Yasumasa Takagi

Development of a scanning tunneling microscope for in situ experiments with a synchrotron radiation hard-X-ray microbeam.

A scanning tunneling microscope dedicated to in situ experiments under the irradiation of highly brilliant hard-X-rays of synchrotron radiation has been developed. In situ scanning tunneling microscopy (STM) observation was enabled by developing an accurate alignment system in ultrahigh vacuum. Despite the noisy conditions of the synchrotron radiation facility and the radiation load around the probe tip, STM images were successfully obtained at atomic resolution. Tip-current spectra were obtained for Ge nano-islands on a clean Si(111) surface by changing the incident photon energy across the Ge absorption edge. A current modification was detected at the absorption edge with a spatial resolution of the order of 10 nm.

Perpendicular magnetic anisotropy at the interface between ultrathin Fe film and MgO studied by angular-dependent x-ray magnetic circular dichroism

Interface perpendicular magnetic anisotropy (PMA) in ultrathin Fe/MgO (001) has been investigated using angular-dependent x-ray magnetic circular dichroism (XMCD). We found that anisotropic orbital magnetic moments deduced from the analysis of XMCD contribute to the large PMA energies, whose values depend on the annealing temperature. The large PMA energies determined from magnetization measurements are related to those estimated from the XMCD and the anisotropic orbital magnetic moments through the spin-orbit interaction. The enhancement of anisotropic orbital magnetic moments can be explained mainly by the hybridization between the Fe 3dz2 and O 2pz states.

Local and Reversible Change of the Reconstruction on Ge(001) Surface between c(4×2) and p(2×2) by Scanning Tunneling Microscopy

The reconstruction on a Ge(001) surface is locally and reversibly changed between c (4×2) and p (2×2) by controlling the bias voltage of a scanning tunneling microscope (STM) at 80 K. It is c (4×2)...

In situ study of an oxidation reaction on a Pt/C electrode by ambient pressure hard X-ray photoelectron spectroscopy

We have constructed an ambient pressure X-ray photoelectron spectroscopy instrument that uses hard X-ray radiation at the high-performance undulator beamline BL36XU of SPring-8. The dependence of the Au 4f peak intensity from Au foil on the ambient N2 pressure was measured. At a photon energy of 7.94 keV, the Au 4f peak intensity maintained 40% at 3000 Pa compared with that at high vacuum. We designed a polymer electrolyte fuel cell that allows us to perform X-ray photoelectron spectroscopy measurements of an electrode under working conditions. The oxidized Pt peaks were observed in the Pt 3d5/2 level of Pt nanoparticles in the cathode, and the peaks clearly depended on the applied voltage between the anode and cathode. Our apparatus can be applied as a valuable in situ tool for the investigation of the electronic states and adsorbed species of polymer electrolyte fuel cell electrode catalysts under the reaction conditions.

Enhancements of Spin and Orbital Magnetic Moments of Submonolayer Co on Cu(001) Studied by X-ray Magnetic Circular Dichroism Using Superconducting Magnet and Liquid He Cryostat

Magnetic properties of 0.4 monolayer Co grown epitaxially on Cu(001) were investigated by X-ray magnetic circular dichroism (XMCD) using our newly constructed ultrahigh vacuum system equipped with a 7 T superconducting magnet and a liquid He cryostat. The angle-dependent XMCD spectra for the saturated magnetization recorded at 6.0 K allowed us to evaluate separately the spin and orbital magnetic moments along the surface parallel and normal directions. Enhancements of the magnetic moments compared with the corresponding bulk values were clearly elucidated: ~15% for the spin magnetic moment and ~96 and ~53% for the orbital magnetic moments along the surface parallel and normal directions, respectively.

Visualizing chemical states and defects induced magnetism of graphene oxide by spatially-resolved-X-ray microscopy and spectroscopy.

This investigation studies the various magnetic behaviors of graphene oxide (GO) and reduced graphene oxides (rGOs) and elucidates the relationship between the chemical states that involve defects therein and their magnetic behaviors in GO sheets. Magnetic hysteresis loop reveals that the GO is ferromagnetic whereas photo-thermal moderately reduced graphene oxide (M-rGO) and heavily reduced graphene oxide (H-rGO) gradually become paramagnetic behavior at room temperature. Scanning transmission X-ray microscopy and corresponding X-ray absorption near-edge structure spectroscopy were utilized to investigate thoroughly the variation of the C 2p(π*) states that are bound with oxygen-containing and hydroxyl groups, as well as the C 2p(σ*)-derived states in flat and wrinkle regions to clarify the relationship between the spatially-resolved chemical states and the magnetism of GO, M-rGO and H-rGO. The results of X-ray magnetic circular dichroism further support the finding that C 2p(σ*)-derived states are the main origin of the magnetism of GO. Based on experimental results and first-principles calculations, the variation in magnetic behavior from GO to M-rGO and to H-rGO is interpreted, and the origin of ferromagnetism is identified as the C 2p(σ*)-derived states that involve defects/vacancies rather than the C 2p(π*) states that are bound with oxygen-containing and hydroxyl groups on GO sheets.

