Orbit-Engineered Anisotropic Magnetoresistive Effect for Constructing a Magnetic Sensor with Ultrahigh Sensitivity.

A strong anisotropic magnetoresistance (AMR) effect induced by spin-orbit coupling is the basis for constructing a highly sensitive and reliable magnetic sensor. Presently, effective AMR enhancement in traditional films focuses on the modulation of the lattice or charge degree of freedom, leading to a general AMR ratio below 4%. Here, we demonstrate a different strategy to strengthen the AMR effect by tuning the orbital degree of freedom. By inserting an oxygen-affinitive Hf layer into a Ta/MgO/NiFe/MgO/Ta multilayer film, Fe-O orbital hybridization at the MgO/NiFe interface was modulated to trigger an effective orbital reconfiguration of Fe. In turn, the number of holes in the in-plane symmetric d orbits of Fe increased substantially, facilitating the s-d electron scattering to enhance the AMR ratio to 4.8%. By further micromachining the film into a Wheatstone bridge, we constructed a sensing element that displayed an ultrahigh sensitivity of 2.7 mV/V/Oe and a low noise detectability of 0.8 nT/√Hz. These findings help to advance the development of orbit-governed AMR sensors and provide an alternative method for tuning other orbit-related physical effects.

Enhanced negative magnetoresistance near the charge neutral point in Cr doped topological insulator.

Negative magnetoresistance (MR) is not only of great fundamental interest for condensed matter physics and materials science, but also important for practical applications, especially magnetic data storage and sensors. However, the microscopic origin of negative MR is still elusive and the nature of the negative MR in magnetic topological insulators has still not been completely elucidated. Here, we report magnetotransport studies on Cr doped (Bi1-x Sb x )2Te3 topological insulator thin films grown by magnetron sputtering. At the temperature of 2 K, a giant negative MR reaching 61% is observed at H = 2 T. We show that the negative MR is closely related to the position of the Fermi level, and it reaches the maximum when the Fermi level is gated near the charge neutral point. We attribute these results to the Coulomb potential due to the random composition fluctuations in Cr doped (Bi1-x Sb x )2Te3. Our results provide a deeper insight into the mechanism of negative MR, and are helpful to realize the quantum anomalous Hall effect in the sputtered Cr-(Bi1-x Sb x )2Te3 thin-film systems by tuning the Fermi level and reducing disorder effects.

A novel purification method of activated carbon-supported carbon nanotubes using a mixture of Ca(OH)2 and KOH as the ablation agent.

Transition metals (Fe, Co, Ni) supported on activated carbons with different pore diameters (<2 nm, 10 nm, 50 nm) to synthesize carbon nanotubes (CNTS) are first investigated in this study. Through several characteristic analyses, Ni supported on 50 nm activated carbon is verified to be the most efficient catalyst among the samples for CNT growth. The optimum conditions for CNT growth are at a growth temperature of 750 °C with a reaction time of 45 min. Furthermore, a novel purification method for CNTs is proposed, in which KOH and Ca(OH)2 powder are pre-mixed with the crude CNTs and CO2 and N2 gas are introduced into this mixture. When KOH and Ca(OH)2 powder are used at a ratio of 2 : 1 under the atmosphere of CO2 and N2 at the temperature of 750 °C for 1 h, almost all of the amorphous carbon is ablated. Compared with KOH powder, the addition of Ca(OH)2 not only advances the ablation effect, but reduces the damage to CNTs.

Anisotropic Magnetoresistance of Nano-conductive Filament in Co/HfO2/Pt Resistive Switching Memory.

Conductive bridge random access memory (CBRAM) has been extensively studied as a next-generation non-volatile memory. The conductive filament (CF) shows rich physical effects such as conductance quantization and magnetic effect. But so far, the study of filaments is not very sufficient. In this work, Co/HfO2/Pt CBRAM device with magnetic CF was designed and fabricated. By electrical manipulation with a partial-RESET method, we controlled the size of ferromagnetic metal filament. The resistance-temperature characteristics of the ON-state after various partial-RESET behaviors have been studied. Using two kinds of magnetic measurement methods, we measured the anisotropic magnetoresistance (AMR) of the CF at different temperatures to reflect the magnetic structure characteristics. By rotating the direction of the magnetic field and by sweeping the magnitude, we obtained the spatial direction as well as the easy-axis of the CF. The results indicate that the easy-axis of the CF is along the direction perpendicular to the top electrode plane. The maximum magnetoresistance was found to appear when the angle between the direction of magnetic field and that of the electric current in the CF is about 30°, and this angle varies slightly with temperature, indicating that the current is tilted.

