B. Y. Zong

Carbon nanowalls and related materials

Size, dimensionality, and shape play important roles in determining the properties of nanomaterials. So far, most of the nanomaterial researches have been focused on zero-dimensional nanoparticles/nanodots and one-dimensional nanowires/nanorods/nanotubes, but very few studies have been carried out on two-dimensional nano-sheets. Starting from carbon, recently we have succeeded in growing a class of nanostructured two-dimensional materials either in the pure forms or in the form of composites with carbon. In this paper, we will first briefly discuss various types of two-dimensional systems and then focus on the formation mechanism of carbon nanowalls and their field-emission and electron transport properties. The use of carbon nanowalls as templates for the formation of other types of nanomaterials will also be discussed.

Angular dependence of the coercivity and remanence of ferromagnetic nanowire arrays

Magnetization properties of magnetic nanowire arrays are studied on various ferromagnetic materials grown in anodic alumina (alumite) and track etched polycarbonate (PCTE) membranes by pulsed electrodeposition. Magnetization curves were measured as functions of wire material, field orientation, and wire length. The coercivity (Hc) and remanent squareness (S) of the various wire arrays were derived from hysteresis loops as a function of angle (θ) between the field and wire axis. For PCTE membranes, Hc(θ) curves for CoNiFe, NiFe, and Co nanowire arrays all show an otherwise-bell-type variation, while they change shapes from the otherwise bell to bell type for Ni nanowire arrays as the wire diameter decreases to 30 nm. These characteristics can be understood based on different magnetization reversal mechanisms of small wires. The effect of magnetostatic interaction among wires on the magnetic properties was examined by changing the wire lengths in alumite membranes. It is found that the interaction reduces H...

Fabrication of a Class of Nanostructured Materials Using Carbon Nanowalls as the Templates

Well-aligned carbon nanowalls with a thickness of a few nanometers and a lateral size in the micrometer range have been grown on various types of substrates. The nanowalls exhibit a remarkably different surface morphology as compared to fullerenes and carbon nanotubes, in particular their two-dimensionality and high surface area. In this work, we focused on the second aspect and developed a templating method to fabricate a class of nanostructured materials based on the novel surface morphology of the carbon nanowalls. These structures may have potential applications in batteries, gas sensors, catalysts, and light-emission/detection, field-emission, and biomedical devices.

Methotrexate-conjugated and hyperbranched polyglycerol-grafted Fe3O4 magnetic nanoparticles for targeted anticancer effects

Superparamagnetic nanoparticles grafted with hyperbranched polyglycerol (HPG) and conjugated with methotrexate (MTX) (MNP-g-HPG-MTX) were synthesized via a sol-gel reaction followed by thiol-ene click chemistry and esterification reaction. The successful grafting of MTX and HPG onto the nanoparticles was confirmed by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) spectroscopy, and UV-visible spectroscopy. The HPG-graft layer confers the magnetic nanoparticles with good dispersibility and stability in aqueous medium and macrophage-evasive property while the MTX acts as a chemotherapeutic drug as well as a tumor targeting ligand. The dose-dependent targeting and anticancer effect of the MNP-g-HPG-MTX nanoparticles were evaluated, and the results showed that depending on the amount of conjugated MTX and the concentration of the incubated nanoparticles, the uptake of MNP-g-HPG-MTX nanoparticles by human head and neck cancer (KB) cells can be eight times or more higher than those by 3T3 fibroblasts and RAW macrophages. As a result, the MNP-g-HPG-MTX nanoparticles are capable of killing ∼50% of the KB cells while at the same time exhibiting low cytotoxicity towards 3T3 fibroblasts and RAW macrophages. Thus, such nanoparticles can potentially be used as active targeting anticancer agents.

Magnetic properties of magnetic nano-wire arrays

The arrays of NiFe and CoNiFe nanowires were grown in anodic alumina (alumite) and track etched polycarbonate (PCTE) membranes with various pore sizes by pulsed electrodeposition. Magnetic properties of the array were studied as functions of wire material, length, and diameter as well as field orientation. In PCTE membranes, as high as 0.92 of remanent squareness (S) was obtained for the array with wire diameter of 30 nm. However, for the array of nanowires in anodic membranes, anisotropy with field applied in and out of plane is very low, and the obtained S is less than 10%. For nanowires in alumite with the same wire length, CoNiFe shows a higher anisotropic field (H/sub k/) than NiFe, leading to larger coercivity (H/sub c/) and S and a lower saturation field. Wire-length dependence was measured for alumite membranes. As wire length increases, H/sub k/, H/sub c/, and S decrease, but the saturation field increases. This magnetic behavior can be qualitatively explained by considering dipolar interactions among nanowires and shape anisotropy of an individual nanowire, but it remains difficult to give a quantitative explanation.

