Sining Dong

Dynamic properties of spin cluster glass and the exchange bias effect in BiFeO 3 nanocrystals

A spin cluster glass behavior and a complicated exchange bias effect are observed in high quality BiFeO(3) nanocrystals grown by a hydrothermal method. The dynamic properties of the spin clusters investigated by measuring the frequency dependences of ac susceptibility show that the relaxation process can be described using a power law with the glass transition temperature T(g) = 57 K, relaxation time constant τ(0) = 4.4 × 10(-10) s, and critical exponent zv = 10.3 ± 1.9, consistent with a three-dimensional Ising spin glass. The exchange bias field (H(EB)) varies non-monotonically with temperature and achieves a minimum at T(g). The abnormal shift of hysteresis loops above T(g) may be interpreted in terms of a Malozemoffs random-field model with a framework of antiferromagnetic core/spin-cluster shell structure and a two-dimensional diluted antiferromagnet in a field (2D-DAFF) model, respectively. The exchange anisotropy of the BiFeO(3) nanocrystals will shed light on a possible application for magnetism related nanosized devices.

Giant magnetodielectric effect in Terfenol-D/PZT magnetoelectric laminate composite

The magnetic field dependence of the dielectric permittivity of Terfenol-D/PbZr(x)Ti(1-x)O(3) magnetoelectric composites in the temperature range from 200 K to 340 K was investigated systematically. It was found that there is a large magnetodielectric effect up to 15% around the electromechanical resonance frequency in a magnetic field of 5 kOe at room temperature. Nonmonotonic variations of dielectric permittivity with magnetic fields are associated with the mechanical energy loss due to magnetic domain wall motion in the magnetostrictive layer Terfenol-D. A numerical modeling is proposed and agrees well with the experimental data. The results are of significance in the development of magnetic-field-tuned electronic devices at room temperature

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.

Tunable dielectric and ferroelectric properties in heteroepitaxial PbZr0.52Ti0.48O3/La0.625Ca0.375MnO3 thin films

In this work, the PbZr0.52Ti0.48O3/La0.625Ca0.375MnO3 (PZT/LCMO) thin films show a large magnetodielectric effect up to 52% at 3 MHz in a field of 0.8 T near the ferromagnetic Curie temperature Tc of LCMO. According to the equivalent RC-circuit fitting, the large magnetodielectric effect is found to be closely related to the interface behaviors between PZT and LCMO, which exhibits impressive magnetodielectric and magnetoresistance effects. Meanwhile, the extrinsic change of the ferroelectric coercive field Ec and remnant polarization Pr can be explained by the variations of voltage drop and space-charge related polarization. These findings improve our comprehension of magnetoelectric coupling in multiferroic heterostructure, and may provide potential application for multifunctional devices in spintronics.

Room temperature multiferroicity in Bi 4.2 K 0.8 Fe 2 O 9+δ

Magnetoelectric multiferroics are materials that have coupled magnetic and electric dipole orders, which can bring novel physical phenomena and offer possibilities for new device functions. In this report, single-crystalline Bi4.2K0.8Fe2O9+δ nanobelts which are isostructural with the high-temperature superconductor Bi2Sr2CaCu2O8+δ are successfully grown by a hydrothermal method. The regular stacking of the rock salt slabs and the BiFeO3-like perovskite blocks along the c axis of the crystal makes the Bi4.2K0.8Fe2O9+δ nanobelts have a natural magnetoelectric–dielectric superlattice structure. The most striking result is that the bulk material made of the Bi4.2K0.8Fe2O9+δ nanobelts is of multiferroicity near room temperature accompanied with a structure anomaly. When an external magnetic field is applied, the electric polarization is greatly suppressed, and correspondingly, a large negative magnetocapacitance coefficient is observed around 270 K possibly due to the magnetoelectric coupling effect. Our result provides contributions to the development of single phase multiferroics.

Large Magnetodielectric Effect in (1-X) La2NiMnO6—(X) La2/3Sr1/3MnO3 Composites

A large magnetodielectric effect in (1-x) La2NiMnO6—(x) La2/3Sr1/3MnO3 composites over wide frequency and temperature ranges is reported. The results indicate that variation of dielectric constants with magnetic fields depends non-monotonously on the molar ratio of two compounds and reaches a maximum up to 20% for x = 0.4 at the frequency of 1 MHz and field of 1 kOe at room temperature. The further analyses suggest that this effect is closely related to the magnetoresistance. These findings provide an alternative strategy to achieve a room temperature magnetodielectric response in multiferroic composites, indicating that it is promising for potential applications.

Effects of magnetic electrode on the ferroelectric properties in heteroepitaxial BiFeO3/La0.625Ca0.375MnO3 thin films

The ferroelectric properties of the BiFeO3/La0.625Ca0.375MnO3 (BFO/LCMO) heterostructures were investigated using different bottom electrode configurations at different magnetic fields and temperatures. It is found that the apparent coercive voltage (Vac) increases linearly with the increase of LCMO resistances for different electrodes, and the extrinsic relative contribution from different LCMO electrodes to the variation of Vac caused by magnetic field can be quantitatively analyzed based on the scenario of voltage drop model. The magnetic field and temperature dependences of the heterostructure coercive voltage (Vac0) obtained by subtracting the voltage drop on LCMO from Vac are closely related to the interface behaviors. These findings not only further elucidate the physics of magnetoelectric coupling in multiferroic heterostructures but also are helpful for designing artificial prototype device.

Structure Evolution and Multiferroic Properties in Cobalt Doped Bi4NdTi3Fe1-xCoxO15-Bi3NdTi2Fe1-xCoxO12-δ Intergrowth Aurivillius Compounds

Here, we report the structure evolution, magnetic and ferroelectric properties in Co-doped 4- and 3-layered intergrowth Aurivillius compounds Bi4NdTi3Fe1-xCoxO15-Bi3NdTi2Fe1-xCoxO12-δ. The compounds suffer a structure evolution from the parent 4-layered phase (Bi4NdTi3FeO15) to 3-layered phase (Bi3NdTi2CoO12-δ) with increasing cobalt doping level from 0 to 1. Meanwhile the remanent magnetization and polarization show opposite variation tendencies against the doping level, and the sample with x = 0.3 has the largest remanent magnetization and the smallest polarization. It is believed that the Co concentration dependent magnetic properties are related to the population of the Fe3+ -O-Co3+ bonds, while the suppressed ferroelectric polarization is due to the enhanced leakage current caused by the increasing Co concentration. Furthermore, the samples (x = 0.1–0.7) with ferromagnetism show magnetoelectric coupling effects at room temperature. The results indicate that it is an effective method to create new multiferroic materials through modifying natural superlattices.

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].