APPLIED PHYSICS LETTERS
VOLUME 78, NUMBER 18
30 APRIL 2001
Vector magnetization imaging in ferromagnetic thin films using soft x-rays Sang-Koog Kima) and J. B. Kortrightb) Materials Sciences Division, Lawrence Berkeley National Laboratory, University of California, One Cyclotron Road, Berkeley, California 94720
Sung-Chul Shin Department of Physics and Center for Nanospinics of Spintronic Materials (CNSM), Korea Advanced Institute of Science and Technology (KAIST), Taejon 305-701, Korea
共Received 15 January 2001; accepted for publication 16 March 2001兲 A magnetization vector M imaging using a transmission x-ray microscope with magnetic circular dichroism 共MCD兲 contrast is demonstrated. MCD images through a semitransparent, 33-nm-thick Fe film are measured at the Fe L 3 edge along three different x-ray propagation directions to determine all three components of the M. The transmission images clearly display the vector nature of complex microstructures, associated with the M reversal between oppositely oriented 180° domains, including not only 90° domains, a Ne´el wall-like structure, and an extended ripple structure, but also a striking feature of localized magnetization spirals with perpendicular components at their cores. These studies have important implications for applications of this technique to better understand the expected features as well as details of domain-wall structures. © 2001 American Institute of Physics. 关DOI: 10.1063/1.1370120兴
The promise and reality of nanoscale magnetic devices continues to prompt the development of imaging techniques able to resolve magnetization structures at submicron resolution.1,2 The general goals in the field of magnetic microscopy have been to improve the resolution and sensitivity to the vector nature of magnetization.1–11 Since magnetization is a vector quantity, imaging its vector character at high resolution is especially important to understand and design properties associated with the spatial correlation of local magnetizations at various length scales.12,13 A variety of magnetic microscopies developed to date, including scanning electron microscopy with polarization analysis,3–5 spinpolarized low-energy electron microscopy,6 Lorenzian transmission electron microscopy,7 electron holography,8 and Kerr microscopy,9 allow investigation of local magnetization directions with their corresponding spatial resolutions, providing full or partial vector information of magnetic domains and their walls. In this letter, full vector imaging of the spatial variation of magnetization is demonstrated using magnetic circular dichroism 共MCD兲 contrast in the soft x-ray range. The use of soft x-rays to image magnetization used a photoemission microscope with MCD contrast.14 The element-specific nature of core-level MCD enables these techniques to image magnetization in individual layers having chemically distinct magnetic species to facilitate studies of coupled thin-film systems.15 In this work, we add the capability of magnetization vector M imaging, and sensitivity to the entire thickness of the thin films because of the transmission geometry used. Here, we image the magnetization structure near 180° domain walls in demagnetized Fe films and find a variety of complex structures, only some of which can be related to a兲
Current address: Department of Physics and CNSM, KAIST, Taejon 305701, Korea; electronic mail:
[email protected] b兲 Electronic mail:
[email protected]
known structures near hybrid domain walls. These initial observations of the vector nature of such fine features in extended films demonstrate the value of this technique to yield deeper insight into the magnetization structure near domain walls. Here, we image magnetization using a photon-based scanning transmission x-ray microscope 共STXM兲 operating at undulator beamline 7.0 at Lawrence Berkeley National Laboratory’s Advanced Light Source.16 The STXM uses a Fresnel zone-plate lens to focus x-rays on the sample with a beam spot size of less than 200 nm in this case. Images are obtained by scanning the sample through the focused spot. Magnetic contrast is obtained using transmission MCD contrast through samples,10,11 with circularly polarized radiation of opposite helicity obtained from an upstream magnetic film acting as a Faraday circular polarizing filter.17 Left- and right-handed elliptically polarized beams are obtained by reversing the filter magnetization 共saturated in plane兲 by mechanical rotation through 180° in this study. MCD contrast ⌬, defined as the difference in absorption coefficient of the left and right circular components, provides the projection of M along the x-ray propagation