S. Y. Park

Metamaterial Absorber for Electromagnetic Waves in Periodic Water Droplets.

Perfect metamaterial absorber (PMA) can intercept electromagnetic wave harmful for body in Wi-Fi, cell phones and home appliances that we are daily using and provide stealth function that military fighter, tank and warship can avoid radar detection. We reported new concept of water droplet-based PMA absorbing perfectly electromagnetic wave with water, an eco-friendly material which is very plentiful on the earth. If arranging water droplets with particular height and diameter on material surface through the wettability of material surface, meta-properties absorbing electromagnetic wave perfectly in GHz wide-band were shown. It was possible to control absorption ratio and absorption wavelength band of electromagnetic wave according to the shape of water droplet–height and diameter– and apply to various flexible and/or transparent substrates such as plastic, glass and paper. In addition, this research examined how electromagnetic wave can be well absorbed in water droplets with low electrical conductivity unlike metal-based metamaterials inquiring highly electrical conductivity. Those results are judged to lead broad applications to variously civilian and military products in the future by providing perfect absorber of broadband in all products including transparent and bendable materials.

Controlled magnetic nanofiber hydrogels by clustering ferritin.

We have fabricated biocompatible nanofiber hydrogels with diverse sizes of ferritin clusters according to the mixing temperature of solutions employing electrospinning. Poly(vinyl alcohol) (PVA) was used as a polymeric matrix for fabricating nanocomposites. By thermal means we controlled the interaction between the host PVA hydrogel and the protein shell on ferritin bionanoparticles to vary the size and concentration of ferritin clusters. The clustering of ferritin was based on the partial unfolding of a protein shell of ferritin. By studying the magnetic properties of the PVA/ferritin nanofibers according to the mixing temperature of the PVA/ferritin solutions, we confirmed that the clustering process of the ferritin was related to changes in the superparamagnetic properties and magnetic resonance imaging (MRI) contrast of the PVA/ferritin nanofibers. PVA/ferritin nanofiber hydrogels with diverse spatial distributions of ferritin nanoparticles are applicable as MRI-based noninvasive detectable cell culture scaffolds and as artificial muscles because of their improved superparamagnetic properties.

Evidence for the charge-ordered state and phase separation at room temperature in half-doped La0.5Ca0.5MnO3 films

The microstructure and the magnetotransport properties of La0.5Ca0.5MnO3 films, prepared by rf magnetron sputtering on a LaAlO3 substrate, have been investigated. The electron-diffraction and the high-resolution electron-microscopy studies reveal the coexistence at room temperature of double-period modulated (charge-ordered) and unmodulated (charge-disordered) phases. It is suggested that the high-temperature fragmentation of film results in the magnetic-phase-separated state at low temperatures.

Controllable switching behavior of defect modes in one-dimensional heterostructure photonic crystals

A dimerlike-positional-correlation heterostructure is used in one-dimensional photonic crystals to introduce the defect mode with perfect transmittance. The switching behavior of the transmittance, at the defect mode, is demonstrated theoretically. When the normal incident beam is tilted at a negligibly small angle, the perfect transmittance peak vanishes. It is found that this condition causing this phenomenon can be easily met and controlled through elucidation on the mechanism. This finding is significant for potential applications in high-precision filters and optical switches.

Magnetic and transport properties driven by lattice strain in La0.7Ca0.3MnO3∕BaTiO3 and La0.7Sr0.3MnO3∕BaTiO3 bilayer films

The microstructure and the magnetic and transport properties of La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3 films deposited on a BaTiO3 layer (LCMO/BTO and LSMO/BTO) and on a LaAlO3(001) single crystal (LCMO/LAO and LSMO/LAO) by rf-magnetron sputtering using “soft” (or powder) targets are investigated. The films grown on BTO demonstrate biaxial tensile in-plane and compressive out-of-plane strains, while the films grown on LAO, in contrast, manifest compressive in-plane and tensile out-of-plane strains. The films with biaxial tensile in-plane lattice strain undergo the magnetic transition at a higher temperature than that for the biaxial compressive case. This argues that the Mn–O–Mn bond-angle variation, controlled by the lattice strain, plays a more important role in the formation of the spin ordering than the attendant modification of the Mn–O bond length. It is shown that the magnetic inhomogeneity, expressed by a significant difference between the field-cooled and zero-field-cooled temperature-dependent magnetization, has a metallurgical rather than an electronic nature, and is controlled by the crystal lattice distortion and the microstructure defects. The observed enhancement of the magnetoresistance effect in the LSMO/BTO bilayer at room temperature makes this object greatly beneficial in the development of new hybrid ferromagnetic/ferroelectric devices.The microstructure and the magnetic and transport properties of La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3 films deposited on a BaTiO3 layer (LCMO/BTO and LSMO/BTO) and on a LaAlO3(001) single crystal (LCMO/LAO and LSMO/LAO) by rf-magnetron sputtering using “soft” (or powder) targets are investigated. The films grown on BTO demonstrate biaxial tensile in-plane and compressive out-of-plane strains, while the films grown on LAO, in contrast, manifest compressive in-plane and tensile out-of-plane strains. The films with biaxial tensile in-plane lattice strain undergo the magnetic transition at a higher temperature than that for the biaxial compressive case. This argues that the Mn–O–Mn bond-angle variation, controlled by the lattice strain, plays a more important role in the formation of the spin ordering than the attendant modification of the Mn–O bond length. It is shown that the magnetic inhomogeneity, expressed by a significant difference between the field-cooled and zero-field-cooled temperature-dependent magnet...

