm above the surface at a certain point within the nanowire, which is at a height much lower than that in a conventional MFM. This measurement was carried out at room temperature in a vacuum near 101 Pa. Figure 1 shows the SEM and MFM images of isolated Ni–Fe nanowires, which were 10, 30, and 50-nm thick. The shape of each nanowire is clearly linear near the center, but is slightly rounded at both edges [Figs. 1(a)–1(c)]. For the
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Fig. 3. (a) MFM image in zero field of 10-nm-thick Ni–Fe nanowire. Points 1– 5 indicate measured points within the nanowire. (b)–(f) Curves of phase vs magnetic field (H) for various points measured by MFSMFM. Magnetic field is applied in the longitudinal direction of the nanowire plane.
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Fig. 4. (a) MFM image in zero field of 30-nm-thick Ni–Fe nanowire. Points 1– 5 indicate measured points within the nanowire. (b)–(f) Curves of phase vs magnetic field (H) for various points measured by MFSMFM. Magnetic field is applied in the longitudinal direction of the nanowire plane.
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10-nm-thick Ni–Fe nanowire [Fig. 1(d)], the black (dark) and white (bright) areas are observed at the left and right edges of the nanowire, respectively. Figure 1(d) demonstrates that the domain wall exists at only the edge of the nanowire, as shown by the arrows, and that the magnetic state is ‘‘C’’ shaped in a zero field, as reported in refs. 18 and 19. This magnetic state can be also reproduced by a micromagnetics simulation.20) On the other hand, for the 30and 50-nm-thick Ni–Fe nanowires [Figs. 1(e) and 1(f)], the black (dark) and white (bright) areas are observed not only at both edges, but also within each nanowire. These results reveal that several domain walls exist within the nanowires, as indicated by the arrows in these figures, and that their magnetic states have a lot of domain walls in a zero field. However, their magnetic states cannot be reproduced by a micromagnetics simulation, because they might be influenced by the shape of the edge in the nanowire and the stray field of the MFM tip in conventional MFM. Figure 2 shows the MOKE loop of some ten thousands of Ni–Fe nanowires with various thicknesses. The magnetic field is applied in the longitudinal direction of the nanowire plane. The MOKE loop changes from a stepped square loop [Fig. 2(a)] to a nearly square loop [Figs. 2(b) and 2(c)] as the thickness of the nanowire increases. This change can be confirmed by a micromagnetics simulation, which also demonstrates that the shape of the magnetization loop in a nanowire changes from a stepped square to nearly square as the thickness of the nanowire increases. This result reveals that the magnetization reversal process in each nanowire differs according to the thickness. In order to clarify the difference in the magnetic state in a nanowire with various thicknesses in detail, the phase curves vs magnetic field (H) at various points in the 10-, 30-, and 50-nm-thick Ni–Fe nanowires were measured by MFSMFM. Figures 3–5 show these results when the magnetic field is applied in the longitudinal direction of the nanowire plane. Here, Figs. 3(a), 4(a), and 5(a) are the same as Figs. 1(d), 1(e), and 1(f), respectively, but the location of each measured point is denoted in Figs. 3(a), 4(a), and 5(a). Points 1– 5 indicate measured points within the nanowire. All the points within the nanowire show marked phase changes as the magnetic field is varied. For the 10-nm-thick Ni–Fe nanowire, points 1 and 5 [Figs. 3(b) and 3(f)] each show a hysteresis loop of the phase. These hysteresis loops are attributed to the magnetization reversal at the edge of the nanowire. At points 2 – 4 [Figs. 3(c)–3(e)], sharp phase increases are observed at approximately þ0:13 and 0:13 kOe, and are derived from the domain wall motion within
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the nanowire. The magnetic fields at which these sharp phase increases are observed do not completely coincide with the switching field at approximately þ0:12 and 0:12 kOe [Fig. 2(a)], because the switching field of each nanowire is widely distributed in the MOKE loop. These results mean that the detailed magnetization reversal process within a nanowire can be observed using MFS-MFM. On the basis of
