Weichuan Huang

Ultrahigh Energy Density in SrTiO3 Film Capacitors

Solid-state dielectric film capacitors with high-energy-storage density will further promote advanced electronic devices and electrical power systems toward miniaturization, lightweight, and integration. In this study, the influence of interface and thickness on energy storage properties of SrTiO3 (STO) films grown on La0.67Sr0.33MnO3 (LSMO) electrode are systematically studied. The cross-sectional high resolution transmission electron microscopy reveals an ion interdiffusion layer and oxygen vacancies at the STO/LSMO interface. The capacitors show good frequency stability and increased dielectric constant with increasing STO thickness (410-710 nm). The breakdown strength (Eb) increases with decreasing STO thickness and reaches 6.8 MV/cm. Interestingly, the Eb under positive field is enhanced significantly and an ultrahigh energy density up to 307 J/cm3 with a high efficiency of 89% is realized. The enhanced Eb may be related to the modulation of local electric field and redistribution of oxygen vacancies at the STO/LSMO interface. Our results should be helpful for potential strategies to design devices with ultrahigh energy density.

Ferroelectric Control of Organic/Ferromagnetic Spinterface

Organic multiferroic tunnel junctions based on La0.6 Sr0.4 MnO3 /poly(vinylidene fluoride) (PVDF)/Co structures are fabricated. The tunneling magneto-resistance sign can be changed by electrically switching the ferroelectric polarization of PVDF barrier. It is demonstrated that the spin-polarization of the PVDF/Co spinterface can be actively controlled by tuning the ferroelectric polarization of PVDF. This study opens new functionality in controlling the injection of spin polarization into organic materials via the ferroelectric polarization of the barrier.

Oxygen vacancy-induced ferromagnetism in Bi4NdTi3FeO15 multiferroic ceramics

Layered Aurivillius compounds with multiferroic properties have attracted much attention due to their rich fundamental physics and great application potential. However, the ferroelectric and magnetic properties are different for these compounds with different synthesis conditions. In this paper, we investigate the structure, ferroelectricity, and magnetism of four-layer Aurivillius-phase multiferroic Bi4NdTi3FeO15. The four-layer structure is confirmed by powder X-ray diffraction and high-angle annular dark field scanning transmission electron microscopy. The ferroelectricity together with dielectric constant can be reduced by vacuum-annealing treatment due to the increase of oxygen vacancy concentration. More interestingly, the ferromagnetism is strongly enhanced by vacuum-annealing and can be obviously suppressed after re-oxidization, which may be associated with Fe3+-O-Fe2+ coupling originated from the variable valence state of Fe with different oxidization conditions. These findings indicate that oxyg...

Interfacial Ion Intermixing Effect on Four-Resistance States in La0.7Sr0.3MnO3/BaTiO3/La0.7Sr0.3MnO3 Multiferroic Tunnel Junctions

A multiferroic tunnel junction (MFTJ), employing a ferroelectric barrier layer sandwiched between two ferromagnetic layers, presents at least four resistance states in a single memory cell and therefore opens an avenue for the development of the next generation of high-density nonvolatile memory devices. Here, using the all-perovskite-oxide La0.7Sr0.3MnO3/BaTiO3/La0.7Sr0.3MnO3 as a model MFTJ system, we demonstrate asymmetrical Mn-Ti sublattice intermixing at the La0.7Sr0.3MnO3/BaTiO3 interfaces by direct local measurements of the structure and valence, which reveals the relationship between ferroelectric polarization directions and four-resistance states, and the low temperature anomalous tunneling behavior in the MFTJ. These findings emphasize the crucial role of the interfaces in MFTJs and are quite important for understanding the electric transport of MFTJs as well as designing high-density multistates storage devices.

Effects of Interface Layers and Domain Walls on the Ferroelectric-Resistive Switching Behavior of Au/BiFeO3/La0.6Sr0.4MnO3 Heterostructures

The electric field effects on the electric and magnetic properties in multiferroic heterostructures are important for not only understanding the mechanisms of certain novel physical phenomena occurring at heterointerfaces but also offering a route for promising spintronic applications. Using the Au/BiFeO3/La0.6Sr0.4MnO3 (Au/BFO/LSMO) multiferroic heterostructure as a model system, we investigated the ferroelectric-resistive switching (RS) behaviors of the heterostructure. Via the manipulation of the BFO ferroelectric polarizations, the nonvolatile tristate of RS is observed, which is closely related to the Au/BFO and BFO/LSMO interface layers and the highly conducting BFO domain walls (DWs). More interestingly, according to the magnetic field dependence of the RS behavior, the negative magnetoresistance effect of the third resistance state, corresponding to the abnormal current peak in current-pulse voltage hysteresis near the electric coercive field, is also observed at room temperature, which mainly arises from the possible oxygen vacancy accumulation and Fe ion valence variation in the DWs.

