Research and Education Center for Materials Science is one of the three centers established in NAIST. It includes two laboratories, which are managed in close connection with Graduate School of Materials Science, NAIST. One group specializes in organic chemistry and the other does in the structure science of inorganic materials like semiconductors, metals and magnetic materials. The two groups accept graduate students from Graduate School of Materials Science. This is the home page of the structure-science group.


♦ MEMBERS
Hiroo HASHIZUME Professor hhashizu@ms.naist.jp
Nobuyoshi HOSOITO Associate Professor hosoito@ms.naist.jp
Motoo Komagata Technician komagata@ms.naist.jp
Kotaro ISHIJI Postdoctoral scientist i-kotaro@ms.naist.jp
Yuichi HAYASAKI Doctoral student h-yuuiti@ms.naist.jp
Tatsuo FUKANO Doctoral student e0987@mosk.tytlabs.co.jp
Ryoma KOKUFU Master student k-ryoma@ms.naist.jp
kenji.KODAMA Master student k-kenji@ms.naist.jp
Takahito MASUI Master student ma-taka@ms.naist.jp
Hironori YAMASAKI Master student y-hirono@ms.naist.jp

Group picture

♦ CURRENT RESEARCH TOPICS
  • Thin films, multilayers and superlattices
  • Surface and interface structures
  • X-ray magnetic scattering
  • Magnetism-structure correlations in thin films
  • Interlayer magnetic interactions and giant magnetoresistance
  • X-ray scattering from rough interfaces
  • Lattice distortions in superlattices
  • Surface crystallography
  • X-ray standing waves
  • Synchrotron X-ray scattering and spectroscopy
♦ ABOUT THE GROUP

         Professor H. Hashizume has been exploring new possibilities of synchrotron X-rays in structure science since 1977. He has developed techniques for X-ray position detectors, time-resolved X-ray diffraction, harmonics-rejection X-ray monochromators, grazing-angle X-ray standing-wave methods, surface X-ray diffraction in ultrahigh vacuum environments, and resonant X-ray magnetic scattering using circularly polarized probing beams. He also contributed to the design, construction, and commissioning of beamline facilities at domestic and overseas synchrotron sources, including Photon Factory, SPring-8 (Japan) and Pohang Light Source (Korea). Important technical contributions have also been made to Polarization station (4-ID-D) at Advanced Photon Source, Argonne National Laboratory (USA). Many of these were successfully operated and have been widely used to determine new structures in various bulk, thin-film and surface materials, which pioneered new research frontiers.
         The group at NAIST was formed in 1999 when H. H. arrived in the position. In the same year, K. Ishiji arrived in the laboratory as the first graduate student, who was followed by others in the succeeding years. In February 2001, the laboratory moved to a new building. Dr. N. Hosoito joined the group in April 2002.
         The research effort by the group is currently focused on the growth and the structure science of magnetic thin films at atomic and electron levels. Though many researches being promoted are fundamental in nature, we are keen to the development of new materials and applications in spin-electronics, nano-technologies, magnetic sensors, and high-density recording devices. Our molecular-beam-epitaxy and sputtering facilities produce high-quality metal multilayers. Sputter-grown Co/Cu samples show clear coupling oscillations with a giant magnetoresistance (GMR) of 20% in the MR (magnetoresistance) ratio on the second peak at room temperature. We characterize and evaluate the structural and magnetic properties of the samples using laboratory facilities, including X-ray reflectometers/diffractometers, scanning-probe AFM/MFM (atomic-force/magnetic-force scanning microscopes), high-resolution transmission electron microscopes, and low-temperature magnetometer-magnetoresistance measurement facilities. Selected samples are brought to third-generation synchrotron sources to investigate magnetic states and structures using resonant X-ray spectroscopy and scattering techniques. The resonance techniques measure X-ray absorptions and scatterings associated with specific electron transitions in magnetic atoms at absorption edges, whereby the extremely bright, energy tunable, highly polarized radiation from undulator sources plays essential roles.
         We are currently interested in the magnetic polarizations of non-magnetic metals in the vicinity of ferromagnetic layers, as well as in the structures of magnetic interfaces. We have observed significant XMCD (X-ray magnetic circular dichroism) and RXMS (resonant X-ray magnetic scattering) signals from magnetic moments induced on Cu 4p electrons in Cu layers sandwiched between Co layers. The signals are as small as 10-4 in flipping ratio, but the use of the rock-in techniques, both analog and digital, and high-count-rate diode X-ray detectors as well has allowed us to observe clear signals from very small magnetic moments induced on 'nonmagnetic' Cu. XMCD signals from a series of Co/Cu samples with varying Cu thicknesses show anomalies at the coupling-oscillation peaks, providing a clue to reveal the electron mechanism of the indirect exchange coupling. RXMS data from similar exchanged-coupled Co/Cu samples evidence oscillatory polarization profiles in the nanometer-thick Cu layers along the out-of-plane direction (J. Phys. Condens. Matter 16, 1915 (2004)). Models based on the RKKY theory adapted to planar geometries provide tentative explanations of the data. Further studies are expected to explore the role of magnetic/non-magnetic interface structures in the indirect exchange coupling. We have already evidenced distinct magnetic and chemical structures at the Fe/Gd interface using the magnetic X-ray diffuse scattering technique (Phys. Rev. B 60, 12234 (1999)). Probing the magnetic moments induced on 'non-magnetic' metals can hardly be feasible with neutrons.
         Spectroscopic and scattering studies using hard X-rays are being paralleled by those using soft X-rays to explore the magnetic properties of localized electrons in 3d and 4f transition metals. The resonance techniques are useful as well to look into the magnetic states of tunnel junctions and nano-magnetic structures, which are key elements in spin-electronics applications. Surface magnetism and strongly correlated systems are also within our future research scope.
         Active@collaborations with domestic and international groups feature the research activity of the group. This partly originates from the interdisciplinary and frontier nature of our research. Joint projects were, and are being, promoted with groups in Kyoto University, Tokyo Institute of Technology, Tohoku University, Chiba University, Chang Won National University (Korea), Argonne National Laboratory (USA), and University of Munich (Germany).

