Room temperature ferromagnetism was observed in both C- and O-implanted GaN films, which were irradiated by 80 keV C/O-ions with respective dose of 5 × 1016 and 2 × 1017 ions/cm2. Positron annihilation spectroscopy was used to explore the magnetic origin and the correlation between the magnetism and structural features. The results reveal that carbon-ions play an important role in the stable ferromagnetism in C-implanted GaN films, while oxygen has no effect on the magnetic properties, even than a weak hysteresis loop was observed in O-implanted sample. This weak ferromagnetism is demonstrated as originated from Ga-related vacancies which induced by implantation. With first-principle calculations, we confirmed that substitutional C-ion at N-site can introduce magnetic moment for 0.8 μB and stabilize ferromagnetic coupling with adjacent Ga-vacancy at room temperature. Moreover, the effect of O-ions was clearly ruled out. Our discussion gives an experimental and theoretical insight of the different origin of ferromagnetism between acceptor and donor non-metal-doped GaN materials.

•The defects of the high temperature phase in hexagonal NiS are mainly Ni-monovacancies.•The defect concentration and the trapping functionality for positrons have almost no changes before and after the phase transition in hexagonal NiS.•The existence of Ni-monovacancies lowers the transition temperature and promotes the transition occurrence.The defect structures of the hexagonal nickel sulfide with different Ni contents (Ni1−ΔS) have been investigated mainly by the positron annihilation lifetime spectroscopy and the temperature-dependent positron Doppler broadening technique. Combining with theoretical calculations, it is convincingly found that there is a high concentration of Ni-monovacancies in Ni1−ΔS not only in the low temperature (LT) phase but also in the high temperature (HT) phase. The defect concentration and the trapping functionality for positrons have almost no changes before and after the phase transition. Besides, the transition temperature decreases and the thermal hysteresis width of the phase transition narrows down with the reduction of Ni contents, since the existence of Ni-monovacancies can lower the energy requirement of the transformation and thus promote the transition occurrence.

•Defect-induced ferromagnetism was observed in N-implanted GaN films.•Ga-vacancy contributed ferromagnetism was confirmed by positron spectrum.•Density functional calculations are in good agreement with experimental results.GaN films prepared on sapphire substrates with thickness of 30 μm, were implanted by nitrogen ions with energy of 80 keV at doses of 5 × 1016 cm−2 and 2 × 1017 cm−2, respectively. An obvious ferromagnetic loop was obtained in the higher dose irradiated GaN film at room temperature, indicating magnetic defects were induced into this film. After irradiation, the films contained lots of Ga vacancies were investigated by slow positron annihilation spectroscopy. With first-principle calculations, we demonstrated that Ga vacancies could lead to an enhancement of magnetic moment for 3 μB in GaN crystal and form ferromagnetic coupling at room temperature between two close range Ga vacancies.

MgO single crystals were implanted with 70 keV C/N/O ions at room temperature with respective doses of 2 × 1016 and 2 × 1017 ions/cm2. All samples with high-dose implantation showed room temperature hysteresis in magnetization loops. Magnetization and slow positron annihilation measurements confirmed that room temperature ferromagnetism in O-implanted samples was attributed to the presence of Mg vacancies. Furthermore, the introduction of C or N played more effective role in ferromagnetic performance than Mg vacancies. Moreover, the magnetic moment possibly occurred from the localized wave function of unpaired electrons and the exchange interaction formed a long-range magnetic order.Graphical abstractHighlights► The room temperature ferromagnetism was observed in C-/N-/O-implanted MgO single crystals. ► Mg vacancies as the origin of room temperature ferromagnetism in O-implanted MgO was demonstrated directly. ► The introduction of C or N play more effective role in ferromagnetic performance than Mg vacancies was proved in experiments. ► Bound magnetic polaron model was employed to explain the magnetic coupling in insulated MgO material.

Virgin MgO single crystals were implanted with 70 keV C/N/O ions at room temperature to a dose of 2 × 1017/cm2. After implantation the samples showed room temperature hysteresis in magnetization loops. The annealing effects on the magnetic properties and induced defects of these samples were determined by vibrating sample magnetometer and positron annihilation spectroscopy, respectively. The experimental results indicate that ferromagnetism can be introduced to MgO single crystals by doping with C, N or introduction of Mg related vacancy defects. However, the Mg vacancies coexistence with C or N ions in the C-/N-implanted samples may play a negative role in magnetic performance in these MgO samples. The rapid increase of magnetic moment in O-implanted sample is attributed to the formation of new type of vacancy defects.

