Zhenhong Mai

Relation between microstructures and magnetic properties upon annealing in Fe50Mn50/Ni80Fe20 films

The annealing-temperature-dependent change in the exchange bias and coercivity is investigated in Fe50Mn50/Ni80Fe20 films. It is interesting to note that, as the annealing temperature increases, the exchange bias first decreases, and then increases, and finally decreases for the case of annealing at the higher temperature. The coercivity will increase upon annealing at moderate temperature, but decrease upon higher-temperature annealing. We can qualitatively interpret the change of the magnetic properties with annealing temperature in connection to the microstructures by x-ray scattering technologies. The results show that both the large exchange bias field and low coercivity of Fe50Mn50/Ni80Fe20 films are dependent of not only the interfacial roughness but also the antiferromagnetic structure.

Nanopipes in undoped AlGaN epilayers

The surface morphologies of nanopipes were imaged using a scanning electron microscope and an atomic force microscope in undoped AlGaN epilayers grown by metal organic vapor phase epitaxy on GaN-based layers, The nanopipes usually appear as dodecagonal and hexagonal pyramidal indentations for larger and smaller sizes, respectively, with a pinhole at each of their centers. The results indicate that {1 1 (2) over bar 1} facets may play an important role as well as {1 0 (1) over bar 1} facets during the nanopipe formation in undoped AlGaN epilayers, Energy dispersive X-ray spectroscopy shows that Al atoms have precipitated more distinctly on the facets of the nanopipes. The image of yellow cathodoluminescence is characterized by a bright ring around the outer region of pyramidal indentations. This is suggested to be a result of competition between the higher concentration of the defects responsible for the yellow luminescence and the thinner epilayer in the facet site

Study of BaTiO3YBa2Cu3O7 − x heterostructural films grown on (0 0 L)SrTiO3 by pulsed laser deposition

A new tetragonal phase of C-N compound was identified by scanning and transmission electron microscopy and X-ray diffraction. The lattice parameters for the new C-N structure are a = 5.65 Angstrom and c = 2.75 Angstrom. The tetragonal C-N grains were found in the carbon-nitrogen films synthesized on Ni(1 0 0) substrates via bias-assisted chemical vapor deposition using a gas mixture of nitrogen and methane, together with alpha- and beta-C3N4, as well as other unknown C-N phases. Transmission electron microscopy and scanning electron microscopy images show that most of the tetragonal C-N grains have perfect single-crystalline tetrahedrons with different sizes, but some of them exhibit irregular shapes. Energy dispersive X-ray analysis gives a relative N:C of 0.8-1.0 in the tetrahedral-grain enriched films. The stoichiometry and atomic arrangement of the tetragonal C-N phase should be determined further.

Mechanisms of Ag as a surfactant in giant magnetoresistance multilayer growth and thermal stability

The mechanisms played by Ag as a surfactant in giant magnetoresistance multilayers were investigated using interface sensitive x-ray anomalous scattering techniques. Analysis on [Cu∕Ni70Co30]20 and [Cu∕Ag∕Ni70Co30]20 multilayers revealed that 6A thick NiCu and 6A thick CuNi3Co intermixing regions are formed at the Ni70Co30-on-Cu interfaces of undoped and Ag-doped multilayers, respectively. The Cu-on-Ni70Co30 interfaces in both multilayers are sharp. Annealing causes severe diffusion across both types of interfaces in the undoped multilayer. But the interfaces in the Ag-doped multilayer do not change significantly upon annealing, except that Ag atoms diffuse into the whole Ni70Co30 layer and some parts of the Cu layer. The results suggest that addition of Ag during the deposition suppresses interfacial intermixing. X-ray diffuse scattering profiles show that the interfacial lateral correlation length of the Ag-doped multilayer is longer than that of the undoped multilayer and does not change significantly ...

Dependence of giant magnetoresistance on the thickness of magnetic and non-magnetic layers in spin-valve sandwiches

Abstract We have performed the calculations of giant magnetoresistance (GMR) versus the thickness of ferromagnetic and/or non-magnetic layers in NiFe/Cu/NiFe and/or Co/Cu/Co sandwiched structures in both cases: (1) interfacial spin-dependent scattering only; (2) the presence of both interfacial and bulk spin-dependent scattering. The results show that, in the presence of both interfacial and bulk spin-dependent scattering, the dependence of the calculated GMR on ferromagnetic and/or non-magnetic thickness is consistent with the experimental results. The comparison of the calculated results in both cases demonstrates the importance of bulk spin-dependent scattering in Co/Cu and/or NiFe/Cu systems. Furthermore, the calculations indicate that the ferromagnetic thickness corresponding to the GMR maximum increases with the increase of the thickness of non-magnetic layer in NiFe/Cu/NiFe and/or Co/Cu/Co sandwiches.

