Qiwen Yao

Dielectric, magnetic, and magnetotransport properties in Sr doped two-dimensional RE2CoO4 (RE=Pr,Eu) compounds

In this work, we report on the studies of dielectric, magnetic, and magnetotransport properties of Sr doped RE2−xSrxCoO4 (RE=Pr or Eu, x=0.25–1.5). These compounds were systematically studied in terms of structure, magnetic, magnetotransport, and dielectric constant measurements. Rietveld refinement indicated that these compounds crystallized in K2NiF4-type structure with space group I4∕mmm. Lattice parameters increase with Sr doping level. The system changes from paramagnetic to ferromagnetic with increasing Sr doping level and finally becomes ferromagnetic with TC of 230K for Pr0.75Sr1.25CoO4 and EuSrCoO4, respectively. The temperature dependence of resistivity indicates that both systems change from semiconductive to metallic with Sr doping. The magnetoresistance (MR) value of 10% at 5K and 8T is found for the EuSrCoO4 compound. Large dielectric constants with values of above 2000 were observed in low frequencies for samples with x around 1 for Pr based compounds.

Enhancement of the in-field Jc of MgB2 via SiCl4 doping

In this work, we present the following important results: 1) We introduce a new Si source, liquid SiCl4, which is free of C, to significantly enhance the irreversibility field (Hirr), the upper critical field (Hc2), and the critical current density (Jc), with little reduction in the critical temperature (Tc). 2) Although Si can not incorporate into the crystal lattice, we found a reduction in the a-axis lattice parameter, to the same extent as for carbon doping. 3) The SiCl4 treated MgB2 shows much higher Jc with superior field dependence above 20 K than undoepd MgB2 and MgB2 doped with various carbon sources. 3) We provide an alternative interpretation for the reduction of the a lattice parameter in C- and non-C doped MgB2. 4). We introduce a new parameter, RHH (Hc2/Hirr), which can clearly reflect the degree of flux pinning enhancement, providing us with guidance for further enhancing Jc. 5) We have found that spatial variation in the charge carrier mean free path is responsible for the flux pinning mechanism in the SiCl4 treated MgB2 with large in-field Jc.

Multivariate Statistical Characterization of Charged and Uncharged Domain Walls in Multiferroic Hexagonal YMnO3 Single Crystal Visualized by a Spherical Aberration-Corrected STEM

A state-of-the-art spherical aberration-corrected STEM was fully utilized to directly visualize the multiferroic domain structure in a hexagonal YMnO3 single crystal at atomic scale. With the aid of multivariate statistical analysis (MSA), we obtained unbiased and quantitative maps of ferroelectric domain structures with atomic resolution. Such a statistical image analysis of the transition region between opposite polarizations has confirmed atomically sharp transitions of ferroelectric polarization both in antiparallel (uncharged) and tail-to-tail 180° (charged) domain boundaries. Through the analysis, a correlated subatomic image shift of Mn-O layers with that of Y layers, exhibiting a double-arc shape of reversed curvatures, have been elucidated. The amount of image shift in Mn-O layers along the c-axis is statistically significant as small as 0.016 nm, roughly one-third of the evident image shift of 0.048 nm in Y layers. Interestingly, a careful analysis has shown that such a subatomic image shift in Mn-O layers vanishes at the tail-to-tail 180° domain boundaries. Furthermore, taking advantage of the annular bright field (ABF) imaging technique combined with MSA, the tilting of MnO5 bipyramids, the very core mechanism of multiferroicity of the material, is evaluated.

Band structures, and magnetic and transport properties of la doped two dimensional Sr2CoO4

In this work, we report on our studies on the band structure calculations, structures, transport, and magnetic properties in two dimensional layer structured perovskite compounds Sr2−xLaxCoO4. Structure refinement results reveal that these compounds crystallized in K2NiF4-type structures with space group I4∕mmm. The temperature dependence of resistivity shows a semiconductorlike behavior over a wide range of temperatures, a metal-insulator transition at 240K, and an upturn at 160K for the x=1, 1.25, and 0.75 samples. A coercive field is about 1T for the sample with x=0.75, while it is about 0.05T for x=0.75 and 0.1T for the x=1.25 samples. A negative field hysteresis magnetoresistance in close correlation with the coercive field has been observed and can be explained by the grain boundary tunneling effect. The first-principles band structure calculations were carried out for Sr1.5La0.5CoO4 and the results indicate that the system is metallic with a high spin polarization which is responsible for the obser...

Strong competition between the δl and δTc flux pinning mechanisms in MgB2 doped with carbon containing compounds

The transport and magnetic properties of 10 wt % malic acid and 5 wt % nanocarbon doped MgB2 have been studied by measuring the resistivity (ρ), critical current density (jc), connectivity factor (AF), irreversibility field (Hirr), and upper critical field (Hc2). The pinning mechanisms are studied in terms of the collective pinning model. It was found that both mean free path (δl) and critical temperature (δTc) pinning mechanisms coexist in both doped MgB2. For both the malic acid and nanocarbon doped samples, the temperature dependence of the crossover field, which separates the single vortex and the small bundle pinning regime, Bsb(T), shows that the δl pinning mechanism is dominant for temperatures up to t(T/Tc)=0.7 but the δTc pinning mechanism is dominant for t>0.7. This tendency of coexistence of the δl and the δTc pinning mechanism is in strong contrast with the pure MgB2, in which the δTc pinning mechanism is dominant over a wide temperature range below Tc. It was also observed that the connectivi...

