C. J. Won

Diode and photocurrent effect in ferroelectric BaTiO3−δ

The leakage current has been regarded as one of the major problems in ferroelectric memories. However, recent studies on the unidirectional electric transport through a ferroelectric single crystal and the giant tunnel electroresistance in ferroelectric tunnel junctions suggest a possibility to utilize this undesirable leakage current. Here, we present the diodelike transport and the significantly enhanced photocurrent effects in oxygen-deficient BaTiO3−δ single crystals, which are mainly dependent on the direction of ferroelectric polarization. Diode effects in the carrier doped conventional ferroelectrics may suggest an alternative way for the nondestructive readout of polarization states in common ferroelectric memories.

Bipolar resistance switching and photocurrent in a BaTiO3-δ thin film

The leakage current in ferroelectric materials has been considered as a problem to be overcome in ferroelectric memory applications. However, recent studies on the polarization-dependent diode effect and the photocurrent in lossy ferroelectric materials suggest the possibility of using this leakage current for a different type of resistive memory. Along this route, we investigated the resistance-switching effect in SrRuO3/BaTiO3-δ/SrRuO3 thin film capacitors. We observed the bipolar resistance switching and the switchable photocurrent, both of which were tuned by the different electric poling. Our finding may suggest an alternative type of nonvolatile ferroelectric memory capable of nondestructive readout.

Effect of antiferromagnetic order on the dielectric properties of Bi2Fe4O9

We investigated the dielectric and magnetic properties of Bi2Fe4O9 single crystals. The dielectric anomalies observed at the magnetic transition temperature indicate that there is a strong coupling between the magnetic and dielectric properties. The anisotropic magnetic contribution of the dielectric constant is discussed in terms of the spin correlation function, ⟨Si⋅Sj⟩, below the antiferromagnetic transition at 240 K.

Antiferromagnetic ordering in Li2MnO3 single crystals with a two-dimensional honeycomb lattice

Li(2)MnO(3) consists of a layered Mn honeycomb lattice separated by a single layer of LiO(6) octahedra along the c-axis. By using single crystal Li(2)MnO(3) samples, we have examined the physical properties and carried out both powder and single crystal neutron diffraction studies to determine that Mn moments order antiferromagnetically at T(N) = 36 K with an ordered magnetic moment of 2.3 μ(B) perpendicular to the ab plane. We have also discovered that about 35% of the full magnetic entropy is released in the supposedly simple paramagnetic phase, indicative of unusual spin dynamics at higher temperature.

Electronic structure of double perovskite A2FeReO6 (A = Ba and Ca): interplay between spin–orbit interaction, electron correlation, and lattice distortion

We have investigated the electronic structure of double perovskites, Ba(2)FeReO(6) (metallic) and Ca(2)FeReO(6) (insulating) using optical and x-ray absorption spectroscopy. By comparing the experimental results with the density functional theory calculations, we found that the electronic structure of Ba(2)FeReO(6) could be determined from the interaction of the electron correlation and spin-orbit coupling. On the other hand, for Ca(2)FeReO(6), the lattice distortion and electron correlation are important in determining the electronic structure. Additionally, the insulating gap in Ca(2)FeReO(6) is realized by the spin-orbit coupling. Our work shows that the subtle interplay of the spin-orbit interaction, electron correlation, and lattice distortion should be taken into account to understand the electronic structure of the 5d transition metal oxides.

Stabilization of ferromagnetic ordering in cobaltite double perovskites of La₂CoIrO₆ and La₂CoPtO₆.

We investigated the local electronic structure and magnetic properties of the cobaltite double perovskites La2CoIrO6 and La2CoPtO6 using Co L2,3-edge x-ray absorption spectroscopy and x-ray magnetic circular dichroism. Despite similarity in the local electronic structure (Co(2+) high-spin states) as well as in the crystal structure (P2(1)/n), only La2CoIrO6 exhibits substantial orbital and spin magnetic moments of Co(2+), whereas they are much weaker in the case of La2CoPtO6. This composition dependence is consistent with the results of magnetization measurements. The details of the mechanism of ferromagnetic ordering in the Co(2+) sublattice in La2CoIrO6 and the lack thereof in La2CoPtO6 are explained in terms of the orbital hybridization of the Co minority-spin t(2g) state and the Ir/Pt j(eff) = 1/2 state.

Magnetoresistance in single crystalline chromium sulfides

We studied the anisotropic magnetic and magnetotransport properties of Cr2S3 single crystals grown by using the vapor transport method. Large magnetoresistance of ∼ 46% was observed in 9 T magnetic field applied perpendicular to the c axis near the Neel temperature TN ≈ 118 K. Comparison of electric and magnetic properties of Cr2S3 crystals with different sulfur deficiencies suggested that the electron doping by the sulfur deficiency does not contribute to weak ferromagnetism. The correlation between the field-dependent magnetization and resistivity was analyzed by the polaron hopping model of magnetotransport in Cr2S3.

Crystal and Magnetic Structures of La2CoPtO6 Double Perovskite

We investigated the crystal structure and magnetic structure in cobaltite-platinate double perovskite of La2CoPtO6, employing various techniques of X-ray diffraction, neutron diffraction, and the extended X-ray absorption fine structure analysis. It is shown that the crystal symmetry is maintained as P21/n in the temperature range of <500 °C, whereas the lattice constants and the Co–Co distances undergo a continuous structural evolution toward the high-symmetry phases with increasing temperature. The Co–O bonds were overall longer and had a larger degree of structural and thermal disorders than the Pt–O bonds. As for the magnetism, an antiferromagnetic order is stabilized in the Co2+ sublattice at temperatures below 28 K. It is demonstrated that owing to the substantial distortions (quantified by a distortion parameter Σ > 0.03 Å) of the Co–Co networks, the system is not subject to spin frustration effect. Details in the magnetic structure are determined; at 12 K, the Co magnetic moment was (0.8, 0, 2.7) Bohr magneton, the magnetic propagation vector was (−0.5, 0, 0.5), and the magnetic symmetry was preferably Γ1(Ag).