Ag Buffer Layer Research Articles

Molecular Beam Epitaxial Growth and Structures of Al/Ag Superlattices

We studied epitaxial growth and structures of Al/Ag superlattices prepared using molecular beam epitaxy. Samples were grown at temperatures of 20, 50, 70 and 100°C, at a growth rate of 0.1 monolayers per second, on 50-nm-thick (100) Ag buffer layers preformed on (100) MgO substrates. In situ reflection high-energy electron diffraction (RHEED) during growth and ex situ X-ray diffraction were performed to analyze the growth behavior and the structures. In all samples, (100) epitaxial growth was confirmed at early stages. Moreover, RHEED intensity oscillations were observed when Al layers were grown on Ag buffer layers. However, (111)-grown domains subsequently appeared and gradually became dominant upon prolonged growth. After sixty Al/Ag bilayer depositions, all surface areas were occupied by (111)-grown domains. Persistence of (100) epitaxial growth was strongly temperature dependent; (100) growth persisted during a few tens of bilayer depositions at 50°C, while at 20 and 70°C the growth orientation changed to (111) much faster. Other features, including the structures, are also described.

Effect of Ag buffer layer to ultrathin Co films on Si[111] surface

The magnetic properties of ultrathin Co/Ag/Si[111] films were studied by surface magneto-optic Kerr effect. A magnetic phase diagram for binary-metal Co/Ag films deposited on Si[111] surfaces has been successfully established. The phase diagram can be divided into four regions: nonferromagnetic Co-Si compound; low Curie temperature Co film; canted out-of-plane anisotropy; in-plane anisotropy. The ferromagnetic inactive layers at the Co/Si interface are formed due to intermixing of Co and Si; and can be efficiently reduced by the Ag buffer layer. We have demonstrated that the inactive layer thickness is reduced from 2.1 ML for Co/Si[111], to 1.0 for Co/2.8 MLAg/Si[111], and 0.0 for Co/5.6 MLAg/Si[111].

In Situ Studies of the Vibrational and Electronic Properties of Si Nanoparticles

AbstractWe report on in situ studies of the vibrational properties of ultra-thin Si layers grown by dc magnetron sputtering in ultrahigh vacuum on amorphous MgO and Ag buffer layers. The average thickness of the Si layers ranged from monolayer coverage up to 200 Å. The interference enhanced Raman scattering technique has been used to study changes in the phonon spectra of Si nanoparticles during the crystallization process. Marked size-dependencies in the phonon density of states of the Si quantum dots and the relaxation of the k-vector conservation condition with decrease in size of the Si nanoparticles have been detected. Electron energy loss spectra have been collected for amorphous and crystallized Si nanoparticles on SiO2 buffer layers and the difference in the onset of the electronic transitions have been found.

プラズマ溶射法で作製したＢｉ２２２３酸化物円筒の超伝導磁気シールド特性

We fabricated a large Bi2223 hollow cylinder as a superconducting magnetic shield through plasma spraying in order to obtain a very low and calm magnetic-field environment where we can measure the weak pulsed magnetic fields caused by human brain activity. As a substrate, we used a pure-nickel hollow cylinder (323.2mm in O. D., 320mm in I. D. and 660mm in length), the outside of which was coated with a Ag buffer layer, as a diffusion barrier, and then a Bi-system oxide layer about 730μm thick, applied through plasma spraying. However the as-sprayed Bi-system oxide layer showed unclear broad X-ray diffraction patterns of amorphous-like crystal structure and no superconducting properties at 4.2K. Therefore, the sprayed Bi-system oxide hollow cylinder was heat treated at 838°C×100h to form a superconducting phase (Bi2223 phase) through solid-state diffusion reaction. The superconducting Bi2223 hollow cylinder showed a shielding factor of 103 at Z/D=0.98 at 79K in a dc field of 4×10-4T, where Z and D are the distance from the edge of the cylinder to the measuring position at the center line of the cylinder, and the inner diameter of the Bi2223 cylinder, respectively. The same shielding factor, of about 103, was obtained in ac fields of 3×10-5T with frequencies below 10Hz.