Magnetic circular dichroism for surface and thin film magnetism: Measurement techniques and surface chemical applications

The technical development of the characterization of magnetic thin films is an urgent subject especially for further improvement of high-density and high-speed recording media. This article focuses attention on the fundamental methodology and recent exploitations of various magnetic circular dichroism (MCD) techniques. First, basic theories and experimental methods of the magneto-optical Kerr effect (MOKE) and X-ray magnetic circular dichroism (XMCD) are described. MOKE is a conventional but usually the most useful method to characterize macroscopic magnetization of metal thin films using visible lasers. Moreover, recent development of MOKE allows one to perform optical microscopic and ultrafast time resolved investigations. XMCD has now become a mature technique by virtue of the developments of soft and hard X-ray synchrotron radiation sources. Since XMCD is based on core absorption spectroscopy, the technique provides information on element specific magnetization. Using the so-called sum rules, one can determine the microscopic spin and orbital magnetic moments. The experimental method and an example using a superconducting magnet system combined with a liquid helium sample cryostat are described. Moreover, by combining XMCD with photoelectron emission microscopy (PEEM), one can perform spatiotemporal measurements, whose spatial resolution reaches several tens of nanometres. Magnetization induced second harmonic generation (MSHG) is also described. This is a unique technique for its inherently high sensitivity to surfaces and interfaces since MSHG is inhibited in the bulk of centrosymmetric crystals. The drastic polarization dependence of MSHG based on the selection rules is also discussed. As a last method addressed in this article, the threshold photoemission MCD technique is reviewed. The technique has recently been proposed and has demonstrated the possibility of an ultrafast spatiotemporal method by combining PEEM. Applications of these various MCD families to surface physical chemistry are described. Here, drastic spin reorientation transitions (SRT) of some metal thin films induced by adsorption of atoms and molecules are discussed from the macroscopic and microscopic points of view. Consequently, future aspects in the MCD techniques and surface and thin film magnetism are addressed.

Rewritable nanopattern on a Ge(001) surface utilizing p(2×2)-to-c(4×2) transition of surface reconstruction induced by a scanning tunneling microscope

We present rewritable, nanometer-scale patterns formed on Ge(001) at 80 K, which are based on the transition between c(4×2) and p(2×2) surface reconstructions induced by a scanning tunneling microscope (STM). We have found that a negative (−0.8 V and 0.5 s) sample bias voltage pulse creates a c(4×2)-reconstructed domain of ∼1.6×2.0 nm2 in a p(2×2)-reconstructed region. Applying the negative pulses at approriate positions, we form an intended pattern of the c(4×2) reconstruction. The course of patterning can be monitored by STM with a small bias voltage (−0.2 V) without affecting the written pattern. The whole region can be initialized to the p(2×2) by a scan with the bias voltage of +0.8 V.

Scanning Tunneling Microscopy Combined with Hard X-ray Microbeam of High Brilliance from Synchrotron Radiation Source

In situ scanning tunneling microscopy (STM) with highly brilliant hard X-ray irradiation was enabled at SPring-8. To obtain a good signal-to-noise ratio for elemental analysis, an X-ray beam with a limited size of 10 µm was aligned to a specially designed STM stage in ultrahigh vacuum. Despite various types of noises and a large radiation load around the STM probe, STM images were successfully observed with atomic resolution. The use of a new system for elemental analysis was also attempted, which was based on the modulation of tunneling signals rather than emitted electrons. Among tunneling signals, tunneling current was found to be better than tip height as a signal to be recorded, because the former reduces markedly the error of measurement. On a Ge nanoisland on a clean Si(111) surface, the modulation of tunneling current was achieved by changing the incident photon energy across the Ge absorption edge.

Nonlocal Manipulation of Dimer Motion at Ge(001) Clean Surface via Hot Carriers in Surface States

Nonlocal one-dimensional motions of a topological defect are induced by electron tunneling through the dangling-bond states on the clean Ge(001) surface using scanning tunneling microscopy below 80 K. The direction of the motion depends both on the energy of the carriers in the surface state and on the distance between the defect and the tunneling point. The results are interpreted using an electronic excitation model by hot carriers injected into the surface states. The critical distance of the motion is anisotropic and consistent with the band structure of the surface states.