Showing papers by "J.-M. Liu published in 2019"

Excellent Ferroelectric Properties of Hf 0.5 Zr 0.5 O 2 Thin Films Induced by Al 2 O 3 Dielectric Layer

Abstract: The ferroelectricity of Hf0.5Zr0.5O2 (HZO) thin films has been usually reported to be induced by metallic capping layer. In this work, we successfully obtained ferroelectricity of HZO thin films induced by ultrathin insulating Al2O3 capping layers. The ferroelectric properties of HZO thin films induced by Al2O3 capping layers with different thickness have been investigated. It was found that the Al2O3(2 nm)/HZO (20 nm) stack shows excellent ferroelectric properties. The Au/Al2O3(2 nm)/HZO (20 nm)/TiN capacitor exhibits a very low leakage current of $\sim {4.2}\times {10}^{\text {-9}}$ A at 4 V and a maximum $2{P}_{r}\approx 32.3~\mu \text{C}$ /cm2 at sweeping voltages between ±8 V. In addition, the capacitor also shows excellent endurance properties up to 108 cycles. Our work demonstrated that dielectric films like Al2O3 can be adopted to be used as capping layer to generate excellent ferroelectric properties of HfO2-based thin films, which will contribute to their future applications in ferroelectric memory and negative capacitance transistors.

Collinear magnetic structure and multiferroicity in the polar magnet Co 2 Mo 3 O 8

Abstract: Among numerous multiferroic phenomena observed in spin frustrated lattice, giant magnetoelectricity in honeycomb lattice ${(\mathrm{Fe},\phantom{\rule{0.28em}{0ex}}\mathrm{Mn})}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ has stimulated great interest and substantial effort in searching for novel members in this 238 family. In this work, we synthesize successfully compound ${\mathrm{Co}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$, a structural analogue of ${\mathrm{Fe}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$, and present a series of characterizations on its structural, magnetic, and electric properties. An antiferromagnetic transition takes place at the Neel temperature ${T}_{N}=39\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ with appearance of electric polarization and dielectric anomaly, which provides clear evidence of simultaneous magnetic and ferroelectric transitions. The neutron powder diffraction (NPD) and magnetic susceptibility data confirm the $c$-axis collinear antiferromagnetic orders and emergent ferroelectric polarization. In particular, such antiferromagnetic order is relatively robust against magnetic field up to 9 T, different from ${\mathrm{Fe}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ with ferrimagnetic transition or ${\mathrm{Mn}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ with spin flop in the low-field region. Our data on single crystals demonstrate the second-order magnetoelectric effect in terms of magnetic field dependence of ferroelectric polarization response, while no linear magnetoelectric response is allowed. It is suggested that ${\mathrm{Co}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ provides a unique platform on which rich multiferroic physics in the presence of collinear magnetic order can be explored.

Ultrafast depinning of domain walls in notched antiferromagnetic nanostructures

Abstract: The pinning and depinning of antiferromagnetic (AFM) domain wall is certainly the core issue of AFM spintronics. In this work, we study theoretically the Neel-type domain wall pinning and depinning at a notch in an antiferromagnetic (AFM) nano-ribbon. The depinning field depending on the notch dimension and intrinsic physical parameters are deduced and also numerically calculated. Contrary to conventional conception, it is revealed that the depinning field is remarkably dependent of the damping constant and the time-dependent oscillation of the domain wall position in the weakly damping regime benefits to the wall depinning, resulting in a gradual increase of the depinning field up to a saturation value with increasing damping constant. A one-dimensional model accounting of the internal dynamics of domain wall is used to explain perfectly the simulated results. It is demonstrated that the depinning mechanism of an AFM domain wall differs from ferromagnetic domain wall by exhibiting a depinning speed typically three orders of magnitude faster than the latter, suggesting the ultrafast dynamics of an AFM system.

A flexible memory with low-voltage and high-operation speed using an Al2O3/poly(α-methylstyrene) gate stack on a muscovite substrate

Abstract: Flexible organic field effect transistors (OFETs) are a promising class of flexible nonvolatile memories. However, fabrication of flexible OFET devices that offer low-voltage and high-speed operation remains a challenge. Here, we report a flexible OFET memory unit based on a novel gate stack consisting of pentacene as a semiconducting channel, poly(α-methylstyrene) as a charge-trapping layer, and amorphous Al2O3 as a charge-blocking layer, deposited sequentially on an Au-coated flexible muscovite substrate. The optimized flexible OFET structure has excellent electrical performance and mechanical flexibility, with an operating voltage as low as −4 V, an operating speed as fast as 100 μs, the largest memory window (1.73 V) provided +6 V, 100 μs program and −4 V, 100 μs erase pulses, a retention time far longer than 104 s, and program–erase endurance greatly exceeding 3000 cycles. The device can endure at least 5000 bending–unbending cycles. Of all flexible OFET nonvolatile memories reported to date, this OFET device offers the best overall electrical and mechanical properties and is extremely promising for future applications in flexible, lightweight, low-power, and high-speed nonvolatile memories.

Effects of Ambient Gases on the Electrical Performance of Solution-Processed C8-BTBT Thin-Film Transistors.

