Sangwoo Ryu

Co-metal clustering as the origin of ferromagnetism in Co-doped ZnO thin films

The origin of ferromagnetism in ZnO-based systems was investigated using Co-doped ZnO thin films as prototypical examples of II–VI-based diluted magnetic semiconductors. In spite of the atomic-scale dissolution of Co ions in wurtzite ZnO, both the magnetization-temperature curve and the magnetization-field curve demonstrated that Zn1−xCoxO thin films were paramagnetic for x⩽0.12. On the other hand, Zn1−xCoxO films with x greater than 0.12 were characterized by the Co-metal clustering and apparently showed room-temperature ferromagnetism. The discrepancy between the zero-field cooling and the field cooling curves further indicates that Co-doped ZnO films (for x>0.12) are superparamagnetic and the observed ferromagnetism originates from the nanometer-sized Co clusters.

Polarized Raman scattering of multiferroic BiFeO3 epitaxial films with rhombohedral R3c symmetry

Highly (111)-oriented rhombohedral BiFeO3 (BFO) thin films were grown on (111) SrTiO3 substrates by pulsed laser deposition. Polarized Raman-scattering study of the (111)-oriented epitaxial BFO thin film with rhombohedral R3c symmetry was carried out by employing two distinct backscattering geometries. The A1-symmetry transverse-optical [A1(TO)] phonons were selectively isolated from the E-symmetry transverse-optical [E(TO)] phonons by employing Y′(ZZ)Y′ polarization configuration in a novel side-view backscattering. By comparing the Y′(ZZ)Y′ spectrum with the Z(X′X′)Z polarization spectrum in a normal backscattering, we were able to assign most of A1 and E-symmetry normal modes of R3c BFO. In addition, we found that there was a negligible LO-TO splitting in the A1-symmetry normal modes.

Epitaxially grown La-modified BiFeO3 magnetoferroelectric thin films

Effects of the La modification on the structure and magnetoferroelectric properties of BiFeO3 (BFO)-based films were examined. On SrTiO3 (001) planes, all the BFO-based films (for x between 0 and 0.15 in Bi1−xLaxFeO3 with the film thickness of 300 nm) were grown epitaxially along [001] direction of tetragonal symmetry. However, the La modification gradually changes the film structure from a monoclinically tilted state to a nontilted tetragonal-like state. Both extended x-ray absorption fine structure and x-ray absorption near edge structure spectra revealed the presence of a strong in-plane compressive stress. Room-temperature magnetization-field curves indicated that the saturation magnetization of these BFO-based epitaxial films increased with the degree of La modification. La-modified BFO film capacitors fabricated on SrRuO3-buffered SrTiO3 (001) substrates showed fatigue-free ferroelectric switching characteristics up to 4×1010 read∕write cycles at a frequency of 1 MHz.

Magnetoelectric coupling of [00l]-oriented Pb(Zr0.4Ti0.6)O3–Ni0.8Zn0.2Fe2O4 multilayered thin films

Multilayered thin films consisting of alternatively stacking Pb(Zr0.4Ti0.6)O3 (PZT) and Ni0.8Zn0.2Fe2O4 (NZFO) layers were fabricated to exploit a strain-mediated coupling of piezoelectricity and magnetostriction. The 450-nm-thick PZT/NZFO multilayer fabricated by pulsed laser deposition showed magnetodielectric effects upon applying a static magnetic field. The magnetoelectric (ME) susceptibility values estimated using these magnetodielectric responses were in the range of 15–30mV∕cmOe at a zero magnetic-field strength and were comparable to those obtained using a more commonly employed “dynamic” ME method.

A Nonvolatile Memory Device Made of a Ferroelectric Polymer Gate Nanodot and a Single-Walled Carbon Nanotube

We demonstrate a field-effect nonvolatile memory device made of a ferroelectric copolymer gate nanodot and a single-walled carbon nanotube (SW-CNT). A position-controlled dip-pen nanolithography was performed to deposit a poly(vinylidene fluoride-ran-trifluoroethylene) (PVDF-TrFE) nanodot onto the SW-CNT channel with both a source and drain for field-effect transistor (FET) function. PVDF-TrFE was chosen as a gate dielectric nanodot in order to efficiently exploit its bipolar chemical nature. A piezoelectric force microscopy study confirmed the canonical ferroelectric responses of the PVDF-TrFE nanodot fabricated at the center of the SW-CNT channel. The two distinct ferroelectric polarization states with the stable current retention and fatigue-resistant characteristics make the present PVDF-TrFE-based FET suitable for nonvolatile memory applications.

