Y. Fukaya

Current-induced spin polarization on metal surfaces probed by spin-polarized positron beam

Current-induced spin polarization (CISP) on the outermost surfaces of Au, Cu, Pt, Pd, Ta, and W nanoscaled films were studied using a spin-polarized positron beam. The Au and Cu surfaces showed no significant CISP. In contrast, the Pt, Pd, Ta, and W films exhibited large CISP (3~15% per input charge current of 105 A/cm2) and the CISP of Ta and W were opposite to those of Pt and Pd. The sign of the CISP obeys the same rule in spin Hall effect suggesting that the spin-orbit coupling is mainly responsible for the CISP. The magnitude of the CISP is explained by the Rashba-Edelstein mechanism rather than the diffusive spin Hall effect. This settles a controversy, that which of these two mechanisms dominates the large CISP on metal surfaces.

New experiment stations at KEK Slow Positron Facility

Recent development of the Slow Positron Facility at the Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK) is reported. The facility, equipped with a dedicated 55 MeV linac, provides a high-intensity, pulsed slow-positron beam. The beam is produced in a production unit at a high tension of up to 35 kV and guided magnetically through a grounded beam line, and then branched using compact branching units in the experiment hall. An overview, some details of three experiments currently conducted and the outlook of the facility are described.

Reflection high-energy positron diffraction: the past 15 years and the future

Reflection high-energy positron diffraction (RHEPD) is the positron counterpart of reflection high-energy electron diffraction (RHEED). Owing to the positive charge of the positron, RHEPD provides a powerful tool with which to determine the structure of the first surface layer. We have been investigating important surface systems concerning their unique electric and magnetic properties and also phase transition phenomena using positron beams (flux: 103~104 e+/sec) with 22Na sources. Currently, we are developing a new RHEPD apparatus with a bright and intense positron beam (flux: 105 e+/sec) based on the LINAC at the Slow Positron Facility, KEK. Here, we summarize the past results and the future prospects of the RHEPD study in the surface science.

Spin polarizations of positron beams generated using electrostatic and magnetic transportation systems with 68Ge and 22Na sources

Spin polarizations of electrostatic positron beams generated using 68Ge and 22Na sources with tungsten moderators were 47 % and 30 %, respectively. A comparable spin polarization (27 %) was obtained with much reduced beam diameter (0.5 mm), when electromagnetic lenses, a 22Na source and a tungsten moderator were used. Replacing the tungsten moderator with a solid neon moderator in this system, the beam flux was significantly enhanced with maintaining the spin polarization. The Doppler broadening of annihilation radiation spectra of polycrystalline Fe measured using the above beams showed clear asymmetry upon field reversal.

Spin-polarized positron annihilation spectroscopy for spintronics applications

Spin-polarized positron annihilation spectroscopy will be useful in studying spintronics materials. Here, we summarize some fundamental aspects of this method for the future spintronics studies.

Diffraction: Determination of Atomic Structure

Abstract Knowledge of the atomic configuration of a material is important for understanding its properties. Among various experimental techniques, the diffraction method determines the atomic positions in material structures in a straightforward way through the interference between the waves that used as the probe. This chapter provides a basic overview of diffraction methods and their applications to determine the structures of surfaces and two-dimensional atomic layers, focusing on the use of electron and positron beams. We start with the well-known Bragg equation to interpret where diffraction spots appear in a pattern, and we then extend it to a more general case by employing 2D reciprocal-lattice rods, the Ewald sphere, and the Laue condition. We introduce atomic scattering factor, refraction effect, and penetration depth to understand how information is obtained from the spot intensities. We also discuss the differences in the diffraction intensities produced by electrons, positrons, and X-rays. We describe both kinematical and dynamical diffraction theories for calculating the diffraction intensities produced by X-rays, electrons, and positrons. After showing the surface sensitivity of positrons, we review some typical examples of structural analyses (Ag/Si(111), In/Si(111), graphene/Co(0001) and graphene/Cu(111), silicene/Ag(111), and germanene/Al(111)) obtained using positron diffraction. Finally, we briefly describe future prospects for surface structure analysis using diffraction methods.

