X. S. Gao

Temperature-dependent and polarization-tuned resistive switching in Au/BiFeO3/SrRuO3 junctions

The relationship between the bipolar resistive switching and the polarization reversal is investigated at various temperatures in the Au/BiFeO3/SrRuO3 structure. It is found that the polarization-induced barrier variation in the Au/BiFeO3 and BiFeO3/SrRuO3 junctions decreases with decreasing temperature. This explains why the resistance-switching ratio decreases with decreasing temperature below 323 K and gives evidence that the polarization modulates the resistance state of the Au/BiFeO3/SrRuO3 structure. Besides, the oxygen vacancies migration and/or the carrier trapping/detrapping mechanisms are also suggested to play a very important role in the resistive switching behavior in this structure as the temperature goes above 323 K.

Nanoscale ferroelectric tunnel junctions based on ultrathin BaTiO3 film and Ag nanoelectrodes

In this work, Ag nanoisland electrodes (nanoelectrodes) have been deposited on top of ultrathin ferroelectric BaTiO3 (BTO) films to form a nanoscale metal-ferroelectric-metal tunnel junction by integrating growth techniques of nanocluster beam source and laser-molecular beam epitaxy. The ultrathin BTO films (∼3 nm thick) exhibit both apparent ferroelectric polarization reversal and ferroelectric tunneling related resistive switching behaviors. The introducing of Ag nanoislands (∼20 nm in diameter) as top electrode substantially enhances the tunneling current and alters the symmetry of I-V hysteresis curves. The enhanced tunneling current is likely due to the reduction in tunneling barrier height and an increase in effective tunneling area by Ag nano-electrodes, while the improved symmetric in I-V curve may be attributed to the variation of electrode-oxide contact geometry.

Defect states and charge trapping characteristics of HfO2 films for high performance nonvolatile memory applications

In this work, we present significant charge trapping memory effects of the metal-hafnium oxide-SiO2-Si (MHOS) structure. The devices based on 800 °C annealed HfO2 film exhibit a large memory window of ∼5.1 V under ±10 V sweeping voltages and excellent charge retention properties with only small charge loss of ∼2.6% after more than 104 s retention. The outstanding memory characteristics are attributed to the high density of deep defect states in HfO2 films. We investigated the defect states in the HfO2 films by photoluminescence and photoluminescence excitation measurements and found that the defect states distributed in deep energy levels ranging from 1.1 eV to 2.9 eV below the conduction band. Our work provides further insights for the charge trapping mechanisms of the HfO2 based MHOS devices.

Significant enhancements of dielectric and magnetic properties in Bi(Fe1−xMgx)O3−x/2 induced by oxygen vacancies

Bi(Fe1?xMgx)O3?x/2 (x?=?0?10%) ceramics were synthesized by high-energy ball milling and solid-state reaction. It was found that a small amount of Mg doping leads to a dramatic enhancement in dielectric permittivity (?two orders of magnitude), along with an apparent improvement in ferromagnetism. The observed significant enhanced dielectric properties may be interpreted by the Maxwell?Wagner relaxation in association with internal barrier layer capacitance. The ferromagnetism can be ascribed to the creation of unbalanced Fe3+ spins and relative long-range coupling mediated by the oxygen vacancies trapped localized electrons.

Enhanced resistive switching effect in Ag nanoparticle embedded BaTiO3 thin films

Ag nanoparticle (NP) embedded BaTiO3 (BTO) thin films on SrRuO3-coated SrTiO3 (STO) substrates are prepared by the integrated nanocluster beam deposition and laser-molecular beam epitaxy. Enhanced resistive switching, up to an ON/OFF ration of 104, has been achieved at low switching voltage (less than 1 V) without a forming voltage. These characteristics make such nanocomposite film very promising for application of low voltage non-volatile random access memory. The enhanced resistive switching effect may be attributed to the charge storage effect of the Ag nanoparticles and easy formation of Ag filament inside the BTO film.

