Jihong Chen

Enhancement of room temperature ferromagnetism in Mn-implanted Si by He implantation

Room temperature ferromagnetism in Mn-implanted p-Si was enhanced dramatically by implantation of He. A 75 nm end-of-range region was found in the sample, with large scale and inhomogeneous damaging but preserved Si lattice periodicity. The end-of-range region shows an intrinsic magnetization of ∼100 emu/cm3. High resolution transmission electron microscopy and x-ray photoelectron spectroscopy measurements indicate that the spin polarization of Si dangling bonds rather than Mn impurity was the major source for the enhanced magnetism.

Synergistic Effects in Reduced-Activation Martensitic Steel Under Single and Sequential Helium/Hydrogen Ion Irradiation

Abstract Single-beam (He+ or H+) and sequential-beam (He+/H+ or H+/He+) irradiation of reduced-activation martensitic steels at 450°C was carried out to investigate helium/hydrogen synergistic effects on the microstructure of the steels. After helium implantation, a very low density of helium bubbles was observed. No void was observed after hydrogen implantation. For the He+/H+ sequential-beam irradiation, the number density of bubbles increased rapidly at low hydrogen dose, and the average size of bubbles increased at higher hydrogen dose. Helium bubbles were also found in the H+/He+ sequential-beam irradiated specimen, but its swelling was smaller than that for the He+/H+ sequential conditions. It was concluded that the effect of hydrogen atoms on the nucleation and growth of bubbles may enhance the diffusion of helium-atom/helium-vacancy clusters. Hydrogen irradiation after helium irradiation can increase the nucleation and growth of helium bubbles, while helium bubbles may enhance the retention of hydrogen atoms to form He-H-vacancy complexes.

Improving Room-Temperature Ferromagnetism in Mn-Implanted Si by High-Temperature Annealing

Mn-implanted Si with postannealing treatment at 873, 973, 1073, and 1173 K exhibited ferromagnetism at room temperature. Ferromagnetism was enhanced as the annealing temperature increased. The largest value of saturation magnetization was found in the sample annealed at 1173 K. X-ray absorption near-edge structure spectra showed no formation of substitutiona-interstitial Mn-Mn dimmers in all samples. On the other hand, high-resolution transmission electron microscopy and glancing incidence X-ray diffraction revealed a variety of MnSi compounds. We propose that the ferromagnetism has the origin from the MnSi1.7 nanoparticles as well as the MnSi 1:1 phase, which possesses strong short-range spin correlation. The formation of antiferromagnetic Mn5Si3 compound in 873 and 973 K annealed samples substantially reduced the magnetism. This paper provides new insights into the understanding of ferromagnetism in Mn-implanted Si diluted magnetic semiconductors.

Chirality dependence of the energetic stability of 4Å carbon nanotubes

First-principles pseudopotential method is used to study the stability of three kinds of 4A single-wall carbon nanotubes, which have been recently fabricated inside zeolite channels. The minimum energy path for unrolling the tube into a flat ribbon is calculated by the nudged elastic band technique. Our calculations suggest that all the three kinds of 4A tubes are stable when the zeolite framework is dissolved by acid, and the relative stability depends strongly on their chiralities. It is expected that the majority of the grown sample is (3,3) tube in free space.