Research progress at the Slow Positron Facility in the Institute of Materials Structure Science, KEK

Recent results at the Slow Positron Facility (SPF), Institute of Materials Structure Science (IMSS), KEK are reported. Studies using the total-reflection high-energy positron diffraction (TRHEPD) station revealed the structures of rutile-TiO2(110) (1×2), graphene on Cu (111) and Co (0001), and germanene on Al (111). First observations of the shape resonance in the Ps− photodetachment process were made using the positronium negative ion (Ps−) station. Experiments using the positronium time-of-flight (Ps-TOF) station showed significant enhancement of the Ps formation efficiency and the energy loss in the Ps formation-emission process. A pulse-stretching section has been implemented, which stretches the positron pulse width from 1.2 μs up to almost 20 ms.

A total reflection high-energy positron diffraction station at the KEK-SPF

A high-intensity mono-energetic positron beam generated by using a linear electron accelerator (linac) provides total reflection highenergy positron diffraction (TRHEPD) researches at Slow Positron Facility (SPF), KEK [1,2]. A pulsed 50-Hz electron beam generated with a dedicated linac (operated at 55 MeV, 0.6 kW) is injected on a Ta converter and causes fast positron-electron pair creation through bremsstrahlung. The positrons showering down on 25 μm-thick W foils, are moderated to thermal energy, and a fraction spontaneously comes out of the foils with an energy of 3 eV owing to the negative work function. The positron converter/moderator assembly is held at an electrostatic voltage of 15 kV for TRHEPD experiment. The emitted positrons are consequently accelerated to 15 keV as they enter the grounded beam-line and are guided by the magnetic field of about 0.015 T to the TRHEPD station. For diffraction experiments, positrons transported by the magnetic field have to be first released into a nonmagnetic region. Since the released positron beam has a large diameter, a brightness-enhancement unit is effectively used to achieve a small-diameter and highly-parallel beam with a sufficient flux. The linac-based positron beam gives about 60 times intensified diffraction pattern from a Si(111)-7x7 reconstructed surface compared to a previous result with a Na-22-based positron beam [3]. An improved signal-to-noise ratio in the obtained pattern due to the intensified beam allowed an observation of clear fractional-order spots in the higher Lauezones, which had not been observed previously. The much intensified beam with the present system allows adjustment of the sample orientation without accumulating the positron signals. With the brightness enhanced beam, several remarkable results have been obtained efficiently by users of this facility. (Everybody is invited to use KEK Slow Positron Facility through approval of his/her research proposal.)

Doppler-broadening of annihilation radiation spectroscopy under high magnetic field using a longitudinally spin-polarized slow positron beam

A spin-polarized positron beam generated using a 22Na source, solid neon moderator and magnetic lens was developed. The beam diameter, beam flux and spin polarization were 0.5 mm, 1×105 e s−1 and 27 %, respectively. An electromagnet system, which can generate ±1 T magnetic field, was also developed and equipped with the beam apparatus. Using this apparatus, Doppler broadening of annihilation radiation spectra under a high magnetic field were measured for bulk polycrystalline Fe, Co and Ni samples and a thin film Co2MnSi sample. These spectra showed a clear asymmetry upon field reversal.

Development of spin-polarized positron source using high energy proton beam

To obtain a highly spin polarized positron beam, 68Ge isotope have been produced using a nuclear reaction of 69Ga(p,2n)68Ge. As target materials, we examined a metal form 69Ga stable isotope and a GaN substrate. By 20 MeV proton irradiation, the production of 68Ge source was confirmed in both targets. The production rates of 68Ge were 0.16 and 0.53 MBq/μA/h for the metal Ga and GaN target, respectively. The spin polarizations of positrons emitted from 68Ge was estimated to be approximately 50 to 70%.