♦ A FEW RECENT RESULTS AND DEVELOPMENTS
1) Magnetic structures of Fe/Gd multilayers

The magnetic interaction between Fe and Gd is antiferromagnetic. Bulk Gd, having a Curie temperature Tc close to room temperature, develops magnetic moments much faster than Fe when cooled. An Fe/Gd multilayer with appropriate Fe and Gd thicknesses, placed in a weak in-plane field H, are in the Fe-aligned state at temperatures T close to Tc, with Fe moments aligned along the external field. As the temperature goes down, magnetic moments grow on Gd and the balance between the exchange energy and the Zeeman energy dictates the system to transform to the twisted state, in which neither Fe nor Gd moments are in line with the applied field. The transition from the Fe-aligned to twisted states defines a boundary in the
T-H phase diagram. At very low temperatures, the large Gd moments dictate the multilayer to be Gd-aligned, with the Gd and Fe moments aligned parallel and antiparallel to the applied field, respectively. The figure below shows how the Gd magnetic structure changes with temperature in a
15[Fe(3.5)/Gd(4.8 nm)] multilayer. Arrows show the sizes and orientations of local Gd magnetic moments across a 4.8 nm-thick Gd layer. The external in-plane field is directed from right to left in this panel. It is seen that large moments are induced on Gd at the interface even at 300 K, where Gd is paramagnetic, and that the magnitude of the induced Gd moment does not virtually depend on temperature, while spontaneous magnetizations grow with decreasing temperature at the core of the Gd layer. The resonant X-ray magnetic scattering at the Gd L edge has allowed the Gd structure to be selectively probed in the presence of the Fe magnetizations. Further details are given in Phys. Rev. B 60, 9596 (1999) and Jpn. J. Appl. Phys. 41, 1331 (2002). 11111111111111111111111111111111111111111111111111111111111111111111111111

Magnetic structures of 4.8-nm-thick Gd layers sandwiched between Fe layers at a low applied field.
Click here for further information.

