Minghui Yu

Room-temperature ferromagnetism of Mn doped ZnO aligned nanowire arrays with temperature dependent growth

Aligned one-dimensional diluted magnetic semiconductor Zn1−xMnxO nanowires were synthesized from a reaction of metallic Zn foil and MnCl2⋅6H2O under oxygen environment at variant temperatures between 750 and 950 °C by a chemical vapor deposition method. The c-axis preferentially grown nanowire arrays are single crystalline wurtzite structure, of which the growing temperature has a significant influence on both morphology and magnetic ordering. Nanowires with the highest aspect ratios were grown at 850 °C, whereas nanowires presenting largest room-temperature ferromagnetism were formed at 950 °C. More Mn2+ substitution in the ZnO lattice was observed at 950 °C, resulting in strong room-temperature ferromagnetism with a saturation magnetization of 0.25 emu/g. At synthesis temperatures of 750 and 850 °C, formation of a ZnMn2O4 room-temperature paramagnetic second phase was found. The nanostructures with different aspect ratios were obtained with the variation of synthesis temperature. The temperature depende...

Fabrication of Mn-doped ZnO diluted magnetic semiconductor nanostructures by chemical vapor deposition

Mn-doped ZnO diluted magnetic semiconductor (DMS) nanostructures were prepared by direct reaction of zinc metal and manganese chloride powder under oxygen environment using chemical vapor deposition method. Several kinds of nanostructures including nanorod, bowl, and cage were obtained at different deposition temperature zones. The morphologies and crystal structures of the as-synthesized nanostructures were characterized using field-emission scanning electron microscopy, transmission electron microscopy, and x-ray diffraction spectroscopy. Superconducting quantum interference device measurement shows that the doped nanorods exhibited low-temperature ferromagnetism at 5K with Curie temperature around 37K, whereas the DMS nanocrystalline bowl/cage structures have room-temperature ferromagnetic behavior.

Structure and magnetic properties of Cr∕Fe-doped SnO2 thin films

Cr- and Fe-doped SnO2 films were deposited on Al2O3 substrates by pulsed-laser deposition. X-ray-diffraction patterns show that the films have rutile structure and grow epitaxially along the (101) plane. The diffraction peaks of Cr-doped samples exhibit a systematic shift toward higher angles with increasing Cr concentration. This indicates that Cr dissolves in SnO2. On the other hand, there is no obvious shift of the diffraction peaks of the Fe-doped samples. The magnetization curves indicate that the Cr-doped SnO2 films are paramagnetic at 300 and 5K. The Fe-doped SnO2 samples exhibit ferromagnetic behavior at 300 and 5K. Zero-field-cooled and field-cooled curves indicate superparamagnetic behavior above the blocking temperature of 100K, suggesting that it is possible that there are ferromagnetic particles in the Fe-doped films. The anisotropic magnetoresistance was not observed in any of the samples.

Microwave absorption of patterned arrays of nanosized magnetic stripes with different aspect ratios

Arrays consisting of nanosized stripes of Permalloy with different length-to-width ratios have been fabricated using electron beam nanolithography, magnetron sputtering, and lift-off process. These stripes have a thickness of 100nm, a width of 300nm, and different lengths ranging from 300nmto100μm. The stripes are separated by a distance of 1μm. Magnetization hysteresis loops were measured using a superconducting quantum interference device susceptometer. Microwave absorption at 9.8GHz was determined by means of ferromagnetic resonance technique. The dependence of the resonant field on the angle between the nanostructure and the in-plane dc magnetic field indicates the presence of uniaxial magnetic anisotropy associated with the aspect ratio of the stripes. A maximum change of the resonant field of 1600Oe was observed in the longest stripes, yet it was only 200Oe for square shaped stripes. The linewidth of the resonant curve varied with the angle, in the range from 120to300Oe. Most of the ferromagnetic re...

Size dependence of static and dynamic magnetic properties in nanoscale square Permalloy antidot arrays

Permalloy antidot arrays with different square hole sizes (1200×1200, 800×800, and 400×400nm2) have been fabricated by means of electron-beam lithography and lift-off techniques. The smaller square hole size results in enhanced remanence and reduced coercivity in the antidot array. Multiple resonance modes were clearly observed for the magnetic field applied normal to the array plane, and double uniform resonance modes occurred when the field deviated more than 30° from the normal to the plane. Two distinct dipolar field patterns with different orientations and magnitudes split the uniform resonance into double resonance modes. The double resonance modes show uniaxial in-plane anisotropy and the easy axes are orthogonal. The magnitude of the induced dipolar anisotropy remains almost constant with changes in the square hole size. The double resonance peaks move to low field with reduction of the square hole size.