Dip-Pen Lithography of Ferroelectric PbTiO3 Nanodots

Dip-pen nanolithography of ferroelectric PTO nanodots is described. This position-controlled dip-pen nanolithography using a silicon nitride cantilever produced an array of ferroelectric nanodots with a minimum lateral dimension of approximately 37 nm on a Nb-doped SrTiO(3) substrate. This minimum-sized PTO dot is characterized by single-domain epitaxial growth with an enhanced tetragonality (c/a ratio) of 1.08.

Magnetoelectric coupling susceptibility from magnetodielectric effect

A multiferroic material with a pronounced degree of the magnetoelectric (ME) coupling at room temperature is of great technological importance. A widely used method of indirectly assessing the ME coupling is based on the magnetodielectric (MD) effect, which accounts for the variation in dielectric permittivity under an applied bias magnetic field. At the present stage, however, there is no systematic guideline on evaluating the ME coupling susceptibility from the MD effect. Here we propose a simple theoretical method of estimating the ME coupling susceptibility using the MD data. This alternative approach is applied to an epitaxially grown La-doped BiFeO3 thin film as an illustrative example.

Polarization switching characteristics of BiFeO3 thin films epitaxially grown on Pt/MgO at a low temperature

An [001]-oriented BiFeO3 (BFO) thin film having a pseudotetragonal symmetry was epitaxially grown on a Pt/MgO (001) substrate. The Pt-buffered MgO substrate enabled us to fabricate an epitaxial heterostructure at a temperature as low as 500 °C. We examined three major criteria for high-density ferroelectric memories using this BFO/Pt film capacitor. The polarization switching experiment has demonstrated that the film is electrically fatigue-free and possesses stable charge-retention characteristics with a reasonably large sensing margin of 22 μC/cm2. These suggest potential applicability of the present BFO/Pt heteroepitaxial film capacitor to nonvolatile memories.

Electric-field-control of magnetic remanence of NiFe2O4 thin film epitaxially grown on Pb(Mg1/3Nb2/3)O3–PbTiO3

We propose an asymmetric bilayer structure in which the magnetic remanence (MR) is controlled by the in-plane strain of the top NiFe2O4 (NFO) layer epitaxially constrained by the bottom Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) substrate. In this asymmetric structure, an electric-field-induced giant piezoelectric strain from the bottom PMN-PT layer is effectively transferred to the top NFO layer. We have shown that the room-temperature magnetic remanence (MR) of the 100-nm-thick NFO layer is enhanced by 46% when an electric-field-induced in-plane compressive strain is about −0.1%. Synchrotron x-ray absorption near edge structure study supports a scenario of the cation-charge redistribution between Ni2+ and Fe3+ ions under the condition of an electric-field-induced in-plane compressive strain.

Electric-field-induced structural modulation of epitaxial BiFeO3 multiferroic thin films as studied using x-ray microdiffraction

An in situ method, called synchrotron x-ray microdiffraction, was introduced to examine the electric-field-induced structural modulation of the epitaxially grown pseudotetragonal BiFeO3 thin film. To evaluate the d spacing (d001) from the measured intensity contour in the 2θ-χ space, the peak position in each diffraction profile was determined by applying two-dimensional Lorentzian fitting. By tracing the change of d spacing as a function of the applied electric field and by examining the Landau free energy function for P4mm symmetry, the authors were able to estimate the two important parameters that characterize the field-induced structural modulation. The estimated linear piezoelectric coefficient (d33) at zero-field limit is 15pm∕V, and the effective nonlinear electrostrictive coefficient (Qeff) is as low as ∼8.0×10−3m4∕C2.