Spin-polarized Positron Annihilation Research Articles

High-density magnetic-vacancy inclusion in Co2MnGa single crystal probed by spin-polarized positron annihilation spectroscopy

Co2MnGa is a Weyl semimetal exhibiting giant anomalous Hall and Nernst effects. Using spin-polarized positron annihilation spectroscopy, we examined a Bridgman-grown Co2MnGa single crystal with a nearly perfect L21-ordered structure and a reference Co2MnAl polycrystal with a Mn–Al-disordered B2 structure. We found that a large amount of magnetic vacancies (more than 100 ppm) were included in the Co2MnGa crystal but not the Co2MnAl crystal. We discuss possible reasons for the inclusion of vacancies, the role of vacancies in the development of the ordered structure, and the electronic states associated with the vacancies. Toward the development of Co2MnGa-based devices, the manners for reducing vacancies as well as the influence of vacancies on the electrical transport properties should be considered.

Gadolinium-implanted GaN studied by spin-polarized positron annihilation spectroscopy

In this study, Gd ion implantation and annealing were performed at ${900}^{\ensuremath{\circ}}\phantom{\rule{0.16em}{0ex}}\mathrm{C}$ for nominally undoped wurtzite GaN grown by metal-organic chemical vapor deposition. Spin-polarized positron annihilation measurements showed that vacancy clusters including at least 12 vacancies per cluster were the major positron-trapping centers and that the electrons in the vacancy clusters were spin-polarized. These observations could be explained by first-principles calculations. The previous speculation about the defect-assisted ferromagnetism of Gd-implanted GaN may be supported if vacancy clusters are considered.

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.

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.

Positron States at Li- and O-adsorbed Fe(001) Ferromagnetic Surfaces Studied by Two-Component Density Functional Theory

The positron states for Li- and O-adsorbed Fe(001) ferromagnetic surfaces are studied by using two-component density functional theory. Positron surface lifetimes and positron binding energies are found to be sensitive to changes in the surface structure and the dipole barrier induced by adatoms, which can be understood by the positron density distribution and surface potential. Spin-dependent positron lifetime fractions are in excellent agreement with spin-polarization fractions at the topmost surface, because the localized positrons at the surface are sensitive to the surface magnetic state. Therefore, the present results show that spin-polarized positron annihilation spectroscopy can extract the outermost surface magnetic state.

Erratum: Local electron-electron interaction strength in ferromagnetic nickel determined by spin-polarized positron annihilation.

Scientific Reports 6: Article number: 20898; published online: 16 February 2016; updated: 13 April 2016 The original version of this Article contained a typographical error in the spelling of the author Josef Andreas Weber, which was incorrectly given as Josef Andreass Weber. This has now been corrected in the PDF and HTML versions of the Article.

Local electron-electron interaction strength in ferromagnetic nickel determined by spin-polarized positron annihilation.

We employ a positron annihilation technique, the spin-polarized two-dimensional angular correlation of annihilation radiation (2D-ACAR), to measure the spin-difference spectra of ferromagnetic nickel. The experimental data are compared with the theoretical results obtained within a combination of the local spin density approximation (LSDA) and the many-body dynamical mean-field theory (DMFT). We find that the self-energy defining the electronic correlations in Ni leads to anisotropic contributions to the momentum distribution. By direct comparison of the theoretical and experimental results we determine the strength of the local electronic interaction U in ferromagnetic Ni as 2.0 ± 0.1 eV.

Spin-Resolved Fermi Surface of the Localized Ferromagnetic Heusler CompoundCu2MnAlMeasured with Spin-Polarized Positron Annihilation

We determined the bulk electronic structure of the prototypical Heusler compound Cu(2)MnAl by measuring the angular correlation of annihilation radiation using spin-polarized positrons. To this end, a new algorithm for reconstructing 3D densities from projections is introduced that allows us to corroborate the excellent agreement between our electronic structure calculations and the experimental data. The contribution of each individual Fermi surface sheet to the magnetization was identified, and summed to a total spin magnetic moment of 3.6±0.5 μ(B)/f.u..

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 22Na 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.

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.

Current-induced spin polarization on a Pt surface: A new approach using spin-polarized positron annihilation spectroscopy

Transversely spin-polarized positrons were injected near Pt and Au surfaces under an applied electric current. The three-photon annihilation of spin-triplet positronium, which was emitted from the surfaces into vacuum, was observed. When the positron spin polarization was perpendicular to the current direction, the maximum asymmetry of the three-photon annihilation intensity was observed upon current reversal for the Pt surfaces, whereas it was significantly reduced for the Au surface. The experimental results suggest that electrons near the Pt surfaces were in-plane and transversely spin-polarized with respect to the direction of the electric current. The maximum electron spin polarization was estimated to be more than 0.01 (1%).

Spin-polarized positron annihilation measurements of polycrystalline Fe, Co, Ni, and Gd based on Doppler broadening of annihilation radiation

Spin-polarized positron annihilation measurements of polycrystalline Fe, Co, Ni, and Gd based on Doppler broadening of annihilation radiation

Magnetic Compton Profile and Spin Polarized Positron Annihilation Radiation in Hcp Cobalt

A spin polarized ab initio band structure, the magnetic Compton profile (MCP) and spin polarized 2-dimensional positron annihilation radiation (SP-2DACAR) for the hcp cobalt are calculated. The calculated MCP is compared with experimental results for polycrystalline cobalt. Agreement between theoretical results and experimental ones is very good. Though there is a little disagreement for the results of SP-2DACAR, the theoretical electronic structure is in good agreement with experiment.

Spin-polarized positron annihilation and magnetic moment in Fe 1−xCo x alloys

A relation between the average magnetic moment and the positron annihilation parameter characterizing the change in angular correlation spectra due to reversal of magnetic field in a spin-polarized positron annihilation experiment is established in Fe 1− x Co x alloys up to 70% of Co. This connection is significant for application of positron annihilation method to the study of magnetic properties of metallic materials.