Wenhai Song

Crystal growth and superconductivity of FeSe_x

In this work, crystals of FeSex have been grown by a flux approach. The crystallization process is divided into two stages. First, stoichiometric polycrystal FeSe0.82 was sintered in a solid state reaction. Then, FeSex crystals with a size about 500 µm were successfully grown in an evacuated sealed quartz tube using a NaCl/KCl flux. The products include two crystal structures: tetragonal and hexagonal. The electronic transport and magnetic property measurements show that FeSex crystals exhibit a superconducting transition at about 10 K.

Enhanced flux pinning in (Bi, Pb)-2223/Ag tapes by slight Pr-doping

(Bi,Pb)2.2Sr2Ca2.2−xPrxCu3Oy silver sheathed tapes with Pr-doping amount of x=0.0, 0.001 and 0.002 have been prepared by powder-in-tube method. X-ray diffraction analyses manifest that the tapes with x=0.001 and 0.002 (doped) have less Pb-rich phase than the tape with x=0.0 (un-doped). Transmission electron microscopy images show that the Pb-rich phase particles of ∼40 nm are embedded in un-doped (Bi,Pb)-2223 grain matrix, while defects induced by Pr-doping with the size smaller than that of the Pb-rich phase particles exist in the doped samples. As compared with the un-doped tape, the magnetic field dependence of critical current densities of the doped tapes are noticeably improved at 77 K, and their irreversibility lines are also shifted to higher temperatures and magnetic fields and activation energies of flux motion increased drastically. The enhanced flux pinning seems to mainly originate from the fine normal-like defects induced by the partial Pr-substitution for Ca.

Enhanced low-field magnetization and magnetoresistance in nano-MgO added La2 3Ca1 3MnO3 composites

We report the significant enhancement of low-field magnetization and magnetoresistance (MR) in La2/3Ca1/3MnO3 (LCMO) compounds with 5 at.% nano-MgO addition in the form of both nanoparticle and nanowire. It is found that the magnetization of the nano-MgO added compounds is highly sensitive to a low applied field below 7 kOe compared with the MgO-free LCMO. In addition, the enhancement of low-field MR is found in nano-MgO added compounds and the enhancement for the nanowire MgO-added composite is more remarkable than that for the nanoparticle MgO-added one. Moreover, two ferromagnetic phases with low-TC and high-TC in nano-MgO added compounds are observed. These results demonstrate that the low-field transport and magnetic properties of LCMO compounds can be effectively manipulated by controlling the morphology and dispersion of the insulating barrier using nano-MgO addition.

Growth of Ca3Co4O9 films: Simple chemical solution deposition and stress induced spontaneous dewetting

Chemical solution deposition as a simple method for preparation of oxide films is used to fabricate misfit structured Ca3Co4O9 thermoelectric films using cheap precursors, Ca and Co acetates, which opens up a simple route to fabricate large-area Co-based oxide thermoelectric films. The results show that when the annealing temperature is higher than 800°C crystallized Ca3Co4O9 films can be obtained on single-crystal LaAlO3 substrates, whereas the Ca3Co4O9 films are amorphous for lower annealing temperatures. Experimental evidence shows that the amorphous Ca3Co4O9 films behave like semiconductors with enhanced resistivity and Seebeck coefficient within the measured temperature range. The magnetoresistance is positive indicating the absence of long-range order of the Co-ion spins. For the crystallized Ca3Co4O9 films, it can be seen that the thinner films show the characteristics of spontaneous dewetting induced by the stress, which suggests that it is possible to prepare nanostructured Ca3Co4O9 films through...

Colossal negative thermal expansion with an extended temperature interval covering room temperature in fine-powdered Mn0.98CoGe

MnM′X (M′ = Co, Ni; X = Ge, Si, etc.) alloys usually present a large volumetric change during the Martensitic (MA) transformation. This offers a great opportunity for exploring new negative thermal expansion (NTE) materials if the temperature interval of NTE can be extended. Here, we report colossal NTE in fine-powdered Mn0.98CoGe prepared by repeated thermal cycling (TC) through the MA transition or ball milling. Both treatments can expand the MA transformation, and thus broaden the NTE temperature window (ΔT). For the powders that have gone through TC for ten times, ΔT reaches 90 K (309 K–399 K), and the linear expansion coefficient (αL) is about −141 ppm/K, which rank among the largest values of colossal NTE materials. The difference between two kinds of treatments and the possible mechanisms of the extended MA transformation window are discussed based on the introduced strain.

