Akihiro Iwase

Magnetic states controlled by energetic ion irradiation in FeRh thin films

Changes in magnetic properties and lattice structure of FeRh films by 180 keV–10 MeV ion (H, He, and I) irradiation are studied. In spite of the irradiation with different ion species and wide range of energies, the changes in magnetization are dominated by solely a single parameter; the density of energy which is deposited through elastic collision between the ions and the samples. For the low deposition energy density, the magnetization increases with increasing the deposition energy density, while the lattice structure remains unchanged. When the deposition energy density becomes larger, however, the magnetization decreases after reaching the maximum value. The decrease in the magnetization accompanies the crystal structure change from B2 to A1. The present results imply that the magnetic state of FeRh films can be designedly controlled by the energetic ion irradiations.

Defect Production by Electron Excitation in Cu and Ag

It has been found in ion-irradiated Cu and Ag thin foils that lattice defects are produced not only through elastic collisions, but also through a process strongly associated with electronic stopping power S e . In the latter process, the defect production cross section is proportional to S e 1.7 in Cu and S e 1.5 in Ag. The nearly quadratic dependence of the defect production cross section on S e suggests that the mutual Coulomb repulsion of ions positively charged by electron excitation causes the defect production in Cu and Ag.

Quantitative control of magnetic ordering in FeRh thin films using 30 keV Ga ion irradiation from a focused ion beam system

We investigated a low-energy ion-beam irradiation process for the magnetic modification of FeRh thin films using a focused ion beam system. Low-energy ion-beam irradiation induced ferromagnetic states in the FeRh thin films at low temperatures, that originally exhibited antiferromagnetism, as effectively as high-energy ion-beam irradiation. Because the energy deposited by the elastic collisions caused by the irradiation determined the magnetic properties of the samples, the magnetic state of the FeRh thin films could be quantitatively controlled. The low-energy ion-beam irradiation using a focused ion beam system is a potential technique to modify the magnetic properties of materials on the nano- and micro-scales, which may lead to a variety of novel spin devices and applications.

Radiation annealing in nickel and copper by 100 MeV iodine ions

Radiation annealing by 100 MeV 127 I-ions is studied in pre-doped Ni and Cu samples through the defect production rate measurements below 10 K. The experimental results are analyzed by using a new model which describes the production and radiation annealing of two or more types of defects. The subthreshold recombination cross section and the concentration of doped-in defects are obtained for each type of defects. In Ni, the defect annihilation corresponding to the stage-I thermal annealing occurs during the I-ion irradiation with the subthreshold recombination cross section of 6.5×10 -12 to 1.4×10 -12 cm 2 . In Cu, the stage-I defects are annihilated with the cross section of 4.6×10 -13 cm 2 . The large radiation annealing in Ni shows the contribution of the electron excitation by irradiating ions to the subthreshold recombination of defects through the electron-phonon interaction.

Micrometer-Sized Magnetic Patterning of FeRh Films Using an Energetic Ion Microbeam

FeRh thin films were irradiated with a focused 10 MeV I 2 ×2 µm2 ion microbeam at intervals of several µm. Following irradiation, the magnetic state at the surface was observed via magnetic force microscopy. The micrometer-sized regions irradiated with the microbeam show a ferromagnetic state while unirradiated regions remain in an anti-ferromagnetic state. Results indicate that an energetic ion microbeam can be used as a tool to produce micrometer-sized modulations of lateral magnetic states of FeRh films.

Defect Production Induced by Primary Ionization in Ion-Irradiated Oxide Superconductors.

Resistivity as a function of ion-fluence has been measured in situ in thin films of oxide superconductor EuBa 2 Cu 3 O y irradiated with various heavy ions in a wide energy range of 90 MeV–3.84 GeV. From the resistivity measurements, the quantities characterizing the irradiation-induced columnar defects, such as the diameter of the columnar defect and the inside-resistivity (the resistivity inside the columnar defect), have been estimated. It is found that they are not scaled with the electronic stopping power, while they are strongly correlated with the primary ionization rate which is the number of atoms primarily ionized by an incident ion per unit path length. This result suggests that the defect production is triggered by Coulomb repulsion of ionized atoms. Based on the results of thermal annealing of irradiated samples, structure of the columnar defects has also been discussed.

Atomic imaging in EBCO superconductor films by an X-ray holography system using a toroidally bent graphite analyzer

X-ray fluorescence holography (XFH) is a new technique enabling the determination of the three-dimensional local atomic structure around a certain element. This method has been applied to analyze the local structure around Cu in 300 nm thin films of EuBa(2)Cu(3)O(7-delta) (EBCO) epitaxially grown on MgO (100) substrate, using the newest system for XFH measurement and high-brilliance synchrotron radiation at SPring-8. Here, the results of a study on the irradiation effect on the local atomic structure of EBCO superconductor with XFH measurements are presented.

Investigation of photoluminescence of Si−O−C(−H) ceramics at an early stage of decarbonization by using high energy excitation

Si−O−C(−H) ceramics with reduced carbon contents were prepared by pyrolyzing polysiloxane particles in hydrogen at temperatures of 750, 800 and 850 °C. Under HeCd laser irradiation (325 nm), the obtained ceramics show broad spectra peaking at 400–415 nm. On the other hand, the excitation on the higher energy region by an ArF excimer laser (193 nm) induces new PL bands located at short wavelength region of 300 and 355 nm. Such high energy PL bands appear prominently in the ceramics synthesized at 750 °C, and are minor in ceramics synthesized at 800 and 850 °C.

Behavior of free volume in ZrCuAl bulk metallic glass after irradiation

Free volumes in Zr50Cu40Al10 bulk metallic glasses irradiated by 100 MeV and 200 MeV Xe ions and 2 MeV electrons at room temperature have been investigated by using positron annihilation techniques, the micro Vickers hardness and X-ray diffraction measurements. No crystallization took place due to both ion and electron irradiations. The coincidence Doppler broadening spectra of annihilation gamma rays also remained unchanged by ion and electron irradiation. Meanwhile, an increase and a decrease of positron lifetimes after electron and ion irradiation, respectively, were observed. This suggests the structural modification of free volumes. The response of positron lifetime to ion and electron irradiations may be connected with the tendency of hardness.

Effect of oxygen partial pressure on the structural and magnetic properties of Ba(Fe0.5Mn0.5)O3−δ epitaxial thin films

Ba(Fe0.5Mn0.5)O3−δ (BFMO) single crystalline films with various amounts of the oxygen deficiencies on SrTiO3(001) substrates have been successfully synthesized by a pulsed laser deposition method. The lattice constant of the films significantly decreased with an increase in the oxygen partial pressure during the deposition process. This suggests that the radius of the Fe and Mn ions that is strongly correlated with their valence state systematically changed. In addition, the oxygen partial pressure has a considerable influence on the magnetic properties of the films: the saturation magnetization of the samples increased with increasing oxygen partial pressure, i.e., with decrease in the amount of the oxygen deficiencies. The maximum saturation magnetization of 43 emu/cc for the epitaxial BFMO films was obtained at 300 K. The results of the x-ray photoelectron spectroscopic analysis revealed that the relative amount of the Fe3+ (3d5) and Mn4+ (3d3) ions increased with decreasing amount of the oxygen defici...