vector k.18 Thus, imaging the vector nature of M simply requires tilting the sample to measure three different projections of M. The magnetization images in Figs. 1共a兲 and 1共b兲 are taken at 45° from the grazing incidence along the x and y axes, respectively, while the MCD image in Fig. 1共c兲 is taken at normal incidence, and thus those images reveal magnetization contrast along those directions from a 33-nm-thick, demagnetized Fe film deposited on a semitransparent SiNx membrane. The image in Fig. 1共a兲 shows the triangular tip of a needle-shaped domain 共dark兲 with magnetization as noted that was growing into the surrounding 180° domain 共light兲 and shows two smaller needle-shaped domains extending well into the surrounding light region, and a variety of intermediate contrast features as well. The image in Fig. 1共b兲
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Appl. Phys. Lett., Vol. 78, No. 18, 30 April 2001
FIG. 1. Soft x-ray MCD images taken at three viewing angles from a 33nm-thick, demagnetized Fe film. Transmission images taken at the Fe L 3 edge with one helicity are divided by images taken with the opposite helicity to yield the differential MCD absorption. White and black arrows indicate approximate directions of local magnetization shown as dark and light contrast at each viewing angle. Images show the same area of the 60 ⫻60 m2 field. Each pixel is spaced 500 nm apart, while the spatial resolution of the scanning microscope set by the beam size is 200 nm. Gray scales do not represent a single absolute intensity in order to show fine contrast features at each viewing angle. An ac demagnetizing field was applied along the x axis.
shows very different contrast behavior, indicating significant magnetization components along the y axis as well, with clear spatial correlation with those shown in Fig. 1共a兲, whereas the image in Fig. 1共c兲 shows very weak contrast, as expected since dipole–dipole interactions yield predominantly in-plane magnetization for this thin film. To better understand the subtle complexity of the mag-
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netic microstructure, especially, in intermediate contrast regions, it is useful to transform the MCD images to obtain all three components of M⫽M x xˆ⫹M y yˆ⫹M z zˆ along the coordinate axes defined in Fig. 1. Each image shown in Fig. 1 represents a linear combination of M x , M y , and M z projected along each ki (i⫽1,2,3), i.e., ki •M, where ki ⫽k ix xˆ ⫹k iy yˆ⫹k iz zˆ. The direction cosine, cos j⫽M j /M, defines each component, where j⫽x, y, and z, and M ⫽ 冑M 2x ⫹M 2y ⫹M z2 . The MCD images shown in Fig. 1 are the spatial distribution of local ⌬ i ⫽C 兺 j⫽x,y,z k i j M j . The proportionality constant C is taken as 1 to obtain the relative magnitude of M over the scanned area, where the M of each pixel is its average if there is a finer variation of the M direction within a given 200 nm resolution through the entire film thickness. By using images taken with three different k, we obtain the spatial variation of both the direction and the magnitude of M. An apparent reduction in M may result from either actual reduction in local M, or its finer spatial variation within the volume element. The latter is a possible origin, in our case, of the reduction in M at the boundaries of opposite 180° domains since the exchange stiffness length for Fe is of the order of 20 nm. The vectorial analysis described above yields a better display of complex spin structures at or near intermediate contrast regions where a relatively significant variation of the M direction is expected. In Fig. 2共a兲 is shown the in-plane vector image displayed by colored arrows for each pixel
FIG. 2. 共Color兲 Images of the spatial variation of M obtained from the image data in Fig. 1. Images shown are resized to 50⫻35 m2. 共a兲 The in-plane component of M while 共b兲 shows the perpendicular components. Arrows and their colors in 共a兲 both represent local magnetization directions in the film plane as indicated by a color wheel of the inset while the gray bar in 共b兲 denotes angles out of plane with 90° corresponding to the in-plane magnetization. Red circled arrows denote leftand right-handed magnetization spirals. Reversed 180° domains, 90° domains, and Ne´el-type domain wall segments are indicated by arrows. Small blue arrows show a ripple structure extending far into the surrounding 180° domains. The observed out-of-plane components are largely determined from the images at 45° incidence in Figs. 1共a兲 and 1共b兲 that are equally sensitive to perpendicular and in-plane moments, not from the very weak contrast of the normal incidence image in Fig. 1共c兲.
