Yoon-Young Choi

Origin of recoil hysteresis loops in Sm–Co∕Fe exchange-spring magnets

Open recoil loops are often interpreted as a consequence of a breakdown in exchange coupling and attributed to the decoupled soft phase in exchange-coupled permanent magnets. However, in element-specific recoil loop measurements on Sm–Co∕Fe exchange spring magnets, the authors found that the open recoil loops were present not only in the soft (Fe) layer but also in the hard (Sm–Co) layer, and were not a consequence of exchange coupling breakdown between the soft and hard layers. Comparison between the experimental results and micromagnetic calculations reveals that the observed open recoil loops originate from the anisotropy variations in the Sm–Co layer.

Enhancement of local piezoresponse in polymer ferroelectrics via nanoscale control of microstructure.

Polymer ferroelectrics are flexible and lightweight electromechanical materials that are widely studied due to their potential application as sensors, actuators, and energy harvesters. However, one of the biggest challenges is their low piezoelectric coefficient. Here, we report a mechanical annealing effect based on local pressure induced by a nanoscale tip that enhances the local piezoresponse. This process can control the nanoscale material properties over a microscale area at room temperature. We attribute this improvement to the formation and growth of β-phase extended chain crystals via sliding diffusion and crystal alignment along the scan axis under high mechanical stress. We believe that this technique can be useful for local enhancement of piezoresponse in ferroelectric polymer thin films.

Fabrication of Vertically Well‐Aligned P(VDF‐TrFE) Nanorod Arrays

By combining the merits of traditional template-assisted methods for polymer nanostructure fabrication, we demonstrate an immersion crystallization process that combines features of polymer crystallization and template removal simultaneously. Well-aligned poly(vinylidene fluoride-trifluoroethylene) copolymer nanorod arrays are prepared for the first time via this simple and convenient new method.

Polymer Piezoelectric Energy Harvesters for Low Wind Speed

We fabricated polymer piezoelectric energy harvesters (PEHs) that can generate electric power at wind speed of less than 4.7 m/s due to their high sensitivity to wind. In order to optimize their operating conditions, we evaluated three distinct PEH operation modes under the boundary conditions of single-side clamping. We found that a PEH connected to an external load of 120 kΩ shows the largest output power of 0.98 μW at 3.9 m/s, with wind incident on its side (mode I). We attribute this result to large bending and torsion involved in this operation mode.

Vertically aligned P(VDF-TrFE) core-shell structures on flexible pillar arrays

PVDF and P(VDF-TrFE) nano- and micro- structures have been widely used due to their potential applications in several fields, including sensors, actuators, vital sign transducers, and energy harvesters. In this study, we developed vertically aligned P(VDF-TrFE) core-shell structures using high modulus polyurethane acrylate (PUA) pillars as the support structure to maintain the structural integrity. In addition, we were able to improve the piezoelectric effect by 1.85 times from 40 ± 2 to 74 ± 2 pm/V when compared to the thin film counterpart, which contributes to the more efficient current generation under a given stress, by making an effective use of the P(VDF-TrFE) thin top layer as well as the side walls. We attribute the enhancement of piezoelectric effects to the contributions from the shell component and the strain confinement effect, which was supported by our modeling results. We envision that these organic-based P(VDF-TrFE) core-shell structures will be used widely as 3D sensors and power generators because they are optimized for current generations by utilizing all surface areas, including the side walls of core-shell structures.

The piezoresponse force microscopy investigation of self-polarization alignment in poly(vinylidene fluoride-co-trifluoroethylene) ultrathin films

We report the self-polarization alignment without external poling in spin-coated poly(vinylidene fluoride-co-trifluoroethylene), P(VDF-TrFE), thin films on transparent and flexible substrates. Piezoresponse force microscopy (PFM) allows the quantitative analysis of preferentially aligned polarization in the ferroelectric thin films. We found that as-received P(VDF-TrFE) thin films on transparent poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) electrodes showed stronger self-polarization alignment than those on indium-tin oxide (ITO) electrodes. The relative ratios for the aligned polarization per unit volume on PEDOT:PSS and ITO electrodes were 18.6% and 4%, respectively.

Imaging Ferroelectric Domains and Domain Walls Using Charge Gradient Microscopy: Role of Screening Charges

Advanced scanning probe microscopies (SPMs) open up the possibilities of the next-generation ferroic devices that utilize both domains and domain walls as active elements. However, current SPMs lack the capability of dynamically monitoring the motion of domains and domain walls in conjunction with the transport of the screening charges that lower the total electrostatic energy of both domains and domain walls. Charge gradient microscopy (CGM) is a strong candidate to overcome these shortcomings because it can map domains and domain walls at high speed and mechanically remove the screening charges. Yet the underlying mechanism of the CGM signals is not fully understood due to the complexity of the electrostatic interactions. Here, we designed a semiconductor-metal CGM tip, which can separate and quantify the ferroelectric domain and domain wall signals by simply changing its scanning direction. Our investigation reveals that the domain wall signals are due to the spatial change of polarization charges, while the domain signals are due to continuous removal and supply of screening charges at the CGM tip. In addition, we observed asymmetric CGM domain currents from the up and down domains, which are originated from the different debonding energies and the amount of the screening charges on positive and negative bound charges. We believe that our findings can help design CGM with high spatial resolution and lead to breakthroughs in information storage and energy-harvesting devices.

Spin-coated ultrathin poly(vinylidene fluoride-co-trifluoroethylene) films for flexible and transparent electronics

We demonstrate spin-cast ultrathin poly(vinylidene fluoride-co-trifluoroethylene), P(VDF-TrFE), films on flexible and transparent substrates. Importantly, the PEDOT:PSS electrode allowed for the superior piezoresponse value (17.14 ± 2.37 pm V−1) and the low coercive voltage (1.76 ± 0.79 V) of the 20 nm thick P(VDF-TrFE) films. The use of PEDOT:PSS allows for the coherent interface between PEDOT:PSS and P(VDF-TrFE), the strong adhesion at the interface and no interfacial interaction when compared to that of indium–tin oxide (ITO).

Ferromagnetic Mn moments at SrRuO3∕SrMnO3 interfaces

Using element-specific, x-ray probes of magnetism the authors observe a net ferromagnetic moment from Mn in SrRuO3∕SrMnO3 (SRO/SMO) superlattice films. It is found that the magnetic behavior of the SRO and SMO layers is significantly modified by their exchange interaction. Bulk magnetometry shows a two-step, easy-axis magnetization reversal process and x-ray measurements confirm that the reversal with higher coercivity involves the magnetization in the SMO layers. The results provide strong evidence for the presence of pinned SRO magnetization at the SRO/SMO interface. Angle-dependent measurements reveal that the net Mn moment is due to a canted antiferromagnetic spin configuration in the SMO layers.

Layer-resolved study of magnetic interaction effects in heterostructure dot arrays

Using polarized x rays we have studied magnetic interactions in a series of patterned single-layer (NiFe and Co) and multilayer (NiFe∕Co and NiFe∕Cu∕Co) heterostructures. Extraction of layer-specific magnetic hysteresis loops from an array of 1‐μm dots allows us to separate the influence of inter- and intralayer interactions. Double layer (NiFe∕Co) dots show evidence of identical vortex formation in both layers while with the spacer layer the direct coupling between the two magnetic layers is removed, and dipolar field contribution becomes significant so that the vortex formation in both layers is suppressed.