Daeyoung Kwon

Structural evolution of a high Tc ferroelectric (x)Bi(Zn1∕2Ti1∕2)O3–(1−x)PbTiO3 solid solution

We have studied the structural phase transition of solid solutions (x)Bi(Zn1∕2Ti1∕2)O3–(1−x)PbTiO3. The temperature evolution of the x-ray diffraction patterns of the θ-2θ scans around {110} has been performed to check the structural phase transitions: The (101)∕(011) peak shifts to a larger value, while the (110) peak shifts to a smaller value with increasing temperature. Finally, the peaks merge to the same value at the Curie temperature, indicating a tetragonal to the cubic phase transition. The structural phase transition temperature Tc coincides with the Curie temperature and increases as the increasing Bi(Zn1∕2Ti1∕2)O3 composition (x). The evolution of the lattice constant, unit cell volume, and tetragonality as functions of temperature and composition is discussed.

Reentrant Insulating State in Ultrathin Manganite Films

The transport and magnetic properties of La0.7Sr0.3MnO3 thin-films grown by pulsed laser deposition on (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 single crystal substrates have been investigated. A systematic series with various thicknesses of La0.7Sr0.3MnO3 was used to establish a phase diagram which showed a clear difference compared to films grown on SrTiO3 substrates, highlighting the importance of film thickness, and substrate strain. At 8 unit cells, the boundary between the metallic and insulating groundstates, a second abrupt metal-insulator transition was observed at low temperatures, which could be tuned by magnetic field, and was interpreted as a signature of electronic phase separation.

Temperature-dependent neutron diffraction study of phase separation at morphotropic phase boundary in (1−x)BiScO3–xPbTiO3

We report the results of temperature-dependent neutron powder diffraction experiments on (1−x)BiScO3–xPbTiO3 (x=0.62, 0.64, and 0.66) within the morphotropic phase boundary. In all samples the monoclinic phase is found to coexist with the tetragonal phase below the ferroelectric transition temperature. The details of structural information demonstrate a delicate balance between the coexisting two phases for x=0.64. Such delicate balance is suggested to be sensitive to an external electric field, thus responsible for the enhanced electromechanical properties observed for x=0.64. Our result reveals that phase separation and the delicate balance between two phases are crucial to understand the unique properties at the morphotropic phase boundary in (1−x)BiScO3–xPbTiO3 ferroelectric systems.

Structural study on the phase separation in Sm1−xCaxMnO3 (0.8≤x≤0.92)

We have investigated the details of phase separation in the electron-doped manganites Sm1−xCaxMnO3 (0.8≤x≤0.92) by using temperature dependent x-ray diffraction. The lattice of the low-temperature orthorhombic phase is distorted to various extents according to the strength of the coexisting monoclinic phase. Meanwhile, the room-temperature lattice becomes more distorted due to the increasing Jahn–Teller active Mn3+ ions by Sm3+ doping. On the other hand, the substitution of Ca2+ with Sm3+ introduces the Mn3+–Mn4+ pairs, which favor the double exchange ferromagnetism. As a result of the competition between the lattice distortion and electron doping level, the modified phase diagram have been presented, including the high-temperature Griffiths phase and the ground state with competing multiphases.

Thermal instability of the half-metallic CrO2 film epitaxially grown on TiO2

We have investigated the thermal instability of the half-metallic CrO2 films epitaxially grown on the (002) and (200) TiO2 (rutile) substrates by chemical vapor deposition. The temperature-dependent x-ray diffraction measurement directly revealed that the epitaxial CrO2 film on TiO2 was decomposed above 250 °C and recrystallized to the heteroepitaxial Cr2O3 film above 300–350 °C. These results will be a guide for a thermal instability parameter in a development of spintronic devices using the half-metallic CrO2 film.

