Effect of interface bonding on the phase-transformation-aided magnetoelectric effect in ferromagnetic/ferroelectric composites

Based on modified constitutive equations and finite element method, calculations have been performed to study the effect of interface bonding on the phase-transition-aided magnetoelectric (ME) response in a new kind of NiMnGa/lead–zirconate–titanate (PZT) multiferroic laminate composites. The results quantitatively show that the ME effect is remarkably dependent on both the interface layer characteristics and the interface layer thickness. Stiffer and thinner interface layers are apt to produce higher ME effect. Calculations are in good agreement with available experimental results. Furthermore, the theoretical approach was improved to consider the enhancement in the magnetostriction of martensites induced by pre-applied opposing stress. Predictions reveal that the usage of single crystal Fe₇Pd₃ as ferromagnetic phase to form magnetoelectric composite with PZT can produce a high ME up to ~1 V/cm Oe.     

Resistive switching memory properties of layer-by-layer assembled enzyme multilayers

The properties of enzymes, which can cause reversible changes in currents through redox reactions in solution, are of fundamental and practical importance in bio-electrochemical applications. These redox properties of enzymes are often associated with their charge-trap sites. Here, we demonstrate that reversible changes in resistance in dried lysozyme (LYS) films can be generated by an externally applied voltage as a result of charge trap/release. Based on such changes, LYS can be used as resistive switching active material for nonvolatile memory devices. In this study, cationic LYS and anionic poly(styrene sulfonate) (PSS) layers were alternately deposited onto Pt-coated silicon substrates using a layer-by-layer assembly method. Then, top electrodes were deposited onto the top of LYS/PSS multilayers to complete the fabrication of the memory-like device. The LYS/PSS multilayer devices exhibited typical resistive switching characteristics with an ON/OFF current ratio above 10(2), a fast switching speed of 100 ns and stable performance. Furthermore, the insertion of insulating polyelectrolytes (PEs) between the respective LYS layers significantly enhanced the memory performance of the devices showing a high ON/OFF current ratio of ~10(6) and low levels of power consumption.     

Giant sharp and persistent converse magnetoelectric effects in multiferroic epitaxial heterostructures

Magnetoelectric coupling between magnetic and electrical properties presents valuable degrees of freedom for applications. The two most promising scenarios are magnetic-field sensors that could replace low-temperature superconducting quantum interference devices, and electric-write magnetic-read memory devices that combine the best of ferroelectric and magnetic random-access memory. The former scenario requires magnetically induced continuous and reversible changes in electrical polarization. These are commonly observed, but the coupling constants thus obtained are invalid for data-storage applications, where the more difficult to achieve and rarely studied magnetic response to an electric field is required. Here, we demonstrate electrically induced giant, sharp and persistent magnetic changes (up to 2.3 x 10(-7) s m(-1)) at a single epitaxial interface in ferromagnetic 40 nm La(0.67)Sr(0.33)MnO(3) films on 0.5 mm ferroelectric BaTiO(3) substrates. X-ray diffraction confirms strain coupling via ferroelastic non-180( composite function) BaTiO(3) domains. Our findings are valid over a wide range of temperatures including room temperature, and should inspire further study with single epitaxial interfaces.     

Resistive switching characteristics of Cu/ZnO0.4S0.6/Al devices constructed on plastic substrates

In this study, Cu/ZnO0.4S0.6Al devices are fabricated on plastic substrates using the sputtering method at room temperature. The ratio of O/S in the zinc oxysulfide thin film is confirmed to be 0.4/0.6 from the Auger depth profiling. The Cu/ZnO0.4S0.6/Al devices show unipolar resistive switching behaviors and the ratio of the measured resistance in the low-resistance state (LRS) to that in the high-resistance state (HRS) is above 10(4). The conduction mechanism of the LRS is governed by Ohm's law. On the other hand, in the HRS, the conduction mechanism at low voltages is controlled by Ohm's law, but that at high voltages results from the Poole-Frenkel emission mechanism. The Ohmic and Poole-Frenkel conduction mechanisms observed in the LRS and HRS support the filament model of unipolar resistive switching. The memory characteristics of the Cu/ZnO0.4S0.6/Al devices are retained for 10(4) sec without any change.     

Resistive switching in \hbox {BiFeO}_3-based heterostructures due to ferroelectric modulation on interface Schottky barriers

Ferroelectric \(\hbox {BiFeO}_3\) (BFO) thin films were deposited on (001) \(\hbox {SrTiO}_3\) substrates buffered with \(\hbox {La}_{0.7}\hbox {Sr}_{0.3}\hbox {MnO}_3\) (LSMO) electrodes. Bipolar resistive switching in Pt/BFO/LSMO heterostructures were observed with high stability and long retention. However, transport characteristics of Pt/BFO/LSMO is highly asymmetric and pronounced resistive switching can only be observed by applying negative reading pulses on the Pt top electrodes, i.e. when the Pt/BFO Schottky barrier is reverse-biased. This resistive switching is discussed in terms of a modulation on the Pt/BFO interface Schottky barrier by the polarization switching in ferroelectric BFO. Comparative studies on Pt/BFO/ \(\hbox {SrRuO}_3\) and Pt/BFO/ \(\hbox {LaNiO}_3\) heterostructures reveal that the work function of the electrode materials and the formation of Schottky barriers are significant to the observed resistive switching behaviors.     

