Tae Sik Cho

Structural evolution of ZnO/sapphire(001) heteroepitaxy studied by real time synchrotron x-ray scattering

The structural evolution during heteroepitaxial growth of ZnO/sapphire(001) by radio-frequency magnetron sputtering has been studied using real-time synchrotron x-ray scattering. The two-dimensional (2D) ZnO(002) layers grown in the initial stage are highly strained and well aligned to the substrate having a mosaic distribution of 0.01° full width at half maximum (FWHM), in sharp contrast to the reported transition 2D layers grown by molecular-beam epitaxy. With increasing film thickness, the lattice strain is relieved and the poorly aligned (1.25° FWHM) three-dimensional (3D) islands are nucleated on the 2D layers. We attribute the 2D–3D transition to the release of the strain energy stored in the film due to the film/substrate lattice mismatch.

Thickness dependence of the crystallization of Ba-ferrite films

The crystallization of Ba-ferrite/sapphire(001) films of various thicknesses has been studied using synchrotron x-ray scattering, field emission scanning electron microscope, and atomic force microscope. In films thinner than 1000 A, Ba-ferrite amorphous precursor was crystallized into perpendicular grains keeping the magnetically easy c-axis normal to the film plane during annealing to 750 °C. In films thicker than 1000 A, however, acicular grains keeping the c-axis parallel to the film plane were grown on top of the perpendicular grains. The behavior of the saturation magnetization and the intrinsic coercivity was consistent with the thickness dependence of the crystallization. We attribute the thickness dependence of the crystallization to the substrate effect, which prefers the growth of the epitaxial, c-axis oriented perpendicular grains near the film/substrate interfacial area.

Structure determination of nanostructured Ni-Co films by anomalous x-ray scattering

Conventional x-ray diffraction failed to provide correct information on alloying of materials made of elements with close lattice parameters, even for elements commonly accepted to have miscibility. Using anomalous x-ray scattering, we showed that nanostructured NiCo films did not necessarily form solid solution as expected from their phase diagram or suggested by the results of conventional x-ray diffraction.

Formation of crystalline Ba-ferrite phase from α-Fe2O3 phase in amorphous precursor

The formation of crystalline Ba-ferrite/sapphire(001) films in an amorphous precursor during annealing was studied in a real-time synchrotron x-ray scattering experiment. From the x-ray integrated intensity and the surface morphology, we found that the crystalline Ba-ferrite phase was formed by solid phase transformation of the intermediate crystalline α-Fe2O3 phase, not by direct crystallization of the amorphous precursor. The solid phase transformation occurred at temperature above 500 °C. The high crystallization temperature of the Ba-ferrite phase was attributed to the activation barrier for the diffusion of Ba ions into the α-Fe2O3 phase.

Structural study of CoCrPt films by anomalous x-ray scattering and extended x-ray absorption fine structure

The correlation of elemental chemistry with the particular long-range order in question cannot be obtained by common conventional characterization techniques. Here we report a study of determining the elemental concentrations of the textured Bragg diffraction peak and the averaged local atomic environment of sputtered CoCrPt films using anomalous x-ray scattering and extended x-ray absorption fine structures. The elemental compositions of the textured peak in these polycrystalline nanostructured films differed from the average global film composition. The higher Cr concentration in the textured peak indicated that a significant amount of Cr did not segregate towards the grain boundaries as a result of the low sputtering temperature and pressure used. The structural observations were consistent with the magnetic results.

Graphitization of ultrathin amorphous carbon films on Si(001) by Ar+ ion irradiation at ambient temperature

We report the graphitization of ultrathin (8 nm) amorphous carbon films on Si(001) by 2 MeV Ar+ ion irradiation at ambient temperature. The resulting graphite films show the 〈001〉 preferred orientation in the film normal direction, but random distribution in the film plane direction. The smooth surface and interface suggest that the activation energy for the graphitization is supplied mostly by the electronic stopping process while Ar+ ions travel through the amorphous carbon film.

Microstructure of epitaxial α-Fe2O3 grains in Ba-ferrite thin films grown on sapphire (001)

We revealed the existence of epitaxial α-Fe2O3 grains in Ba-ferrite thin films on sapphire (001) using synchrotron x-ray scattering. The antiferromagnetic α-Fe2O3 grains were formed during the crystallization of amorphous Ba-ferrite films grown on sapphire (001) by radio frequency sputtering deposition. The crystal domain size of the α-Fe2O3 grains was about 250 A in the film plane, similar to that of the Ba-ferrite grains. The in-plane crystalline axis of the α-Fe2O3 was aligned to that of sapphire, while the Ba-ferrite film was rotated by 30° in the film plane. We confirm that the existence of these antiferromagnetic α-Fe2O3 grains greatly degraded magnetic properties of the Ba-ferrite films.

Long-range order and short-range order study on CoCrPt/Ti films by synchrotron x-ray scattering and extended x-ray absorption fine structure spectroscopy

The long-range order (LRO) and short-range order (SRO) of CoCrPt/Ti/NiP perpendicular media films were studied by synchrotron x-ray scattering and extended x-rayabsorption fine structure(EXAFS)spectroscopy in order to correlate their effects with the magnetic properties. The thickness of the magnetic media films and the NiP seed layers were kept constant, whereas the Ti underlayer thickness was varied. The increase of out-of-plane coercivity and squareness was related to the increase in the crystallinity and texture of CoCrPt (002), which first increased and then decreased with increasing Ti thickness. The best magnetic properties were observed for the magnetic film grown on the 30 nm Ti underlayer. The improvement was associated with the local atomic environment in which a relatively ordered Co, and disordered Cr and Pt existed. This suggests that a higher degree of Co and Cr phase segregation contributed to the enhancement of the magnetic properties. The LRO and SRO results are consistent with the magnetic properties.

Inhibition of abnormal acicular grain growth of nanostructured Ba–ferrite films by secondary α-Fe2O3 phase

We have studied the inhibition of abnormal acicular grain growth of the Ba–ferrite phase in the crystallization of Ba–ferrite films using real time synchrotron x-ray scattering, field emission scanning electron microscope, and vibrating sample magnetometer. Amorphous Ba–ferrite film on a SiO2 substrate is naturally crystallized into the abnormal acicular Ba–ferrite grains (major axis of ∼1000 nm in the in-plane direction). However, on α-Al2O3 substrate, fine grain sizes of the primary Ba–ferrite phase (∼70 nm) are obtained during crystallization. Nucleation of the α-Fe2O3 phase on the α-Al2O3 surface at an early stage induces the inhibition of the abnormal acicular grain growth of the Ba–ferrite phase. Because of the grain size refinement of the magnetic Ba–ferrite phase, the intrinsic coercivity of Ba–ferrite/α-Al2O3 film enhances 4.4 kOe, much larger than that of Ba–ferrite/SiO2 film (1.9 kOe). We suggest that the secondary α-Fe2O3 phase act as a useful inhibitor to abnormal grain growth in Ba–ferrite f...

Phase Differentiation and Characterization of Nanostructured Composites by Synchrotron Radiation Techniques

The complexity of nanostructured materials presents challenging difficulties in characterization using conventional techniques. For example, conventional x-ray diffraction may not provide accurate information on the structure (solid solution or phase separation) of nanostructured materials. Complementary advanced characterization methods are often required in the detailed understanding of structures. In this paper we report our work on characterization of two nanostructured systems, namely, AgNi powder and NiCo films, using synchrotron radiation techniques of x-ray diffraction, anomalous x-ray scattering and x-ray absorption spectroscopy.