Abstract: We performed a systematic study of the influence of environmental conditions on the electrical performance characteristics of solution-processed 2,7-dioctyl [1] benzothieno[3,2-b][1]-benzothiophene (C8-BTBT) thin-film transistors (TFTs). Four environmental exposure conditions were considered: high vacuum (HV), O2, N2, and air. The devices exposed to O2 and N2 for 2 h performed in a manner similar to that of the device kept in HV. However, the device exposed to air for 2 h exhibited significantly better electrical properties than its counterparts. The average and highest carrier mobility of the 70 air-exposed C8-BTBT TFTs were 4.82 and 8.07 cm2V-1s-1, respectively. This can be compared to 2.76 cm2V-1s-1 and 4.70 cm2V-1s-1, respectively, for the 70 devices kept in HV. Furthermore, device air stability was investigated. The electrical performance of C8-BTBT TFTs degrades after long periods of air exposure. Our work improves knowledge of charge transport behavior and mechanisms in C8-BTBT OTFTs. It also provides ideas that may help to improve device electrical performance further.

Landau-Lifshitz-Bloch equation for domain wall motion in antiferromagnets

Abstract: In this work, we derive the Landau-Lifshitz-Bloch equation accounting for the multidomain antiferromagnetic (AFM) lattice at finite temperature, in order to investigate the domain wall motion, the core issue for AFM spintronics. The continuity equation of the staggered magnetization is obtained using the continuum approximation, allowing an analytical calculation of the domain wall dynamics. The influence of temperature on the static domain wall profile is investigated, and the analytical calculations agree well with the numerical simulations on temperature-gradient-driven domain wall motion, confirming the validity of this theory. Furthermore, the decrease of the acceleration and the increase of the saturation velocity of the domain wall with the increase of temperature are uncovered for a fixed gradient. Moreover, it is worth noting that this theory could be also applied to dynamics of various wall motions in an AFM system. The present theory represents a comprehensive approach to the domain wall dynamics in AFM materials, a crucial step toward the development of AFM spintronics.

Oxygen vacancy mediated conductivity and charge transport properties of epitaxial Ba0.6La0.4TiO3−δ thin films

Abstract: We report on the effects of the oxygen vacancy (VO) regarding the microstructure, conductivity, and charge transport mechanisms of epitaxial Ba0.6La0.4TiO3−δ (BLTO) films. The VO concentration can be largely regulated from 21.5% to 37.8% by varying the oxygen pressure (PO2) during film deposition. Resistivity-temperature and Hall effect measurements demonstrate that the BLTO films can be tuned remarkably from an insulator to a semiconductor, and even to a metallic conductor by regulating the VO concentration. The role of VO concentration in the charge transport mechanism is clarified. For films with low VO concentration, the charge transport is dominated by variable range hopping (VRH) at low temperatures, and it shows small polaron (SP) hopping at high temperatures. For films with high VO concentration, the carrier transport remains VRH at low temperatures, while it changes to SP hopping at moderate temperatures, and is dominated by thermal phonon scattering at high temperatures. Furthermore, the lower starting temperature of SP hopping for films with higher VO concentrations indicates that VO favors electron-phonon coupling. Different charge transport mechanisms are assumed to be due to different VO-induced defect energy levels in the BLTO films, which has been verified by their soft x-ray absorption spectroscopy results.

Successive electric-polarization switches in the S =1 /2 skew chain Co 2 V 2 O 7 induced by a high magnetic field

Abstract: We report successive electric-polarization $(P)$ switches in the $S=1/2$ quantum magnet ${\mathrm{Co}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}$ by application of a magnetic field along the $b$ axis, where a 1/2-magnetization plateau is seen at 5.4--11.6 T. Polarization reversal appears at $\ensuremath{\sim}5$ T from $\ensuremath{-}P\ensuremath{\parallel}b$ to $+P\ensuremath{\parallel}b$, leading to an irreversible magnetoelectric history involved with a memory effect, while the polarization flop in fields of 12--17 T is identified by a transition from the $P\ensuremath{\parallel}b$ to $P\ensuremath{\parallel}ac$ plane, different from those reported previously. These intriguing magnetoelectric phenomena are owing to the unique nature of the skew-chain-like magnetic structure of ${\mathrm{Co}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}$ and can be understood by a change in symmetry of the magnetic order in applied fields. The emergent ferroelectricities deviating from the half-plateau state may arise from magnon Bose-Einstein condensation in this quantum magnet.

Absence of ferroelectricity in double-perovskite Y2CoMnO6 single crystals

Abstract: The rare-earth double-perovskite R2CoMnO6 family has been reported to be promising multiferroics where the predicted ferroelectric polarization along the b-axis is induced by the Co2+/Mn4+ collinear magnetic order. In this work, we report our characterizations on the structural, magnetic, dielectric, and ferroelectric behaviors of flux-grown Y2CoMnO6 single crystals. A weak ferromagnetic transition with the Curie point TC ∼ 70 K is evident in combination with a remarkable magnetic hysteresis loop along the c-axis. The earlier reported dielectric anomaly at the magnetic transition is not detected in our single crystals. In addition, the peak of each electric current curve as a function of temperature, probed along different crystallographic directions, shows serious shifting with the temperature ramping rate, excluding the intrinsic ferroelectric contribution. The measured data are not in conformity with theoretical predictions and earlier experiments on polycrystalline samples. In addition, no reliable sign for magnetoelectric coupling can be detected. The present work seems to exclude the existence of magnetism-driven ferroelectricity in Y2CoMnO6.