Strain relief of heteroepitaxial bcc-Fe(001) films

The strain relief of heteroepitaxial bcc-Fe(001) films, deposited at 520-570 K onto MgO(001), has been investigated by scanning tunneling microscopy. In accordance with real-time stress measurements, the tensile misfit strain is relieved during coalescence of flat, mainly 2-3 monolayers (ML) high Fe islands at the high thickness of approximately 20 ML. To accommodate the misfit between merging strain-relaxed islands, a network of 1/2[111] screw dislocations is formed. A strong barrier for dislocation glide--which is typical for bcc metals--is most likely responsible for the big delay in strain relief of Fe/MgO(001), since only the elastic energy of the uppermost layer(s) is available for the formation of an energy-costly intermediate layer.     

NiO-thickness dependent magnetic anisotropies in Fe/NiO/Au(001) and Fe/NiO/MgO(001) systems

The in-plane magnetic anisotropy of Fe/NiO bilayers was studied quantitatively as a function of NiO thickness using the magneto-optical Kerr effect with a rotating field. For NiO thicker than the ordering transition thickness, the total in-plane fourfold anisotropy of the Fe layer decreases with NiO thickness in Fe/NiO/Au(001), but increases in Fe/NiO/MgO(001). Our result indicates that the exchange coupling in an Fe/NiO bilayer might induce an additional in-plane fourfold anisotropy, and the opposite thickness dependent behaviors may be attributed to the different Ni²⁺ antiferromagnetic spin orientations for NiO films grown on Au(001) and MgO(001) surfaces.     

Ferromagnetism of MnAs studied by heteroepitaxial films on GaAs(001)

Thin epitaxial films of MnAs--promising candidates for the spin injection into semiconductors--are well known to undergo simultaneously a first-order structural and magnetic phase transition at 10-40 degrees C. The evolution of stress and magnetization of MnAs/GaAs(001), both measured quantitatively with our cantilever beam magnetometer at the coexistence region of alpha-MnAs and beta-MnAs, reveal an orthorhombically distorted unit cell of the ferromagnetic phase, which provides important clues on the origin of ferromagnetism in MnAs.     

Oscillatory behavior of the resistance of three-layer heteroepitaxial Mo/Nb/Mo (001) films

Electrical resistance of a three-layer heteroepitaxial Mo(35 nm)/Nb(d _Nb)/Mo(35 nm) (001) film was found to exhibit oscillatory behavior as a function of thickness d _Nb of the ultrathin Nb interlayer. The oscillation period is equal to one Nb monolayer (0.16 nm). The resistance minimum of such a film corresponds to the interlayer thickness with a half-completed monolayer and is equal to the resistance of an individual Mo film of thickness 70 nm equivalent to the total thickness of the three-layer film. In this case, the three-layer film conducts as if it is a unified layer. The resistance of a three-layer film with completed Nb monolayers is maximum and equal to the resistance of two individual parallel-connected Mo films of half thickness (35 nm) each. These results are explained by the strong influence of the built-in potential, whose magnitude and scattering properties are periodic functions of the interlayer thickness.     

Ultrafast transport of laser-excited spin-polarized carriers in Au/Fe/MgO(001)

Hot carrier-induced spin dynamics is analyzed in epitaxial Au/Fe/MgO(001) by a time domain approach. We excite a spin current pulse in Fe by 35 fs laser pulses. The transient spin polarization, which is probed at the Au surface by optical second harmonic generation, changes its sign after a few hundred femtoseconds. This is explained by a competition of ballistic and diffusive propagation considering energy-dependent hot carrier relaxation rates. In addition, we observe the decay of the spin polarization within 1 ps, which is associated with the hot carrier spin relaxation time in Au.     

Solid-state synthesis in Ni/Fe/MgO(001) epitaxial thin films

Solid-state synthesis in Ni/Fe/MgO(001) bilayer epitaxial thin films has been studied experimentally. The phase sequence Fe/Ni→(～350°C)Ni₃Fe→(～400°C)NiFe→(～ 550°C)γ_par is formed as the annealing temperature increases. The crystal structure in the invar region consists of epitaxially intergrown single-crystal blocks consisting of the paramagnetic γ_par and ferromagnetic NiFe phases, which satisfy the orientation relationship [100](001)NiFe ∥ [100](001) γ_par. It has been shown that the nucleation temperatures of the Ni₃Fe, NiFe, and γ_par phases coincide with the temperatures of solid-state transformations in the Ni-Fe system.     

Scanning tunneling microscopy investigation of CoO/Fe(001) and Fe/CoO/Fe(001) layered structures

We report on the nanometer scale morphology of CoO thin films grown on top of Fe(001) substrates from the early stages of interface formation (few atomic layers), and on the surface topography of Fe/CoO/Fe(001) layered structures. The growth of the CoO films is dominated by formation of islands up to about 5 nominal atomic layers, then it proceeds in the layer-plus-island regime. The surface topography of thin Fe films grown on top of the CoO/Fe systems is strongly influenced by the morphology of the latter. Moreover, we observe a strong relationship between the growth mode and the chemical interactions at the CoO/Fe interface, since thick layers of iron oxides develop only below the CoO islands, as an effect of the proximity between Fe and Co atoms. We finally discuss possible implications of our observations on the magnetic properties of these layered magnetic structures.     

