Y. W. Yin

Coexistence of tunneling magnetoresistance and electroresistance at room temperature in La0.7Sr0.3MnO3/(Ba, Sr)TiO3/La0.7Sr0.3MnO3 multiferroic tunnel junctions

Tunnel junctions composed of two ferromagnetic electrodes separated by a ferroelectric barrier were fabricated from epitaxial La0.7Sr0.3MnO3/Ba0.95Sr0.05TiO3/La0.7Sr0.3MnO3 trilayers. Typical R−H curves with sharp-switched resistance states (magnetic parallel and antiparallel) of magnetic tunnel junctions have been observed up to room temperature. After applying a poling voltage, which reverses the barrier polarization, both the parallel and antiparallel resistance states will switch to different values. Clear tunneling magnetoresistance and tunneling electroresistance, hence the four resistance states have been observed at room temperature.

Current-voltage characteristics of La2-xSrxCuO4/Nb-doped SrTiO3 heterojunctions

[Li, X. G.] Univ Sci & Technol China, Hefei Natl Lab Phys Sci Microscale, Dept Phys, Hefei 230026, Peoples R China. Acad Sinica, Int Ctr Mat Phys, Shenyang 110015, Peoples R China.;Li, XG (reprint author), Univ Sci & Technol China, Hefei Natl Lab Phys Sci Microscale, Dept Phys, Hefei 230026, Peoples R China;lixg@ustc.edu.cn

Angle-resolved vortex glass transition and pinning properties in BaFe1.8Co0.2As2 single crystals

The vortex phase transition in BaFe1.8Co0.2As2 single crystals was studied systematically via measuring the current–voltage (I-V) characteristics by rotating the crystals in magnetic fields from H // c to H // ab. The angle-resolved I-V curves clearly show a three-dimensional vortex glass (VG) scaling collapse. The angular dependent VG transition field Hg and critical current density Jc can be approximately described by the anisotropic Ginzburg-Landau model, and no peaks representing c-axis correlated pinning were recorded for H // c, which reveals that the vortex pinning is mainly caused by randomly distributed uncorrelated-defects. While in the vicinity of H // ab, Jc deviates from the model due to the in-plane intrinsic pinning. Based on the above results, the angle-resolved vortex phase diagram is constructed, indicating the anisotropic superconducting properties of the system.

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.

Effect of carrier density and valence states on superconductivity of oxygen annealed Fe1.06Te0.6Se0.4 single crystals

The variations of carrier density and valence states in oxygen annealed Fe1.06Te0.6Se0.4 single crystals were studied systematically. It was found that the carrier density nH increases after oxygen annealing by Hall coefficient measurements. The X-ray photoelectron spectroscopy experiments reveal that the oxygen annealing changes Fe0 and Te0 states to Fe2+/3+ and Te4+, respectively, while the valence variation of Se is negligible. Our results indicate that the improvement of superconductivity, such as the zero resistance transition temperature Tczero, shielding and Meissner fraction value 4πχ and upper critical field Hc2, could be closely related to the proper manipulation of nH and the valence states by oxygen annealing in the system.

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.

Stripe order related in-plane fourfold symmetric superconductivity in La1.45Nd0.4Sr0.15CuO4 single crystal

The in-plane fourfold symmetry of the upper critical field Hc2 in a stripe-ordered La1.45Nd0.4Sr0.15CuO4 single crystal has been observed through measuring the out-of-plane resistivity ρc by rotating the crystal around its c-axis in magnetic fields parallel to the ab-plane of the crystal. It was found that the upper critical field Hc2 takes maxima (minima) for magnetic fields along Cu-O-Cu (Cu-Cu) direction, suggesting an intrinsic fourfold superconductivity. The results should be closely related to the special structure of the charge stripes which are along Cu-O-Cu direction and rotated 90° in adjacent CuO2 planes, and the fourfold symmetry behaviors in Hc2 and ρc can be well described by a scaling method. The stripe phase can also lead to a fourfold vortex pinning effect which is confirmed by the angular dependence of the irreversibility field Hirr and the activation energy U for flux motion.

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.

Spin structure transition in La(1.6-x)Nd(0.4)Sr(x)CuO(4) superconductors

The field and temperature dependencies of the spin structures in the normal states of La(1.6-x)Nd(0.4)Sr(x)CuO(4) (x = 0.10, 0.12, and 0.15) single crystals have been studied by measuring the magnetoresistance and susceptibility. A negative magnetoresistance appears just below the spin-ordering temperature for the magnetic fields parallel to the CuO(2) plane, which can be attributed to the spin-flop transition of the special spin structure in the normal state of the system. The anisotropic variations of susceptibilities with temperature for all the three specimens can be described in the framework of the crystal-field theory. The well fitted broad peaks of the in-plane susceptibilities χ(ab) for the specimens suggest that the susceptibilities are dominated by Nd(3+), and thus the spin reorientation of Cu(2+) in the CuO(2) plane can not be observed from the study of the susceptibility.

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.