Ag films on [formula omitted]: from clean surfaces to atomic Ga structures

We prepare 150 nm thick epitaxial Ag buffer layers on Fe-precovered GaAs(001) at 380 K. Post-annealing at 570 K improves the morphology of the films, as observed by scanning tunneling microscopy (STM), without changing the chemical composition at the surface. The main type of defects are screw dislocations. Nevertheless we achieve a mean terrace width of 36 nm. At temperatures above 620 K, Ga atoms diffuse through the Ag buffer layer to the surface and form an overlayer. Depth profiles and quantitative analysis of X-ray photoemission data yield surface coverages of up to 0.3 ML and Ga concentrations within the Ag bulk of a few at%. The GaAs substrate acts as a source of Ga atoms even at temperatures much lower than the GaAs vacuum decomposition temperature of 890 K. STM images show that in some areas the overlayer consists of Ga tetramers. Their size and orientation are determined by the separation vector between next-nearest neighbour atoms of the Ag(001) substrate. In other areas the tetramers line up to form parallel rows of Ga pairs. This row structure is reflected in the low-energy electron diffraction (LEED) patterns as a (4√2 × √2)R45° superstructure. Based on STM and LEED data we propose a structure model with two Ga atoms per surface unit cell. Finally we predict a long-range interaction between rows and a relaxation of the GaGa separation within the rows.

The effect of Ag diffusion on properties of BiPbSrCaCuO thin films

Superconducting BiPbSrCaCuO films have been evaporated on Ag/MgO and MgO substrates (with and without Ag buffer layer) by the electron-beam technique. The influence of annealing temperature on the crystalline structure and superconducting properties of BiPbSrCaCuO films was investigated by X-ray diffraction, X-ray fluorescence spectroscopy, scanning electron microscopy, critical temperature, critical current density and room temperature resistivity measurements. It was shown that the annealing of both types of films at 835°C resulted in formation of the mixed Bi-2223 and Bi-2212 phases with a high degree of preferential orientation with the c-axis perpendicular to the substrates. Annealing of films on Ag/MgO substrates is additionally accompanied by Ag diffusion from the buffer layer into BiPbSrCaCuO films. The higher rate of crystallization of the Bi-2223 and Bi-2212 phases, the higher degree of c-axis orientation, the higher dense surface morphology, the reduced lattice parameter c (by 0.6–0.8%) the reduced room temperature resistivity (2–3 times), the significantly enhanced critical temperature ( T c=106 K at R=0) and the critical current density ( J c = 1.1 × 10 4 A/cm 2 at 77 K) were observed for the Ag-doped films (on Ag/MgO substrates), in comparison with those for the undoped films (on MgO substrates). The temperature dependence of the Ag diffusion coefficient in Bi-2212 films in the range 650–800°C was described by the relation D=2.2×10 −5 exp(−1.2/ kT).

Magnetic properties of exchange-coupled Fe/FeO bilayers

The magnetic, structural and microstructural properties of sputtered Fe thin films and Fe\Fe–O bilayers were studied as a function of the Fe layer thickness, the type of the Fe oxide and the substrate used. Two different ways to prepare the oxide layers were used; postdeposition oxidation and reactive sputtering. Postdeposition oxidation produced films with mixed Fe–oxides (FeO, Fe3O4, Fe2O3); however reactive sputtering led to bilayers with controlled stoichiometry, Fe\FeO, Fe\Fe3O4, and Fe\FE2O3, respectively. The coercivity of both the Fe films and fE\Fe–O bilayers, deposited on substrates with or without Cr buffer layer, was found to increase with decreasing Fe film thickness. The coercivity of the samples deposited on a Ag buffer layer was much lower and did not change substantially with the Fe film thickness. The presence of the Fe–oxide layer led to a large increase of coercivity. This is attributed to the higher anisotropy of the oxide and to exchange coupling of Fe–oxide with the softer Fe layer.

