Ji Shi

Modulation of Photocatalytic Properties by Strain in 2D BiOBr Nanosheets

BiOBr nanosheets with highly reactive {001} facets exposed were selectively synthesized by a facile hydrothermal method. The inner strain in the BiOBr nanosheets has been tuned continuously by the pH value. The photocatalytic performance of BiOBr in dye degradation can be manipulated by the strain effect. The low-strain BiOBr nanosheets show improved photocatalytic activity. Density functional calculations suggest that strain can modify the band structure and symmetry in BiOBr. The enhanced photocatalytic activity in low-strain BiOBr nanosheets is due to improved charge separation attributable to a highly dispersive band structure with an indirect band gap.

Controlling the magnetic anisotropy of CoPt∕AlN multilayer films

The magnetic anisotropy of sputter-deposited CoPt∕AlN multilayer films has been studied in contrast with the multilayer films consisting of magnetic and noble metals. It has been found that the as-deposited multilayer film shows enhanced in-plane magnetic anisotropy compared with CoPt alloy film of equivalent thickness. However, upon thermal annealing in a vacuum, the film undergoes a smooth transition of magnetic anisotropy from in-plane to perpendicular direction; moreover, the anisotropy field and coercivity can be controlled by controlling the annealing temperature. Structural characterization has revealed that the perpendicular magnetic anisotropy of the annealed CoPt∕AlN multilayer films is mainly of magnetoelastic origin.

Controlled polarity of sputter-deposited aluminum nitride on metals observed by aberration corrected scanning transmission electron microscopy

The polarity determination process of sputter-deposited aluminum nitride (AlN) on metals has been analyzed using aberration corrected atomic resolution scanning transmission electron microscope. Direct growth of c-axis orientated AlN on face centered cubic metals (fcc) (111) with the local epitaxy has been observed, and the polarity was determined at the AlN/metal interface. We found that the AlN polarity can be controlled by the base metal layer: N-polarity AlN grows on Pt(111) while Al-polarity AlN forms on Al(111). Based on these results, the growth mechanism of AlN on metals is discussed.

Enhancement of perpendicular coercivity for CoPt top layer in CoPt/AlN multilayer structure

The magnetic behavior of sputter deposited AlN20 nm/[CoPt2 nm/AlN2 nm]5/CoPt(x) (x as the thickness of the top CoPt layer) multilayer structure has been studied. It has been found that the magnetic anisotropy of the structure strongly depends on thermal annealing. With increasing the annealing temperature, the film changes from the in-plane magnetic anisotropy to the perpendicular anisotropy. Especially, for the top CoPt layer, the perpendicular coercivity increases much rapidly compared with that of the CoPt layers inside the base multilayer, when increasing the annealing temperature to 500 °C or above. The coercivity difference between the top CoPt layer and the base multilayer caused in this way results in an antiparallel alignment state during the magnetization process. The perpendicular magnetization and the coercivity enhancement for the CoPt top layer are correlated with the change in the residual stress inside this layer.

Preparation and characterization of Ni-C composite films

Ni-C composite films were prepared using a codeposition method, their structure and electrical properties were investigated. Depending on experimental conditions, two typical structures are found for as-deposited Ni-C films: i) amorphous Ni-C alloy film; ii) granular film with Ni3C granules and inter-granule amorphous carbon. The amorphous Ni-C films show ohmic conduction behaviour with very high resistivity. On the other hand, granular films with high carbon content show semiconductive characteristics. The electrical property and conduction behaviour are correlated with the film structure. Besides, the crystallization behaviour of the amorphous Ni-C film was also studied.

Strong perpendicular exchange bias in sputter-deposited CoPt/CoO multilayers

Strong perpendicular exchange bias was realized in sputter-deposited CoPt/CoO multilayers. The as-deposited multilayer shows strong perpendicular magnetic anisotropy (PMA) which remains up to a critical CoPt layer thickness as thick as 4 nm. After perpendicular field cooling, the multilayer with antiferromagnetic CoO and ferromagnetic CoPt interfaces exhibits large perpendicular exchange bias of 1730 Oe. Strong PMA in the CoPt/CoO multilayer is mainly attributed to the positive magnetoelastic energy due to the remarkable in-plane tensile stress originating from the local epitaxial growth. The mechanism for the exchange bias was explained in terms of the interfacial spin structure.

