Haitao Yang

Controlled synthesis of highly ordered CuO nanowire arrays by template-based sol-gel route

The highly ordered CuO nanowire arrays of composite-oxides were synthesized within a porous anodic aluminum oxide(AAO) template by a citrate-based sol-gel route. A vacuum system was applied to draw the gel into the template pores, which conquers the only driving force of this technique-capillary action, then the gel was thermally treated to prepare desired CuO nanowires. The results of scanning electron microscopy(SEM) indicate that the CuO nanowires are very uniformly assembled and parallel to each other in, the pores of the anodic aluminum oxide(AAO) template membranes. The results of X-ray diffraction(XRD) and the selected-area electron diffraction(SAED) indicate that the CuO nanowires are monoclinic-type crystalline structure. Furthermore, X-ray photoelectron spectroscopy (XPS) demonstrates that the stoichiometric CuO is formed.

Exceeding natural resonance frequency limit of monodisperse Fe3O4 nanoparticles via superparamagnetic relaxation

Magnetic nanoparticles have attracted much research interest in the past decades due to their potential applications in microwave devices. Here, we adopted a novel technique to tune cut-off frequency exceeding the natural resonance frequency limit of monodisperse Fe3O4 nanoparticles via superparamagnetic relaxation. We observed that the cut-off frequency can be enhanced from 5.3 GHz for Fe3O4 to 6.9 GHz forFe3O4@SiO2 core-shell structure superparamagnetic nanoparticles, which are much higher than the natural resonance frequency of 1.3 GHz for Fe3O4 bulk material. This finding not only provides us a new approach to enhance the resonance frequency beyond the Snoeks limit, but also extend the application for superparamagnetic nanoparticles to microwave devices.

Integrating giant microwave absorption with magnetic refrigeration in one multifunctional intermetallic compound of LaFe 11.6 Si 1.4 C 0.2 H 1.7

Both microwave absorption and magnetocaloric effect (MCE) are two essential performances of magnetic materials. We observe that LaFe11.6Si1.4C0.2H1.7 intermetallic compound exhibits the advantages of both giant microwave absorption exceeding −42 dB and magnetic entropy change of −20 Jkg−1K−1. The excellent electromagnetic wave absorption results from the large magnetic loss and dielectric loss as well as the efficient complementarity between relative permittivity and permeability. The giant MCE effect in this material provides an ideal technique for cooling the MAMs to avoid temperature increase and infrared radiation during microwave absorption. Our finding suggests that we can integrate the giant microwave absorption with magnetic refrigeration in one multifunctional material. This integration not only advances our understanding of the correlation between microwave absorption and MCE, but also can open a new avenue to exploit microwave devices and electromagnetic stealth.

Synthesis of graphene/α-Fe2O3 composites with excellent electromagnetic wave absorption properties

A novel three-dimensional (3D) composite based on reduced graphene oxide (rGO)/Fe2O3 was prepared by a one-pot hydrothermal method. Fe2O3 nanoparticles were either attached on the surface of graphene sheets or coated uniformly in the graphene sheets. The resulting composite is found to self-assemble to form a 3D network via hydrothermal treatment. Our results indicate that the as-prepared Fe2O3 nanoparticles show a porous morphology, which results in the rGO/Fe2O3 composites exhibiting excellent microwave absorbing properties in the range of 2–16 GHz and are therefore expected to be a promising candidate as microwave absorbing materials.

Tuning magnetic anisotropies of Fe films on Si(111) substrate via direction variation of heating current

We adopted a novel method to tune the terrace width of Si(111) substrate by varying the direction of heating current. It was observed that the uniaxial magnetic anisotropy (UMA) of Fe films grown on the Si(111) substrate enhanced with decreasing the terrace width and superimposed on the weak six-fold magnetocrystalline anisotropy. Furthermore, on the basis of the scanning tunneling microscopy (STM) images, self-correlation function calculations confirmed that the UMA was attributed mainly from the long-range dipolar interaction between the spins on the surface. Our work opens a new avenue to manipulate the magnetic anisotropy of magnetic structures on the stepped substrate by the decoration of its atomic steps.

