Q. J. Jia

Microstructures and resistivity of cuprate/manganite bilayer deposited on SrTiO3 substrate

Thin YBa2Cu3O7-δ/La0.67Ca0.33MnO3 (YBCO/LCMO) films were grown on SrTiO3(STO) substrates by magnetron sputtering technique. The microstructures of the bilayers were characterized and a standard four-probe technique was applied to measure the resistivity of the samples. The interdiffusions at the YBCO/LCMO and LCMO/STO interfaces formed two transient layers with the thickness of about 3 and 2 nm, respectively. All the bilayers were well textured along the c axis. At low temperature, the superconductivity can only be observed when the thickness of YBCO is more than 25 nm. When the thickness of YBCO is less than 8 nm, the bilayers show only ferromagnetism. The superconductivity and ferromagnetism perhaps coexist in the bilayer with the YBCO thickness of 12.5 nm. These interesting properties are related to the interaction between spin polarized electrons in the manganites and the cooper pairs in the cuprates.

Strain and magnetic anisotropy of as-grown and annealed Fe films on c(4x4) reconstructed GaAs (001) surface

Fe films with the different thicknesses were grown on c(4x4) reconstructed GaAs (001) surfaces at low temperature by molecular-beam epitaxy. Well-ordered bcc structural Fe epitaxial films are confirmed by x-ray diffraction patterns and high-resolution cross-sectional transmission electron microscopy images. A large lattice expansion perpendicular to the surface in Fe film is observed. In-plane uniaxial magnetic anisotropy is determined by the difference between magnetizing energy along [110] and [110] directions, and the constant of interfacial uniaxial magnetic anisotropy is calculated to be 1.02x10(-4) J m(-2). We also find that magnetic anisotropy is not obviously influenced after in situ annealing, but in-plane strain is completely changed.

Three-dimensional strain state and spacer thickness-dependent properties of epitaxial Pr0.7Sr0.3MnO3/La0.5Ca0.5MnO3/Pr0.7Sr0.3MnO3 trilayer structure

Epitaxial colossal magnetoresistive trilayer structures consisting of ferromagnetic metallic Pr0.7Sr0.3MnO3 (PSMO) and antiferromagnetic insulator La0.5Ca0.5MnO3 (LCMO) were fabricated on (001)-oriented single crystal MgO substrates using pulsed laser deposition technique. The evolution of three-dimensional strain states and electrical and magnetic transport properties of PSMO/LCMO/PSMO trilayers have been studied as a function of LCMO spacer thickness and lattice strain. When the thickness of LCMO spacer is 6 nm, lattice strain in the trilayer begins to be relaxed. Furthermore, trilayers with thickness of LCMO spacer up to 36 nm are not fully strain relaxation. The unit cell volume of the films is not conserved and exhibits the variation with LCMO layer thickness. Strain relaxation states are determined by bulk strain (eB) and Jahn–Teller (eJT) strain together. The electrical and magnetic transport properties, including metal-insulator transition temperatures TMI and saturation magnetization MS, also sho...

Anomalous strains in the cubic-phase GaN films grown on GaAs (001) by metalorganic chemical vapor deposition

Strains in cubic GaN films grown on GaAs (001) were measured by a triple-axis x-ray diffraction method. Residual strains in the as-grown epitaxial films were in compression, contrary to the predicted tensile strains caused by large lattice mismatch between epilayers and GaAs substrates (20%). It was also found that the relief of strains in the GaN films has a complicated dependence on the growth conditions. We interpreted this as the interaction between the lattice mismatch and thermal mismatch stresses. The fully relaxed lattice constants of cubic GaN are determined to be 4.5038 +/- 0.0009 Angstrom, which is in excellent agreement with the theoretical prediction of 4.503 Angstrom

Characterization of surface and interface structure of YBa2Cu3O7−δ-based trilayer with La0.67Ca0.33MnO3 spacer

Thin YBa2Cu3O7-delta/La0.67Ca0.33MnO3/YBa2Cu3O7-delta (YBCO/LCMO/YBCO) trilayers on (001)-oriented SrTiO3 (STO) substrates were fabricated by magnetron sputtering technique. The trilayers are well c-axis oriented confirmed by x-ray diffraction. The surface and interface structure of the trilayers were investigated by grazing incident x-ray reflectivity. The results showed that there exists diffusion at the YBCO/LCMO interfaces with the diffusion length of several nanometers. The root-mean-square (rms) roughness of surface and interface in the trilayers varied with the thickness of YBCO layer. The rms roughness of surface was consistent with observation by atomic force microscopy. A further analysis indicated that the rms roughness was correlative to the misfit strain relaxation in film

Effects of oxidation by O2 plasma on formation of Ni∕Au ohmic contact to p-GaN

Oxidation of Ni∕Au (5nm∕10nm) contact to p-GaN layer was performed by O2 plasma in a reactive ion etching system. The structural characteristics of the Ni∕Au p-GaN for different oxidation time were investigated by x-ray diffraction (XRD) measurements, using an intense synchrotron x-ray source. The XRD measurements indicated that the grains of nickel oxide polycrystalline in the contact were grown continually when the oxidation time increased in 10min. However, Au showed amorphouslike and the intensities of Bragg diffraction peaks were hardly changed when oxidation time increased to 10min. The nickel oxide formed by O2 plasma without sequent thermal annealing did not reduce the specific contact resistance (ρc) to p-GaN, but it took an important role in lowering ρc followed by thermal annealing in N2 at 500°C for 10min. Optical transmission spectra confirmed that the nickel was easy to be oxidized and few interdiffusions occurred at the metal interface in O2 plasma ambient. Finally, the mechanism of oxidati...

