Yukuai Liu

Non-volatile 180° magnetization reversal by an electric field in multiferroic heterostructures.

The deterministic rotation of magnetization by electric fields is a challenging issue for future low-power spintronics. In a Co/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 multiferroic heterostructure, piezostrain-mediated, macroscopically maneuverable, and non-volatile magnetization reversal without an applied magnetic field is demonstrated. This, combined with the presented phase-field simulations, is of practical relevance for designing prototype devices.

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.

Coaction and distinguishment of converse piezoelectric and field effects in La0.7Ca0.3MnO3/SrTiO3/0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 heterostructures

The volatile and nonvolatile electroresistances related to the converse piezoelectric induced strain and ferroelectric field effects are improved in La0.7Ca0.3MnO3/SrTiO3/0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 heterostructures by inserting a SrTiO3 buffer layer. Due to the coaction of the strain and field effects, the tri-resistance states are observed, and the relative contributions of the two effects on the resistance changes can be quantitatively distinguished by a programmable control of the polarization electric fields in “ON” and “OFF” modes, respectively. Our results indicate that the well-designed heterostructure exhibits potential for application in multifunctional devices.

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.

Small torsional piezoelectric fiber actuators with helical electrodes

Small piezoelectric fibers wound with helical electrodes on their outer surface will produce torsional displacement. A piece of piezoelectric tube with an outer diameter of 1mm, a wall thickness of 0.1mm, and an effective axial length of 40mm was used as a prototype actuator’s body, while two pieces of copper wires of 40μm diameter were used as the electrodes. After poled with 2kV∕mm electric field strength, the prototype actuator produces 1.7° torsional angle when it was driven from −500to500V. A resonant frequency as high as 10kHz was observed on this actuator.

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.

Irreversibility line and thermally activated flux flow in La1.6−xNd0.4SrxCuO4 films

The resistive transition broadening of the c-axis oriented La1.6−xNd0.4SrxCuO4 epitaxial films (x=0.1, 0.12, 0.14, and 0.16) has been systematically investigated under magnetic fields up to 14 T for both H⊥c and H‖c configurations. For H‖c, the irreversibility line corresponds to a melting line in high temperature and low field regions, whereas it shows a rapid increase following a decoupling theory in low temperature and high field regions. It is found that the in-plane resistivity ρab below the mean-field transition temperature Tc(H) follows Arrhenius-type thermally activated flux flow model ρ(T)=ρ0exp{−U0(H)[1−T∕Tc(H)]n∕kBT}, where n=3 for H‖c and n=1.5 for H⊥c. For H‖c, the field dependence of activation energy follows a power law, i.e., U0(H)∝H−α(α∼1), which can be interpreted in terms of thermal depinning of vortices in a quasi-two-dimensional superconductor. For H⊥c, a logarithmic dependence of activation energy is observed, which results from the plastic creep of flux lines associated with the int...

Colossal anisotropic resistivity and oriented magnetic domains in strained La0.325Pr0.3Ca0.375MnO3 films

Magnetic and resistive anisotropies have been studied for the La0.325Pr0.3Ca0.375MnO3 films with different thicknesses grown on low symmetric (011)-oriented (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 substrates. In the magnetic and electronic phase separation region, a colossal anisotropic resistivity (AR) of ∼105% and an anomalous large anisotropic magnetoresistance can be observed for 30 nm film. However, for 120 nm film, the maximum AR decreases significantly (∼2 × 103%) due to strain relaxation. The colossal AR is strongly associated with the oriented formation of magnetic domains, and the features of the strain effects are believed to be useful for the design of artificial materials and devices.

Transport and stability properties of the charge ordered state for Sm1-xCaxMnO3 (0.4 <= x <= 0.8)

The structure and transport properties of Sm1-xCaxMnO3 with 0.4less than or equal toxless than or equal to0.8 were investigated. With decreasing temperature from 350 K, cooperative Jahn-Teller (JT) distortion develops, leading to the formation of a charge ordered (CO) state and anomalies in the lattice parameters and resistivities. At low temperature the magnetoresistance effect originates from the polarization of the localized t(2g) spins under the magnetic field and decreases with the increase in Ca2+ content x, whereas the stability of the CO state at the CO transition temperature T-CO strongly depends on both Ca2+ content and cooperative JT distortion. These interesting phenomena sufficiently confirm the close relationship among charge, lattice, spin, and orbital degrees of freedom

Manipulation of morphologies and magnetic properties for Bi4.2K0.8Fe2O9+δ nanostructures

In this paper, Bi4.2K0.8Fe2O9+δ nanostructures with different morphologies, such as dispersible nanobelts, urchin-like and floriated micronanostructures with dendrites in different sizes, were controllably synthesized by a surfactant-free hydrothermal method. The temperatures of the KOH solutions in the co-precipitation step and the hydrothermal reactions had great effects on not only the morphologies but also the magnetic properties of the products. It was found that the low-temperature-fabricated urchin-like samples had a higher magnetization value. The special cores of the urchin-like nanostructures may play a critical role in the enhanced magnetizations and spin-glass dynamic behaviors. These results provide a simple way to manipulate the morphology of low-dimensional Bi4.2K0.8Fe2O9+δ nanostructures as well as their related magnetic properties.