結晶方位の異なるＣｏ薄膜における磁気光学効果

Measurements of polar magneto-optical (MO) Kerr spectra of hcp-CO films with various orientations fabricated by the molecular beam epitaxy (MBE) method were carried out. Substrate used here was MgO(100) and (111) surfaces with Au and Ag buffer layers. The CO films were covered with 20A Au cap layer to prevent from oxidation. The MO Kerr spectra were different to each other between the (1120) and (0001) planes in the low-energy region, below 4.0 eV. This suggests that the electronic states are modified by the magnetization directions through a spin-orbit coupling.

Magnetic anisotropy and magnetization reversal in Fe films grown on GaAs (001) and (110)

We present the results of a systematic study of the evolution of magnetic anisotropies of epitaxial Fe films grown on GaAs (001) and (110) substrates and also on epitaxial Ag buffer layers. The anisotropy was studied in-situ with the magneto-optic Kerr effect (MOKE). This provides a qualitative thickness dependence of the in-plane magnetic anisotropy for single films on the same substrate surface, thus avoiding any uncertainties due to different substrate morphologies. These in-situ studies are complemented by ex-situ MOKE, Brillouin Light Scattering (BLS), and Lorentz microscopy measurements which reveal the relation between the magnetization reversal processes and magnetic anisotropy behaviour on a macroscopic and microscopic scale.

Effects of Metal Buffer Layer on the Epitaxial Growth of YBa2Cu3O7-y. Superconducting Films by Dipping-Pyrolysis Process

AbstractSuperconducting Yba2Cu3O7 -x(YBCO) films were prepared by dipping-pyrolysis process on SrTiO3 (001) or on MgO (001) substrates covered with a metal (Au or Ag) buffer layer, and the effects of these metal layers on the growth of YBCO films were investigated. Epitaxial growth of YBCO was found to be suppressed by the Au buffer layer, which crystallized to exhibit (111) orientation, after heat treatment at 700°C and higher. On the other hand, epitaxial YBCO films were obtained starting with Ag buffer layer by heat treatment at 700°C and higher, after which most of Ag was accumulated to particles. It was also found that the annealing temperature required for the epitaxial growth of YBCO films on MgO was lowered to 850°C by adopting the Ag buffer layer.

Molecular-beam-epitaxial growth and magnetic properties of (111)Pt/Co/Ag, Pt/Co, and Ag/Co/Pt superlattices

The structural and the magnetic properties of (111)Pt/Co/Ag, Pt/Co, and Ag/Co/Pt superlattices (SLs) were studied. The samples were grown at 373 K on Ag buffer layers preformed on 7×7-(111)Si substrates by molecular-beam epitaxy. Reflection high-energy electron diffraction indicated that in-plane lattice constants of Co, Pt, and Ag layers changed continuously during layer growth in Pt/Co/Ag SLs with thin Co layers and Pt/Co SLs. The effective magnetic anisotropy energy Keff of a Pt/Co/Ag SL was similar to K̄eff, an average of Keff values of a Pt/Co and an Ag/Co SL with the same Co layer thicknesses. This suggests that the origins of perpendicular magnetic anisotropy in the Pt/Co system are different from those in the Au/Co system investigated earlier, in which the difference between the Au/Co and Au/Co/Ag SLs is small in Keff value.

Magnetic and magneto-optic properties of sputtered Co/Ni multilayers

We have investigated the magnetic and magneto-optic properties of Co/Ni multilayers deposited on Ag and Au buffer layers. The samples with Au buffer layers show perpendicular magnetic anisotropy, but those with Ag buffer layers do not. The structure and degree of crystalline alignment of the buffer layer are evidently crucial to development of perpendicular magnetic anisotropy. We also present the results of polar Kerr rotation measurements as a function of wavelength and layer thickness of the multilayers.

