A. Kawasuso

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 105u2005A/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.

Total reflection high-energy positron diffraction: An ideal diffraction technique for surface structure analysis

It is shown that the reflection high-energy positron diffraction (RHEPD) pattern from a Si(111)-(7 × 7) reconstructed surface for the total reflection condition, that is, the total reflection high-energy positron diffraction (TRHEPD) pattern, does not contain contributions from atoms in the bulk. Now, a method of observing the diffraction pattern formed only by the atoms on the topmost surface by a straightforward measurement of a bulk sample is available.

Enhanced damage buildup in C+-implanted GaN film studied by a monoenergetic positron beam

Wurtzite GaN films grown by hydride vapor phase epitaxy were implanted with 280u2009keV C+ ions to a dose of 6u2009×u20091016u2009cm−2. Vacancy-type defects in C+-implanted GaN were probed using a slow positron beam. The increase of Doppler broadening S parameter to a high value of 1.08–1.09 after implantation indicates introduction of very large vacancy clusters. Post-implantation annealing at temperatures up to 800u2009°C makes these vacancy clusters to agglomerate into microvoids. The vacancy clusters or microvoids show high thermal stability, and they are only partially removed after annealing up to 1000u2009°C. The other measurements such as X-ray diffraction, Raman scattering and Photoluminescence all indicate severe damage and even disordered structure induced by C+-implantation. The disordered lattice shows a partial recovery after annealing above 800u2009°C. Amorphous regions are observed by high resolution transmission electron microscopy measurement, which directly confirms that amorphization is induced by C+-implantation. The disordered GaN lattice is possibly due to special feature of carbon impurities, which enhance the damage buildup during implantation.

Defects and acceptor centers in ZnO introduced by C + -implantation

ZnO single crystals were implanted with 280xa0keV C+ to a dose of 6xa0×xa01016xa0cm−2. Positron annihilation measurements reveal a large number of vacancy clusters in the implanted sample. They further agglomerate into larger size or even microvoids after annealing up to 700xa0°C, and are fully removed at 1200xa0°C. X-ray diffraction, photoluminescence, and Raman scattering measurements all indicate severe damage introduced by implantation, and the damaged lattice is partially recovered after annealing above 500xa0°C. From room temperature photoluminescence measurements, an additional peak at around 3.235xa0eV appears in the implanted sample after annealing at 1100xa0°C, which is much stronger than that of the free exciton. From the analysis of low temperature photoluminescence spectra, this peak is mostly a free electron to acceptor (e,A0) line which is probably associated with CO.

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 573u2009K. 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.

A study of defects in electron- and ion-irradiated ZrCuAl bulk glassy alloy using positron annihilation techniques

Free volume changes in Zr50Cu40Al10 bulk glassy alloys irradiated by 200 and 2.5 MeV Xe ions, 180 keV He ions, and 2 MeV electrons were investigated at room temperature using positron annihilation lifetime and Doppler broadening techniques. In addition, a slow positron beam was used to probe the change in free volume in the 180 keV He ion-irradiated sample. X-ray diffraction revealed that no crystallization took place in any of the irradiated samples. The Doppler broadening spectra from the annihilated gamma rays remained essentially constant in all ion-irradiation cases; however, an extremely minor change of positron mean lifetime was detected in each case. For electron- and He ion-irradiated samples the positron lifetime increased, and the opposite was seen in heavy-ion irradiated samples. The Doppler broadening S parameter increased with He-ion radiation dose, and the depth profile correlated well to the damage profile.

Doppler broadening of annihilation radiation of some single-element materials from the second to the sixth periods

The Doppler broadening of annihilation radiation spectra of some single-element materials from the second to the sixth periods measured by the coincidence technique have been compared with the theoretical calculation based on the projector-augmented wave (PAW) method. Having appropriate valence electron configurations the calculation well reproduces the measured spectra.