Seungwoo Song

Structurally tailored hexagonal ferroelectricity and multiferroism in epitaxial YbFeO3 thin-film heterostructures.

Multiferroics have received a great deal of attention because of their fascinating physics of order-parameter cross-couplings and their potential for enabling new device paradigms. Considering the rareness of multiferroic materials, we have been exploring the possibility of artificially imposing ferroelectricity by structurally tailoring antiferromagnets in thin-film forms. YbFeO(3) (YbFO hereafter), a family of centrosymmetric rare-earth orthoferrites, is known to be nonferroelectric (space group Pnma). Here we report that a YbFO thin-film heterostructure fabricated by adopting a hexagonal template surprisingly exhibits nonferroelastic ferroelectricity with the Curie temperature of 470 K. The observed ferroelectricity is further characterized by an extraordinary two-step polarization decay, accompanied by a pronounced magnetocapacitance effect near the lower decay temperature, ~225 K. According to first-principles calculations, the hexagonal P6(3)/mmc-P6(3)mc-P6(3)cm consecutive transitions are primarily responsible for the observed two-step polarization decay, and the ferroelectricity originates from the c-axis-oriented asymmetric Yb 5d(z(2))-O 2p(z) orbital hybridization. Temperature-dependent magnetization curves further reveal an interesting phenomenon of spontaneous magnetization reversal at 83 K, which is attributed to the competition between two distinct magnetocrystalline anisotropy terms, Fe 3d and Yb 4f moments.

Four-states multiferroic memory embodied using Mn-doped BaTiO3 nanorods.

Multiferroics that show simultaneous ferroic responses have received a great deal of attention by virtue of their potential for enabling new device paradigms. Here, we demonstrate a high-density four-states multiferroic memory using vertically aligned Mn-doped BaTiO3 nanorods prepared by applying the dip-pen nanolithography technique. In the present nanorods array, the polarization (P) switching by an external electric field does not influence the magnetization (M) of the nanorod owing to a negligible degree of the P-M cross-coupling. Similarly, the magnetic-field-induced M switching is unaffected by the ferroelectric polarization. On the basis of these, we are able to implement a four-states nonvolatile multiferroic memory, namely, (+P,+M), (+P,-M) ,(-P,+M), and (-P,-M) with the reliability in the P and M switching. Thus, the present work makes an important step toward the practical realization of multistate ferroic memories.

On the failure indices of quadratic failure criteria for optimal stacking sequence design of laminated plate

The quadratic failure criterion, which is intended to predict fracture, may be used as an object function for optimal stacking sequence design of laminated plate. However, calculations using a symmetric laminated plate demonstrate that Tsai-Wu theory may give incorrect optimum predictions under uniaxial loading.

Bandgap Tuning with Thermal Residual Stresses Induced in a Quantum Dot

Lattice distortion induced by residual stresses can alter electronic and mechanical properties of materials significantly. Herein, a novel way of the bandgap tuning in a quantum dot (QD) by lattice distortion is presented using 4-nm-sized CdS QDs grown on a TiO2 particle as an application example. The bandgap tuning (from 2.74 eV to 2.49 eV) of a CdS QD is achieved by suitably adjusting the degree of lattice distortion in a QD via the tensile residual stresses which arise from the difference in thermal expansion coefficients between CdS and TiO2. The idea of bandgap tuning is then applied to QD-sensitized solar cells, achieving ≈60% increase in the power conversion efficiency by controlling the degree of thermal residual stress. Since the present methodology is not limited to a specific QD system, it will potentially pave a way to unexplored quantum effects in various QD-based applications.

Implementing Room-Temperature Multiferroism by Exploiting Hexagonal-Orthorhombic Morphotropic Phase Coexistence in LuFeO3 Thin Films

Room-temperature multiferroism in LuFeO3 (LFO) films is demonstrated by exploiting the orthorhombic-hexagonal (o-h) morphotrophic phase coexistence. The LFO film further reveals a magnetoelectric coupling effect that is not shown in single-phase (h- or o-) LFO. The observed multiferroism is attributed to the combination of sufficient polarization from h-LFO and net magnetization from o-LFO.

Anomalous domain periodicity observed in ferroelectric PbTiO3 nanodots having 180° stripe domains.