Exchange bias induced by the fully strained La2/3Ca1/3MnO3 dead layers

A pure compressively strained La2/3Ca1/3MnO3 (LCMO) dead layer grown on (001)-oriented LaAlO3 substrate can show all the rich phenomenon of large bias field shift, coercive field enhancement, and high blocking temperature. The obtained exchange bias field (∼350 Oe) and the enhanced coercivity of about 1160 Oe at 5 K under 500 Oe cooling field are superior to that have been reported in LCMO-based ferromagnetic/antiferromagnetic superlattices or nanoscale systems. Our results clearly demonstrate that the inhomogeneous magnetic dead layer of LCMO can induce a strong exchange bias effect, which may be exploited as a very simple structure for spin-valve device application.

Exchange bias induced by surface inhomogeneities in epitaxial Pr 0.7 Sr 0.3 MnO 3 / La 0.5 Ca 0.5 MnO 3 bilayer film

The epitaxial growth of bilayer film made of ferromagnetic metal material Pr0.7Sr0.3MnO3 and charge-ordering insulating material La0.5Ca0.5MnO3 on single crystalline SrTiO3 (001) substrate. High resolution X-ray diffraction and atomic force microscopy analyses show the single crystalline nature of the film. Further atomic force microscopy measurement shows the presence of non-stoichiometric large particulates at the film surface, imparting an overall inhomogeneous composition to the film. This implied that the variation of crystalline structure occurred due to inhomogeneous composition. Magnetic measurement results indicate that the inhomogeneous composition could cause inhomogeneity of magnetic phase which in turn reduce ferromagnetism and enhance exchange bias effect.

Effect of interlayer-induced asymmetrical stress on magnetotransport properties of epitaxial [Pr 0.7 Sr 0.3 MnO 3 /La 0.5 Ca 0.5 MnO 3 ] 20 superlattice

Perovskite manganite R_1-xA_xMnO₃ (R=rare earth, A=alkaline earth) has been attracting great research interesting due to its magnetoelectronic phenomena such as colossal magnetoresistance (CMR), charge-orbital ordering, and metal-insulator transitions, etc.[1,2] In manganite superlattice, interfacial effects including cation intermixing, charge transfer, exchange coupling, strain and defect formation are crucial in determining the magnetic and transport properties of superlattice. [3-5] In this work, all-manganite [Pr_0.7Sr_0.3MnO₃/La_0.5Ca_0.5MnO₃]₂₀ superlattices were epitaxially fabricated on (001)-oriented single crystal MgO substrates with 24 nm La_0.5Ca_0.5MnO₃ buffer layer by using plused laser deposition. All the as-grown superlattices are of single phase and single orientation (see Fig.1). We obtained various stress at interface by varying relative layer thickness. As the thickness of Pr_0.7Sr_0.3MnO₃ layer and La_0.5Ca_0.5MnO₃ layer is not identical, interlayer-induced asymmetrical stress will occur and result in lower metal-insulator (MI) transition temperature and larger magnetoresistance (MR) in a wider temperature range. This result indicated that [Pr_0.7Sr_0.3MnO₃/La_0.5Ca_0.5MnO₃]₂₀ has opportunities for practical application in magnetic sensor and magnetic memory material. Furthermore, the percolation model has been used to quantitatively understand the transport mechanism of superlattice (see Fig.2). Our results demonstrated that inter-layer-induced asymmetrical stress accompanied with phase separation and charge ordering could strongly influence the magnetotransport properties of superlattice.

Magnetostriction and microstructure of melt-spun Fe77Ga23 ribbons prepared with different wheel velocities

A set of stacked ribbons with the composition of Fe77Ga23 were prepared with different wheel velocities of 7 m/s, 12.5 m/s and 25 m/s (named as S-7, S-12.5 and S-25, respectively). High resolution X-ray diffraction patterns of these ribbons show that all the ribbons present the disordered A(2) structure, whereas an additional modified-DO3 phase is detected in S-12.5 and S-25. S-25 has stronger (100) texture than other two samples. Ga K-edge extended X-ray absorption fine structure results indicate that both bond distance and the number of Ga atoms in the second neighbor shell around Ga decrease with increasing wheel velocity. No Ga cluster is detected in the studied ribbons. A short-range ordering Ga-rich phase and large local strain have no obvious influence on magnetostriction of S-7. It is believed that both the (100) texture and the additional modified-DO3 phase play a positive role in magnetostrictive properties of Fe77Ga23 ribbons.