2) Magnetic and chemical interfaces in Fe/Gd multilayers

Electron scattering at layer interfaces plays essential roles in the magneto-transport in thin-film magnetism. X-ray diffuse scattering techniques are unique in the ability to probe interface structures buried under the surface. Using this feature in resonant magnetic scattering experiments, one can separately investigate the chemical and magnetic interfaces using a circularly polarized probing X-ray beam of alternating helicities, + and -. Sum intensity I(+)+I(-),due to the charge scattering, provides
information on the chemical structure, whereas difference intensity I(+)-I(-) as a function of scattering vector q carries magnetic information. The figure shows the sum (a) and difference (b) diffuse

profiles observed in the transverse scans across the first (qz=1.47 nm-1) and the second (qz=2.15 nm-1) superlattice Bragg peaks of the same Fe/Gd multilayer as studied in the preceding section at the Gd L edge. The weaker qx dependences of the difference profiles than the sum profiles indicate that the magnetic interface is smoother than the chemical interface in this sample. A fit to a Born-approximation scattering theory give 23 and 142 nm for the in-plane correlation lengths of the random roughness at the chemical and magnetic interfaces, respectively. See Phys. Rev. B 60, 12234 (1999) for further details.

3) XMCD anomalies at the coupling-oscillation peak in Co/Cu multilayers

For exchanged-coupled Co/Cu multilayers, the RKKY (Ruderman-Kittel-Kasuya-Yosida) theory tells that an oscillatory magnetic polarization is induced on nearly free electrons in the Cu layer through magnetic interactions with the ferromagnetic Co layers at the interface. The polarization propagates across the Cu layer and interacts with another Co layers, thereby giving rise to the magnetic coupling between the Co layers. These electrons are confined in quantum-well (QW) states, which were proven to be spin polarized in photoemission measurements. Another evidence for the spin-polarized Cu spacer was provided by X-ray magnetic circular dichroism (XMCD) measurements. We have collected XMCD data from
six sputter-grown 50[Co(1.2 nm)/Cu(tCu)] multilayers, with tCu ranging from 1.4 to 2.3 nm, at the Co K and the Cu K absorption edges. The samples span the second peak of the coupling oscillation with
a peak MR ratio reaching 20% at tCu=1.8 nm (left figure). In zero applied fields, samples Co/Cu3, Co/Cu4 and Co/Cu5 are in the antiferromagnetic coupling regime, while Co/Cu1 and Co/Cu6 are in the ferromagnetic coupling regime. The Co and Cu dichroism spectra showed very similar profiles in



each sample, suggesting that the Cu 4p electrons are spin-polarized through hybridization with the magnetic Co 4p electrons at the interface. We found that the Cu dichroism, about one-fifth as large as the Co dichroism, showing a peak of 0.04% in /jump slightly above the absorption edge, scales very well to the Co dichroism at arbitrary in-plane field strengths applied on the samples. The use of the analog lock-in technique allowed us to measure Cu XMCD signals as small as 0.01% with a high signal-to-noise ratio. In the right figure, the ratios of (/jump)~tCu for Cu to /jump for Co are plotted versus tCu for the six samples, where a negative anomaly is seen in the AFC (antiferromagnetically coupled) samples. (/jump)~tCu represents an integrated magnetization, namely the moment of a Cu layer, while (Co XMCD) scales to the mean Co magnetization. The broken line in the right figure shows an integral of P(z)+P(tCu-z) over z, where P(z) is the polarization profile in the Cu layers calculated from the RKKY theory of exchange coupling adapted to a planar geometry. A more complete description of the work will be published shortly. Click here for some further information.