Nanometer-scale ferroelectric dots and tubes have received a great deal of attention owing to their potential applications to nonvolatile memories and multi-functional devices. As for the size effect of 180° stripe domains in ferroelectric thin films, there have been numerous reports on the thickness-dependent domain periodicity. All these studies have revealed that the domain periodicity (w) of 180° stripe domains scales with the film thickness (d) according to the classical Landau-Lifshitz-Kittel (LLK) scaling law (w ∝ d1/2) down to the thickness of ~2 nm. In the case of PbTiO3 nanodots, however, we obtained a striking correlation that for the thickness less than a certain critical value, dc (~35 nm), the domain width even increases with decreasing thickness of the nanodot, which surprisingly indicates a negative value in the LLK scaling-law exponent. On the basis of theoretical considerations of dc, we attributed this anomalous domain periodicity to the finite lateral-size effect of a ferroelectric nanodot with an additional effect possibly coming from the existence of a thin non-ferroelectric surface layer.

Mode coupling between nonpolar and polar phonons as the origin of improper ferroelectricity in hexagonal LuMnO3

Currently, the most puzzling problem associated with the hexagonal LuMnO3 (h-LMO) is a large temperature-gap between the structural phase transition to the polar P63cm phase at ∼1290 K and the emergence of the spontaneous polarization at a substantially reduced temperature, ∼750 K. Interestingly, this large temperature-gap is not limited to h-LMO but is a universal phenomenon valid for other rare-earth manganites. We have examined this important issue by exploiting first-principles calculations. It is shown that the structural phase transition to the polar P63cm phase from the nonpolar P63/mmc phase of h-LMO is mediated by the freezing-in of the zone-boundary K3 phonon. However, the ferroelectric polarization remains at a negligibly small value until the amplitude of the K3 phonon reaches a certain critical value above which the coupling of the polar Γ2− mode with the nonpolar K3 mode is practically turned on. This coupling-induced polarization explains the observed temperature-gap in h-LMO as well as other rare-earth manganites.

Piezoelectrically enhanced exchange bias in multiferroic heterostructures

We propose a multiferroic tri-layer structure in which two magnetic layers are epitaxially constrained on a bottom piezoelectric substrate. In this structure, the exchange bias is observed due to interface spin coupling between the two magnetic layers. We also show that the exchange bias is significantly modulated by the application of an external electric field at room temperature, and the modulation in exchange bias is reproducible and reversible.

Pulsed laser deposition of Pb(Zr0.52Ti0.48)O3 thin film on cobalt ferrite nano-seed layered Pt(111)/Si substrate: effect of oxygen pressure

The effect of oxygen pressure during pulsed laser deposition of Pb(Zr0.52Ti0.48)O3 (PZT) thin films on CoFe2O4 nano-seed layered Pt(111)/Si substrate was investigated. The PZT film deposited at oxygen pressure lower than 25 mTorr is identified as both perovskite and pyrochlore phases and the films deposited at high oxygen pressure (50-100 mTorr) show the single-phase perovskite PZT that has a perfect (111)-orientation. In addition, the film deposited at PO2 of 50 mTorr has a uniform surface morphology, whereas the film deposited at PO2 of 100 mTorr has a non-uniform surface morphology and more incompacted columnar cross-section microstructure. The polarization of film deposited at 100 mTorr is higher than that deposited at 50 mTorr, but shift of the hysteresis loop along the electrical field axis in the film deposited at PO2 of 100 mTorr is larger than that of the film deposited at PO2 of 50 mTorr.

Polar P63cm phase as a marginally stable ground-state structure of InMnO3: First-principles study

To resolve a dispute associated with the ferroelectricity in hexagonal InMnO3 (h-IMO), we have examined the ground-state structure by exploiting density-functional theory calculations. It is shown that the ferroelectric phase is marginally stable over the nonpolar phase for a wide range of the external pressure. However, the computed Kohn-Sham energy predicts an interesting crossover from the polar state to the nonpolar state beginning at a compressive strain of ?1%. The partial density of states (PDOS) supports our previous finding that the In 4d-O 2p hybridization is the main bonding mechanism directly related to the manifestation of ferroelectricity in h-IMO. In addition, the computed PDOS does not show any evidence of the In 5s-O 2p orbital overlapping which had been asserted to be the main bonding interaction in the nonpolar phase.