Shoichi Okamoto

Large magnetization induced in single crystalline iron films by high‐dose nitrogen implantation

Nitrogen ions were implanted in 200‐nm‐thick single crystalline iron films for a stoichiometric concentration of Fe16 N2 (11.1 at.u2009%) by way of a multiple implantation. X‐ray diffraction revealed that the as‐implanted films were composed of 70% nitrogen martensite and 30% residual α‐Fe. A slight amount of α‘‐martensite, that is, Fe16 N2, was detected but the principal constituent of the nitrogen martensite was α’‐martensite. The saturation magnetic moment increased by 13.4 % with the implantation. Annealing the implanted films at 150u2009°C for 2 h in vacuum resulted in an increase in a constituent of the α‘ phase, but affected the magnetic moment only slightly. It was deduced that the saturation magnetization of the nitrogen martensite is ∼257 emu/g at room temperature. It was also found that the implantation induced a perpendicular magnetic anisotropy due to the characteristics of the nitrogen martensite.

Formation of ferromagnetic iron nitrides in iron thin films by high‐dose nitrogen ion implantation

Synthesis of thin films of Fe16N2 has been investigated by use of a high‐dose nitrogen ion implantation into thin iron films. Nitrogen molecular ions of N+2 were accelerated to 150 keV and implanted into α‐iron films with a thickness of 200 nm. As a result of an implantation with the dose of 4.0×1016 (N2/cm2), 35% of the α‐Fe was transformed into an iron nitride with a body‐centered‐tetragonal (bct) structure. The c axis of the bct lattices was oriented perpendicularly to the film plane. It was found that nitrogen implantation into iron films gives rise to both an increase in the spontaneous magnetization and a perpendicular anisotropy. These facts are explained on the basis of the bct nitride formation and the c‐axis orientation. Conversion electron Mossbauer spectroscopy was used to study hyperfine structures and magnetic properties of the implanted films. Three different Fe sites corresponding to the bct nitride were recognized in a spectrum of the implanted film. The maximum hyperfine field was 372.8 ...

Nitrogen‐implantation‐induced transformation of iron to crystalline Fe16N2 in epitaxial iron films

Crystalline phase Fe16 N2 has been found in nitrogen‐implanted epitaxial iron films on mirror‐polished (100)u2009MgO substrates. Implantations of N+2 at an acceleration voltage of 140 keV transformed the bcc iron into a partially ordered nitrogen martensite with a body‐centered‐tetragonal (bct) structure. The amount of the ordered part was increased to some extent by an annealing treatment in vacuum at 150u2009°C for 2 hr. An implantation with a dose of 4×1016 N2/cm2 transformed 60% iron in a film into the bct martensite. The constituent of the ordered phase (Fe16 N2 ) in the bct martensite was 16 wt.u2009%. After the annealing, the constituent increased to 24%. It is proposed that a formation of lower temperature and metastable compound such as Fe16 N2 can be made available through energetic ion implantation.

Resonant Raman scattering and free-exciton emission in CuGaS2 crystals

Origins of lines appearing in the exciton region spectra of CuGaS2 crystals at low temperatures under 4579, 4765 or 4880 A excitation of an Ar+ laser are discussed in terms of emissions from upper- and lower-branch excitonic polaritons (UBPs and LBPs) and multi-phonon Raman processes. The line at 2.500 eV is an emission from LBPs. The line at 2.504 eV seen in our samples under 4765 A excitation is shown to be interpretable as a three-phonon Raman process in resonance with UBP states, while the line reported at the same energy by Shirakata is likely to be an emission from UBPs.

Resistometric and Mössbauer studies on structural relaxation in (Ni,Fe)75Si10B15 metallic glasses

The structural relaxation in (Ni,Fe)75Si10B15 metallic glasses has been studied by measuring resistivity changes and Mossbauer spectra. The sign and magnitude of resistivity changes due to the relaxation depend strongly on the Ni/Fe ratio. The isothermal resistivity changes fit well the nonlinear kinetic form, indicating that the fluctuation of local environments of Ni and Fe atoms is very large and Ni and Fe atoms in Ni‐Fe‐Si‐B metallic glasses are mobile compared with Co and Fe atoms in Co‐Fe‐Si‐B metallic glasses. The compositional dependence and kinetic parameters for reversible resistivity changes and Mossbauer spectra strongly support that the reversible structural relaxation corresponds to the reversible short‐range ordering between Ni and Fe atoms.

Optical and Electrical Characterization of CuGaS2 Grown by Vapor Phase Epitaxy

Investigations were made on band-edge-region photoluminescence and the electric resistivity of CuGaS2 epitaxial films on GaAs and GaP substrates, which were prepared by vapor phase epitaxy using metal chlorides and H2S gas. The similar temperature dependence was observed on the resistivity and the intensity of 2.40 eV emission, giving nearly the same activation energy of about 0.05 eV. This value suggests the existence of an acceptor shallower than the acceptor of effective mass approximation. A proposed candidate of this shallow acceptor is an interstitial chlorine atom coming from source materials.