So Ra Lee

The magnetic properties of Tb-Fe-(B) thin films fabricated by rf magnetron sputtering

Abstract The magnetic properties of giant magnetostrictive TbFe and TbFeB thin films are systematically investigated. The films were fabricated by rf magnetron sputtering using a composite target which consists of an Fe plate and Tb chips. The introduction of B to the films was done by using an Fe plate containing B. The microstructure, examined by X-ray diffraction, mainly consists of an amorphous phase and, at high Tb contents, a pure Tb phase also exists. A progressive change in the direction of anisotropy from the perpendicular to in-plane occurs as the Tb content increases from 35 to 68 at.% for the B-free TbFe films and from 44 to 67 at.% for the TbFeB thin films. The boundary at which the anisotropy change occurs shifts significantly towards to higher Tb contents when B is introduced to the films. The saturation magnetization exhibits a maximum at the Tb contents of 42 and 48 at.% and the values are about 5800 and 5100 G for the TbFe and TbFeB thin films, respectively. The coercive force, measured in the easy direction, decreases monotonically with the Tb content and ranges from 7.5 to 100 Oe when the films with in-plane anisotropy are considered. The magnetic softness of the present film which exhibits the coercive force of below 10 Oe and a typical squared-loop shape, and saturates at a low field of 1 kOe, is considered to be the best, to our knowledge, among this kind of thin films.

Phase change induced degradation of amorphous vanadium oxide cathode thin film during charge-discharge

Amorphous vanadium oxide (V2O5) is a very good candidate as material for cathode thin film since it has a relatively high capacity. In addition, the room temperature deposition process is valuable in V2O5 thin film fabrication. Due to these advantages, much effort to grow amorphous V2O5 thin film has been made. In this research, we successfully grew amorphous V2O5 thin film using room temperature sputtering. Based on a Li/LiPON/V2O5 full cell structure, charge-discharge performances were measured according to cycling number. Even though the full cell structure showed an average capacity of 15 μAh/cm2 over more than 500 cycles, a capacity fade was shown after a few cycles. Many reports revealed that the phase change of V2O5 from amorphous to crystalline made this kind of capacity fade. In order to investigate this phenomenon, high-resolution transmission electron microscopy (HRTEM) was employed. The as-deposited V2O5 thin film consisted of a homogeneous amorphous without any grain-boundary and/or polycrystalline island. However, the microcrystalline V2O5 phase was randomly distributed in the amorphous V2O5 thin film matrix after 450 cycles by cross sectional TEM (XTEM). That is, some amorphous phase in the V2O5 thin film matrix changed to the crystalline phase. This crystalline phase strongly prevented the extraction of Li ions during the charge process, which induced the irreversible diffusion of Li ions from cathode to anode. From this result, a high efficiency thin film battery based on amorphous V2O5 can be fabricated by preventing amorphous-crystal phase transformation during cycling.