Solid-State Synapse Based on Magnetoelectrically Coupled Memristor

Brain-inspired computing architectures attempt to emulate the computations performed in the neurons and the synapses in the human brain. Memristors with continuously tunable resistances are ideal building blocks for artificial synapses. Through investigating the memristor behaviors in a La0.7Sr0.3MnO3/BaTiO3/La0.7Sr0.3MnO3 multiferroic tunnel junction, it was found that the ferroelectric domain dynamics characteristics are influenced by the relative magnetization alignment of the electrodes, and the interfacial spin polarization is manipulated continuously by ferroelectric domain reversal, enriching our understanding of the magnetoelectric coupling fundamentally. This creates a functionality that not only the resistance of the memristor but also the synaptic plasticity form can be further manipulated, as demonstrated by the spike-timing-dependent plasticity investigations. Density functional theory calculations are carried out to describe the obtained magnetoelectric coupling, which is probably related to the Mn-Ti intermixing at the interfaces. The multiple and controllable plasticity characteristic in a single artificial synapse, to resemble the synaptic morphological alteration property in a biological synapse, will be conducive to the development of artificial intelligence.

Anisotropic transport property anomaly in K0.8Fe1.65Se2 crystal

The anisotropic transport properties of K0.8Fe1.65Se2 crystal have been investigated with the angle θ (between the magnetic field H and c-axis) varied from 0° to 180°. It is found that the angle dependencies of the mixed state resistivities near Tc0 show a small anomalous peak on the resistive valley for H//ab, probably due to the kinked vortex structure with a lower pinning energy in the phase-separated crystal. In the normal state, the resistivities increase with increasing angles and take a maximum at H//ab, which may be related to the larger resistivity when carriers move along the c-axis driven by Lorentz force.

Multiferroic properties of neodymium and cobalt co-doped four-layer Aurivillius compounds

Single-phase multiferroic materials have attracted much attention in recent years due to their rich fundamental physics and great application potential^[1]. In the last few years, the bismuth-based Aurivillius phase compound, which is formed by stacking fluorite-like (Bi₂O₂)²⁺ layers and per-oskite-like (A_n-1B_nO_3n+1)^2-, is one of the most promising candidates of single-phase multiferroic materials by doping magnetic cations at B site^[2]. Recently, co-doped Aurivillius compounds have been reported to present an enhanced ferroelectric or ferromagnetic properties^[3].

Electric-Field-Controlled Nonvolatile Magnetization Rotation and Magnetoresistance Effect in Co/Cu/Ni Spin Valves on Piezoelectric Substrates

Electric-field control of magnetism is a key issue for the future development of low-power spintronic devices. By utilizing the opposite strain responses of the magnetic anisotropies in Co and Ni films, a Co/Cu/Ni/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) spin-valve/piezoelectric heterostructure with ∼7 nm Cu spacer layer was properly designed and fabricated. The purely electric-field-controlled nonvolatile and reversible magnetization rotations in the Co free layer were achieved, whereas the magnetization of the Ni fixed layer was almost unchanged. Accordingly, not only the electroresistance but also the electric-field-tuned magnetoresistance effects were obtained, and more importantly at least six nonvolatile magnetoresistance states in the strain-tuned spin valve were achieved by setting the PMN-PT into different nonvolatile piezo-strain states. These findings highlight potential strategies for designing electric-field-driven multistate spintronic devices.

Electric-field-controlled nonvolatile magnetic switching and resistive change in La0.6Sr0.4MnO3/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (011) heterostructure at room temperature

Control over nonvolatile magnetization rotation and resistivity change by an electric field in La0.6Sr0.4MnO3 thin films grown on (011) oriented 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 substrates are studied. By utilizing an in-plane strain induced by a side ferroelectric switching with pulsed electric fields from −2.5 kV/cm to +5 kV/cm along [011¯], a nonvolatile and reversible 90°-rotation of the magnetic easy-axis is achieved, corresponding to −69.68% and +174.26% magnetization switching along the [100] and [011¯] directions, respectively. The strain induced nonvolatile resistivity change is approximately 3.6% along the [011¯] direction. These findings highlight potential strategies for electric-field-driven spintronic devices.