4) Development of a digital lock-in system for measurement of resonant magnetic scatterings using high-count-rate diode X-ray detectors at synchrotron sources

Resonant X-ray magnetic scattering (RXMS) using circularly polarized X-rays is a powerful probe to explore partial magnetic structures of compound materials, alloys, and nanostructures. RXMS has the element and electron-shell specificity the same as X-ray magnetic circular dichroism (XMCD). At third-generation synchrotron sources, scattering intensities I(+) and I(-) observed from metal multilayers for the + and - helicities of the probing X-rays are highly intense, often exceeding 108 cps, which cannot be coped with by conventional NaI(Tl) counter. The weakest measurable RXMS, which scales to I(+)-I(-), is practically limited by the high count-rate capability of a detector. Our experience shows that with a standard NaI counter, this limit is located not far from 1~10-3 in flipping (or asymmetry) ratio
[I(+)-I(-)]/[I(+)+I(-)]. This is just enough to explore the magnetism of 3d transition metals at the K absorption edges. A faster X-ray detector is required to probe weaker magnetism of enonmagneticf metals. We commissioned an APD (avalanche photodiode) detector to measure weak RXMSs from induced magnetic polarizations in Cu layers in exchange-coupled Co/Cu multilayers. The detector can count X-rays at rates of 107 photons per second, giving good estimates of the RXMSs at superlattice Bragg peaks
in a reasonably short time when count losses due to the time structure of synchrotron X-rays
are corrected for. Data are collected in a dual-channel gated fast counter SR400 in synchronism with the periodic rotary oscillation (frequency f) of a diamond phase shifter which generates a circularly polarized X-rays beam of alternating helicities (left figure). The count time in SR400 is defined by Gate open in the right figure. Typically, we use 10 and 390 ms for Gate delay and Gate open, respectively, for f=1 Hz. This digital lock-in system has allowed us to measure RXMSs as small as 1~10-4 in flipping ratio from a Co/Cu multilayer at the Cu K edge. The system is thoroughly described in J. Phys. Condens. Matter 16, 1915 (2004). Also see here. 11111111111111111111111111111111111111111111111111111111111



5) Resonant X-ray magnetic scattering from induced Cu polarizations in exchange-coupled Co/Cu multilayers
The left figure below shows RXMS (resonant X-ray magnetic scattering) superlattice Bragg peaks observed from a sputter-grown 50[Co(1.2)/Cu(3.9 nm)] multilayer at the K absorption edge of Cu. These peaks originate in the interference of resonant magnetic scattering from Cu and the charge scatterings from Co and Cu. The pure magnetic scattering from Co makes no significant contribution. Red line shows a fit to the data calculated from the oscillatory model magnetization profile in the Cu layers derived from the RKKY (Ruderman-Kittel-Kasuya-Yosida) theory of exchange coupling adapted to a planar geometry (right figure). Ideally smooth Co/Cu interfaces were assumed. The Cu polarization function P(z)+P(tCu-z) is truncated near the interfaces since the asymptotic form of P(z) diverges at z=0. The profile is featured by large positive polarizations at the interfaces, accompanied by comparably large negative polarizations inside. The smaller positive and negative peaks seen further inside demonstrate the oscillatory nature of P(z). The Cu polarization patterns emanating from the two interfaces interfere constructively in this sample where tCu=3.9 nm. Read a more complete story in J. Phys. Condens. Matter 16, 1915 (2004). Also see here.



Short stories on other research topics
         Interface structures in Si/SiGe superlattices grown on vicinal Si(111) substrates (Japanese)
         Three dimensional strain relaxation in the As/Si(001)-(2~1) surface (Japanese )
         Structure determination of submonatomic Sn layers in Fe/Cr(Sn)Cr magnetic multilayers (English, pdf)
         Resonant X-ray magnetic scattering and its application to thin-film magnetism   Part1   Part2     (English, PowerPoint)
         Development of new X-ray magnetic scattering techniques using third-generation synchrotron sources (Japanese, pdf)
Recent publications (English, Japanese)

♦ EQUIPMENTS
Molecular-beam epitaxy facility, Sputtering facility, Scanning-probe microscope (AFM, MFM), Vibrating-sample magnetometer/magnetoresistance measurement facility, X-ray reflectometer, Beryllium-window cryostat with superconducting magnet, Unltrahigh vacuum X-ray scattering chamber , X-ray standing-wave facility, Diamond X-ray phase shifter, CCD X-ray image detector, Position-sensitive X-ray detectors (linear, curved, area), Four-circle goniometer, Rotating-anode X-ray generators, Diamond-blade crystal slicing machine

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