功能梯度铁电铁磁复合材料弯曲模态下的磁电效应

基于磁致伸缩相与压电相的本构方程,应用弹性力学的方法,建立了功能梯度铁电铁磁复合材料弯曲模态下的磁电耦合静态力学模型。假设铁电和铁磁材料的物理参数均为沿厚度方向的线性或指数函数,分析计算了由PZT作为铁电材料和CoFe2O4作为铁磁材料的双层复合材料的磁电效应。结果表明,在弯曲模态下,磁电电压系数出现两个峰值。负梯度的铁电(或铁磁)材料提高磁电效应,正梯度的铁电(或铁磁)材料降低磁电效应。同号梯度的铁电铁磁材料对磁电效应的影响更大。     

Resistive switching effects in oxide sandwiched structures

Resistive switching (RS) behaviors have attracted great interest due to their promising potential for the data storage. Among various materials, oxide-based devices appear to be more advantageous considering their handy fabrication and compatibility with CMOS technology, though the underlying mechanism is still controversial due to the diversity of RS behaviors. In this review, we focus on the oxide-based RS memories, in which the working mechanism can be understood basically according to a so-called filament model. The filaments formation/rupture processes, approaches developed to detect and characterize filaments, several effective attempts to improve the performances of RS and the quantum conductance behaviors in oxide-based resistive random access memory (RRAM) devices are addressed, respectively.     

Unexpectedly low barrier of ferroelectric switching in HfO2 via topological domain walls

Fluorite-structure ferroelectrics — in particular the orthorhombic phase of HfO₂ — are of paramount interest to academia and industry because they show unprecedented scalability down to 1-nm-thick size and are compatible with Si electronics. However, their polarization switching is believed to be limited by the intrinsically high energy barrier of ferroelectric domain wall (DW) motions. Here, by unveiling a new topological class of DWs, we establish an atomic-scale mechanism of polarization switching in orthorhombic HfO₂ that exhibits unexpectedly low energy barriers of DW motion (up to 35-fold lower than given by previous conjectures). These findings demonstrate that the nucleation-and-growth-based mechanism is feasible, challenging the commonly held view that the rapid growth of the oppositely polarized domain is impossible. Building on this insight, we describe a strategy to substantially reduce the coercive fields in HfO₂-based ferroelectric devices. Our work is a crucial step towards understanding the polarization switching of HfO₂, which could provide a means to solve the key problems associated with operation speed and endurance.     

Domain switching in polycrystalline ferroelectric ceramics

Ferroelectric ceramics are widely used as sensors and actuators for their electro-mechanical properties, and in electronic applications for their dielectric properties. Domain switching--the phenomenon wherein the ferroelectric material changes from one spontaneously polarized state to another under electrical or mechanical loads--is an important attribute of these materials. However, this is a complex collective process in commercially used polycrystalline ceramics that are agglomerations of a very large number of variously oriented grains. As the domains in one grain attempt to switch, they are constrained by the differently oriented neighbouring grains. Here we use a combined theoretical and experimental approach to establish a relation between crystallographic symmetry and the ability of a ferroelectric polycrystalline ceramic to switch. In particular, we show that equiaxed polycrystals of materials that are either tetragonal or rhombohedral cannot switch; yet polycrystals of materials where these two symmetries co-exist can in fact switch.     

Studies on dielectric and magnetoelectric behavior of 25% CMFO ferrite and 75% BZT ferroelectric multiferroic magnetoelectric composites

In the present work, multiferroic magnetoelectric (ME) composites of ferrite and ferroelectric phases are prepared. Here, the magnetostrictive (ferrite) phase, Co_1.2 − yMn_yFe_1.8O₄ (y = 0.0 to 0.4) i.e. CMFO is synthesized by chemical combustion route and the piezoelectric (ferroelectric) phase, BaZr_0.08Ti_0.92O₃ i.e. BZT is synthesized by conventional ceramic method. Frequency dependent dielectric constant measurement from 20 Hz-1 MHz at room temperature shows usual dielectric dispersion behaviour, which may be attributed to the Maxwell-Wagner type interfacial polarization. Temperature dependent dielectric constant measurement shows two dielectric maxima, one below 100 °C and the second above 500 °C. The dielectric maxima below 100 °C corresponds to the transition temperature of the ferroelectric phase and that of above 500 °C corresponds to the transition temperature of the ferrite phase of the ME composites. It is observed that as Mn content increases in the cobalt ferrite, the phase transition temperature of the ferrite phase decreases. The static magnetoelectric voltage coefficient was measured as a function of intensity of the applied dc magnetic field. These magnetoelectric composites may have possible applications in magnetic field sensing probes and linear ME devices.     