Cubic to tetragonal phase transformation in cold-compressed Pd nanocubes.

Pd nanocubes with an average side length of approximately 10 nm were compressed up to 24.8 GPa in a diamond-anvil cell (DAC). In situ synchrotron X-ray diffraction was used to monitor structural changes, and a face-centered cubic (fcc) to face-centered tetragonal (fct) distortion was observed for the first time. This novel discovery not only provides new insights into the pressure-induced behavior of faceted nanocrystals of palladium and other noble metals but also gives guidance for finding new phases in close-packed metals.

Pressure-induced cubic to monoclinic phase transformation in erbium sesquioxide Er(2)O(3).

Cubic Er(2)O(3) was compressed in a symmetric diamond anvil cell at room temperature and studied in situ using energy-dispersive X-ray diffraction. A transition to a monoclinic phase began at 9.9 GPa and was complete at 16.3 GPa and was accompanied by a approximately 9% volume decrease. The monoclinic phase was stable up to at least 30 GPa and could be quenched to ambient conditions. The normalized lattice parameter compression data for both phases were fit to linear equations, and the volume compression data were fit to third-order Birch-Murnaghan equations of state. The zero-pressure isothermal bulk moduli (B(0)) and the first-pressure derivatives (B(0)') for the cubic and monoclinic phases were 200(6) GPa and 8.4 and also 202(2) GPa and 1.0, respectively. Ab initio density functional theory calculations were performed to determine optimized lattice parameters and atom positions for the cubic, monoclinic, and hexagonal phases of Er(2)O(3). The calculated X-ray spectra and predicted transition pressure are in good qualitative agreement with the experimental results.

Synthesis of multi-walled and bamboo-like well-crystalline CNx nanotubes with controllable nitrogen concentration (x = 0.05-1.02).

The multi-walled and bamboo-like well-crystalline CNx nanotubes with controllable nitrogen concentration (x = 0.05-1.02) were synthesized. The stoichiometry of as-prepared CNx (x congruent with 1.02) is close to that of the theoretically predicted graphite-like CN.

Synthesis of nitrogen-doped carbon nanostructures by the reactions of small molecule carbon halides with sodium azide.

Nitrogen-doped carbon nanostructures including particles, whiskers, square frameworks, lamellar layers, hollow spheres, and tubular structures have been successfully synthesized by designed direct chemical reactions of small molecule carbon halides (such as CCl4, C2Cl6) and nitridation reagent NaN3 in the absence of any templates and catalysts. The N/C ratios of the as-prepared CNx nanostructures (0.01 approximately 0.33) are strongly and systematically related to the reaction temperatures and the choice of carbon sources, as well as the presence or absence of the solvent. The Raman spectra indicate that the approaching graphitization process has occurred as the reaction temperature increases. The possible reaction mechanisms for the formation of the hollow structures are tentatively discussed according to the experimental results. This strategy provides an alternative route to synthesize nitrogen-doped carbon nanostructures and is expected to open up a new route for the synthesis of carbon nitrides.

Large-area synthesis of high-quality beta-MnO(2) nanowires and the mechanism of formation through a facile mineralization process.

High-quality large-area beta-MnO(2) nanowires can be easily synthesized using KNO(3) as the mineralizing agent in a process of mineralization from Mn(NO(3))(2) aqueous solution. The morphological evolution of the beta-MnO(2) nanowires and the influences of mineralizing agents and their concentrations on the morphology of the final products were investigated in detail. KNO(3) was the best mineralizing agent among KNO(3), NaNO(3), KCl, and NaCl. More interestingly, when the concentration of the mineralizing agent KNO(3) was not saturated, only irregular short faceted nanorods, instead of nanowires, can be observed. Finally, the formation mechanism is discussed.

Synthesis of carbon nitride nanotubes with the C(3)N(4) stoichiometry via a benzene-thermal process at low temperatures.

In this communication, we first report the direct synthesis of high-quality carbon nitride nanotubes (CNNTs) with inner diameters of 50-100 nm and wall thicknesses of 20-50 nm with the C(3)N(4) stoichiometry on a high-yield of 40%via a simple benzene-thermal process involving the reaction of C(3)N(3)Cl(3) with NaN(3) in a Teflon-lined autoclave at 220 [degree]C without using any catalyst or template.

Synthesis of high quality inorganic fullerene-like BN hollow spheres via a simple chemical route.

High quality inorganic fullerene-like boron nitride hollow spheres (100-200 nm) have been successfully synthesized via a simple chemical route with a 30-40% yield of BN hollow spheres.