Materials Science Forum Vol. 815

Abstract: The photovoltaic effect in Ag/Bi_0.9La_0.1FeO₃/La_0.7Sr_0.3MnO₃ heterostructures was investigated and the short circuit photocurrent was found to be strongly dependent on the polarization orientation and oxygen vacancies (V_Os) distribution. The photocurrent direction was switched accompanying polarization switching. Besides, according to manipulate the V_Os accumulated at either the Ag/Bi_0.9La_0.1FeO₃ or the Bi_0.9La_0.1FeO₃/La_0.7Sr_0.3MnO₃ interface by electric pulses, obvious enhancement of photovoltaic effects was obtained. These results can be explained well using the concepts of drift current and diffusion current controlled by the combination of oxygen vacancies and polarization. This work provides deep insights into the nature of photovoltaic effects in ferroelectric films, and will facilitate the advanced design devices combining spintronic, electronic, and optical functionalities.

Abstract: To investigate the epitaxial strain effect on local electronic structure of FeO₆ octahedron of BiFeO₃ epitaxial film, polarization-dependent Fe L₂₃-edge x-ray absorption spectroscopy studies were performed on both tetragonal (T)-like BFO/LAO and rhombohedral (R)-like BFO/STO epitaxial films. Charge transfer multiplet theory based fittings were also performed to reveal the local electronic structure difference. Due to dramatic structural difference caused by epitaxial strain between these two samples, significant electronic structure differences were observed between these two specimens. For BFO/LAO, anisotropic electronic structure appears in vertically-elongated FeO₆ octahedron and an additional shift of Fe ion off the central position is suggested. For BFO/STO, electronic structure is almost isotropic.

Abstract: Pure BiFeO₃ ceramics and BiMn_xFe_1-xO₃ (x=0.1, 0.15, 0.2, and 0.25) ceramics were prepared by sol-gel method. A structural phase transition from rhombohedral R3c to triangle R3m occurred in Mn-doped BFO ceramics. Mn-doped BFO ceramics exhibited better crystallinity, larger dielectric constant as well as smaller dielectric loss. Besides, a small nonlinearity of magnetization-field curves M(H) was observed, the remanent magnetization increased with the increase of manganese content, and all the BiMn_xFe_1-xO₃ ceramics showed the typical antiferromagnetic, which could be attributed to the effective decrease in the concentration of oxygen vacancies, and the corresponding structural transition.

Abstract: FeGa/PZT/FeGa magnetoelectric device was prepared and the ME effect examined. FeGa alloys was made by different processing method. The results showed that the domains and texturing of FeGa had a great impact on both magnetic and ME performance. The permeability and d₃₃ (the differential of magnetostrictive coefficient) of FeGa differed with processing. It was found that FeGa rolled sheets had good toughness, d₃₃ could reach 2.7 ppm/Oe, and magnetic permeability reached 180 H/m. The device made of FeGa rolled sheets also had a large output (18 V/cm Oe) and low bias field (~ 96 Oe).

Abstract: To investigate the mechanism of self-bias magnetoelectric effect in magnetoelectric composite materials, a synchronous characterization technique was developed to characterize the magnetoelectric effect, the magnetostrictive effect, and the magnetic hysteresis loop by one-time test. The results of a magnetoelectric composite consisting of hybrid ferromagnetic phases showed that the obvious magnetoelectric hysteresis behavior was found with significant self-bias magnetoelectric effect. In addition, after demagnetizing, the residual magnetic polarization became zero and the magnetoelectric effect disappeared at the same time. Since the ferromagnetic phases were separated from each other, the mechanism of self-bias magnetoelectric effect mainly resulted from static magnetic coupling instead of build-in magnetic field. It was concluded that the synchronous characterizing technique was quite helpful when analyzing the mechanism of magnetoelectric behavior.