Numerical analysis of ferroelectric/ferroelastic domain switching in ferroelectric ceramics

A numerical approach predicting the behavior of ferroelectric ceramics under electric field and mechanical loading is proposed in this paper. In the model, macroscopic properties of ferroelectric ceramics are determined by microscopic structures. Ferroelectric ceramics are seen to be composed of many domains with different orientations, and domain switching is the source of the nonlinear constitutive behavior of the ferroelectric ceramics. Numerical calculations based on the model were carried out, and the computational results are compared with the experimental results, which shows the two sets of results consist with each other. The calculation approach can provide a guidance for the ceramics component design.     

铁电陶瓷中电畴翻转研究进展

电畴为铁电陶瓷固有的独特微观组织特征之一,铁电陶瓷的许多性能均与其密切有关.综述了铁电陶瓷中的电畴结构,系统介绍了电场、机械作用引起的电畴翻转,概述了电畴翻转对铁电陶瓷断裂韧性的影响及其研究进展.     

NiFe/IrMn多层膜制备及MgO掺杂研究

采用磁控溅射制备了Ta/NiFe/IrMn/Ta薄膜,研究了反铁磁IrMn的溅射功率和铁磁层NiFe厚度对多层膜交换偏置场的影响。在反铁磁IrMn中插入MgO,发现MgO含量对交换偏置场有一定影响。随着MgO含量的增加,多层膜的交换偏置场逐渐增大,当MgO的含量约为2.5%交换偏置场达到最大值。随着MgO含量进一步增加交换偏置场下降。在IrMn中插入适量的MgO可以有效地增加交换偏置场。     

Modeling of charge switching in ferroelectric capacitors

To simulate charge switching in ferroelectric capacitors, a pair of exponential growth and decay currents is mapped to the process of polarization reversal. This is based on the fact that these exponential currents [i.e., i = I(m) e(t/tau) (t < or = 0) and i = I(m) e(-t/tau) (t > or = 0)], are completely specified by two constants I(m) and tau and each accommodates an integral charge Q = I(m) x tau. Equating this charge to the remanent spontaneous polarization allows for the modeling of switching current. For practical circuit simulations for charge switching, this modeling of switching current is simplified to an exponential decay current whose integral charge is set equal to the total reversed spontaneous polarization. This is because an exponential decay current can be conveniently implemented by charging a series resistor and capacitor (RC) circuit with a pulse-voltage source. The voltage transitions of the pulse source are associated with the polarization reversal and can be controlled with a noninverting Schmitt trigger that toggles at the positive and negative coercive voltages of a ferroelectric capacitor. The final circuit model incorporates such electrical and geometrical parameters as capacitance, remanent spontaneous polarization, coercive field, electrode area, and film thickness of a ferroelectric, thin-film capacitor.     

Perpendicular magnetic anisotropy in amorphous ferromagnetic CoSiB/Pt multilayers

Magnetic anisotropy properties of amorphous ferromagnetic CoSiB/Pt multilayers with perpendicular magnetic anisotropy (PMA, K(u)) were systematically investigated as a function of CoSiB layer thickness (t(coSiB)) and Pt layer thickness (t(Pt)). In two series of [CoSiB t(coSiB)Pt t(P1)]5 multilayers, the perpendicular coercivity (H(c)) increased to reach a maximum and then decreased with further increase in both t(coSiB) and t(Pt), due to intermixing of CoSiB/Pt interfaces. Particularly, using the amorphous soft magnetic CoSiB, the coercivity became very sensitive to the CoSiB thickness. These multilayer films exhibited a high K(u) of 2 x 10(6) erg/cc and a high H(c) of 360 Oe with marked squareness. It was found that even after annealing at 350 degrees C, the CoSiB/Pt multilayers had a high PMA and their H(c) increased.     

Colors of graphene and graphene-oxide multilayers on various substrates

We investigated the colors of graphene and graphene-oxide multilayers that were deposited on various dielectric layers. In particular, the effects of the material thickness, the types of dielectric layers, and the existence of a back silicon substrate were analyzed. The colors of graphene-oxide layers on a SiO2/Si substrate were found to periodically change as the material thickness increased. However, the colors of graphene layers on the same substrate became saturated without a similar periodic change. The calculated colors corresponding to the material thicknesses were verified by optical microscopy and profilometry. We believe that these results demonstrate the possibility of utilizing color as a simple tool for detecting and estimating the thicknesses of graphene and graphene-oxide multilayers.