Giant negative thermal expansion covering room temperature in nanocrystalline GaNxMn3

Nanocrystalline antiperovskite GaNxMn3 powders were prepared by mechanically milling. The micrograin GaNxMn3 exhibits an abrupt volume contraction at the antiferromagnetic (AFM) to paramagnetic (PM) (AFM-PM) transition. The temperature window of volume contraction (ΔT) is broadened to 50 K as the average grain size (⟨D⟩) is reduced to ∼30 nm. The corresponding coefficient of linear thermal expansion (α) reaches ∼ −70 ppm/K, which are comparable to those of giant NTE materials. Further reducing ⟨D⟩ to ∼10 nm, ΔT exceeds 100 K and α remains as large as −30 ppm/K (−21 ppm/K) for x = 1.0 (x = 0.9). Excess atomic displacements together with the reduced structural coherence, revealed by high-energy X-ray pair distribution functions, are suggested to delay the AFM-PM transition. By controlling ⟨D⟩, giant NTE may also be achievable in other materials with large lattice contraction due to electronic or magnetic phase transitions.

Structural, magnetic and dielectric properties of the Aurivillius phase Bi6Fe2−xMnxTi3O18 (0 ≤ x ≤ 0.8)

The n = 5 Aurivillius phase ceramics Bi6Fe2−xMnxTi3O18 (BFMTO) (0 ≤ x ≤ 0.8) were synthesized with a conventional solid-state reaction method. All samples can be indexed with an orthorhombic structure with the space group B2cb. The magnetic measurements indicate that these ceramics are predominantly paramagnetic with the presence of short-range antiferromagnetic interactions and a weak ferromagnetic ordering state, implying that the predicted Fe3+–O–Mn3+ 180° ferromagnetic superexchange interaction based on the Goodenough–Kanamori rule might not be achieved in BFMTO ceramics through Mn substitution for Fe in the n = 5 Aurivillius phase. The dielectric loss of the x = 0.3 and 0.4 samples demonstrates the relaxation process and the rather large activation energy (2.63 eV for the x = 0.3 sample and 2.10 eV for the x = 0.4 sample) implies that this relaxation process is not due to the thermal motion of oxygen vacancies. The 0.5 ≤ x ≤ 0.8 samples exhibit a paraelectric–ferroelectric phase transition and the ferroelectric Curie temperature decreases upon increasing the doping level of Mn.

BiFeO3 thin films prepared on metallic Ni tapes by chemical solution deposition: effects of annealing temperature and a La0.5Sr0.5TiO3 buffer layer on the dielectric, ferroelectric and leakage properties

In this work, BiFeO3 (BFO) thin films were prepared on metallic Ni (200) tapes with and without a La0.5Sr0.5TiO3 (LSTO) buffer layer at different temperatures by chemical solution deposition. The effects of the annealing temperature as well as the LSTO buffer layer on the dielectric, leakage and ferroelectric properties have been studied in detail. The crystallite size, dielectric constant and leakage current density increase, while the coercive field decreases with increasing annealing temperature. The BFO thin films deposited directly on the Ni tapes are prone to wrinkling, while the wrinkles are smoothed by introducing a thin LSTO buffer layer. Decreased compressive microstrain as well as improved ferroelectric and leakage properties are observed in the BFO thin films deposited on the LSTO buffered Ni tapes. The results will provide an instructive route to optimize BiFeO3-based thin films on metallic tapes by chemical solution deposition methods.

Enhanced remnant polarization in ferroelectric Bi6Fe2Ti3O18 thin films

For practical applications of ferroelectric memory, a large remnant polarization in nontoxic Pb-free ferroelectric materials is required. We prepared Bi6Fe2Ti3O18 thin films on Pt/Ti/SiO2/Si (100) substrates by chemical solution deposition using different annealing processes and found that a well-defined ferroelectric hysteresis loop with a rather large remnant polarization (Pr ≈ 21.5 μC cm−2) occurs in Bi6Fe2Ti3O18 thin films prepared by rapid thermal annealing in air. This value of remnant polarization is superior to those of other ferroelectric Aurivillius compounds such as Bi2WO6, Bi3TiNbO9, Bi4Ti3O12, Bi5FeTi3O15, and Bi6Fe2Ti3O18 thin films prepared by chemical solution deposition and pulsed laser deposition. The results of X-ray diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy and ferroelectric P–E hysteresis loops revealed that the grain size, surface morphologies, oxygen vacancies and lattice distortion play very important roles in the determination of the remnant polarization. The present study provides an effective route to prepare layered Aurivillius thin films with a large remnant polarization by chemical solution deposition.

Low-field magnetoresistance in nanostructured Sr2FeMoO6∕CeO2 composites

Nanosized (Sr2FeMoO6)1−x∕(CeO2)x composites were prepared by Sr2FeMoO6 (SFMO) obtained by sol-gel and nanosized CeO2 powders. The magnetoresistance (MR) of composites is explored as a function of the molybdate/insulator composition and magnetic field. The enhancement of the low-field magnetoresistance is observed with increasing CeO2. The MR ratio at 10K with H=5kOe is 7.9% and 10.2% for the x=0.05 and 0.3 samples, which are 1.1 times and 1.7 times as large as that for pure SFMO, respectively. The enhanced MR was attributed to the spin-dependent tunneling at the interface of grain boundaries.