Abstract: Spheric SiO₂ nanoparticles with the sizes distribution between 20-80 nm dispersed in nonaqueous solvent were prepared under the assistance of surfactant. Shear emulsification time was a key role for the dispersion of nanoparticles and the optimal value was 30 min with the smallest median size of 51 nm. The relationship between fluid viscosity and dispersion condition was researched, and the results demonstrated that the additional volume arising from aggregation interspaces enhanced shear fore and opposite resistance and finally the viscosity increased.

Abstract: W/TiNiNb shape memory alloy laminated composites were fabricated by vacuum hot press, forging and rolling. The microstructure, transformation behavior, mechanical behavior and damping capacity of the laminated composite were investigated by SEM, DSC, DMA and bending test, respectively. The results showed that the W layer and the TiNiNb shape memory alloy layer in the composite was about 15 μm and 5 μm, respectively. The TiNiNb alloy in the composite exhibited the reversible martensite transformation. The composite also had high damping capacity (tanδ=0.03). The three-point bending test showed various plateaus in the stress–strain curve due to delamination processes, which are suitable for enhancing the fracture toughness of the laminates. The flexure strength of the laminated composite was 1260 MPa.

Abstract: nanocrystalline cellulose (NCC) was prepared from micro-crystalline cellulose (MCC) by strong acid hydrolysis. The characteristics of such particle were studied by transmission electron microscopy, X-ray diffraction and Fourier transform infrared spectroscopy. Electro-rheological fluids (ERF) were prepared by dispersing NCC and MCC in methyl-silicone oil, and their ER effects were measured. Experimental results indicated that NCC ERF exhibited a remarkable ER effect. The highest static shearing stress of NCC ERF (3.5 g/ml) was 5.1 kPa at the room temperature under a 4 .2 kV/mm electric field, increased about 5.5 times compared to MCC ERF, and sedimentation of NCC ERF was not observed even after 60 days.

Abstract: The microstructure, phase transformation and mechanical property of Ni-Co-Mn-In alloy fabricated by powder metallurgical method were studied in the present investigation. The initial polycrystalline alloy was prepared by arc melting and then milled to fine particles, finally the powder particles were sintered by spark plasma sintering (SPS) method to make the bulk alloy with refined grains. The size of as-milled particles was much smaller than the grain size of the initial arc-melted alloy. The particles can be successfully densified to form a compact bulk by SPS. The initial arc-melted alloy presented a good martensitic transformation behavior, whereas the martensitic transformation of the as-sintered bulk alloy disappeared. The compressive strength and fracture strain of the as-sintered alloy greatly enhanced as compared to the initial arc-melted alloy due to the grain refinement and strengthening of grain boundaries.

Abstract: A practical way to manipulate the magnetic anisotropy of magnetostrictive FeGa thin films grown on flexible polyethylene terephthalate (PET) substrates is introduced in this study. The effect of film thickness on magnetic properties and magnetostriction constant of polycrystalline FeGa thin films was investigated. The anisotropy field H_k of flexible FeGa films, i.e., the saturation field determined by fitting the hysteresis curves measured along the hard axis, was enhanced with increasing the tensile strain applied along the easy axis of the thin films, but this enhancement via strain became unconspicuous with increasing the thickness of FeGa films. In order to study the magnetic sensitivity of thin films responding to the external stress, we applied different strains on these films and measure the corresponding anisotropy field. Moreover, the effective magnetostriction constant of FeGa films was calculated from the changes of both anisotropy field and external strain based on the Villari effect. A Neel’s phenomenological model was developed to illustrate that the effective anisotropy field of FeGa thin films was contributed from both the constant volume term and the inverse thickness dependent surface term. Therefore, the magnetic properties for the volume and surface of FeGa thin films were different, which has been verified in this work by using vibrating sample magnetometer (VSM) and magneto-optic Kerr effect (MOKE) system. The anisotropy field contributed by the surface of FeGa film and obtained by MOKE is smaller than that contributed by the film volume and measured by VSM. We ascribed the difference in H_k to the relaxation of the effective strain applied on the films with increasing the thickness of films.