Recent studies on the Rashba effect on surfaces are reviewed. The Rashba effect refers to the k-dependent spin splitting of valence bands due to spin-orbit coupling in two-dimensional systems under out-of-plane electric field. After the physical mechanism of the Rashba effect is briefed, experimental and theoretical studies since the surface Rashba effect was first demonstrated for Au(111) in 1996 are surveyed with an emphasis placed on the microscopic origin of the giant Rashba spin splitting on surfaces covered with monolayer films of heavier elements. Most recently, giant Rashba spin splitting was realized on the surface of semiconductors, which serves a possibility of spintronic application of the surface Rashba effect.
Precise characterization of physical properties in nanometer-scale materials is interesting not only in terms of low-dimensional physics but also in application to devices. Due to the reduced dimensionality and symmetry, these systems possess various interesting properties that cannot be found in the bulk. In this article, focusing on epitaxial ultrathin bismuth films formed on a silicon substrate, we introduce an intriguing interplay of the quantum size and relativistic effects in reciprocal space. Utilizing spin- and angle-resolved photoemission spectroscopy, we observed clear Rashba-split nature of the surface-state bands in these Bi films which is a relativistic effect. However, the band dispersion did not follow the simple Rashba picture and the spin-splitting was lost where they overlapped with the bulk projection. From first-principles calculations, this is explained as a change in the nature of the band- splitting into an even-odd splitting induced by the quantum size effect.
We report magnetic circular dichroism (MCD) on ultrathin films using lasers as excitation sources. By using threshold photoemission, we can obtain high MCD asymmetry. On the other hand, when the laser photon energy is far away from the threshold, the MCD asymmetry is small. The MCD asymmetry depends also on the magnetization, and MCD is larger for the perpendicularly magnetized films. The reason for the enhanced MCD asymmetry near the threshold is spontaneous momentum and energy selection in the photoelectron excitation. The MCD dependence on the magnetization direction is explained by the polarization change of reflected light, which is negligible for x-ray MCD. Also shown is the photoemission electron microscope observation of magnetic domains using lasers.
We investigate the magnetic structures of one-dimensional Fe nanowires grown on a Au(788) surface by x-ray magnetic circular dichroism. The long-range ferromagnetic order breaks into spin-blocks at the elevated temperature due to enhanced fluctuation in the 1D system. The average block size rapidly decreases as the temperature increases, following the Arrhenius law ruled by exchange energy and the number of antiparallel spin pairs at the boundaries. We also found a gradual change in magnetization processes depending on the coverage: from site-by-site motion of spin-block boundaries toward pinning of the boundaries at the narrow positions of the wires resulting in flips of giant spins inside the blocks.
A method for measuring the surface spin polarization using a spin-polarized metastable helium atom (He*) beam under high external magnetic fields has been developed. Recently, we have combined a spin-polarized He* beam generated using a hexapole magnet with a 5 T superconducting magnet. The apparatus has been used for studying the spin polarization of an Fe/MgO(100) surface oxidized at room temperature. A high negative spin polarization has been found at around the Fermi level when the surface was exposed to an excess amount of O2 gas (>100 L) at room temperature. This may be understood if assuming the formation of Fe3O4 on the surface.
Present status of the magnetic imaging technique using a combination of the X-ray microbeam with the X-ray magnetic circular dichroism is presented together with future prospects such as the development of a three-dimensional magnetic analysis. After the introduction to the X-ray microfucusing optics, the magnetic imaging of a wedge-shaped Fe/Ni/Cu(100) film is shown as an example for the microbeam XMCD technique. A possibility for a combination of the X-ray microbieam with the depth-resolved XMCD tequnique is briefly mentioned.
Perovskite-type manganites have various electronic and magnetic properties by changing their compositions and temperature. It is known that an La0.6Sr0.4MnO3 thin film has a half metallic property, which is expected for the application to magnetic tunneling junctions. To obtain a yield rate of a TMR device accompanied with high-performance of a TMR effect, a single magnetic domain should be made in a La1-xSrxMnO3 thin film. Photoelectron emission microscopy (PEEM) is a powerful tool to directly perform spatial mapping of magnetic domains, combining with using synchrotron radiation beam. We can obtain useful images in terms of a element-selective electronic structure and/or a magnetism. We have performed the direct observation of magnetic domain structures on the microstructures of the La0.6Sr0.4MnO3 thin films using photoelectron emission microscope with synchrotron radxation in order to clarify their magnetic domain formations. The magnetic domain formation depends on a subtle balance between the step-induced uniaxial anisotropy and the shape anisotropy. We reveal that a cooperative effect between the step-induced uniaxial and shape anisotropies makes a single magnetic domain in a patterned LSMO thin film on the micron scale. On the contrary, the competitive effect generates a multi-magnetic domain. This findings provides useful information on the improvement of TMR devices in La1-xSrxMnO3 thin films epitaxialy fabricated on a substrate with step structures.
Spin-split energy band structure appears in the surface state due to a broken space inversion symmetry at the surface boundary as well as a strong spin-orbit coupling. Spin- and angle-resolved photoemission spectroscopy (spin-resolved ARPES) has been applied to the study of the spin-split energy band structures of non-magnetic solid surfaces. First of all, some explanation of the spin-splitting mechanism for free electrons, so-called Rashba effect, will be given. Next, our spin-resolved ARPES system equipped with the compact Mott-type spin polarimeter will be introduced. We will show how the surface Rashba effect is observed from the spin-resolved ARPES with the recent result for the surface states of Bi(111). Here, we have observed strong and anti-symmetric spin polarizations with respect to the center of surface Brillouin zone in the surface states of Bi(111).
In order to minimize contributions to global warming, it is important to develop a Perfluorocompounds (PFCs) abatement system that can remove PFCs effectively with low electric power. We have developed a new PFCs abatement system consisting mainly of a 2 MHz ICP plasma source and two CaO columns operated at low pressure. The CaO agent developed for this system has a specific surface area of 60-80 m2/g and a grain diameter of 2-6 mm, simultaneously. Reactive fluorinated compounds are immobilized in the CaO columns without a water scrubber to prevent the recombination of fluorocarbons. Stable compounds such as CF4 are decomposed by the 2 MHz ICP plasma before introducing in the second CaO column. When the emissions from a fluorocarbon film chemical vapor deposition process chamber were treated by this abatement system, PFCs removal efficiency and CO2 equivalent removal efficiency was 99.96% and 93.5%, respectively.