Yalu Zuo

The enhanced microwave absorption property of CoFe2O4 nanoparticles coated with a Co3Fe7–Co nanoshell by thermal reduction

CoFe(2)O(4) nanoparticles were fabricated by a sol-gel method and then were coated with Co(3)Fe(7)-Co by means of a simple reduction process at different temperatures under 2% H(2) with the protection of argon to generate the dielectric-core/metallic-shell structure. The optimum reflection loss (RL) calculated from permittivity and permeability of the 80 wt% CoFe(2)O(4)/Co(3)Fe(7)-Co and 20 wt% epoxy resin composites reached - 34.4 dB, which was much lower than that of unreduced CoFe(2)O(4) and epoxy resin composites, at 2.4 GHz with a matching thickness of 4.0 mm. Moreover the RL exceeding - 10 dB in the maximum frequency range of 2.2-16 GHz was achieved for a thickness of composites of 1.0-4.5 mm with 600u2009°C thermal reduction process. The improved microwave absorption properties are a consequence of a proper electromagnetic match and the enhanced magnetic loss besides its dielectric loss due to the existence of the core/shell structure in CoFe(2)O(4) composites. Thus, the reductive CoFe(2)O(4) nanoparticles have great potential for being a highly efficient microwave absorber.

Relationship between the surface chemical states and magnetic properties of CeO2 nanoparticles

Hydrogen reduction treatments with variations in isothermal reduction temperature, duration, and partial pressure of hydrogen (PH2) are carried out on CeO2 nanoparticles (NPs) to investigate the relationship between the surface chemical states and magnetic properties of the samples. The phenomenon of surface reduction degree dependence of the ferromagnetism of CeO2 NPs is observed. Semiquantitative calculated concentrations of surface Ce3+ ions from x-ray photoelectron spectroscopy data demonstrate that ferromagnetism does not relate to the surface oxygen vacancies but to the surface Ce3+/Ce4+ pairs. The higher the number of surface Ce3+/Ce4+ pairs, the more robust the ferromagnetism appears.

Room temperature ferromagnetism in Sn1−xVxO2 films prepared by sol-gel method

The structure and magnetic properties of Sn1−xVxO2 (x=0.02–0.22) thin films fabricated on Si (111) substrate using a sol-gel method and spin coating technique have been investigated. All the samples have pure rutile polycrystalline structure and exhibit room temperature ferromagnetism. The magnetic moment per V reaches 2.92μB for the Sn0.98V0.02O2 film and drops rapidly as V content is increased. X-ray photoelectron spectroscopy study reveals that vanadium is in V4+ chemical state. Various annealing treatments were performed to explore the origin of the ferromagnetism. It is found that the ferromagnetism of Sn0.98V0.02O2 film disappears after annealing in a rich-oxygen atmosphere and occurs again after annealing in a low vacuum condition. Furthermore, an annealing in Sn vapor leads to the decrease in ferromagnetism. These results confirm that the oxygen vacancies play a critical role in introducing ferromagnetism of Sn1−xVxO2 films; therefore, the origin of the ferromagnetism in our samples can be underst...

Microwave absorption properties of amorphous iron nanostructures fabricated by a high-yield method

Amorphous Fe nanoparticles and a nanonecklace were synthesized at room temperature by an aqueous reduction procedure, which provided a simple and potential method for volume production of ferromagnetic materials. The morphology was examined by scanning electron microscopy and transmission electron microscopy. The amorphism of Fe nanoparticles and the nanonecklace was confirmed by x-ray diffraction and electron diffraction patterns in transmission electron microscopy. The complex permittivity and permeability behaviour of amorphous iron nanoparticles/paraffin wax (NPPW) and nanonecklace/paraffin wax (NCPW) composites was investigated in 0.1–18xa0GHz by a coaxial method. The strongest reflection loss values of NPPW and NCPW calculated from permittivity and permeability reached −53.2xa0dB and −47.8xa0dB at 6.4xa0GHz and 4.6xa0GHz with matching thicknesses of 2.4xa0mm and 2.3xa0mm, respectively. Moreover, the frequency ranges of microwave absorption exceeding 90% were around 4.9–8.8xa0GHz and 3.7–6.1xa0GHz for NPPW and NCPW, respectively. Comparing the microwave absorption property with crystallized Fe nanostructures, we may conclude that the relatively high resistivity and low permittivity of amorphous Fe nanostructures are favourable for impedance matching, and consequently result in the attracting microwave absorption property of amorphous Fe nanostructures. Thus, amorphous iron nanoparticles and the nanonecklace prepared by a high-yield method have great potential to be a highly efficient microwave absorber.

Tunable cut-off frequency by in-plane uniaxial anisotropy in (Fe66.9Co33.1)86.8Sm13.2 films

(Fe66.9Co33.1)86.8Sm13.2 (FeCoSm) thin films with thickness from 110 to 330?nm are fabricated on silicon (1?1?1) substrates by magnetron co-sputtering under ambient condition. The as-deposited FeCoSm thin film is amorphous when the thickness is less than 150?nm. When the thickness increases further, the film becomes partially crystallized. The maximum in-plane uniaxial anisotropy field is up to 1200?Oe. After the films are heat treated in vacuum with a magnetic field, the in-plane uniaxial anisotropy field decreases with an increase in annealing temperature and becomes zero after annealing at 500??C. The dynamic magnetic properties of the FeCoSm films are also investigated in the range 0.1?9?GHz. Based on the one-terminal micro-strip transmission-line perturbation method and an analysis by the Landau?Lifshitz equation, the anisotropy field and the resonance frequency of the FeCoSm_films are in the range 50?1200?Oe and 1.8?12.1?GHz, respectively. The anisotropy field and resonance frequency of the film can be controlled by varying the film thickness or annealing temperature, this means that FeCoSm films have great potential in high-frequency applications.