Energy storage and polarization switching kinetics of (001)-oriented Pb0.97La0.02(Zr0.95Ti0.05)O3 antiferroelectric thick films

For antiferroelectric (AFE) energy storage, the stability of energy storage density and conversion efficiency against wide temperature (T) range and broad frequency (f) band is highly preferred. In this work, we investigate the energy storage and associated kinetics of polarization switching in (001)-textured AFE Pb0.97La0.02(Zr0.95Ti0.05)O3 (PLZT 2/95/5) thick films prepared by sol-gel method. A recoverable energy storage density (Wre) of ∼26.8 J/cm3 and an energy conversion efficiency (η) as high as ∼62.5% have been obtained under an electric field of 1.85 MV/cm and room temperature. Both the Wre and η are only weakly T-dependent up to 280 °C and weakly f-dependent ranging from 20 Hz to 10 kHz. The high frequency stability originates from the rapid polarization switching as identified by the nucleation-limited-switching theory, suggesting a characteristic switching time as short as ∼3 ns, favorable for applications in pulse energy storage.

Dielectric and magnetic properties of BiFe1-4x/3TixO3 ceramics with iron vacancies: Experimental and first-principles studies

BiFe1-4x/3TixO3 (x = 0-0.2) ceramics with Fe vacancies created by nonstioichiometric compositions were synthesized to study their crystal structures, dielectric, and ferromagnetic behaviors. X-ray diffraction and Raman spectroscopy analysis reveal a structure transition from rhombohedral to orthorhombic phases at x = 0.2. Dielectric measurement shows that the dielectric loss is significantly reduced in the Ti-doped BiFeO3 samples. With increasing x concentration, the remanent magnetization (Mr) first increases and then decreases. The maximal Mr of 0.13 emμ/g is obtained at x = 0.05. Furthermore, theoretical calculations based on the density-functional theory prove that the Ti-doping does enhance the lattice constants, band gap, and magnetization. These results show that the Ti-doped BiFeO3 with Fe vacancies could enhance resistivity and magnetism, implying a possible improvement in multiferroic behavior.

Influence of Co:Mn ratio on multiferroicity of Ca3Co2−xMnxO6 around x∼1

A series of polycrystalline Ca3Co2−xMnxO6 with 0.93<x<1.07 are prepared in order to investigate the effect of Co:Mn ratio around x∼1.0 on the multiferroicity. It is found that the ferroelectric polarization P at low temperature shows significant dependence on the Co:Mn ratio, forming a V-shaped pattern with the lowest point at x∼0.99. It is proposed that the suppression of P with the Co:Mn ratio approaching to the best Co/Mn order originates from the disruption of the long range spin order.

Room temperature multiferroic and magnetodielectric properties in Sm and Sc co-doped BiFeO3 ceramics

In this work, Sm and Sc co-doped Bi1−x Sm x Fe1−y Sc y O3 (x = 0.00–0.20; y = 0.03) ceramics are fabricated by a rapid liquid phase sintering method, in order to develop single-phase multiferroics with large magnetization and polarization. X-ray diffraction and Raman spectroscopic studies reveal that the ceramics are single-phase with a structural transition from rhombohedral to orthorhombic structures near x = 0.15. Electric and magnetic measurement results indicate that the transition significantly enhances the multiferroic properties, which stems from the Sm/Sc doping induced collapse of space-modulated spin structure and internal structural distortion. At an optimized composition of Bi0.85Sm0.15Fe0.97Sc0.03O3 (x = 0.15), a remanent polarization of 16.5 μC cm−2, a magnetization 0.2020 emu g−1, and a magnetodielectric effect of 0.46% can be obtained. These results clearly demonstrate a potential application for Sm/Sc doped BiFeO3 ceramics in the field of multiferroic devices.

Analysis of the alternating current conductivity and magnetic behaviors for the polycrystalline Y-type Ba0.5Sr1.5Co2(Fe1-xAlx)12O22 hexaferrites

Co2Y hexaferrites have attracted intensive interests due to its potential high temperature magnetoelectricity as the single phase multiferroics. Numerous efforts have been paid to enhance their magnetoelectric properties at high temperatures through increasing the magnetic transition temperature and decreasing the conductivity. In this work, we investigated the conductivity and magnetic properties of the polycrystalline Ba0.5Sr1.5Co2(Fe1−xAlx)12O22 (0 ≤ x ≤ 0.12) hexaferrites and found that Al-doping has important effects on both the conductivity and magnetic properties. The underlying physical mechanisms were also systematically analyzed. Most importantly, a very much enhanced resistivity (over 10 MΩ cm), and a high magnetic transition temperature (∼346 K) have been obtained at a doping amount of x = 0.04. These improvements are very promising for achieving significant magnetoelectric effect at room temperature. The current research can provide the basic understanding of the Y-type ferrites for future ap...