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Appl. Phys. Lett., Vol. 78, No. 18, 30 April 2001
whose direction and colors both represent the in-plane direction of local M, while Fig. 2共b兲 shows the perpendicular component displayed by gray scales indicating angles out of plane, where 90° corresponds to the in-plane direction. The dominant feature in this region is a needle-shaped 180° domain growing into an oppositely oriented 180° domain, and these images reveal many subtle features whose vector nature is observed here by means of MCD microscopy. Also evident are 90° domains and Ne´el wall-like segments identified by their characteristic in-plane structure. The ripple structure extending far into the surrounding 180° domain is characterized by magnetization components canted at roughly 15°–45° from the dominant direction. Most interesting are the magnetization spirals 共indicated by red circular arrows兲 in which fully closed spirals are clearly resolved along with the out-of-plane components at their cores, as seen in Fig. 2共b兲. This rich variety of features, especially at or near the domain boundaries in the x – y plane, attests to a very complex set of nonlocal interactions and dynamics in the reversal of this sample. The out-of-plane components of M are extremely weak in most regions of the 180° domains, with notable features spatially isolated at well-defined portions of the domain wall and at the characteristic cores of the magnetization spirals, as seen in Fig. 2共b兲. The largest perpendicular components, where M is as much as 20° out of plane, are found only at each core of the magnetization spirals, with M less than 1° out of plane only several microns away. It was found that the core width of the magnetic vortices in laterally confined circular dots is several tens of nm order of magnitude,19,20 being much less than the current spatial resolution. Thus, we can expect larger perpendicular components strongly localized at the cores with a lateral resolution of better than the current resolution of 200 nm. The apparent reduction in M at the cores also supports this possibility. The magnetization spirals with their perpendicular components at the cores in the extended film studied here not only are apparently similar to magnetic vortices with Bloch cores recently observed in laterally confined systems such as patterned dots or bits,19–21 but are also partially similar to a cross-tie wall4,22 and an asymmetric Bloch wall23 in extended films. Both structures have Ne´el and Bloch wall character. Even though there is a partial similarity in magnetization spirals between extended and confined films, there are also subtle differences. Our observation of the magnetization spirals not only suggests a close relation between these structures in extended films and vortex structures in laterally confined films but also stimulates continuing studies of details of their similarity and difference. Improved spatial resolution anticipated in future STXMs with the vector imaging capability, element specificity, and both surface and bulk sensitivity will allow detailed studies and comparisons of quasistatic three-dimensional M structures involved in the reversal process for both types of structures under an applied magnetic field.24,25 In conclusion, this demonstration of imaging the spatial variation of the direction of the magnetization vector reveals both strong and subtle features over a broad range of length