Demonstration of magnetoelectric memory cell in (110) [Pb(Mg1/2Nb2/3)O3]0.68-[PbTiO3]0.32/Ru/FeCo heterostructures

An electric-field pulses driven magnetoelectric memory cell in a single layered ferromagnetic thin film was fabricated by direct-current magnetron sputtering Ru/Fe65Co35 on ferroelectric (110) [Pb(Mg1/2 Nb2/3)O3]0.68-[PbTiO3]0.32 (PMN-PT) substrates. The magnetization in the orthogonal directions can be reset by the positive/negative electric fields pulse in PMN-PT/Ru/FeCo heterostructures due to the strain mediated converse magnetoelectric effect. The high (low) resistance state was realized under the negative (positive) electric fields pulse due to the anisotropy magnetoresistance of FeCo films. Then, a non-volatile magnetic memory cell with resistance and electric field, respectively, as the media and writing field was realized.

Tunable in-plane uniaxial anisotropy and the magnetization reversal mechanism of patterned high-frequency soft magnetic FeTa strips

FeTa films with thickness of 110 nm are fabricated on glass substrates by magnetron sputtering, and then a series of strips is designed on the FeTa films by conventional optical lithography and the ion beam etching method. Patterned FeTa strips show a tunable in-plane uniaxial magnetic anisotropy property in contrast with the magnetic isotropic property of as-deposited FeTa thin films. The magnetization reversal mechanism of the patterned FeTa strips is investigated via the in-plane angular dependences of magnetization and coercivity. The angular dependence of coercivity (ADC) is explained well in terms of the two-phase model, giving good quantitative agreement with the experimentally measured M-shaped ADC curve. The domain structure and spatial resolution magneto-optical Kerr effect measurement indicate that the smaller the strip width, the stronger will be the anisotropy field. Regarding the dynamic magnetic properties, a transformation from Debye dispersion spectrum for strips with weak anisotropy to natural resonance spectrum for strips with strong anisotropy is finally obtained. The tunable in-plane anisotropy fields of the FeTa strips result in tunable high-frequency soft magnetic properties by altering the strip width, indicating that patterned FeTa strips have great potential in high-frequency soft magnetic application fields.

Controlling the microwave characteristic of FeCoB-ZnO granular thin films deposited by obliquely sputtering

A series of FeCoB-ZnO soft magnetic granular films deposited at different oblique angles were prepared by magnetron sputtering system. A variable in-plane uniaxial magnetic anisotropy field from 27.6u2009Oe to 212u2009Oe and an adjustable ferromagnetic resonance frequency from 1.89 GHz to 5.3u2009GHz were obtained in the as-deposited films just by increasing the oblique angle from 15° to 56°. Frequency line-width and effective Gilbert damping factor were both insensitive to the different oblique angles (αeff decreased from 0.036 to 0.03 and Δf decreased from 1.49 to 1.27), which almost satisfied the requirement that fFMR could be tuned independently in a certain frequency range. Besides, the change of dynamic magnetic anisotropy field versus oblique angle was illustrated and analyzed quantitatively compared with the static magnetic anisotropy.

Electric field induced magnetic anisotropy transition from fourfold to twofold symmetry in (001) 0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3/Fe0.86Si0.14 epitaxial heterostructures

The epitaxial growth of FeSi film on (001) 0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 (PMN-0.32PT) was fabricated by sputtering and confirmed by high-resolution transmission electron microscopy. A fourfold symmetric angular remanent magnetization curve of as-deposited FeSi thin film is well fitted theoretically by considering the cubic magnetocrystalline anisotropy. We found that the fourfold anisotropy decreases slightly when an electric field (E) is applied on the Pt/PMN-0.32PT/FeSi/Ta heterostructures with Pt layer as the positive electrode. However, a magnetic anisotropy transition from fourfold anisotropy to twofold anisotropy occurs under negative E. The strain-electric field curve suggests that the observed different variation trend of magnetic anisotropy results from the asymmetric strain response on the polarity of E. Moreover, once the transition happens, it was irreversible unless the heterostructures are heated above the phase transition temperature of PMN-0.32PT.

Reduction of magnetic damping constant of FeCo films by rare-earth Gd doping

Magnetic damping constant (α) is one of the key parameters to determine the critical current density of spin-transfer-torque devices and the switching time of magnetization for ultra-high-frequency devices. In this work, Gd doped FeCo films were fabricated to investigate α based on the ferromagnetic resonance technique. Gd doping not only can efficiently decrease the magnetic inhomogeneity and the extrinsic part of α but also the Lande g-factor and intrinsic part of α. The obtained α was roughly proportional to (g-2)2 and the magnetic anisotropic constant, indicating that the decreased spin-orbit interaction decreases α by Gd doping.