D. Y. Geng

Microwave-absorption properties of ZnO-coated iron nanocapsules

[Liu, X. G.] Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, Shenyang 110016, Peoples R China. Chinese Acad Sci, Int Ctr Mat Phys, Shenyang 110016, Peoples R China.;Liu, XG (reprint author), Chinese Acad Sci, Inst Met Res, Shenyang Natl Lab Mat Sci, 72 Wenhua Rd, Shenyang 110016, Peoples R China;liuxg@imr.ac.cn

Microwave-absorption properties of FeCo microspheres self-assembled by Al2O3-coated FeCo nanocapsules

FeCo microspheres, self-assembled by Al(2)O(3)-coated FeCo nanocapsules, were prepared by means of the arc-discharge technique to investigate application in electromagnetic-wave absorption. For the FeCo microspheres, a reflection loss (RL) exceeding -20 dB was obtained in the frequency range of 10.9-15.6 GHz for an absorber thickness of 1.2-2.0 mm. An optimal RL of -30.8 dB was found at 11.4 GHz for an absorber thickness of 2.0 mm. The excellent microwave-absorption properties are a consequence of a proper electromagnetic match in the microstructure of a strong natural resonance and of the shape anisotropy.

Dual nonlinear dielectric resonance and strong natural resonance in Ni/ZnO nanocapsules

The electromagnetic characteristics of Ni/ZnO nanocapsules were studied at 2–18 GHz. The dual nonlinear dielectric resonance and strong natural resonance at 16.6 GHz contribute to excellent electromagnetic absorption. A reflection loss (RL) exceeding −20 dB was calculated in 14–18 GHz for an absorber thickness of 2.05 mm, and RL exceeds −10 dB in the whole X-band (10–12.4 GHz) and the whole Ku-band (12.4–18 GHz) for a thickness of 2.50 mm. The equivalent circuit model was used to explain the dual nonlinear dielectric resonance, which is ascribed to a cooperative consequence of the core/shell interfaces and the dielectric ZnO shells.

Broadband electromagnetic-wave absorption by FeCo/C nanocapsules

Electromagnetic-wave absorption by FeCo/C nanocapsules has been investigated. In contrast to earlier reported materials, including other nanocapsules, the absorption amplitude of FeCo/C nanocapsules is found not to decrease with increasing absorption-layer thickness. A reflection loss (RL) exceeding −20 dB can be obtained for all frequencies within the 2–18 GHz range by choosing an appropriate layer thickness between 1.6 and 8.5 mm. The broadest bandwidth (RL values exceeding −10 dB) from 10 to 18 GHz, covering half of the X-band and the whole Ku-band, is obtained for a 2 mm layer.

Enhanced natural resonance and attenuation properties in superparamagnetic graphite-coated FeNi 3 nanocapsules

Electromagnetic (EM) characteristics of superparamagnetic graphite-coated FeNi(3) nanocapsules were studied at 2-18 GHz. Compared with FeNi(3) nanoparticles coated by an amorphous oxide layer, the natural resonance and attenuation properties of the graphite-coated FeNi(3) nanocapsules were dramatically enhanced, due to the coating of the graphite. Graphite layers can restrain the growth of FeNi(3) nanocapsules, increase the resistivity, enhance the resonance frequency, keep the real part of permeability almost constant at high frequency and increase the magnetic loss. As a result of enhanced natural resonance and attenuation properties, the FeNi(3)/C nanocapsules exhibit good EM absorption properties.

Unconventional exchange bias in oxide-coated manganese nanoparticles

We report unconventional exchange bias in oxide-coated manganese nanoparticles, in which the Curie temperature of Mn3O4 shell is lower than the Neel temperature of the antiferromagnetic core. The coercivity (873 kA/m) of the nanoparticles, which is more than four times greater than that of bulk Mn3O4, has been enhanced significantly. A considerable enhancement in Curie temperature compared to the bulk was also observed for Mn3O4 in nanoscale. An exchange bias field as large as 400 kA/m was observed due to the strong interfacial exchange coupling. A simple phenomenological model is given to understand these phenomena in this ferri/antiferromagnetic system

Giant room-temperature magnetocaloric effect in Mn1−xCrxAs

A giant magnetocaloric effect was observed at room temperature in Mn1−xCrxAs compounds with x=0.006 and 0.01. The Cr dopant reduces (or even eliminates) the large thermal hysteresis of MnAs, while it lowers the first-order transition temperature from 313K for MnAs to 265K for Mn0.99Cr0.01As. Near the Curie temperature, a magnetic field induces a first-order phase transition from a ferromagnetic hexagonal phase to a paramagnetic orthorhombic phase, leading to a maximum value of ΔSM of 20.2J∕kgK at 267K for a 5T field change for Mn0.99Cr0.01As. The study on the Mn1−xCrxAs system may open an important field in searching proper materials for room-temperature magnetic refrigeration.

Magnetostriction and anisotropy compensation in TbxDy1-xPr0.3(Fe0.9B0.1)(1.93) alloys

Structure and magnetostriction of Dy0.7-xTbxPr0.3(Fe0.9B0.1)(1.93) (0less than or equal toxless than or equal to0.70) alloys have been studied. The easy magnetization direction of the Laves phase in the Dy0.7-xTbxPr0.3(Fe0.9B0.1)(1.93) alloys with 0less than or equal tox axis, while that of the Laves phase in those alloys with 0.25less than or equal toxless than or equal to0.70 lies along axis, as determined by means of x-ray crystallography study. When x is increased from 0.15 to 0.25, the change of the easy magnetization direction from to axis is detected also by Mossbauer spectra, in good agreement with the results of x-ray crystallography. The Laves phase Tb0.25Dy0.45Pr0.3(Fe0.9B0.1)(1.93) has a large spontaneous magnetostriction (lambda(111)approximate to1850 ppm) and a low anisotropy at room temperature, which could make it a good candidate material for magnetostriction applications

Al2O3 coated α-Fe solid solution nanocapsules prepared by arc discharge

Al2O3 coated alpha-Fe solid solution nanocapsules are prepared by arc-discharging a bulk AlNiCo permanent magnet. The size of the nanocapsule is in range of 3-300 nm and the thickness of the shell is 1-6 nm. Al atoms in the AlNiCo magnet form the shell of amorphous Al2O3 to prevent the nanocapsules from further oxidation. The magnetic properties of saturation magnetization J(s) = 85 A m(2)/kg and coercive force H-j(c) = 27.5 kA/m are achieved for the nanocapsules

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