J. S. Ahn

Bio: J. S. Ahn is an academic researcher from Pusan National University. The author has contributed to research in topics: Phonon & Charge ordering. The author has an hindex of 18, co-authored 41 publications receiving 2877 citations. Previous affiliations of J. S. Ahn include Seoul National University & Rutgers University.

Electric polarization reversal and memory in a multiferroic material induced by magnetic fields.

Abstract: Ferroelectric and magnetic materials are a time-honoured subject of study and have led to some of the most important technological advances to date. Magnetism and ferroelectricity are involved with local spins and off-centre structural distortions, respectively. These two seemingly unrelated phenomena can coexist in certain unusual materials, termed multiferroics1,2,3,4,5,6,7,8,9,10,11. Despite the possible coexistence of ferroelectricity and magnetism, a pronounced interplay between these properties has rarely been observed6,12. This has prevented the realization of multiferroic devices offering such functionality13. Here, we report a striking interplay between ferroelectricity and magnetism in the multiferroic TbMn2O5, demonstrated by a highly reproducible electric polarization reversal and permanent polarization imprint that are both actuated by an applied magnetic field. Our results point to new device applications such as magnetically recorded ferroelectric memory.

Mid-infrared properties of a VO 2 film near the metal-insulator transition

Abstract: A ${\mathrm{VO}}_{2}$ film was grown on a sapphire(0001) substrate using pulsed laser deposition. The film showed a first-order metal-insulator (MI) transition and its dc conductivity started to increase drastically near 68 \ifmmode^\circ\else\textdegree\fi{}C and changed by three orders of magnitude. Mid-infrared transmittance and reflectance spectra of the ${\mathrm{VO}}_{2}$ film were measured between 20 \ifmmode^\circ\else\textdegree\fi{}C and 90 \ifmmode^\circ\else\textdegree\fi{}C. Using the intensity transfer-matrix method, the frequency-dependent dielectric constant ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$(\ensuremath{\omega}) and the conductivity ${\mathrm{\ensuremath{\sigma}}}_{\mathit{f}}$(\ensuremath{\omega}) of the film were obtained between 1600 and 4000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ from the measured transmittance and reflectance spectra. With the ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$(\ensuremath{\omega}) and ${\mathrm{\ensuremath{\sigma}}}_{\mathit{f}}$(\ensuremath{\omega}) spectra, mid-infrared properties of the ${\mathrm{VO}}_{2}$ film near the MI transition region were investigated in detail. Above 78 \ifmmode^\circ\else\textdegree\fi{}C, ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$(\ensuremath{\omega})0 and d${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$/d\ensuremath{\omega}g0, which is a typical metallic behavior. In particular, ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$(\ensuremath{\omega}) and ${\mathrm{\ensuremath{\sigma}}}_{\mathit{f}}$(\ensuremath{\omega}) at 88 \ifmmode^\circ\else\textdegree\fi{}C were analyzed in terms of extended Drude model in which the frequency-dependent scattering rate and the effective mass could be obtained. The mean free path of charge carriers in the dc limit was estimated to be larger by an order of magnitude than the previously reported value, i.e., 4 \AA{}. Below 74 \ifmmode^\circ\else\textdegree\fi{}C, ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$(\ensuremath{\omega})g0 and d${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$/d\ensuremath{\omega}\ensuremath{\approxeq}0, which is characteristic of an insulator. Interestingly, ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$ in the insulating region increased as the temperature approached the MI transition temperature. To explain this anomalous behavior, the MI transition of the ${\mathrm{VO}}_{2}$ film was modeled with coexistence of metallic and insulating domains and their dynamic evolution. Then the behaviors of ${\mathrm{\ensuremath{\epsilon}}}_{\mathit{f}}$(\ensuremath{\omega}) and ${\mathrm{\ensuremath{\sigma}}}_{\mathit{f}}$(\ensuremath{\omega}) were explained using the effective medium approximation, which is a mean-field theory predicting a percolation transition. This work clearly demonstrates that the transport and optical properties near the MI transition region are strongly influenced by the connectivity of the metallic domains. \textcopyright{} 1996 The American Physical Society.

Thermal conductivity of geometrically frustrated, ferroelectric YMnO3: extraordinary spin-phonon interactions.

Abstract: The thermal conductivity of the magnetically frustrated, ferroelectric ${\mathrm{Y}\mathrm{M}\mathrm{n}\mathrm{O}}_{3}$ exhibits an isotropic suppression in the cooperative paramagnetic state, followed by a sudden increase upon magnetic ordering. This unprecedented behavior without an associated static structural distortion probably originates from the strong dynamic coupling between acoustic phonons and low-energy spin fluctuations in geometrically frustrated magnets. The replacement of magnetic Ho for Y at the ferroelectrically active site results in an even larger effect, suggestive of the strong influence of multiferroicity.

Exchange interaction effects on the optical properties of LuMnO3.

Abstract: We have measured the optical conductivity of single crystal LuMnO3 from 10 to 45000 cm(-1) at temperatures between 4 and 300 K. A symmetry allowed on-site Mn d-d transition near 1.7 eV is observed to blueshift ( approximately 0.1 eV) in the antiferromagnetic state due to Mn-Mn superexchange interactions. Similar anomalies are observed in the temperature dependence of the TO phonon frequencies which arise from spin-phonon interaction. We find that the known anomaly in the temperature dependence of the quasistatic dielectric constant epsilon(0) below T(N) approximately 90 K is overwhelmingly dominated by the phonon contributions.

Ferroelectricity driven by Y d0-ness with rehybridization in YMnO3.