Positron annihilation spectroscopy has been used to investigate CuInSe2 solar cell thin films. The films were grown on Mo-coated soda lime glass substrates by the electrochemical deposition processing technique. As-grown samples are found to contain large concentration of vacancy defects. The selenium (Se) atmosphere and sulfur (S) atmosphere annealing of as-grown samples at 800 K can dramatically reduce the number of vacancy defects and the film becomes crystalline. In addition, a defect layer of about 50 nm thickness was observed at the surface of the CuInSe2 thin film. This layer results from the electrochemical deposition method, but the defect concentration in the defect layer can be greatly reduced by annealing in selenium atmosphere. The Doppler broadening line shape parameter correlation plot provided evidence that the positron trapping defect states where in three samples.Highlights► As-grown CuInSe2 thin films contain large concentration of defects. ► A defect layer of about 50 nm exists in the CuInSe2 thin film surface. ► The defect concentration in the defect layer can be greatly reduced.

This paper presents experimental methods and results of measuring backward sputtering induced by fast neutrons. Backward sputtering yields of 10 materials Mg, Al, Sc, V, Fe, Co, Cu, Zr, Au and type 316 stainless steel have been measured and compared with forward sputtering yields. The typical value of backward sputtering yield is about 10−8–10−10 atoms per neutron, which is approximately 1–2 orders smaller than that of forward sputtering. The present results have been compared with other experimental results and have been explained using sputtering theories.

•The defects of the high temperature phase in hexagonal NiS are mainly Ni-monovacancies.•The defect concentration and the trapping functionality for positrons have almost no changes before and after the phase transition in hexagonal NiS.•The existence of Ni-monovacancies lowers the transition temperature and promotes the transition occurrence.The defect structures of the hexagonal nickel sulfide with different Ni contents (Ni1−ΔS) have been investigated mainly by the positron annihilation lifetime spectroscopy and the temperature-dependent positron Doppler broadening technique. Combining with theoretical calculations, it is convincingly found that there is a high concentration of Ni-monovacancies in Ni1−ΔS not only in the low temperature (LT) phase but also in the high temperature (HT) phase. The defect concentration and the trapping functionality for positrons have almost no changes before and after the phase transition. Besides, the transition temperature decreases and the thermal hysteresis width of the phase transition narrows down with the reduction of Ni contents, since the existence of Ni-monovacancies can lower the energy requirement of the transformation and thus promote the transition occurrence.

This paper proposes a novel methodology of MeV positrons generation for PAS application. Feasibility of this proposal analyzed by G4Beamline and Transport have shown reasonable success. Using 2 Hz, 1.6 GeV, 100 ns and 1.5 μC/bunch proton bunches for bombarding a graphite target, about 100 ns e+ bunches are generated. Quasi-monochromatic positrons in the range of 1–10 MeV included in these bunches have a flux of >107/s, peak brightness of 1014/s. A magnetic-confinement beamline is utilized to transport the positrons and a “Fast Beam Chopper” is unprecedentedly extended to chop those relativistic bunches. The positron beam can be finally characterized by the energy range of 1–10 MeV and bunch width from one hundred ps up to 1 ns. Such ultrashort bunches can be useful in tomography-type positron annihilation spectroscopy (PAS) as well as other applications.

•Interfacial defects and magnetization of Bi7Fe2.75Co0.25Ti3O21 were studied.•Interfacial defects disappear slowly with the increase of annealing temperature.•Saturation magnetization decreased with increasing the annealing temperature.•Higher concentration of interfacial defects bring higher saturation magnetization.This paper investigated the effect of the annealing temperature on the interfacial defects and the magnetization of a single-phase multiferroic Bi7Fe2.75Co0.25Ti3O21. With the increase of annealing temperature, the average thickness of the nonaplates increased from 80 to 180 nm. But the magnetic property measurement shows that the saturation magnetization gradually decreases with the increase of the annealing temperature correspondingly. Positron annihilation measurements reveal that the interfacial defects disappear obviously when the annealing temperature increased, which is found to agree well with the variation of saturation magnetization. The results suggest that with the higher concentration of interfacial defects may bring about higher saturation magnetization for the Aurivillius phase material, opening a window to improve the magnetic performance through controlling the concentration of interfacial defects.