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scales that are associated with the magnetization reversal process. By adding vector imaging to other attributes of transmission x-ray magneto-optical effects, this emerging technique of magnetic microscopy is expected to be of value in both fundamental and applied studies of magnetic structure and interactions in a variety of materials. The authors appreciate G. Meigs and T. Warwick for their valuable assistance in the experiments. The work at LBNL was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Science Division, of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. One of the authors 关S.-K.K. 共partially兲兴 and one of the authors 共S.-C.S.兲 were supported by the Korean Ministry of Science and Technology through the Creative Research Initiatives Project. A. Hubert and R. Scha¨fer, Magnetic Domains 共Springer, Berlin, 1998兲. E. D. Dahlberg and J.-G. Zhu, Phys. Today 48 , 34 共1995兲. 3 M. R. Scheinfein, J. Unguris, M. H. Kelley, D. T. Pierce, and R. J. Celotta, Rev. Sci. Instrum. 61, 2501 共1990兲. 4 Y. Lee, A. R. Koymen, and M. J. Haji-Sheikh, Appl. Phys. Lett. 72, 851 共1998兲. 5 M. R. Scheinfein, J. Unguris, R. J. Celotta, and D. T. Pierce, Phys. Rev. Lett. 63, 668 共1989兲, and references therein. 6 T. Duden and E. Bauer, Phys. Rev. B 59, 474 共1999兲. 7 A. C. Daykin, J. P. Jakubovics, and A. K. Petford-Long, J. Appl. Phys. 82, 2447 共1997兲. 8 M. R. McCartney and Y. Zhu, J. Appl. Phys. 83, 6414 共1998兲, and references therein. 9 R. Mattheis, D. Berkov, and N. Gorn, J. Magn. Magn. Mater. 198–199, 216 共1999兲. 10 J. B. Kortright, S.-K. Kim, H. Ohldag, G. Meigs, and A. Warwick, X-Ray Microscopy: Proceedings of the Sixth International Conference, edited by W. Meyer-Ilse, T. Warwick, and D. T. Attwood 共AIP, New York, 2000兲, p. 49. 11 P. Fischer, T. Eimu¨ller, G. Schu¨lz, P. Guttmann, G. Schmahl, K. Pruegl, and G. Bayreuther, J. Phys. D 31, 649 共1998兲. 12 J. Unguris, R. J. Celotta, and D. T. Pierce, Phys. Rev. Lett. 79, 2734 共1997兲. 13 E. Y. Vedmedenko, H. P. Oepen, A. Ghazali, J.-C. S. Le´vy, and J. Kirschner, Phys. Rev. Lett. 84, 5884 共2000兲. 14 J. Sto¨hr, Y. Wu, B. D. Hermsmeier, M. G. Samant, G. R. Harp, S. Koranda, D. Dunham, and B. P. Tonner, Science 259, 658 共1993兲. 15 F. Nolting, A. Scholl, J. Sto¨hr, J. W. Seo, J. Fompeyrine, H. Siegwart, J.-P. Loquet, S. Anders, J. Lning, E. E. Fullerton, M. F. Toney, M. R. Scheinfein, and H. A. Padmore, Nature 共London兲 405, 767 共2000兲. 16 T. Warwick, K. Franck, J. B. Kortright, G. Meigs, M. M. Moronne, S. Myneni, E. Rotenberg, S. Seal, W. Steele, H. Ade, A. L. Garcia, S. Cerasari, J. Denlinger, S. Hayakawa, A. P. Hitchcock, T. Tyliszczak, E. G. Rightor, H.-J. Shin, and B. P. Tonner, Rev. Sci. Instrum. 69, 2964 共1998兲. 17 J. B. Kortright, S.-K. Kim, T. Warwick, and N. V. Smith, Appl. Phys. Lett. 71, 1446 共1997兲. 18 V. Chakarian, Y. U. Idzerda, G. Meigs, E. E. Chaban, J.-H. Park, and C. T. Chen, Appl. Phys. Lett. 66, 3368 共1995兲. 19 T. Shinjo, T. Okuno, R. Hassdorf, K. Shigeto, and T. Ono, Science 289, 930 共2000兲. 20 J. Raabe, R. Pulwey, R. Sattler, T. Schweinbo¨ck, J. Zweck, and D. Weiss, J. Appl. Phys. 88, 4437 共2000兲. 21 J. Shi, S. Tehrani, T. Zhu, Y. F. Zheng, and J.-G. Zhu, Appl. Phys. Lett. 74, 2525 共1999兲. 22 M. Lo¨hndorf, A. Wadas, H. A. M. van den Berg, and R. Wiesendanger, Appl. Phys. Lett. 68, 3635 共1996兲. 23 I. L. Prejbeanu, L. D. Buda, U. Ebels, and K. Ounadjela, Appl. Phys. Lett. 77, 3066 共2000兲. 24 R. P. Cowburn, D. K. Koltsov, A. O. Adeyeye, M. E. Welland, and D. M. Tricker, Phys. Rev. Lett. 83, 1042 共1999兲. 25 T. Pokhil, D. Song, and J. Nowak, J. Appl. Phys. 87, 6319 共2000兲. 1 2
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