Size-dependent microwave absorption properties of Fe3O4 nanodiscs

Fabrication of uniform magnetic nanodiscs with tunable size is of great importance for the study of their size dependent microwave properties. In this work, uniform Fe3O4 nanodiscs with different sizes are successfully synthesized by hydrogen-wet reduction of α-Fe2O3 nanodisc templates. The thickness of the Fe3O4 discs are around 30 nm and the diameters could be adjusted between 80–500 nm by controlling the amount of water in the reaction. The dynamic permittivity and permeability of Fe3O4 nanodisc/paraffin composite (20 vol%) were measured in the frequency range of 0.1–18 GHz. It is found that a larger diameter leads to a higher permittivity, lower permeability and higher resonance frequency as a consequence of stronger shape anisotropy. Moreover, excellent microwave absorption performances are achieved by these nanodiscs, which exhibit wide effective absorption (RL < −10 dB) with wide frequency bandwidth. Furthermore, small nanodiscs show superior microwave absorption properties over large nanodiscs because of lower permittivity which results in a better impedance match. The study not only presents an effective method to fabricate high quality Fe3O4 nanodiscs with tunable sizes, but also could be of great help for designing magnetic nanodisc based microwave absorbers.

A study of multirow-per-track bit patterned media by spinstand testing and magnetic force microscopy

We propose and demonstrate a concept of multirow-per-track bit patterned media (BPM) recording to overcome the problems encountered with the conventional one-row-per-track design. Focused ion beam was used on continuous granular perpendicular magnetic media to fabricate the prototype structures consisting of birow tracks with sub-100-nm single domain magnetic islands with the two rows of islands to be interleaved along the track direction, as well as an additional nonmagnetic spacer band between adjacent birow tracks for further bit aspect ratio (BAR)≥2 adjustment. Readback from such birow tracks with a two-row-wide read head was performed by dynamic spinstand testing. The proposed concept BPM provides many advantages including higher linear recording density (under the same lithographic limit), therefore enabling a higher data rate and a greater BAR≥2 for better integration with head design and servocontrol, as well as allowing the use of wider thus larger recording heads to improve writing efficiency fo...

Investigation of magnetic proximity effect in Ta/YIG bilayer Hall bar structure

In this work, the investigation of magnetic proximity effect was extended to Ta which has been reported to have a negative spin Hall angle. Magnetoresistance (MR) and Hall measurements for in-plane and out-of-plane applied magnetic field sweeps were carried out at room temperature. The size of the MR ratio observed (∼10−5) and its magnetization direction dependence are similar to that reported in Pt/yttrium iron garnet, both of which can be explained by the spin Hall magnetoresistance theory. Additionally, a flip of magnetoresistance polarity is observed at 4 K in the temperature dependent measurements, which can be explained by the magnetic proximity effect induced anisotropic magnetoresistance at low temperature. Our findings suggest that both magnetic proximity effect and spin Hall magnetoresistance have contribution to the recently observed unconventional magnetoresistance effect.

Electrodeposition of granular FeCoNi films with large permeability for microwave applications

A simple methodology to fabricate soft magnetic FeCoNi granular films from cheap salt solutions viaelectrodeposition at room temperature is demonstrated. With the addition of a small quantity of organic and inorganic additives into the solutions, the FeCoNi nano-granular films possess ultra-high permeability, large resistivity, and other desirable magnetic properties for gigahertz microwave applications. Typically, the films have a coercivity of less than 10 or 20 Oe along the hard or easy axis, respectively, with a saturation flux density of up to 2.43 T. The magnetic permeability and resistivity are correspondingly up to a magnitude order of about 103 and 10−4 Ω cm. These soft FeCoNi films also show a big anisotropic field of more than 50 Oe and a very small magnetostriction of <10−5. They can be potentially applied to microwave absorption as well as other applications.

Switching Probability Distribution of Bit Islands in Bit Patterned Media

In this paper, we report magnetic force microscopy (MFM) observations of switching probability of individual bit islands in bit patterned media. The switching probability (p) of each island was measured by repeated reversal tests at the same experimental conditions for each switching field (SF). It was found that there are ~60% of islands with for (which is approximately the average remnant coercivity, Hcr of the patterned islands) while the rest of the islands are either switched every time (for magnetically softer islands) or never switched (for magnetically harder islands). The observed statistical behavior of is an indication of thermal fluctuation during switching when magnetostatic energy (due to the applied external field) is comparable to magnetic anisotropy energy. As SF is decreased or increased away from Hcr (11 kOe to 9.5 kOe or 11 kOe to 12.5 kOe), percentage of islands with becomes smaller [narrower switching probability distribution (SPD)], due to less dipolar interaction/ variations among islands [which also lead to less switching field distribution (SFD) broadening]. Our results provide insights on the effects of statistical switching behavior of bit islands on the write errors in bit patterned media recording.