M. Maekawa

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.

Vacancy-induced ferromagnetism in ZnO probed by spin-polarized positron annihilation spectroscopy

We investigated the ferromagnetism of ZnO induced by oxygen implantation by using spin-polarized positron annihilation spectroscopy together with magnetization measurements. The magnetization measurements showed the appearance of ferromagnetism after oxygen implantation and its disappearance during post-implantation annealing at temperatures above 573 K. The Doppler broadening of annihilation radiation (DBAR) spectrum showed asymmetry upon field reversal after oxygen implantation. The obtained differential DBAR spectrum between positive and negative magnetic fields was well-explained with a theoretical calculation considering zinc vacancies. The disappearance of the field-reversal asymmetry of the DBAR spectrum as a result of annealing agreed with the observations of ferromagnetism by magnetization measurements. These results suggest the radiation-induced zinc vacancies to be the source of the observed ferromagnetism of ZnO.

Effect of magnetic field on positron lifetimes of Fe, Co and Ni

Positron lifetime spectra of Fe, Co and Ni were measured under magnetic field using a (22)Na source. Very small but distinguishable difference of positron lifetime upon magnetic field reversal was observed suggesting the existence of two bulk lifetimes associated with majority and minority spin electrons. Using two spin-dependent Fe bulk lifetimes, the difference Doppler broadening of annihilation radiation spectra between majority and minority spin electrons were also examined. Agreement between experiment and theory indicates that spin-polarized positron annihilation spectroscopy may have potential in investigation of spin-aligned electron momentum distribution.

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.

Free-volume structure of fluoropolymer-based radiation-grafted electrolyte membranes investigated by positron annihilation lifetime spectroscopy

In the field of polymer-electrolyte-membrane (PEM) fuel cell technology, the structures of free-volume holes in the PEMs are very important because they are correlated to the supplied-gas crossover phenomenon, which sometimes deteriorates the cell performance. In this study, we investigated the size and location of free-volume holes in the crosslinked-polytetrafluoroethylene (cPTFE) based radiation-grafted PEMs by positron annihilation lifetime (PAL) spectroscopy. For comparison, the base cPTFE and polystyrene grafted films were also measured. From the analysis of PAL spectra, it was found that there were free-volume holes with different radius of 0.28-0.30 nm and 0.44-0.45 nm. The smaller holes should be located in both PTFE crystallites and poly(styrene sulfonic acid) grafts, while the larger holes are considered to exist in amorphous PTFE phases.

Vacancy Generation in Si During Solid?Liquid Transition Observed by Positron Annihilation Spectroscopy

The Doppler broadening of annihilation radiation (DBAR) measurements were performed on Czochralski and floating-zone-grown Si crystals near the melting point using a positron microbeam. Below 1380 °C, the DBAR spectra showed essentially no change. In a very narrow temperature range near the melting point, the peak intensities of the DBAR spectra (S parameter) decreased by approximately 1% suggesting an increase in material density. Upon further heating, the S parameter markedly increased until melting. This indicates the formation of thermal vacancies. Compared with theoretical calculation, both monovacancies and divacancies are considered to be formed.

Spin-Polarized Positron Annihilation Study on Some Ferromagnets

We briefly review the spin-polarized positron annihilation experiments on some ferromagnets (Fe, Co, Ni, Gd, Co2MnSi, Co2MnAl and NiMnSb) using positron beams generated with 68Ge-68Ga sources. The differential DBAR spectra between majority and minority spin electrons are well interpreted by the first principles band structure calculation. This further provides information about the half-metallicity of the Heusler alloys. The surfaces of Fe, Co and Ni are more negatively spin-polarized, that is, there are more majority than minority spin electrons. To explain the observed spin polarization quantitatively, detailed theoretical calculations and further experiments are required.

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.

Reflection high-energy positron diffraction study on the first surface layer

Reflection high-energy positron diffraction (RHEPD) is a powerful tool for studying surface structure. In particular, the topmost surface layer can be observed, facilitated by the characteristic of total reflection for positrons. A previous RHEPD study on a Ag surface, using a 22Na-based beam, is revisited and the analysis detailed.