Abstract: We investigated electronic structure of hexagonal multiferroic YMnO3 using the polarization dependent x-ray absorption spectroscopy (XAS) at O K and Mn L(2,3) edges. The spectra exhibit strong polarization dependence at both edges, reflecting anisotropic Mn 3d orbital occupation. Moreover, the O K edge spectra show that Y 4d states are strongly hybridized with O 2p ones, resulting in large anomalies in Born effective charges on off-centering Y and O ions. These results manifest that the Y d(0)-ness with rehybridization is the driving force for the ferroelectricity, and suggest a new approach to understand the multiferroicity in the hexagonal manganites.

Multiferroic and magnetoelectric materials

Abstract: A ferroelectric crystal exhibits a stable and switchable electrical polarization that is manifested in the form of cooperative atomic displacements. A ferromagnetic crystal exhibits a stable and switchable magnetization that arises through the quantum mechanical phenomenon of exchange. There are very few 'multiferroic' materials that exhibit both of these properties, but the 'magnetoelectric' coupling of magnetic and electrical properties is a more general and widespread phenomenon. Although work in this area can be traced back to pioneering research in the 1950s and 1960s, there has been a recent resurgence of interest driven by long-term technological aspirations.

Revival of the Magnetoelectric Effect

Abstract: Recent research activities on the linear magnetoelectric (ME) effect?induction of magnetization by an electric field or of polarization by a magnetic field?are reviewed. Beginning with a brief summary of the history of the ME effect since its prediction in 1894, the paper focuses on the present revival of the effect. Two major sources for 'large' ME effects are identified. (i) In composite materials the ME effect is generated as a product property of a magnetostrictive and a piezoelectric compound. A linear ME polarization is induced by a weak ac magnetic field oscillating in the presence of a strong dc bias field. The ME effect is large if the ME coefficient coupling the magnetic and electric fields is large. Experiments on sintered granular composites and on laminated layers of the constituents as well as theories on the interaction between the constituents are described. In the vicinity of electromechanical resonances a ME voltage coefficient of up to 90?V?cm?1?Oe?1 is achieved, which exceeds the ME response of single-phase compounds by 3?5 orders of magnitude. Microwave devices, sensors, transducers and heterogeneous read/write devices are among the suggested technical implementations of the composite ME effect. (ii) In multiferroics the internal magnetic and/or electric fields are enhanced by the presence of multiple long-range ordering. The ME effect is strong enough to trigger magnetic or electrical phase transitions. ME effects in multiferroics are thus 'large' if the corresponding contribution to the free energy is large. Clamped ME switching of electrical and magnetic domains, ferroelectric reorientation induced by applied magnetic fields and induction of ferromagnetic ordering in applied electric fields were observed. Mechanisms favouring multiferroicity are summarized, and multiferroics in reduced dimensions are discussed. In addition to composites and multiferroics, novel and exotic manifestations of ME behaviour are investigated. This includes (i) optical second harmonic generation as a tool to study magnetic, electrical and ME properties in one setup and with access to domain structures; (ii) ME effects in colossal magnetoresistive manganites, superconductors and phosphates of the LiMPO4 type; (iii) the concept of the toroidal moment as manifestation of a ME dipole moment; (iv) pronounced ME effects in photonic crystals with a possibility of electromagnetic unidirectionality. The review concludes with a summary and an outlook to the future development of magnetoelectrics research.

Multiferroics: a magnetic twist for ferroelectricity

Abstract: Magnetism and ferroelectricity are essential to many forms of current technology, and the quest for multiferroic materials, where these two phenomena are intimately coupled, is of great technological and fundamental importance. Ferroelectricity and magnetism tend to be mutually exclusive and interact weakly with each other when they coexist. The exciting new development is the discovery that even a weak magnetoelectric interaction can lead to spectacular cross-coupling effects when it induces electric polarization in a magnetically ordered state. Such magnetic ferroelectricity, showing an unprecedented sensitivity to ap plied magnetic fields, occurs in 'frustrated magnets' with competing interactions between spins and complex magnetic orders. We summarize key experimental findings and the current theoretical understanding of these phenomena, which have great potential for tuneable multifunctional devices.

Berry phase effects on electronic properties

Abstract: Ever since its discovery, the Berry phase has permeated through all branches of physics. Over the last three decades, it was gradually realized that the Berry phase of the electronic wave function can have a profound effect on material properties and is responsible for a spectrum of phenomena, such as ferroelectricity, orbital magnetism, various (quantum/anomalous/spin) Hall effects, and quantum charge pumping. This progress is summarized in a pedagogical manner in this review. We start with a brief summary of necessary background, followed by a detailed discussion of the Berry phase effect in a variety of solid state applications. A common thread of the review is the semiclassical formulation of electron dynamics, which is a versatile tool in the study of electron dynamics in the presence of electromagnetic fields and more general perturbations. Finally, we demonstrate a re-quantization method that converts a semiclassical theory to an effective quantum theory. It is clear that the Berry phase should be added as a basic ingredient to our understanding of basic material properties.

Multiferroic magnetoelectric composites: Historical perspective, status, and future directions

Abstract: Multiferroic magnetoelectric materials, which simultaneously exhibit ferroelectricity and ferromagnetism, have recently stimulated a sharply increasing number of research activities for their scientific interest and significant technological promise in the novel multifunctional devices. Natural multiferroic single-phase compounds are rare, and their magnetoelectric responses are either relatively weak or occurs at temperatures too low for practical applications. In contrast, multiferroic composites, which incorporate both ferroelectric and ferri-/ferromagnetic phases, typically yield giant magnetoelectric coupling response above room temperature, which makes them ready for technological applications. This review of mostly recent activities begins with a brief summary of the historical perspective of the multiferroic magnetoelectric composites since its appearance in 1972. In such composites the magnetoelectric effect is generated as a product property of a magnetostrictive and a piezoelectric substance. A...