Precipitating Ordered Skyrmion Lattices from Helical Spaghetti

Magnetic skyrmions have been the focus of intense research due to their potential applications in ultra-high density data and logic technologies, as well as for the unique physics arising from their antisymmetric exchange term and topological protections. In this work we prepare a chiral jammed state in chemically disordered (Fe, Co)Si consisting of a combination of randomly-oriented magnetic helices, labyrinth domains, rotationally disordered skyrmion lattices and/or isolated skyrmions. Using small angle neutron scattering, (SANS) we demonstrate a symmetry-breaking magnetic field sequence which disentangles the jammed state, resulting in an ordered, oriented skyrmion lattice. The same field sequence was performed on a sample of powdered Cu2OSeO3 and again yields an ordered, oriented skyrmion lattice, despite relatively non-interacting nature of the grains. Micromagnetic simulations confirm the promotion of a preferred skyrmion lattice orientation after field treatment, independent of the initial configuration, suggesting this effect may be universally applicable. Energetics extracted from the simulations suggest that approaching a magnetic hard axis causes the moments to diverge away from the magnetic field, increasing the Dzyaloshinskii-Moriya energy, followed subsequently by a lattice re-orientation. The ability to facilitate an emergent ordered magnetic lattice with long-range orientation in a variety of materials despite overwhelming internal disorder enables the study of skyrmions even in imperfect powdered or polycrystalline systems and greatly improves the ability to rapidly screen candidate skyrmion materials

A deep investigation into the structure of carbon dots

Since their discovery, carbon dots (CDs) have been a promising nanomaterial in a variety of fields including nanomedicine. Despite their potential in this area, there are many obstacles to overcome for CDs to be approved for biomedical use. One major hindrance to CDs’ approval is related to their poorly defined structure. Herein a structural study of CDs is presented in order to rectify this shortcoming. The properties of three CDs which have significant promise in biomedical applications, black CDs (B-CDs), carbon nitride dots (CNDs), and yellow CDs (Y-CDs), are compared in order to develop a coherent structural model for each nanosystem. Absorption coefficients were measured for each system and this data gave insight on the level of disorder in each system. Furthermore, extensive structural characterization has been performed in order to derive structural information for each system. This data showed that B-CDs and CNDs are functionalized to a greater degree and are also more disordered and amorphous than Y-CDs. These techniques were used to develop a structural model consistent with the obtained data and what is known for carbonic nanostructures. These models can be used to analyze CD emission properties and to better understand the structure-property relationship in CDs

Neural Spintronics: Noninvasive Augmentation of Brain Functions

Understanding the complexity of the brain ultimately requires insight into the decoding of local microcircuit functionality by noninvasive approaches. Recently, the new field of Spintronics is attracting a lot of attention with its noninvasive abilities to sense the magnetic field of neurons and to modulate their firing with spintronics devices. The two emerging tools are transcranial magnetic stimulation (TMS) and magnetic encephalography (MEG). The proposed nano-TMS device will use magnetic nanowires—the electromagnetic coils’ nanoscale cousins—to generate focused and programmable magnetic fields. Preliminary theoretical calculations show that proposed devices can provide programmable, focused stimulation for noninvasive neuromodulation of neural microcircuits with unprecedented high spatial and temporal resolutions. The nano-MEG is based on a simple version of the magnetometer capable of imaging the neural connections in the brain. The proposed magnetometer will realize the simple quantum limit (SQL) of the ferromagnetic resonance (FMR) of a “YIG” oscillator and/or spin-torque nano-oscillators (STNO) using a phase-locked loop (PLL) synchronized to a quartz clock. This micro-to-nano-metric technology is comparable with silicon integrated circuits and promises a “laboratory on a chip” approach to MEG that permits millions of detectors to be used. The design is aimed at reducing the massive magnetic screening associated with the usual superconducting quantum interference devices (SQUID) or optically pumped magnetometers (OPM)

Magnetotransport properties of polycrystalline and epitaxial chromium dioxide nanowires

Temperature dependent magnetotransport measurements were performed on polycrystalline and epitaxial chromium dioxide ( Cr O 2 ) nanowires fabricated using the selective-area growth technique. Polycrystalline nanowires showed a negative temperature coefficient of resistivity at low temperatures because of strong grain boundary scattering. The magnetoresistance (MR) value exhibited a width dependence, reaching a maximum of 20% for a 150 nm wide wire. In contrast, the MR response of single crystal Cr O 2 wires was mainly determined by magnetocrystalline and shape anisotropy

Rapid thermal annealing study of magnetoresistance and perpendicular anisotropy in magnetic tunnel junctions based on MgO and CoFeB

The tunneling magnetoresistance and perpendicular magnetic anisotropy in CoFeB(1.1-1.2 nm)/MgO/CoFeB(1.2-1.7 nm) junctions were found to be very sensitively dependent on annealing time. During annealing at a given temperature, decay of magnetoresistance occurs much earlier compared to junctions with in-plane magnetic anisotropy. Through a rapid thermal annealing study, the decrease of magnetoresistance is found to be associated with the degradation of perpendicular anisotropy, instead of impurity diffusion as observed in common in-plane junctions. The origin of the evolution of perpendicular anisotropy as well as possible means to further enhance tunneling magnetoresistance is discussed

Development of Red-Emissive Carbon Dots for Bioimaging through a Building Block Approach: Fundamental and Applied Studies

In recent years, many researchers have struggled to obtain carbon dots (CDs) that possess strong photoluminescence in the red region of light. Success in this area has been limited, although the past few years have brought several promising reports on this topic. The most successful efforts in this area still seem to struggle from a lack of dispersibility/reduced emission in water. This work endeavors to understand the formation process of CDs that do not possess strong performance in an aqueous environment and to improve their capabilities in bioimaging. -Phenylenediamine ( -PDA) is used along with various precursors in several different solvents (varying acidic and oxidative strengths) to understand the formation process behind the structure leading to red emission that is sensitive to water. These results showed that the combination of acid properties and oxidation is essential for this process, and the important reactions are oligomerization of -PDA and the crosslinking of these oligomers to form aromatic structural segments of CDs. These CDs are shown to be capable of quantitatively detecting water in organic solvents. Additionally, we have shown that conjugation with transferrin remarkably enhances the biocompatibility of these CDs. Transferrin-conjugated CDs with better biocompatibility were applied to bioimaging studies of neuroblastoma cell lines with N- and non-N- gene amplification, for the first time. Furthermore, CDs showed versatile bioimaging capability toward a highly aggressive neuroblastoma subgroup of tumors. The importance of creating red-emissive CDs has been well established, and this work is an important step toward understanding their formation and realizing their use in biological systems

Periodic magnetic domains in single-crystalline cobalt filament arrays

Magnetic structures with controlled domain wall pattern may be applied as potential building blocks for three-dimensional magnetic memory and logic devices. Using a unique electrochemical self-assembly method, we achieve regular single-crystalline cobalt filament arrays with specific geometric profile and crystallographic orientation, and the magnetic domain configuration can be conveniently tailored. We report the transition of periodic antiparallel magnetic domains to compressed vortex magnetic domains depending on the ratio of height to width of the wires. A "phase diagram" is obtained to describe the dependence of the type of magnetic domain and the geometrical profiles of the wires. Magnetoresistance of the filaments demonstrates that the contribution of a series of 180 domain walls is over 0.15% of the zero-field resistance ρ(H=0). These self-assembled magnetic nanofilaments, with controlled periodic domain patterns, offer an interesting platform to explore domain-wall-based memory and logic devices