Facile synthesis of hollow nano-spheres and hemispheres of cobalt by polyol reduction

The hydrophilic hollow nano-spheres and hemispheres of Co are synthesized via ethylene glycol reduction of cobalt acetate in the presence of PVP and Pd nano-particle seeds. The dimensions of the hollow core can be tuned from 100 to 300 nm by controlling the amount of Pd nano-particle seeds. The morphology of the hollow materials strongly depends on the molar ratio of the amide unit in PVP over Co and the M(w) of PVP. The hollow structure is formed when the ratio falls in the range 1-1.5 and the M(w) is over 40,000. Based on the experimental data, a possible formation mechanism of Co hollow spheres is proposed.

Ultrafast demagnetization enhancement in CoFeB/MgO/CoFeB magnetic tunneling junction driven by spin tunneling current

The laser-induced ultrafast demagnetization of CoFeB/MgO/CoFeB magnetic tunneling junction is exploited by time-resolved magneto-optical Kerr effect (TRMOKE) for both the parallel state (P state) and the antiparallel state (AP state) of the magnetizations between two magnetic layers. It was observed that the demagnetization time is shorter and the magnitude of demagnetization is larger in the AP state than those in the P state. These behaviors are attributed to the ultrafast spin transfer between two CoFeB layers via the tunneling of hot electrons through the MgO barrier. Our observation indicates that ultrafast demagnetization can be engineered by the hot electrons tunneling current. It opens the door to manipulate the ultrafast spin current in magnetic tunneling junctions.

Electrosynthesis of Al/Pb/α-PbO2 composite inert anode materials

The alpha-PbO2 deposition layers were prepared on the surface of Al/Pb substrates by constant current electrosynthesis from an alkaline bath, and Al/Pb/alpha-PbO2 composite inert anode materials were obtained. The effects of the bath composition and bath temperature on the electrosynthesis of alpha-PbO2 were investigated by means of anodic polarization method, the phase structures and surface microstructures of Al/Pb and alpha-PbO2 deposition layers were tested by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The experimental data have shown that the process of alpha-PbO2 formation have several stages. The appropriate conditions can effectively improve the formation rate of alpha-PbO2 and avoid the occurrence of oxygen evolution reaction. The alpha-PbO2 deposition layer obtained in alkaline bath possesses rhombic structure, and it is composed of well developed spherical unit cells.

Multifunctional multisegmented Co/CoPt3 heterostructure nanowires

Multisegmented (MS) Co/CoPt3 nanowires (NWs) have been successfully fabricated by a template-assisted pulsed electrodeposition process and the length of each Co and CoPt3 segment is controlled by the pulse durations. The MS NWs show both the characteristic surface plasmon absorption and catalytic properties of CoPt3 and the strong ferromagnetic properties of Co that are affected by the interactions between Co and CoPt3. Furthermore, the ferromagnetic, optical, and catalytic properties can be well modulated by mainly changing the length of the Co and CoPt3 segments, respectively. Such multifunction of MS nanowires can be exploited for recyclable catalysts and bioassays based on their magnetic-catalytic and magnetic-optical properties, respectively.

Magnetic anisotropy of ultrathin Fe films grown on vicinal Si (111)

We have investigated magnetic anisotropy of ultrathin Fe films grown on vicinal Si (111) with 4° miscut towards [11-2] direction. Spin reorientation transition (SRT) from out-of-plane to in-plane proceeds in a wider thickness range than on flat substrates. Meanwhile, the easy axis of in-plane uniaxial magnetic anisotropy varies from [11-2] to [-110] with an intermediate state of approximate four-fold symmetry. The evolution of magnetic anisotropy is attributed to competition of surface magnetic anisotropy, first-order magnetocrystalline anisotropy, and step induced magnetic anisotropy from symmetry breaking and dipolar interactions.