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these results, it can be seen that the domain wall motion dominates the magnetization reversal process of a 10-nmthick Ni–Fe nanowire. For the 30-nm-thick Ni–Fe nanowire, points 1, 3, and 5 [Figs. 4(b), 4(d), and 4(f)] each display a hysteresis loop. These hysteresis loops are attributed to the magnetization reversal at the edge of the nanowire and around the center of the nanowire. At point 2 [Fig. 4(c)], weak and sharp jumps of the phase are observed at approximately þ0:12 and 0:12 kOe, respectively, while point 4 [Fig. 4(e)] displays a sharp phase jump at approximately þ0:12 kOe, which is similar to that of point 2. These jumps nearly coincide with the starting field of the magnetization reversal at approximately þ0:12 and 0:12 kOe [Fig. 2(b)]. These jumps are derived from the domain wall motion within the nanowire. Furthermore, at point 4 [Fig. 4(e)], a weak plateau area of the phase is observed between 0:12 and 0:20 kOe. This plateau originates from domain wall pinning within the nanowire. This plateau area slightly differs with the switching field area in the MOKE loop of the nanowire [Fig. 2(b)]. The reason for this is that the distribution of the switching field in some ten thousands of nanowires is included in the MOKE loop of the nanowire. These results indicate that the change in the domain wall within the nanowire can be observed in detail using MFS-MFM. On the basis of these results, it can be seen that the domain wall motion along with domain wall pinning dominate the magnetization reversal process of a 30-nm-thick Ni–Fe nanowire. For the 50-nm-thick Ni–Fe nanowire, points 1, 3, and 5 [Figs. 5(b), 5(d), and 5(f)] each display a weak hysteresis loop of the phase, which is attributed to the magnetization reversal at the edge of the nanowire and around the center of the nanowire. At point 2 [Fig. 5(c)], positive and negative plateau areas are observed between 0:01 and þ0:17 kOe and between þ0:02 and 0:19 kOe, respectively. At point 4 [Fig. 5(e)], positive and negative plateau areas are observed between 0:10 and 0:18 kOe and between þ0:08 and þ0:18 kOe, respectively, which are similar to those of point 2. These plateau areas are derived from not only the domain wall motion but also the domain wall pinning within the nanowire; the nucleation and annihilation of the Bloch wall within the nanowire occurs. These areas are different from the switching areas at approximately þ0:10–þ0:30 kOe and 0:10–0:30 kOe [Fig. 2(c)]. These differences are due to the fact that the MOKE loop exhibits the average of the loops in some ten thousands of nanowires. These results mean that the detailed magnetization reversal process within the nanowire can be observed using MFS-MFM. On the basis of these results, it can be seen that domain wall motion and domain wall pinning dominate the magnetization reversal process of a 50-nm-thick Ni–Fe nanowire, and that this behavior is the same as that in a 30-nm-thick Ni–Fe nanowire. Accordingly, the magnetization reversal process changes can be understood from the domain wall motion to both the domain wall motion and the domain wall pinning as the thickness of the Ni–Fe nanowire increases.
In summary, we found that the domain wall motion within the nanowire mainly dominates in the magnetization reversal process of a 10-nm-thick Ni–Fe nanowire, but in addition to the domain wall motion, domain wall pinning within the nanowire plays an important role in the magnetization reversal process of both 30- and 50-nm-thick Ni–Fe nanowires. Hence, it is concluded that a detailed magnetization reversal process at various points within a Ni–Fe nanowire can be directly observed using MFS-MFM. This work was partly supported by a Grant-in-Aid for Scientific Research (S), Exploratory Research and Encouragement of Young Scientists (B) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. This study was supported by Priority Assistance for the Formation of Worldwide Renowned Centers of Research—The Global COE Program (Project: Center of Excellence for Advanced Structural and Functional Materials Design) from the MEXT.
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