Su Jae Kim

Reversible ferromagnetic spin ordering governed by hydrogen in Co-doped ZnO semiconductor

We report a reversible manipulation of short-range spin ordering in Co-doped ZnO through hydrogenation and dehydrogenation processes. In both magnetic-circular dichroism and superconducting quantum interference device measurements, the ferromagnetism was clearly induced and removed by the injection and ejection of hydrogen, respectively. The x-ray photoelectron spectroscopy results and the first-principles electronic structure calculations consistently support the dependence of the ferromagnetism on the hydrogen position and the contribution of transition metal ions. The results suggest the ferromagnetic interaction between Co ions can be reversibly controlled by the hydrogen-mediated intrinsic spin ordering in Co doped ZnO.

Reproducible manipulation of spin ordering in ZnCoO nanocrystals by hydrogen mediation

Through a simple post hydrogen plasma treatment, we show the strong inducement of the room temperature ferromagnetism in Zn0.9Co0.1O:H nanocrystals fabricated by sol-gel process. X-ray diffraction, high resolution transmission electron microscopy, and neutron high resolution powder diffraction measurements of Zn0.9Co0.1O nanocrystals before and after hydrogenation process confirmed that there are no structural changes in nanocrystal and any creation of magnetic secondary phase. Nevertheless, the clear ferromagnetic hysteresis loop of the hydrogenated Zn0.9Co0.1O nanocrystal was observed in superconducting quantum interference device. These results indicate that the ferromagnetism can be derived from a short-range spin ordering by hydrogen mediation in Co doped ZnO.

Fabrication of the best conductor from single-crystal copper and the contribution of grain boundaries to the Debye temperature

The scientific community has attempted to fabricate superconductors that work at room temperature; however, this goal has not yet been achieved. Considering miniaturization of devices and the requirement of low-power consumption in electronics, it is important to enhance room-temperature conductivity. We prepared the best conductor using crystallization and enhanced its conduction using high-pressure annealing. Single-crystal copper showed a conductivity enhancement of 12–14% after treatment under high pressure and temperature compared with that of oxygen-free copper, which has the highest conductivity for a metal. The contribution of grain boundaries to the Debye temperature was calculated and the phonon excitation was maximized at a lower temperature in grain-free copper. This study aimed to develop a new conductor, in which phonon–electron scattering is reduced at high temperatures. Single-crystal copper could be used for large-scale production of Bitter magnets to generate a high magnetic field and for applications of crystal engineering.

Temperature-dependent oxygen behavior of LixNi0.5Mn1.5O4 cathode material for lithium battery

We have investigated the temperature-dependent oxygen behavior in the lithium battery cathode LixNi0.5Mn1.5O4 (LNMO) materials in the temperature range 30-1000 °C. As the temperature increases, oxygen release occurs and the change of crystal structures from the face centered cubic spinel at 30 °C to other phases follows. The amount of released oxygen and the changed crystalline phases are dependent on Li content and temperature. These phenomena are reversible against temperature in air, but not in vacuum and argon gas environments. This study illustrates the important role of temperature and atmospheric environments in synthesizing the LNMO battery materials.