Dalong Zhang

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.

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.

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 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.

Electric field manipulated nonvolatile and reversible 90°-rotation of the magnetic easy-axis in LSMO/PMN-PT (011) multiferroic heterostructures

Using pure electric voltages instead of magnetic fields or large currents to manipulate magnetisms in multiferroic heterostructures is a goal for future low-power spintronics such as electric-writing magnetic-reading memories. Usually, the electric manipulation to magnetism shows a volatile effect which cannot be used for information storage,[1] because the magnetic variation vanishes after the electric field removal. Recently, the piezostrain-mediated non-volatile 90° and 180° magnetization rotations are realized in the Co/0.7Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ multiferroic heterostructures at room temperature.[2] However, the non-volatile magnetization rotations in perovskite manganese systems, which are of significance for the design of perovskite magnetic random access memories such as perovskite magnetic tunneling junctions, are still rarely studied. Here we report an in-situ electric field manipulated nonvolatile and reversible 900-rotation of the magnetic easy-axis (MEA) in La_0.7Sr_0.3MnO₃/0.7Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ (LSMO/PMN-PT) multiferroic heterostructure.

Resistive switching in Au/BiFeO 3 /La 0.6 Sr 0.4 MnO 3 heterostructures

The ferromagnetically coupled ferroelectric domain walls (DWs), which may be more conductive, in multiferroic material BiFeO3 (BFO), make it possible to realize intrinsic multiferroelectricity in single phase material and open a chance for combining spintronics and ferroelectricity in multifer-roic systems.[1] From the aspect of conductivity, the conductive DWs will influence the macroscopic transport of ferroelectric materials, and thus be coupled with ferroelectric resistive switching, which arises from the modulation of band alignment and contact resistance in the interface between ferroelectric and electrode layers when ferroelectric polarization is switched. In the present work, a room temperature resistive switching behavior of Au/BiFeO3/La0.6Sr0.4MnO3 (BFO/LSMO) heterostructures accompanied with DWs conductance effect is reported.

Kondo effect on the transport properties in the multiferroic tunnel junction of La 0.7 Sr 0.3 MnO 3 /BaTiO 3 /La 0.7 Sr 0.3 MnO 3

The Kondo effect, one of the most interesting topics in condensed matter physics, shows significant influences on the transport behaviors in magnetic tunnel junctions (MTJs). As we know that there is a competition between electron tunneling and Kondo scattering in MTJs. However, it is not clear how Kondo effect will affect the transport properties of a multiferroic tunnel junction (MFTJ), a MTJs with a ferroelectric material as the tunnel barrier. As a ferroelectric polarization may manipulate the Kondo effect, we here attempt to investigate these issues by studying the temperature-dependent and polarization-tuned transport properties in the La0.7Sr0.3MnO3/BaTiO3/ La0.7Sr0.3MnO3 (LSMO/BTO/LSMO) MFTJ.