New Multiferroic Materials: Bi2FeMnO6

The term “ferroic” was introduced by Aizu in 1970, and presented a unified treatment of certain symmetry-dictated aspects of ferroelectric, ferroelastic, and ferromagnetic materials. Ferroelectric materials possess a spontaneous polarization that is stable and can be switched hysteretically by an applied electric field; antiferroelectric materials possess ordered dipole moments that cancel each other completely within each crystallographic unit cell. Ferromagnetic materials possess a spontaneous magnetization that is stable and can be swithched hysteretically by an applied magnetic field; antiferromagnetic materials possess ordered magnetic moments that cancel each other completely within each magnetic unit cell. By the original definition, a single-phase multiferroic material is one that possesses more than one ‘ferroic’ properties: ferroelectricity, ferromagnetism or ferroelasticity. But the classification of multiferroics has been broadened to include antiferroic order. Multiferroic materials, in which ferroelectricity and magnetism coexist, the control of magnetic properties by an applied electric field or, in contrast, the switching of electrical polarization by a magnetic field, have attracted a great deal of interest. Now we can classify multiferroic materials into two parts: one is single-phase materials; the other is layered or composite heterostructures. The most desirable situation would be to discover an intrinsic single-phase multiferroic material at room temperature. However, BiFeO3 is the only known perovskite oxides that exhibits both antiferromagnetism and ferroelectricity above room temperature. Thus, it is essential to broaden the searching field for new candidates, which resulted in considerable interest on designed novel single phase materials and layered or composite heterostructures.

Reduction of superconducting transition temperature and flux pinning in Al and C codoped MgB2 with insight from first-principles calculations

We report for the first time the Al and C codoping effect on the structures and superconductivity in MgB2. It was found that both the lattice parameters and the Tc decreased monotonically with increasing doping level of both Al and C. The Tc dropped to 27.5 and 7K for x=0.2 and for x=0.4, respectively. The reduction of Tc for the codoping was found to be quicker than for individual Al doping and slower than for individual C doping at the same doping levels. First-principles calculations indicated that the observed decrease in Tc for the codoped MgB2 can be understood in terms of a band filling effect due to the electron doping by both Al and C. However, it is suggested that other factors may also play a role in the Tc reduction in the real Mg1−xAlxB2−xCx samples. Furthermore, the Al and C codoping also reduces Jc and weakens flux pinning in MgB2.

Magneto-optical imaging of the magnetization process in colossal magnetoresisitive lanthanum manganite

To gain insight into the origin of the colossal reduction of resistance in response to magnetic field in colossal magnetoresistance manganite, the magnetic field induced transition in ferromagnetic La0.7Ca0.3MnO3 was studied using a high-resolution magneto-optical imaging (MOI) technique. The MO images were captured in various magnetic fields over a wide temperature range for both highly dense samples with strong-link grain boundaries and porous samples with weak-link boundaries. Formation and evolution of magnetic domains as a function of field or temperature were clearly observed around and far below the Curie temperature TC=240K. Ferromagnetic areas tend to grow to large sizes and finally join together at the expense of paramagnetic areas as the field increases or temperature decreases for strong-link samples. A sharp magnetoresistive transition is observed when the sample changes from a paramagnetic insulator to a metallic ferromagnetic phase in the vicinity of TC. In contrast, the porous samples show...

EFFECT OF NANO-Y-ZrO2 ADDITION ON THE MICROSTRUCTURE AND CRITICAL CURRENT DENSITY OF MgB2 SUPERCONDUCTORS

Polycrystalline MgB2 samples with 0, 5, 10, and 20 wt% nano-Y-ZrO2 (YSZ) powder addition were prepared by short time, as little as several minutes, or long time in-situ reaction process. The phases, microstructures and flux pinning behaviors were characterized using X-ray diffractometry (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscope (TEM) and Magnetic Measurements. Results indicated that nano-YSZ particles included in MgB2 grains. Samples doped with 10wt% YSZ powders showed new record of critical current density Jc as high as 1×106 and 4×106A/cm2 in low magnetic fields at 30 and 20K, respectively. However, the Jc drops faster compared to that made by long time sintering samples. It is proposed that the improved Jc in low fields was due to the enhanced density of the sample, which was caused by the YSZ nano-particle inclusion.

Band structure, magnetic, and transport properties of two dimensional compounds Sr2−xGdxCoO4

The layered perovskite compound Sr2−xGdxCoO4 has not yet been subjected to detailed study so far. In this report, structures, transport, magnetic properties, and first principle calculations will be reported for the two dimensional compounds Sr2−xGdxCoO4 (x = 0.5, 0.75, 1, 1.25). Rietveld refinement revealed that these compounds are crystallized in K2NiF4-type structures with space group I4/mmm. It was found that the lattice parameter c decreases as x increases. Through the Curies Weiss fitting of the temperature dependent magnetization, it was found that the Sr1.25Gd0.75CoO4 sample exhibits a weak ferromagnetic to paramagnetic transition at about 62 K, with a Curie constant of 0.113 emu K/mol. Band structure calculations indicated that electrons of these compounds are spin polarized at the Fermi level. The 2-D Variable Range Hopping model fitting indicated that the two dimensional variable range hopping mechanism could be used to account for the conducting mechanism for these samples.