Enhanced Jahn–Teller distortion in the orthorhombic phase of La0.15Ca0.85MnO3 and Y0.15Ca0.85MnO3

We report the results of neutron powder diffraction experiments on La0.15Ca0.85MnO3 and Y0.15Ca0.85MnO3. After an incomplete orthorhombic-monoclinic phase transition, the residual orthorhombic phase undergoes an enhancement of Jahn–Teller distortion of MnO6 octahedra. The degree of Jahn–Teller distortion is strongly influenced by coexisting monoclinic phase. Furthermore, ferromagnetism is closely associated with this Jahn–Teller distortion, rather than the buckling effect in the orthorhombic phase. Despite the low concentration of Jahn–Teller active Mn3+ ions, our result indicates an important role of Jahn–Teller effect on physical properties of the lightly electron-doped manganites.

Growth and structural characterization of (0.2)Bi(Zn1/2Ti1/2)O3 -(0.8)PbTiO3 epitaxial thin films by off-axis rf sputtering

We report the epitaxial growth of ferroelectric (0.2)Bi(Zn1∕2Ti1∕2)O3–(0.8)PbTiO3 thin films. Despite its high c∕a ratio, the off-axis rf magnetron sputtering yields the epitaxial growth of high-quality thin films on (001) SrTiO3 substrates. The structural development of the film with increasing film thickness has been examined by using the synchrotron x-ray diffraction and x-ray reflectivity. The experimental results can be explained in terms of the strain relaxation process associated with the misfit dislocations between the thin film and the substrate.

Comparison of structural and electric properties of PbZr0.2Ti0.8O3 and CoFe2O4/PbZr0.2Ti0.8O3 films on (100)LaAlO3

In this work, we investigated structural, electrical, and magnetic properties of ferroelectric PbZr0.2Ti0.8O3 (PZT) and ferrimagnetic/ferroelectric [CoFe2O4(CFO)/PZT] bilayers grown on (100)LaAlO3 (LAO) substrates supplied with bottom 50 nm thick LaNiO3 electrodes. Interestingly, structural and electrical properties of the PZT layer exhibited remarkable changes after the top-layer CFO deposition. X-ray diffraction data suggested that both the c- and a-domains exist in the PZT layer and the tetragonality of the PZT decreases upon the top-layer deposition. A variation in the electrical properties of the PZT layer upon the CFO deposition was investigated by polarization versus voltage (P-V), capacitance versus voltage (C-V), and capacitance versus frequency (C-f) measurements. The CFO deposition induced a slight decrease of the remnant polarization and more symmetric behavior of P-V loops as well as led to the improvement of fatigue behavior. The tentative origin of enhanced fatigue endurance is discussed ba...

Epitaxial growth and piezoelectric characterization of the (1―x)BiScO3―(x)PbTiO3 ultrathin film

We studied ferroelectric properties of morphotropic phase boundary (1−x)BiScO3−(x)PbTiO3 (BSPT, x = 0.64) epitaxial thin films on SrTiO3 (001) and Nb:SrTiO3 (001). BSPT thin films with various thicknesses were deposited using off-axis radio frequency magnetron sputtering. By analyzing x-ray data of BSPT thin films with various thicknesses, we confirmed that films thinner than ∼22 nm were in a strained state. Films thicker than ∼22 nm were in a relaxed state because of the strain relaxation mechanism caused by misfit dislocation formation. Clear piezoresponses and polarization reversal phenomena can be observed in the ultrathin limit down to 8 nm through Piezo Force Microscope experiments. The piezoresponse data as a function of thickness correlates with the structural modification of thin films.

Epitaxial growth and characterization of Eu0.5Sr0.5CoO3 thin films by off-axis sputtering

We report the epitaxial growth and physical properties of Eu0.5Sr0.5CoO3 (ESCO) thin films deposited on (001) LaAlO3 (LAO) and (001) SrTiO3 (STO) substrates by off-axis rf sputtering. The magnetic properties of a grown film are governed by the crystallinity of the thin film and strain effects due to the substrate. The temperature-dependent resistivity of an optimized ESCO thin film on a LAO substrate shows a characteristic sudden decrease near the ferromagnetic transition temperature, indicating metallic double-exchange-like behavior, while the resistivity of ESCO on a STO substrate displays insulatinglike behavior because of substrate strain. These results suggest that optimized ESCO film on LAO is ideal as a bottom electrode for strained dielectric and ferroelectric heterostructures.