Song Ma

Broadband microwave absorption of CoNi@C nanocapsules enhanced by dual dielectric relaxation and multiple magnetic resonances

Dual dielectric relaxation of the permittivity and multiple magnetic resonances of the permeability (including one natural resonance and two exchange resonance modes) are observed in CoNi@C nanocapsules in the same 5–17u2009GHz frequency range which leads to a better electromagnetic-wave absorption than earlier reported for nanocomposites. A reflection loss (RL) exceeding −25u2009dB is obtained in a wide frequency range of 5–17 GHz when an appropriate absorber thickness between 2 and 4.8u2009mm is chosen. For a 2u2009mm absorber layer, a RL value exceeding −10u2009dB is achieved in the broad frequency range 12–18 GHz, which covers the whole Ku-band.

Optimal electromagnetic-wave absorption by enhanced dipole polarization in Ni/C nanocapsules

Electromagnetic-wave (EMW) absorption by Ni/C nanocapsules with similar permeability but different permittivity mainly due to differences in the graphite-shell thickness has been investigated. The optimal working frequency could appear at S-band and C-band and considerable strong EMW absorption was achieved. For the optimal Ni/C nanocapsules, a reflection loss exceeding −20u2009dB was reached from 2.6 to 8.2u2009GHz with a maximum value of −40u2009dB at 3u2009GHz. The improved absorption can be attributed to an optimal electromagnetic match and an enhanced dipole polarization upon increasing of shell thickness.

Tailoring the topology of an artificial magnetic skyrmion

Despite theoretical predictions, it remains an experimental challenge to realize an artificial magnetic skyrmion whose topology can be well controlled and tailored so that its topological effect can be revealed explicitly in a deformation of the spin textures. Here we report epitaxial magnetic thin films in which an artificial skyrmion is created by embedding a magnetic vortex into an out-of-plane aligned spin environment. By changing the relative orientation between the central vortex core polarity and the surrounding out-of-plane spins, we are able to control and tailor the system between two skyrmion topological states. An in-plane magnetic field is used to annihilate the skyrmion core by converting the central vortex state into a single domain state. Our result shows distinct annihilation behaviour of the skyrmion core for the two different skyrmion states, suggesting a topological effect of the magnetic skyrmions in the core annihilation process.

Synthesis of PVP-coated ultra-small Fe3O4 nanoparticles as a MRI contrast agent

Ultra-small Fe3O4 nanoparticles were prepared by using the coprecipitation method, in which the polyvinylpyrrolidone (PVP) serves as a stabilizer. The nanoparticles were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), infra spectrum (IR), X-ray photoelectron spectroscopy (XPS) and in vivo magnetic resonance imaging (MRI) test. The results showed that the particles’ size was determined by the dripping rate and that PVP molecules played the role of preventing the aggregation and restricting the size of Fe3O4 nanoparticles. The Fe3O4 nanoparticles with diameter from 6.5 to 1.9xa0nm obviously exhibited negative contrast enhancement and concentrated at the target area guided by a permanent magnet.

In situ oxidation of carbon-encapsulated cobalt nanocapsules creates highly active cobalt oxide catalysts for hydrocarbon combustion.

Combustion catalysts have been extensively explored to reduce the emission of hydrocarbons that are capable of triggering photochemical smog and greenhouse effect. Palladium as the most active material is widely applied in exhaust catalytic converter and combustion units, but its high capital cost stimulates the tremendous research on non-noble metal candidates. Here we fabricate highly defective cobalt oxide nanocrystals via a controllable oxidation of carbon-encapsulated cobalt nanoparticles. Strain gradients induced in the nanoconfined carbon shell result in the formation of a large number of active sites featuring a considerable catalytic activity for the combustion of a variety of hydrocarbons (methane, propane and substituted benzenes). For methane combustion, the catalyst displays a unique activity being comparable or even superior to the palladium ones.