One-step synthesis of magnetic hollow silica and their application for nanomedicine

Magnetic nanoparticles are usually present in the form of magnetic carriers and used in nanomedicine and biosystem. In this paper, magnetic hollow silica (MHS) nanoparticles were fabricated by a one-step synthesis of Fe3O4 nanoparticles and then coating of silica on nanosized spherical calcium carbonate under alkaline conditions, in which nanosized calcium carbonate (CaCO3, 25–60nm) was used as a scarified template, tetraethoxysilane as a precursor, and Fe3O4 nanoparticles (∼5nm), formed in the initial reaction stage, as magnetic agents. The as-synthesized nanoparticles were immersed in a weak acetic acidic solution to remove CaCO3, forming MHS carriers. The nanostructures of the MHS carriers were characterized by scanning electron microscope, transmission electron microscope, and x-ray diffraction. Superconducting quantum interference device measurement exhibited that the MHS nanoparticles were superparamagnetic. Toxicity was tested for MHS carriers using rat brain microvascular endothelial cells. The ce...

High-frequency characterization of Permalloy nanosized strips using network analyzer ferromagnetic resonance

We report on the dynamic properties of Permalloy nanostrips at gagahertz frequencies. The thickness of the strips is 100nm, strip width is 300nm, strip spacing is 1μm, and length is 0.3–100μm; aspect ratios are 1:1, 1:2, 1:3, 1:5, 1:10, and 1:333. The dynamic behavior was studied by network analyzer ferromagnetic resonance (FMR) using Permalloy strips on a coplanar waveguide in flip-chip geometry. The FMR mode frequencies (fr) can be controlled by the aspect ratio as well as by the applied magnetic field (H). In longer strips (1:10 and 1:333), the excitation frequencies show a soft mode behavior (Heff=990Oe) when the field is along the hard axis. However, along the easy axis (along the strip length), fr increases with applied field. At a field of 3kOe, fr values are almost independent of aspect ratio along the easy axis except for the 1:1 strip. Along the hard axis, the frequencies are strongly dependent upon the aspect ratio. We also observed that the frequency linewidths of the strips are dependent on t...

Structure and magnetic properties of pure and Gd-doped HfO2 thin films

Pure HfO2 and Gd-doped HfO2 thin films have been grown on different single crystal substrates (silicon, R-Al2O3, and LaAlO3) by pulsed laser deposition. X-ray diffraction (XRD) patterns show that the pure HfO2 thin films are of single monoclinic phase. Gd-doped HfO2 films have the same XRD pattern except that their diffraction peaks have a shift toward lower angles, which indicates that Gd dissolves in HfO2. Transmission electron microscopy images show a columnar growth of the films. Very weak ferromagnetism is observed in pure and Gd-doped HfO2 films on different substrates at 300 and 5K, which is attributed to either impure target materials or signals from the substrates. The magnetic properties do not change significantly with postdeposition annealing of the HfO2 films. In addition to the films, HfO2 powders were annealed in pure hydrogen flow, and a ferromagnetic signal was not observed.

Large room-temperature spin-dependent tunneling magnetoresistance in a Fe3O4-polymer composite system

Powders of polymer and α‐Fe2O3 nanoparticles were pressed into pellets. The samples were annealed at 250°C in pure hydrogen flow. X-ray diffraction spectra indicate that there is a complete phase transformation from α‐Fe2O3 to Fe3O4. Giant negative magnetoresistance (MR) was observed at room temperature and the MR ratio is over 14% in an applied field of 5.5T. The maximum MR ratio is higher than 22% at 130K. The drastic enhancement of the MR ratio is attributed to the fact that the polymer is an excellent barrier material and, more importantly, prevents the oxidation of the surface of Fe3O4. Our results clearly suggest that there is a high degree of spin polarization at room temperature for half-metallic Fe3O4.

Fabrication of magnetic porous hollow silica drug carriers using CaCO3∕Fe3O4 composite nanoparticles and cationic surfactant double templates

Magnetic porous hollow silica nanosphere is a new class of structured nanomaterials for drug delivery. In this paper, we report a synthesis of magnetic porous hollow silica nanospheres (MPHSNs) using CaCO3∕Fe3O4 composite nanoparticles and cationic surfactant double templates. Fe3O4 nanoparticles were first mixed into CaCO3 using rotating packed bed forming CaCO3∕Fe3O4 composite nanoparticles. Tetraethoxysilane was then added as precursor to form silica layer on the surface of CaCO3∕Fe3O4 composite nanoparticles, while hexadecyltrimethylammonium bromide was used as a second template to direct the formation of porous silica shells. After the calcination of the surfactants and etching away CaCO3, MPHSNs were formed with the magnetite nanoparticles remaining in the cores. Transmission electron microscopy was applied for the nanostructure determination. The pore size can be measured by micromeritics analyzer. Magnetic properties of MPHSNs were measured by a superconducting quantum interface device. Zero-field...