Lixin Yuan

Controlled synthesis of anatase TiO2 nanotube and nanowire arrays via AAO template-based hydrolysis

A novel and facile method was successfully developed to fabricate nanotube (NT) and nanowire (NW) arrays through an anodic aluminum oxide (AAO) template-based Ti(OC4H9)4 hydrolysis process. The wall thickness of the TiO2 NTs can be controlled by adjusting the concentration of Ti(OC4H9)4 solutions. A continuous series from thin-walled NTs to completely filled NWs were synthesized using Ti(OC4H9)4 solutions of different concentrations from 0.1 to 2.0 M. The TiO2 NTs and NWs in the AAO membrane transformed from amorphous to nanocrystalline anatase completely after annealing at 650 °C, which is higher than the phase transformation temperature of TiO2 powder. Optical spectra of the synthesized TiO2 NT and NW arrays/AAO composite nanostructures were characterized and showed a blue shift of the optical absorption edge with the decrease of NT wall thickness, due to the nanoscale effect and shape dependent transformation.

Magnetic properties of in-plane oriented barium hexaferrite thin films prepared by direct current magnetron sputtering

In-plane c-axis oriented Ba-hexaferrite (BaM) thin films were prepared on a-plane (112¯0) sapphire (Al2O3) substrates by DC magnetron sputtering followed by ex-situ annealing. The DC magnetron sputtering was demonstrated to have obvious advantages over the traditionally used RF magnetron sputtering in sputtering rate and operation simplicity. The sputtering power had a remarkable influence on the Ba/Fe ratio, the hematite secondary phase, and the grain morphology of the as-prepared BaM films. Under 80 W of sputtering power, in-plane c-axis highly oriented BaM films were obtained. These films had strong magnetic anisotropy with high hysteresis loop squareness (Mr/Ms of 0.96) along the in-plane easy axis and low Mr/Ms of 0.03 along the in-plane hard axis. X-ray diffraction patterns and pole figures revealed that the oriented BaM films grew via an epitaxy-like growth process with the crystallographic relationship BaM (101¯0)//α-Fe2O3 (112¯0)//Al2O3 (112¯0).

Dual band microwave ferromagnetic resonance absorption in annealed cobalt nanowire arrays

In this letter, we present a dual band microwave absorption phenomenon in 60 nm diameter Co ferromagnetic nanowire arrays annealed at 500 °C in nitrogen atmosphere. The frequency dependent microwave response of the wires is obtained for applied field parallel to the nanowire axis, below 4 kOe and above saturation magnetization up to 8 kOe, in steps of 1 kOe. For applied magnetic fields above saturation magnetization, two sets of absorption peaks are observed, while below saturation, only one single peak is obtained. Combining structural characterization and ferromagnetic resonance measurements, it is proven that the two ferromagnetic resonance peaks are associated with a coexistence of two kinds of nanopillars with different crystal structure in annealed nanowires, one for nanopillars with hexagonal close packed structure at higher ferromagnetic resonance frequency, and the other with face center cubic structure.