Yong-Chan Cho

Magnetism depending on carrier in (Zn,Co)O

The ferromagnetic properties of (Zn,Co)O were observed by a magnetometer at low temperatures. This is, however, inconsistent with the results from the magneto-optical measurements. They reflect the presence of Co metal precipitates in (Zn,Co)O. On the other hand, the formation of Co metal precipitates was strongly suppressed by the additional Cu doping. It appears that the formation of Co metal precipitates is related to the carrier concentration in the system.

Structural Properties and Phase Transitions of Bi4 −x La x Ti3O12 Crystals

The measurement of P-E loops, dielectric, thermo-mechanical and micro-structural properties was performed on ferroelectric Bi4 − x La x Ti3O12 (BLT) crystals in order to clarify their ferroelectricity and to provide an explanation for the phase transition with La incorporated. The grown c-axis Bi4.3Ti3O12 and Bi3.65La0.47Ti3O12 crystals showed ferroelectricity but the P-E hysteresis loop of the c-axis Bi2.76La1.67Ti3O12 crystal was not fully saturated. BLT crystals tend to change from a monoclinic to an orthorhombic structure with increasing La concentration because they substitute La for Bi with a lone-pair electron. From the measurement of physical properties under varying temperatures, the ferroelectric phase transition of BLT crystals was showed to occur at temperature close to 500°C.

Analysis of the ferroelastic domains and phase transition in the K2SO4 crystal

Abstract The domain structures of the β-K2SO4 crystal were analyzed by group theory. We obtained the permissible kinds of domain association and domain walls from the results of the group theory. It is suggested from the analysis that the (110) and (130) planes are Wmb walls. The value of the spontaneous strain of K2SO4 wast = 6.32 × 10−3 at room temperature and also its temperature dependence was observed.

Phase-dependence of dielectric constants on ferroelastic domain switching stress for Pb3(PO4)2

Abstract The dependence of the dielectric constants on the applied sinusoidal mechanical field for Pb3(PO4)2 was investigated near the transition temperature. 180°C. In the ferroelastic phase, the dielectric constant showed periodic change depending on the applied stress, which is caused only by the ferroelastic domain switching. The increase and decrease of the dielectric constants under the tensional and compressive stress are attributed to the orientation change of domain under the stress. At the maximum and the minimum stress, the dielectric constants show no more sharp increase and decrease, because near these stresses possible domains were already switched to another orientation state. The dielectric change of the ferroelastic compounds under the sinusoidal stress is defined as a ferroelastic piezo-displacement.

THE COUPLING BETWEEN AN ELECTRIC FIELD AND MECHANICAL STRESS IN THE INCOMMENSURATE PHASE

Abstract The coupling between an electric field and mechanical stress was investigated, focusing on the incommensurate (IC) phase of K2ZnCl4 with the ferroelectric commensurate (C) phase. The coupling by both external fields is derived theoretically by the Landau theory. In order to prove the theoretical results, the electric field dependence of dielectric constants was studied under the mechanical stress of 0 Mpa, 1.5 MPa and 2.5 MPa. Under the stress of 2.5 Mpa, the electric field dependence of X shows the anomalous behavior from the IC phase, and it corresponds to the phase transition from the IC phase to the pseudo-C phase. Such a behavior suggests the coupling between the polarization and the strain in the IC phase.

The Studies of Ferroelastic Phase Transitions and Domain Walls in KMgCl3 Crystal

The ferroelastic phase transitions of KMgCl 3 crystal grown by the Czochralski method were investigated by direct observation using a polarizing microscope with varying temperature. The results showed that the KMgCl 3 crystal undergoes two pure ferroelastic phase transitions from cubic prototypic phase to tetragonal, and to orthorhombic structure at about 312 °C and 224 °C, respectively, and can be classified into m3mFmmm(p) species. The permissible domain walls of this species were analyzed by both the strain tensor and group theoretical method.