Y. A. Park

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.

Photo-carrier control of exchange bias in BiFeO3/La2/3Sr1/3MnO3 thin films

We report the facile control of exchange bias in BiFeO3/La2/3Sr1/3MnO3 (BFO/LSMO) thin films on an SrTiO3 (STO) substrate using light irradiation. Illumination with weak red light (λ: 630 nm, intensity: ∼1 mW/cm2) reduced the exchange bias field (HE) of BFO/LSMO from +30 Oe in the dark to −2 Oe with red light. In accompanying the decrease of HE, the resistance of BFO/LSMO significantly increased. These results were attributed to the reduction in the hole-doping ratio of LSMO and the weakened exchange coupling between Fe and Mn spins at the interface, resulting from photo-injected electrons from the STO substrate. Successive turning on/off of red light gives rise to cyclical change of corresponding HE, which should be useful for applications like photo-controlled tunneling magnetoresistance devices.

Observation of intriguing exchange bias in BiFeO3 thin films

We have reported an intriguing exchange bias in BiFeO3 thin films on SrTiO3 substrates. While the field cooling temperature (∼300 K) is far below the Neel temperature of BiFeO3 (∼640 K), we clearly observe the exchange bias (∼−225 Oe) at 10 K. The exchange bias becomes strong with lowering temperature and depends on the applied magnetic field direction during cooling, irrespective of crystallographic directions. Magnetic force microscopy measurements reveal the existence of a ferri/ferromagnetic material, probably γ-Fe2O3, at the grain boundary of antiferromagnetic BiFeO3, forming a core-shell like structure. Based on the uncompensated spins and strong anisotropy of antiferromagnetic layer, we have explained the observed exchange bias.

Suppressed magnetoelectric effect in epitaxially grown multiferroic Pb(Zr0.57Ti0.43)O3–Pb(Fe2/3W1/3)O3 solid-solution thin films

We report on the epitaxial growth of single-phase [Pb(Zr0.57Ti0.43)O3]0.8[Pb(Fe2/3W1/3)O3]0.2 (PZT–PFW) solid-solution thin films using pulsed laser deposition. X-ray diffraction measurements reveal that the films have a tetragonal structure. The films exhibit ferroelectric properties and weak ferromagnetic responses at room temperature. The magnetoelectric effects were investigated; the nonlinear magnetocapacitance coefficient, β33, was measured and found to be comparable to those of multiferroic hexagonal manganites, but at least two orders of magnitude smaller than that for polycrystalline PZT–PFW films.

Application of magnetic fields for a low temperature growth of high-quality SrRuO3 thin films

We report the low temperature growth of high-quality SrRuO3 thin films by applying moderate magnetic fields (Heclipse) during the growth. The lattice constants, the temperature dependence of resistivity and the ferromagnetic transition temperature of the film grown under Heclipse = 2500?Oe at a temperature as low as 400??C were quite similar to the reported values for the film grown at high temperatures of 700?800??C under no magnetic field. Moreover, we found that the magnetic coercive field decreased significantly with increasing Heclipse, which should be quite useful for a coercivity tunable top electrode for magnetic tunnel junctions.