Rongkun Zheng

Role of point defects in room-temperature ferromagnetism of Cr-doped ZnO

Polycrystalline CrxZn1−xO films and powders are prepared by cosputtering and sol-gel method, respectively. While room-temperature ferromagnetism is found in as-deposited films, the powders exhibit paramagnetism. Comparison of the structural and magnetic properties of the as-deposited, annealed, and powdered samples indicates that the interstitial zinc, together with Cr doping, plays an important role in the ferromagnetic origin of Cr:ZnO. The ferromagnetism in films can be described by bound magnetic polaron models with respect to defect-bound carriers.

The effect of mesoporous bioactive glass on the physiochemical, biological and drug-release properties of poly(DL-lactide-co-glycolide) films

Poly(lactide-co-glycolide) (PLGA) has been widely used for bone tissue regeneration. However, it lacks hydrophilicity, bioactivity and sufficient mechanical strength and its acidic degradation by-products can lead to pH decrease in the vicinity of the implants. Mesoporous bioactive glass (MBG) with highly ordered structure (pore size 2-50nm) possesses higher bioactivity than non-mesoporous bioactive glass (BG). The aim of this study is to investigate the effect of MBG on the mechanical strength, in vitro degradation, bioactivity, cellular response and drug release of PLGA films and optimize their physicochemical, biological and drug-delivery properties for bone tissue engineering application. The surface and inner microstructure, mechanical strength and surface hydrophilicity of MBG/PLGA and BG/PLGA films were tested. Results indicated that MBG or BG was uniformly dispersed in the PLGA films. The incorporation of MBG into PLGA films significantly improved their tensile strength, modulus and surface hydrophilicity. MBG/PLGA resulted in an enhanced mechanical strength, in vitro degradation (water absorbance, weight loss and ions release), apatite-formation ability and pH stability in simulated body fluids (SBF), compared to BG/PLGA. MBG/PLGA and BG/PLGA films enhanced human osteoblastic-like cells (HOBs) attachment, spreading and proliferation compared to PLGA. HOBs differentiation was significantly upregulated when cells were cultured on 30 MBG/PLGA for 14 days, compared to 30 BG/PLGA. MBG/PLGA enhanced the accumulative release of dexamethazone (DEX) at early stages (0-200h) compared to BG/PLGA, however, after 200h, DEX-release rates for MBG/PLGA was slower than that of BG/PLGA. The contents of MBG in PLGA films can control the amount of DEX released. Taken together, MBG/PLGA films possessed excellent physicochemical, biological and drug-release properties, indicating their potential application for bone tissue engineering by designing 3D scaffolds according to their corresponding compositions.

Fabrication and magnetic properties of ultrathin Fe nanowire arrays

Ultrathin Fe nanowire (about 5 nm in diameter) arrays have been fabricated by electrodeposition using anodic porous alumina templates. These ultrathin nanowires exhibited uniaxial anisotropy and a quite large coercivity (4190 Oe) at 5 K. In addition, the field needed to saturate the magnetization, when the field was applied perpendicularly to the easy axis, was much larger than the shape anisotropy field (2πM S ). This saturation field increased with decreasing temperature. We believed that this enhanced saturation field was mainly due to the contribution of the surface spins.

Large room-temperature spin-dependent tunneling magnetoresistance in polycrystalline Fe3O4 films

Polycrystalline Fe3O4 films have been prepared by reactive sputtering at room temperature. Transmission electron microscopy images show that the films consist of quite uniform Fe3O4 grains well separated by grain boundaries. It was found that the tunneling of spin-polarized electrons across the antiferromagnetic coupled grain boundaries dominates the transport properties of the films. Magnetoresistance (MR) {=[ρ(H)−ρ(0)]/ρ(0)} shows linear and quadratic magnetic-field dependence in the low-field range when the field is applied parallel and perpendicular to film plane, which is similar to the behaviors observed in the epitaxial Fe3O4 films consisting of a large fraction of antiferromagnetic antiphase domain boundaries. At 300 K, the size of the MR reaches −7.4% under a 50-kOe magnetic field, which is a very large MR for polycrystalline Fe3O4 films.

Exchange bias and the origin of magnetism in Mn-doped ZnO tetrapods

Wurtzite-type ZnO tetrapod nanostructures were prepared by evaporating Zn metal under humid argon flow. After the fabrication, Mn was doped into ZnO nanostructures by diffusion at 600°C. The average concentration of Mn was determined to be 8.4mol% by x-ray fluorescence. X-ray diffraction patterns obtained from the doped and undoped samples are almost the same. High-resolution transmission electron microscopy observations reveal the existence of surface layers. Magnetic measurements show that the sample has a very large coercivity HC=5500Oe at 5.5K and a Curie temperature TC=43K, which may suggest that ferrimagnetic (Zn,Mn)Mn2O4 exists at the surface. Exchange bias is clearly observed below 22K. Exchange bias is attributed to the exchange interaction between ferrimagnetic (Zn,Mn)Mn2O4 and spin-glass-like (or antiferromagnetic) phase in manganese oxides.

