Effect of Storage Conditions on the Long-Term Stability of Bactericidal Effects for Laser Generated Silver Nanoparticles.

Silver nanoparticles (AgNPs) are widely used as antibacterial agents, but their antibacterial durability and the influence by storage conditions have not been thoroughly investigated. In this study, AgNPs were produced using a picosecond laser and stored under three different conditions: daylight, dark and cold (4 °C). The antibacterial effects of the laser AgNPs were examined against Escherichia coli in either a 14-day interval (frequent air exposure) or a 45-day interval (less frequent air exposure) using a well-diffusion method until the antibacterial effects disappeared. Results showed that the antibacterial activity of the laser generated AgNPs lasted 266 to 405 days. Frequent air exposure increased particle oxidation as measured by high-angle annular dark-field detector for scanning transmission electron microscopy (HAADF-STEM) and X-ray energy dispersive (EDX) spectroscopy, and reduced the antibacterial duration by about 13 weeks. Compared to the chemically produced AgNPs, the antibacterial effect of the laser AgNPs lasted over 100 days longer when tested in the 45-day interval, but was susceptible to oxidation when frequently exposed to the air. The laser generated AgNPs had lower antibacterial activity when stored in cold compared to that stored at room temperature. This study demonstrated the long lasting antibacterial durability of the laser generated AgNPs. Such information could help design future medical applications for the AgNPs.

Minor hysteresis patterns with a rounded/sharpened reversing behavior in ferromagnetic multilayer.

Hysteresis of ferromagnetic system exhibits a fundamental stimulus-response behavior, thereby casting all the important macromagnetic system parameters such as coercivity, nucleation field, saturation magnetization, and hysteresis loss. Recently, increasing attention has been paid to exploration of relatively less understood minor loop behavior, since faster operation of magnetic devices is inevitably accompanied by minor hysteresis behavior from cycling among unsaturated ferromagnetic states. Here, we report our microscopic investigation of unusual minor hysteresis loop behavior, represented by rounded or sharpened response of minor hysteresis loop of (CoFeB/Pd)4 multilayer film. It is observed that rounded and sharpened response in the minor hysteresis response could be manifested under proper conditions. The minor loop behavior has been systematically investigated by direct microscopic magnetic domain observation using magneto-optical Kerr microscopy. The rounded response of magnetization at the reversing external field along the minor hysteresis curve, so far neglected or considered as one of 'unusual' behaviors, has been found to be elaborately controllable by tuning the reversing field strength and the field sweep rate for multilayers with low repeat numbers. Variable roundedness of the minor hysteresis loop is understandable based on the analysis of magnetic domain dynamics such as domain nucleation and the domain wall velocity.

Field-driven single domain wall motion in ferromagnetic nanowires.

We present a Lorentz microscopy study of polycrystalline permalloy 2D nanostructures with a thickness of 20 nm. Each structure was designed as a single domain wall trap. The trap comprises two horizontal nanowires with an in-plane dimension of 200 × 1000 nm2, and three tilted pads with different shapes. These structures allow us to create head-to-head domain walls, and these created walls can propagate in the structures by an external magnetic field. These designed traps were simulated using the micro-magnetic OOMMF simulation software. Those nanostructures were also patterned using electron beam lithography and focussed-ion beam techniques. This aims to determine the geometric parameters required to propagate a single magnetic domain wall in these structures reproducibly. Among the studied structures with one and two field directions, we found that the motion of a domain wall can be reproducibly driven by two alternative field directions in a trap which consists of the two horizontal nanowires and three 90°-tilted ones. We investigated systematically the viability of both single field and sequential switching of two field directions. Lorentz microscopy and micro-magnetic simulation results indicate that the propagation of a domain wall is strongly affected by the precise shape of the corner sections linking the trap elements, and the angles of the horizontal nanowires and tilted pads. Domain wall pinning and transformation of wall chirality are strongly correlated to the trap geometries. Our results are vital to design an optimal trap which supports a reproducible domain wall motion. This might also support a greater understanding of domain wall creation and propagation in magnetic nanowires which are of interest for concepts of high-density and ultrafast nonvolatile data storage devices, including racetrack memory and magnetic logic gates.

Exploring characteristics of the corner sections of a domain wall trap nanostructure with the two-field direction method.

A 2D polycrystalline permalloy domain wall trap nanostructure with a thickness of 20 nm was studied. The structure was alternatively designed and patterned using QCAD/L-Edit software and focused-ion beam technique. With this design, a magnetic domain wall can be created and propagated with a sequence of two-field directions in a Lorentz microscopy. The trap consists of two horizontal nanowires and three 90°-tilted ones. Each nanowire has an in-plane dimension of 200 × 1000 nm2. The trap corners were curved to allow a created domain wall that easily moves through the structure. A head-to-head domain-wall aims to create using a continuous field, this created wall can be propagated in the trap using a sequence of two-field directions. The designed trap was simulated using the Object Oriented Micro-Magnetic Framework software. Lorentz microscopy and simulation results indicate that the propagation of a domain wall is strongly affected by the precise roughness behavior of the trap elements. Domain wall pinning and transformation of wall chirality are sensitively correlated to the corner sections of the trap structure and field directions at a certain regime. Using the two-field direction method enables us to explore characteristics of the corner sections of the patterned trap nanostructure. This study is vital to fabricate an optimal nano-trap which supports a reproducible domain wall motion. This also suggests a useful method for the domain wall propagation using sequences of two-field directions. This work provides a better understanding of wall creation and propagation in polycrystalline permalloy curved nanowires which are of interest for concepts of nonvolatile data storage devices.