Deposition and properties of NiFe2O4 thin films (abstract)

We have investigated the properties of NiFe2O4 thin films prepared by laser ablation of a stoichiometric NiFe2O4 target. Textured polycrystalline films were obtained on a-SiO2 as well as on various substrates with Au, Ag, Pt, and MgF2 buffer layers. Epitaxially oriented films were obtained on MgO, (11̄02)-oriented Al2O3, (112̄0)-oriented Al2O3, Y-stabilized ZrO2 (YSZ), and SrTiO3, although the crystalline quality of the films varied. Contamination by diffusion from the substrate and strains induced by both lattice constant mismatch and differential thermal expansion degraded the magnetic properties of the films, and in some cases decreased the electrical resistivity as well. By choosing the right substrate (YSZ), temperature (600 °C), and PO2 (0.01 mT), we are able to prepare epitaxial films with bulk saturation magnetization (Ms=270 G) and fairly low anisotropy (K∼105 erg/cm3) as inferred by torque magnetometry. These films and bilayers are expected to be useful in a variety of fundamental investigations as well as having the potential for technological applications.

Magnetoresistance and Preferred Orientation in Fe–Mn/Ni–Fe/Cu/Ni–Fe Sandwiches with Various Buffer Layer Materials

Magnetoresistance effects and film structures have been investigated in [Fe–Mn/Ni–Fe/Cu/Ni–Fe/buffer layer/Si] sandwiches with various buffer layer materials. The sandwich with Nb, Ta, Ti, Zr or Hf buffer layer has strong (111) texture. Cu, Ag, Au and Cr buffer layers do not cause strong (111) texture. When the sandwich has strong (111) texture, the Ni–Fe layer neighboring the Fe–Mn antiferromagnetic layer is strongly exchange-biased by the Fe–Mn layer, and thus the sandwich shows a high magnetoresistance ratio.

X-Ray Diffraction Measurements of Lattice Strains in Co/Pd(001) Superlattice Films

Lattice spacings in fcc-Co/Pd(001) superlattice films grown on GaAs(001) substrates with a Ag(001) buffer layer were measured using a four-circle diffractometer. For samples with Co layers thinner than 9 Å and Pd layers thinner than 12 Å, the observed in-plane (110) lattice spacings were all 2.71±0.01 Å, indicating 8.0% lattice expansion of Co layers and 1.5% lattice contraction of Pd layers. The (002) lattice spacings along the growth direction were estimated from a profile fitting method, and they were 1.60±0.02 Å for Co layers and 1.98±0.02 Å for Pd layers. This means that Co layers are contracted nearly 10%, while Pd layers are expanded 2% along the growth direction of the superlattice films. The estimated large lattice strains are successfully applied to explain the large magnetovolume contribution to the magnetic anisotropy reported previously.

Perpendicular magnetic anisotropy and structures of epitaxial Co/Ag and Co/Au metallic superlattices

The magnetic anisotropy and structures of Co/Ag and Co/Au metallic superlattices (SLs) grown by molecular-beam epitaxy were studied. In the Co/Ag SLs, the total magnetic anisotropy energy Keff was larger for the flatter Co/Ag interfaces and for the smaller strains of the Ag buffer layers. Particularly at 5 K, the perpendicular magnetic anisotropy was observed in the Co/Ag SLs with the nominal Co layer thickness t being 1 monolayer, and was strongly temperature dependent. The dependence of Keff⋅t on t was linear both at 5 and 300 K. The difference of the magnetic anisotropy in the Co/Ag and the Co/Au SLs was discussed in terms of structural characteristics. The origin of the perpendicular magnetic anisotropy was suggested to be the Néel-type surface anisotropy in the Co/Ag SLs and to be mainly the magnetoelastic anisotropy in the Co/Au SLs.