Giant exchange bias and the vertical shifts of hysteresis loops in γ-Fe2O3-coated Fe nanoparticles

We fabricated core/shell-structured Fe nanoparticles, in which the α-Fe core is about 5 nm in diameter and the γ-Fe2O3 shell is about 3 nm thick, and systematically studied their structural and magnetic properties. The magnetic hysteresis (M–H) loops, measured at low temperatures, after the particles were cooled from 350 K in a 50 kOe field, show significant shifts in both horizontal and vertical directions. It has been found that the exchange-bias field can be as large as 6.3 kOe at 2 K, and that the coercive field is also enhanced greatly in the field-cooled (FC) loops. The large exchange bias and vertical shifts of the FC loops at low temperatures may be ascribed to the frozen spins in the shells. A simple model is proposed to interpret the observations.

Structural, optical and magnetic properties of Co-doped ZnO nanorods with hidden secondary phases

Co-doped ZnO nanorods (composition: Zn(0.955)Co(0.045)O) were grown by a simple surfactant-assisted hydrothermal technique. The morphological, structural, optical and magnetic properties of the as-prepared nanorods were investigated by means of scanning electron microscopy, high-resolution transmission electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, micro-Raman spectroscopy, micro-cathodoluminescence, and vibrating sample magnetometry (VSM). The results showed that the sample had rod-like morphology and that the preferential growth direction was along the c axis. While Co was successfully doped into the ZnO wurtzite lattice structure as revealed by several characterization techniques, hidden secondary phases of Zn(y)Co(3-y)O(4) (0≤y≤1) were also clearly detected by the micro-Raman spectroscopic technique. We propose that the predominant diffusion-limited Ostwald ripening crystal growth mechanism under the hydrothermal coarsening yielded such phase segregation. VSM results showed that the nanorods displayed relatively weak room-temperature ferromagnetism. We suggest that the origin of the ferromagnetism is probably due to the presence of the mixed cation valence of Co via a d-d double-exchange mechanism rather than the real doping effect. It is essential to control the crystal growth mechanism and defect states associated with the ferromagnetism in order to realize the intrinsic diluted magnetic semiconductors.

Self‐Assembly and Self‐Orientation of Truncated Octahedral Magnetite Nanocrystals

Monodispersed magnetite (Fe3O4) nanoparticles were synthesized. Transmission electron microscopy study shows that the nanoparticles are in the shape of Wagner-Seitz crystals. The magnetite nanoparticles self-assemble into body-centered cubic superlattice, in which the nanoparticles have the same crystallographic orientations. Shape plays a very critical role in controlling the orientation of the nanoparticles in the superlattice. Both the self-assembly and self-orientation of nanoparticles are important for technical applications. This can also act as a complement to conventional lithography techniques.

Chemical synthesis of narrowly dispersed SmCo5 nanoparticles

In this article we report a chemical synthetic means for generating a high Ku magnetic material—narrowly dispersed nanoparticles of SmCo5. Using Co2(CO)8 and Sm(acac)3 as the precursors under air-free conditions, we produced SmCo5 nanoparticles according to the procedure reported by Sun et al. [Science 287, 1981 (2000)] but with some modifications. The nanoparticles, with diameters of 6–8 nm, have a SmCo5 composition, as indicated by transmission electron microscopy, electron diffraction, and x-ray photoelectron spectroscopy. The magnetization measurement of the nanoparticles, exhibits superparamagnetism, which is blocked for temperatures below ∼110 K, suggesting Ku to be ∼2.1×106 erg/cm3 for the as-prepared particles.

Role of structural defects on ferromagnetism in amorphous Cr-doped TiO2 films

Amorphous CrxTi1−xO2 films with different Cr concentrations of 0⩽x⩽0.16 were prepared by cosputtering method at room temperature. All as-deposited samples show hysteresis behavior from 2to340K and the Curie temperatures are well above 390K. The saturation magnetization is about 3.21×10−1μB∕Cr for x=0.05 at 340K and decreases with increasing Cr dopant. After annealing at temperature above 300°C, the films crystallized into anatase structure and lost their ferromagnetic property. The results indicate that the ferromagnetism in amorphous Cr-doped TiO2 films is intrinsic and the structural defects play an important role in the ferromagnetism of Cr:TiO2 system.