Effect of inserting a ferromagnetic layer on the giant magnetoresistance of spin-valve sandwiches

By extending a previous semi-classical model, we investigate the effects of inserting a second ferromagnetic material at the interface or in the interior of the ferromagnetic layer in NiFe/Cu/NiFe and/or Co/Cu/Co sandwiched structures on their giant magnetoresistance (GMR). The calculated GMR was found to be consistent with experimental results, indicating that our model is applicable even for complicated spin-valve multilayers. Moreover, some theoretical predictions are given in this letter that allow us to propose ways to optimize spin-valve structures.

Effect of annealing on the microstructure of Ni80Fe20/Cu multilayers

[Ni80Fe20/Cu](15) multilayers grown by DC-magnetron sputtering and annealed at different temperatures and/or times were investigated by low- and high-angle X-ray diffraction. Structural parameters such as superlattice period, interplane distance, average multilayer coherence length and interfacial roughness were obtained. It was found that as the annealing temperature increases the superlattice period, interplane distance, average multilayer coherence length decrease, while (1 1 1) preferred orientation of the superlattices was improved slightly. The interfacial roughness increases with increasing annealing temperature and/or time. A significant intermixing layer located in the interlayer region between the Ni80Fe20 and Cu layers was revealed by simulating the high-angle X-ray diffraction profiles. The thickness of the intermixing layer increases as the annealing temperature or annealing time increases

Characterization of InxGa1−xAs/InP epilayers by X-ray double crystal rocking curve peak profile analysis

Abstract Katos statistical theory of X-ray dynamical diffraction was generalized to the case of diffraction by epitaxial layers and was applied to analyze the X-ray double crystal rocking curves of In x Ga 1−x As InP single epilayers in this paper. It is shown that the statistical theory provides us with quantitative information about the perfectness as well as thicknesses and compositions of the epilayer. Moreover, some confusing experimental values can be interpreted by taking into account the effect of diffuse scattering. The statistical theory can help us to obtain much more information which is significant for improving the quality of the epilayer through optimizing the growth conditions.

The interfacial structure and degradation mechanism of the GMR effect in Co90 Fe10/Cu and Ni70Co30/Cu magnetic multilayers

We investigated the microstructures of Co90Fe10/Cu and Ni70Co30/Cu multilayers before and after annealing at 285 °C for 2 h, as well as the influence of microstructures on the thermal degradation of the GMR (giant magnetoresistance) effect, using innovative x-ray anomalous scattering techniques. It is found that there exists a significant asymmetry between the interfaces on both sides of the Cu layers during the deposition. The diffusion only occurs at the Cu/Ni70Co30 and Cu/Co90Fe10 interfaces while the Ni70Co30/Cu and Co90Fe10/Cu interfaces are sharp. After annealing at 285 °C, the Co90Fe10/Cu interface is still sharp, but the Ni70Co30/Cu interface forms a CuNi2Co intermixing region. X-ray diffuse scattering profiles performed with x-ray energy close to the Fe, Co, Ni and Cu K edges show that the local lateral environments of Fe, Co, Ni and Cu at the interfaces are different for both the as-deposited multilayers. After annealing, the interfacial lateral correlation lengths and the fractal exponent of the Ni70Co30/Cu multilayer decrease significantly and the difference of the interfacial lateral correlation lengths of Co, Ni and Cu becomes smaller. However, those of the Co90Fe10/Cu multilayer do not change. This indicates that the main cause of thermal degradation of the GMR effect is the compositional intermixing at the multilayer interface, which has more influence on the GMR effect than the atom diffusion along the grain boundary.

Study of Ni80Fe20/Fe50Mn50 superlattice microstructures by transmission electron microscopy and x-ray diffraction

The detailed microstructures of Ni80Fe20/Fe50Mn50 superlattices have been characterized using both x-ray diffraction techniques and transmission electron microscopy. The obvious layered structure, typical column structure and twins which exist in Ni80Fe20/Fe50Mn50 superlattices were observed through performing transmission microscopy. By combining the technique of low-angle x-ray reflectivity (specular and off-specular scans) with the anomalous scattering effect and high-angle x-ray diffraction (using conventional x-ray), we quantitatively analysed the microstructural variation as a function of annealing temperature. It is found that the lateral correlation length, the (111) peak intensity of the superlattices and the average multilayer coherence length all increase with a rise in annealing temperature. The correlated roughness slightly decreases after annealing, while moderate annealing can decrease the root-mean-square roughness at the interfaces of Ni80Fe20/Fe50Mn50 superlattices. The obtained microstructural knowledge will be helpful in understanding the magnetic properties of the Ni80Fe20/Fe50Mn50 exchange bias system.