A new spin-and angle-resolved photoemission spectrometer was developed adopting the very-low-energy-electron-diffraction (VLEED) type spin polarimeter. The Fe(001)p(1×1)-O film grown on MgO(001) crystal for the VLEED target yields significantly high spin-resolving power, the effective Sherman function of 0.40±0.02, with long lifetime and stability compared with the conventional Fe(001) target. Under the favor of high resolving power, approximately 100 times higher efficiency than that of conventional Mott type spin polarimeter, the figure of merit of 1.9±0.2×10-2 was achieved. Owing to this high efficiency, quite high energy-resolution (ΔE∼30 meV) and high angular resolution (Δθ∼±0.7o) can be realized with this new spin-polarized photoemission spectrometer. The simplified ways of preparation and revitalization of the target make the VLEED spin polarimeter much more convenient and feasible for the spin polarized photoemission spectroscopy. This technique will open new insight in the magnetism of surfaces and nano-structures and their applications for the spintronics in the future.
The Kondo effect observed for surface magnetic impurities is successfully controlled using magnetic metal multilayers. Low-temperature scanning tunneling spectroscopy of Co adatoms on Cu/Co/Cu(100) multilayers reveals the Kondo resonance, whose analysis based on the Fano line shape gives the Kondo temperatures TK. TK varies between 60 K and 157 K as the Cu overlayer thickness changes from 20 to 5 atomic layers. This is attributed to the modulation of the density of states at the Fermi level due to formation of quantum well states in the Cu overlayer.
We discuss the detection of a single spin by monitoring the tunneling current of the scanning tunneling microscope in the presence of a magnetic field. We have measured high frequency signals in the tunneling current of scanning tunneling microscopy for a submonolayer oxide thin film on the Si(111)-7×7 surface. We demonstrate that the signal is related to the Larmor precession of the electron spin associated with a dangling bond. The detected precession frequency possesses a broad distribution (line-width is comparable with that observed by conventional electron spin resonance) and a split near the maxima, both of which are attributed to the inhomogeneity of the g factor of a single spin. With site-specific measurement, we found that the spin signal was detected on the bright Si adatom in which oxygen atoms occupy the backbonds and weakened the metallic nature of the Si(111)-7×7 surface. The measured Larmor frequency corresponded to g∼2.00. On the other hand, no peak was detected on the dark adatom that is tied with an oxygen atom at the on-top site. This is due to the disappearance of the Si dangling bond through the chemical bond formation with the oxygen atom.
We report on laser photoemission electron microscopy (PEEM) using magnetic circular dichroism (MCD). Although MCD asymmetry in valence band is generally very small, it can be enhanced using threshold photoemission. We give experimental results for Ni/Cu(001) to show enhanced MCD effect near the threshold. Possible reasons for the mechanism of the enhancement are its energy and angle limited measurement of a specific electron energy state in the threshold photoemission, which can be achieved without an energy analyzer. As an application of the enlarged MCD using laser, we show magnetic domain imaging using PEEM. Due to its high brilliance, the laser MCD PEEM can measure the magnetic domain in a real time, but its explosive photoemission in a short period would deteriorate the special resolution. Finally we show our effort to make time resolved MCD-PEEM experiment in sub ps time resolution, whose time resolution would be much better than that of the state-of-art experiment using synchrotron with MCD-PEEM.
Molecular adsorptions affect on the magnetism of magnetic thin films. We have observed anomalous magnetic states in gas adsorbed Fe thin films. In this report, we present the CO or NO adsorption effects on the magnetism of 2-4 ML Fe thin films on Cu(001), which are typical magnetic thin films. The Fe(2 ML)/Cu(001) film does not change its magnetic structure upon CO adsorption, whereas the CO/Fe(3 and 4 ML)/Cu(001) films turned their easy axes of magnetization to in plane. Depth resolved XMCD measurements give us detailed information on the depth profile of the magnetic structures. The surface two layers of the CO/Fe(4 ML)/Cu(001) film lose the magnetization, and only the bottom two layers keep the magnetization. NO adsorbed Fe films have the in-plane easy axes of magnetization. Moreover, the magnetization of the topmost layer of the NO/Fe/Cu(001) films aligns in the opposite direction to the other layers. Thus, the antiferromagnetism in the surface layers has appeared in the NO/Fe/Cu(001) films.
We applied in-situ electrochemical scanning tunneling microscopy (in-situ EC-STM) to observe phospholipid layers over thiol-modified gold substrates as a model biological cell membrane. Our model cell membrane was 1,2-dihexanoyl-sn-glycero-3-phosphocholine, introduced on a monolayer of 1-octanethiol modified Au(111), in a neutral 0.05 M NH4ClO4 buffer solution. The lipid molecules formed a fluidic layer at the open-circuit potential. By cycling the electrode potential, the lipid layer reversibly changed over between the fluidic phase and a striped/grainy structure. This structural change might involve partial decomposition and oligomerization of phospholipids. This method will contribute for molecular biology by revealing the nanometer-scale structure of cell membrane.
We have developed a method to detect specific proteins with a high sensitivity using a gel electrophoresis method and force measurement of atomic force microscopy (AFM). Biotinylated proteins were separated by electrophoresis and fixed with cross-linking chemicals on the gel, followed by direct force measurement between the biotinylated proteins on the gel and a streptavidin modified tip of an AFM cantilever. We were able to achieve a high enough sensitivity to detect the picogram order of the biotinylated proteins by evaluating the frequency of the interaction force larger than 100pN in the force profile, which corresponds to the rupture force of interaction between streptavidin and biotin.