Magnetic and Microwave-absorption Properties of Graphite-coated (Fe, Ni) Nanocapsules

The structure, magnetic and microwave-absorption properties of graphite-coated (Fe, Ni) alloy nanocapsules, synthesized by the arc-discharge method, have been studied. High-resolution transmission electron microscopy shows that the nanocapsules have a core/shell structure with (Fe, Ni) alloy as the core and graphite as the shell. All (Fe, Ni) alloy nanocapsules/paraffin composites show good microwave-absorption properties. The optimal reflection loss (RL) was found for (Fe(70)Ni(30))/C nanocapsules/paraffin composites, being -47.84 dB at 14.6 GHz for an absorber thickness of 1.99 mm, while the RL values exceeding -10 dB were found in the 12.4-17.4 GHz range, which almost covers the K(u) band (12.4-18 GHz). For (Fe(70)Ni(30))/C nanocapsules/paraffin composites, RL values can exceed -10 dB in the 11.4-18 GHz range with an absorber thickness of 1.91 mm, which cover the whole K(u) band.

Electromagnetic-wave absorption properties of FeCo nanocapsules and coral-like aggregates self-assembled by the nanocapsules

FeCo nanocapsules with FeCo alloy as core and amorphous Al(2)O(3) as shell have been synthesized by a modified arc-discharge technique. Three-dimensional coral-like aggregates are self-assembled by the nanocapsules in the arc-discharge process. The FeCo nanocapsules are ferromagnetic at room temperature. The electromagnetic-wave absorption properties of FeCo nanocapsules were investigated in the frequency range from 2 to 18 GHz. A reflection loss exceeding -20 dB was obtained in the frequency range of 5.2-15.9 GHz for absorber thicknesses of 2-4 mm. An optimal reflection loss of -44.8 dB was reached at 12.8 GHz for an absorber thickness of 3 mm. As a result, these FeCo nanocapsules may be applied in high-frequency electromagnetic-wave absorption

High Curie temperature Bi1.85Mn0.15Te3 nanoplates

Bi(1.85)Mn(0.15)Te(3) hexagonal nanoplates with a width of ~200 nm and a thickness of ~20 nm were synthesized using a solvothermal method. According to the structural characterization and compositional analysis, the Mn(2+) and Mn(3+) ions were found to substitute Bi(3+) ions in the lattice. High-level Mn doping induces significant lattice distortion and decreases the crystal lattice by 1.07% in the a axis and 3.18% in the c axis. A high ferromagnetic state with a Curie temperature of ~45 K is observed in these nanoplates due to Mn(2+) and Mn(3+) ion doping, which is a significant progress in the field of electronics and spintronics.

Microwave absorption properties of Ni/(C, silicides) nanocapsules

The microwave absorption properties of Ni/(C, silicides) nanocapsules prepared by an arc discharge method have been studied. The composition and the microstructure of the Ni/(C, silicides) nanocapsules were determined by means of X-ray diffraction, X-ray photoelectric spectroscopy, and transmission electron microscope observations. Silicides, in the forms of SiOx and SiC, mainly exist in the shells of the nanocapsules and result in a large amount of defects at the ‘core/shell’ interfaces as well as in the shells. The complex permittivity and microwave absorption properties of the Ni/(C, silicides) nanocapsules are improved by the doped silicides. Compared with those of Ni/C nanocapsules, the positions of maximum absorption peaks of the Ni/(C, silicides) nanocapsules exhibit large red shifts. An electric dipole model is proposed to explain this red shift phenomenon.

Synthesis of a new type of GdAl2 nanocapsule with a large cryogenic magnetocaloric effect and novel coral-like aggregates self-assembled by nanocapsules

A new type of GdAl2 nanocapsule with single-phase intermetallic compound GdAl2 as the core and amorphous Al2O3 as the shell has been synthesized by the arc-discharge technique with modified strategies. Meanwhile, novel three-dimensional coral-like hierarchical branching macro-aggregates were self-assembled by disordered nanocapsules synthesized simultaneously in the arc-discharge process. The GdAl2 nanocapsules display superparamagnetic properties between their blocking temperature of 100 K and Curie temperature of 162 K. The magnetocaloric effect of the GdAl2 nanocapsules was measured between 5 and 165 K. The absolute value of the change of magnetic entropy of the GdAl2 nanocapsules sharply increases with decreasing temperature and reaches 14. 5Jk g −1 K −1 at 5 K in magnetic fields varying from 0 to 50 kOe. As a result, this new type of nanocapsule can prospectively be applied in cryogenic magnetic refrigerator devices.