Development of magnetic anisotropies in ultrathin epitaxial films of Fe(001) and Ni(001)

Ultrathin films, bcc Fe(001) on Ag(001), fcc Fe(001) on Cu(001) and Fe/Ni(001) bilayers on Ag, were grown by molecular beam epitaxy. A wide range of surface science tools were employed to establish the quality of epitaxial growth. Ferromagnetic resonance and Brillouin light scattering were used to extract the magnetic properties. Emphasis was placed on the study of magnetic anisotropies. Large uniaxial anisotropies with easy axis perpendicular to the film surface were observed in all ultrathin structures studied. These anisotropies were particularly strong in fcc Fe and bcc Fe films. In sufficiently thin samples the saturation magnetization was oriented perpendicularly to the film surface in the absence of an applied field. It has been demonstrated that in bcc Fe films the uniaxial perpendicular anisotropy originates at the film interfaces. In situ measurements indentified the strength of the uniaxial perpendicular anisotropy constant at the Fe/vacuum, Fe/Ag and Fe/Au interfaces asK _us = 0.96, 0.63, and 0.3 ergs/cm² respectively. The surface anisotropies deduced for [bulk Fe/noble metal] interfaces are in good agreement with the values obtained from ultrathin films. Hence the perpendicular surface ansiotropies originate in the broken symmetry at abrupt interfaces. An observed decrease in the cubic anisotropy in bcc Fe ultrathin films has been explained by the presence of a weak 4th order in-plane surface anisotropy,K _1S=0.012 ergs/cm². Fe/Ni bilayers were also investigated. Ni grew in the pure bcc structure for the first 3–6 ML and then transformed to a new structure which exhibited unique magnetic properties. Transformed ultrathin bilayers possessed large inplane 4th order anisotropies far surpassing those observed in bulk Fe and Ni. The large 4th order anisotropies originate in crystallographic defects formed during the Ni lattice transformation.     

Characterization of defect type and dislocation density in double oxide heteroepitaxial CeO2/YSZ/Si(001) films

In order to qualitatively and quantitatively analyze the structural defects including the defect types and their concentrations in oxide heteroepitaxial films, a new X-ray rocking-curve width-fitting method was used in the case of doubleCeO₂/YSZ/Si (YSZ=yttria-stabilized ZrO₂) films that were prepared by pulsed laser deposition. Two main defect types, angular rotation and oriented curvature, were found in both CeO₂ and YSZ. Dislocation densities of CeO₂ and YSZ, which were obtained from the angular rotations, are functions of the YSZ thickness. A distinct two-step correlation between dislocation densities of CeO₂ and YSZ was found that as the dislocation density of YSZ is higher than 2.4×10¹¹ cm^-2, the dislocation density of CeO₂ shows a high sensitivity with that of YSZ compared with the low relativity in lower dislocation density (<2.4×10¹¹ cm^-2). In addition, YSZ always has higher dislocation densities and oriented curvatures than CeO₂ in each specimen, which can be attributed to the smaller mosaic domain sizes in YSZ than in CeO₂ as observed by high-resolution transmission electron microscopy. Received: 12 August 2002 / Accepted: 14 August 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +81-3/5734-3369, E-mail: chun_hua_chen@hotmail.com     

Subsurface As-layer formation in Au films deposited on GaAs(001)

Au deposited at room temperature on MBE grown GaAs (001) has been studied by UPS, XPS and AES. At Au coverages below ～2Å the adsorption interaction spreads the Au (6s) states density over the width of the GaAs valence band, while at higher coverages the Au overlayer has metallic properties. For the thick Au overlayers a subsurface As-rich region is established. It is suggested that this has a stabilizing influence on the continuous Au overlayer.     

An angle-scanned photoelectron diffraction (XPD) study of the growth and structure of ultrathin Fe films on Au(001)

We report a study of the growth and structure of Fe films on Au(001) at room temperature using angle-resolved photoelectron spectroscopy (ARXPS, AlK) and polar-scan photoelectron diffraction (XPD, AlK), exploiting the forward scattering (FS) enhancement of photoelectrons along atomic chains. The structure of the Fe 3p and 2p XPD polar diagrams and the development of the FS features with film growth evidence that Fe grows pseudomorphically in a nearly perfect layer-by-layer mode with bcc (001) structure rotated by 45° about the surface normal. At least up to 4 and probably up to 6 monolayers Fe, a segregated Au monolayer (surfactant layer) exists on top of the Fe film. This follows from the comparison of a simple model for the development of the substrate and film FS enhancements with the experimental data. By using angular shifts of the Fe 3p and Fe 2p bcc-[111] and bcc-[101] FS peaks we could determine the Au(on top)---Fe and Fe---Fe interlayer distances for 1 and 2 ML thick films to be 1.71(0.04) Å and 1.48(0.08) Å, respectively, showing that very thin films have a slightly expanded bcc structure (bct). The regular bcc angle positions are observed above 4–6 ML.     

Au/Cu(001)异质外延岛演化的分子动力学研究

利用分子动力学弛豫方法模拟了Au/Cu(001)异质外延生长初期Au异质外延岛的形貌演化，分析了Au外延岛演化过程中的局域应力及与基体结合能随表面岛尺寸的变化. 研究结果表明：当异质外延岛小于7×7时，外延岛原子分布呈现赝Cu点阵形貌；当外延岛达到8×8后，外延岛内开始出现失配位错，失配位错数量随外延岛尺寸的增加而增加. 局域压力分析指出，外延岛上原子之间的近邻环境不同导致了所受应力的差异，而外延岛的形变则是由外延岛原子的应力分布所决定. 研究还发现，失配位错的产生导致错位原子与基体原子之间的结合强度减弱，但相对增加了非错位原子与基体原子之间的结合强度. 关键词： 异质外延 表面形貌 局域压力 分子动力学模拟