DC-sputtered high tc superconducting YBCO thin films on silicon substrates

We have prepared thin films of YBCO on Si (100) and on Si with Ag-buffer layer by DC-magnetron sputtering. Post-deposition annealing at 1173K was necessary in order to obtain superconducting films. The films on silicon have no superconducting transition down to 50k. The films on silicon with Ag-buffer layer have a broad superconducting transition with Tonset=95K and Tco=78K. The X-ray diffraction studies show that films are oriented with c-axis perpendicular to the substrate. The critical current density for these films is aboat 10, 000 A/sq.cm at 77K.

Structures of Co/Ag metallic superlattices grown by molecular beam epitaxy

Co/Ag metallic superlattices were epitaxially grown onto Ag buffer layers pre-deposited on (0001)sapphire or (111)Si substrates by molecular beam epitaxy. In-situ time-resolved reflection high-energy electron diffraction observations during superlattice growth revealed the structures of Ag and Co layers. Ag layers had an FCC structure with growth direction 〈111〉 FCC. On the other hand, the structure of Co layers on Ag layers was initially close-packed with random stacking normal to the close-packed plane and after about 7 monolayer growth it was changed to FCC at temperatures below 323 K and to HCP at above 323 K. Interface roughness was deduced from the period of time when both patterns from Co and from Ag were being observed. Ex-situ X-ray diffraction (XRD) measurements and high-resolution transmission electron microscopy observations disclosed the atomic-level structures, and showed that the interfaces between Co layers and Ag layers were wavy at 273 K and the Co layers grown at 373 K discontinuous. A wave-model and an island-model were proposed to interpret the experimental XRD profiles. XRD with 4-circle diffractometer revealed that Ag layers on sapphire had a local lattice distortion, but that Ag on Si did not.

Microstructure and magnetic anisotropy of ultrathin Co/Pt multilayers grown on GaAs (1̄1̄1̄) by molecular-beam epitaxy

Multilayers of (Co3 Å, Pt15 Å)x, x=15 or 30 repeats, with or without a 200 Å silver buffer layer, were grown on GaAs (111) substrates by molecular-beam epitaxy. Vibrating sample magnetometry measurements confirmed that the samples with the Ag buffer layer show strong uniaxial magnetic anisotropy perpendicular to the surface. The perpendicular anisotropy exhibited by these metallic superlattices is discussed in terms of the microstructure of the overall multilayer stack, as well as the structural characteristics of the Co interface layer. Samples grown on the Ag buffer layer show strong (111) texture with 30–40-nm-size twin-related grains. These grains, correspond to the two possible (111) stacking sequence for an fcc lattice, i.e., double positioning. However, direct growth on GaAs (111) results in randomly oriented 10–20 nm grains. All samples exhibit a repeat period of 1.83 nm in both low-angle reflectivity and high-angle Θ–2Θ x-ray scattering measurements. In addition, transverse scans through the low-angle multilayer Bragg peaks show the interfaces to be diffuse in nature indicative of considerable in-plane inhomogeneity and/or compound formation. High-resolution electron microscopy measurements of cross sections compared with image simulations confirm that the interface layer is diffuse and its stoichiometry is such that the Co occupation is less than 40%. Redistribution of Co should then extend over at least four monolayers. The nanostructure of the samples grown with the Ag buffer layer comprises an eight atomic layer repeat with the Co interface layer diffuse over four monolayers. The microstructure is strongly (111) textured with columns of twin related 30-nm-sized grains. It is suggested that the combination of interdiffusion, highly oriented but twin-related columnar growth, small grain size with a possible nanometer-scale second phase may be the key to the understanding of the perpendicular anisotropy observed in these (111) superlattices.

Metal buffer layers and Y-Ba-Cu-O thin films on Pt and stainless steel using pulsed laser deposition

A versatile pulsed laser deposition chamber was employed for fabricating metal buffer layers and superconducting YBaCuO thin films on metallic substrates. Ag buffer layers were found to improve the resistive transition behavior for superconducting films on Pt and stainless steel. As-deposited YBaCuO films with Tc (R=0) at 84 K were produced on stainless steel using in situ laser-deposited Ag buffer layers. The critical current density was measured to be approximately 103 A/cm2 at 67 K.