In Situ TEM Studies of Nanostructured Thermoelectric Materials: An Application to Mg-Doped Zn4 Sb3 Alloy.

We demonstrate an advanced approach using state of the art in situ transmission electron microscopy (TEM) to understand the interplay between nanostructures and thermoelectric (TE) properties of high-performance Mg-doped Zn4 Sb3 TE systems. By using the technique, microstructure and crystal evolutions of TE material have been dynamically captured as a function of temperature from 300 K to 573 K. On heating, we have clearly observed precipitation and growth of a Zn-rich secondary phase as nanoinclusions in the matrix of primary Zn4 Sb3 phase. Elemental mapping by STEM-EDX spectroscopy reveals enrichment of Zn in the secondary Zn6 Sb5 nanoinclusions during the thermal processing without decomposition. Such nanostructures strongly enhances phonon scattering, resulting in a decrease in the thermal conductivity leading to a zT value of 1.4 at 718 K.

Synthesis, Characterizations of Superparamagnetic Fe3O4-Ag Hybrid Nanoparticles and Their Application for Highly Effective Bacteria Inactivation.

In recent years, outbreaks of infectious diseases caused by pathogenic micro-organisms pose a serious threat to public health. In this work, Fe3O4-Ag hybrid nanoparticles were synthesized by simple chemistry method and these prepared nanoparticles were used to investigate their antibacterial properties and mechanism against methicilline-resistant Staphylococcus aureus (MRSA) pathogen. The formation of dimer-like nanostructure of Fe3O4-Ag hybrid NPs was confirmed by X-ray diffraction and High-resolution Transmission Electron Microscopy. Our biological analysis revealed that the Fe3O4-Ag hybrid NPs showed more noticeable bactericidal activity than that of plain Fe3O4 NPs and Ag-NPs. We suggest that the enhancement in bactericidal activity of Fe3O4-Ag hybrid NPs might be likely from main factors such as: (i) enhanced surface area property of hybrid nanoparticles; (ii) the high catalytic activity of Ag-NPs with good dispersion and aggregation stability due to the iron oxide magnetic carrier, and (iii) large direct physical contacts between the bacterial cell membrane and the hybrid nanoparticles. The superparamagnetic hybrid nanoparticles of iron oxide magnetic nanoparticles decorated with silver nanoparticles can be a potential candidate to effectively treat infectious MRSA pathogen with recyclable capability, targeted bactericidal delivery and minimum release into environment.

Visualization of vortex core polarity in NiFe nanodots by tilted Fresnel images.

We illustrate an approach which allows determining the out-of-plane component of the vortex core (polarity) in NiFe nanodots using Fresnel imaging in Lorentz electron microscopy. Using tilted Fresnel images, contribution of the polarity is introduced into the Fresnel image. However, this contribution is relatively small and a difference image from two symmetrically tilted Fresnel images must be used to eliminate the strong contribution from the in-plane curling magnetization and non-magnetic contrast. The sense of the polarity appears as a bipolar white-black contrast in the difference image on the tilt axis. A vortex core with a diameter of 16.5 ± 2.5 nm is experimentally measured. Image tilting, displacement and geometrical distortion may disturb the difference image, and hence subtraction of the difference image must be aligned by cross-correlation. The method is also justified by a study of the observed contrast characteristic due to misalignment. The method is confirmed to be superior to similar approach with direct interpretation of information and more information subtracted.

The microstructure, high performance magnetic hardness and magnetic after-effect of an alpha- FeCo/Pr(2)Fe(14)B nanocomposite magnet with low Pr concentration.

In this paper, a systematic investigation of the microstructure, high performance magnetic hardness as well as novel magnetic memory effect of the Pr(4)Fe(76)Co(10)B(6)Nb(3)Cu(1) nanocomposite magnet fabricated by conventional melt-spinning followed by annealing at temperatures ranging from 600 to 700 degrees C in Ar gas for nanocrystallization are presented and discussed. Transmission electron microscopy (TEM) observation confirms an ultrafine structure of bcc-Fe(Co) as a magnetically soft phase and Pr(2)Fe(14)B as a hard magnetic phase with a spring-exchange coupling in order to form the nanocomposite state. Electron diffraction analysis also indicates that the Co atoms together with Fe atoms form the Fe(70)Co(30) phase with a very high magnetic moment (2.5 mu(B)), leading to a high saturation magnetization of the system. High magnetic hardness is obtained in the optimally heat-treated specimen with coercivity H(c) = 3.8 kOe, remanence B(r) = 12.0 kG, M(r)/M(s) = 0.81 and maximum energy product (BH)(max) = 17.8 MG Oe, which is about a 25% improvement in comparison with recent results for similar compositions. High remanence and reduced remanence are the key factors in obtaining the high performance with low rare-earth concentration (only 4 at.%). High-resolution TEM analysis shows that there is a small amount of residual amorphous phase in the grain boundary, which plays a role of interphase to improve the exchange coupling. Otherwise, in terms of magnetic after-effect measurement, a magnetic memory effect was observed for the first time in an exchange-coupled hard magnet.