Interface roughness induced asymmetric magnetic property in sputter-deposited Co/CoO/Co exchange coupled trilayers

The effect of interface roughness on magnetic properties of exchange coupled polycrystalline Co/CoO(tAF)/Co trilayers has been investigated by varying antiferromagnetic layer (CoO) thickness. It has been found that the upper CoO/Co interface becomes rougher with increasing CoO layer thickness, resulting in stronger exchange bias of the upper interface than the lower one. The interfacial exchange coupling is strengthened by the increase of defect-generated uncompensated antiferromagnetic spins; such spins form coupling with spins in the Co layer at the interface. As a result, the CoO layer thickness dependence of exchange bias is much enhanced for the upper Co layer. The transition from anisotropic magnetoresistance to isotropic magnetoresistance for the top Co layer has also been found. This could be attributed to the defects, probably partial thin oxide layers, between Co grains in the top Co layer that leads a switch from spin-orbit scattering related magnetoresistance to spin-dependent electron scatter...

Perpendicular magnetic anisotropy of Co–TiN composite film with nano-fiber structure

Co–Ti–N films have been prepared by sputter deposition of Co and Ti in Ar+N2 atmosphere. Upon thermal anneal at elevated temperatures, Co (face-centered cubic) and TiN were formed in the film and separated from each other. Fiber-like microstructure developed with Co nano-fibers vertical to the substrate surface, and with their lateral size being less than 10 nm. The magnetic anisotropy of such films depends strongly on the film thickness. The Co–TiN films with their thickness above 100 nm show perpendicular magnetic anisotropy, which is explained in terms of shape anisotropy. Considering their microstructure, it is concluded that the diameter to length ratio of Co nano-fibers is an important factor controlling the magnetic anisotropy. For the Co–TiN film to show perpendicular magnetic anisotropy, the diameter to length ratio has to be smaller than 0.07 according to the experimental results. TiN in the films plays an important role in separating Co nano-fibers and thus to reduce the lateral magnetic intera...

Roles of L10 ordering in controlling the magnetic anisotropy and coercivity of (111)-oriented CoPt ultrathin continuous layers in CoPt/AlN multilayer films

Chemical ordering of (111)-oriented CoPt ultrathin continuous layers in CoPt/AlN multilayer structure has been investigated. It is found that upon thermal annealing at 600 °C, CoPt layers undergo ordering transformation even when the CoPt layers are as thin as 2 nm. The degree of ordering increases with increasing the layer thickness. For the 600 °C annealed CoPt/AlN multilayer film with 2 nm thick CoPt layers, due to the ordering, out-of-plane coercivity is increased to 3330 Oe, while large perpendicular magnetic anisotropy constant of 8.1 × 106 erg/cm3 is exhibited. These coercivity and anisotropy energy are appropriate for the practical use as magnetic recording media. It is clear that for such a structure, ordering transformation does not impair the perpendicular magnetic anisotropy induced by the interface and magnetoelastic effects. The easy axis of magnetization still lies in the layer normal direction, i.e., [111] direction, although for L10 structure it is supposed to lie in the [001] direction. ...

Thickness-Dependent Perpendicular Magnetic Anisotropy of CoPt Top Layer on CoPt/AlN Multilayer

The magnetic anisotropy of the CoPt top layer on a CoPt/AlN multilayer has been studied. The layered structures were sputter deposited on fused quartz substrates and subsequently annealed at 500 C in a vacuum. CoPt layers are of a disordered fcc structure and highly (111) textured. It has been found that the CoPt top layer is perpendicular magnetic anisotropic and shows an enhanced coercivity in comparison with the bottom CoPt/AlN multilayer. A critical (maximum) thickness of 6 nm is found for the perpendicular anisotropy in the CoPt top layer. The structural results indicate that the CoPt top layer has experienced a tensile strain under a certain thickness. However, when the thickness of the CoPt top layer is above 6 nm, the inplane CoPt lattice parameter begins to decrease as the thickness increases, indicating that the CoPt top layer cannot tolerate the elastic energy and shrinks to release this energy. The simultaneous changes of the magnetic anisotropy and the inplane lattice parameter with the thickness strongly suggest that the thickness-dependent magnetic anisotropy of the CoPt top layer is correlated with the elastic strain through the magnetoelastic effect.