Experimental Realization of Tunable Metamaterial Hyper-transmitter

We realized the tunable metamaterial hyper-transmitter in the microwave range utilizing simple planar meta-structure. The single-layer metamaterial hyper-transmitter shows that the transmission peak occurs at 14 GHz. In case of the dual-layer one, it is possible to control the transmission peak from 5 to 10 GHz. Moreover, all the transmission peaks reveal transmission over 100%. We experimentally and theoretically investigated these phenomena through 3-dimensional simulation and measurement. The reason for being over 100% is also elucidated. The suggested hyper-transmitter can be used, for example, in enhancing the operating distance of the electromagnetic wave in Wi-Fi, military radar, wireless power transfer and self-driving car.

Fabrication and analysis of one-dimensional defect-induced ultrawide photonic band gaps

An approach named the defect-induced extension of photonic band gaps (PBGs) is proposed, in which a stratified periodic structure is combined with another stratified structure containing defect layers. For comparison, three structures composed of TiO2 and SiO2 alternating layers, perfect, five-defect and combined-photonic crystals (PCs), were fabricated by using an electron-beam evaporation system. The measured PBG width of combined-PCs extended to 370 nm at the normal incidence, nearly three times more than that of perfect ones, 130 nm, but a few ripples are observed within the PBG. In order to elucidate the origin of the ripples, the influence of the refractive-index contrast on the reflectance was analyzed theoretically. When the refractive-index contrast nH∕nL≥1.7, the ripples are eliminated. More importantly, an overdoubled omnidirectional PBG is realized at nH∕nL=2.56, covering the visible region completely. This approach is considered to be based on the light localization and a heterostructure resu...

Exchange bias in self-organized Nd0.5Sr0.5MnO3 bilayer film

The self-organized Nd0.5Sr0.5MnO3 bilayer film with a homogeneous chemical composition and different microstructures has been prepared by rf-magnetron sputtering. The top layer of the film has a columnlike nanostructure while the bottom one forms an epitaxial crystal structure. The magnetic properties of the film mimic the behavior of superparamagnetic particles grown onto the ferromagnetic layer. An exchange-bias effect was observed at a temperature below the blocking one, even in the present ferromagnetic hybrid system.

Nonclassical magnetic dynamics and negative exchange bias in Nd0.5Sr0.5MnO3 films

Amorphous, nanocluster, and self-organizing bilayer Nd0.5Sr0.5MnO3 films are prepared by rf magnetron sputtering. The amorphous film turns out to be a typical paramagnet with free motion of the individual Mn spins, the magnetic properties of which are well described by the Curie-Weiss approximation. The nanocluster film manifests magnetic properties mimicking superparamagnetic particles with a nonclassical magnetic dynamics. The unique shape of the hysteresis loops, which have hysteretic lobes at high magnetic field but are nonhysteretic as the field crosses zero, suggests that each particle (nanocluster) is a closure magnetic domain (or magnetic vortex) rather than a single domian. At the same time, a blocked to unblocked transition was observed with increasing temperature, as in the usual superparamagnet. The self-organizing bilayer film demonstrates a negative exchange bias, which is typical for the ferromagnet/antiferromagnet hybrid system in spite of the fact that both layers in our case have a ferro...

Magnetic and transport properties of charge ordered La0.5Ca0.5MnO3 and La0.4Ca0.6MnO3 films

The magnetic and transport properties of La0.5Ca0.5MnO3 and La0.4Ca0.6MnO3 films with different thickness, prepared by rf-magnetron sputtering with the use of a so-called “soft” (or powder) target on a LaAlO3 substrate, are investigated. Electron-diffraction and high-resolution electron microscopy (HREM) studies show that the charge-ordered phase is observed at room temperature for all films. Both the paramagnetic-to-ferromagnetic transition at TC≈250K upon cooling and the appearance of an antiferromagnetic (AFM) phase at TN≲140K are observed in the La0.5Ca0.5MnO3 films, while the La0.4Ca0.6MnO3 films exhibit the AFM transition only, at the same temperature, except for a small ferromagnetic (FM) response from a “dead” layer. It is shown that the volume fraction of the FM phase in the La0.5Ca0.5MnO3 film does not exceed 30% and that the FM phase coexists with the AFM phase at low temperature. All films manifest an exponential temperature dependence of the resistance, with no evidence of the metal-insulator...