In situ magneto-optical Kerr effect study of uncovered Fe films on ZnSe(0 0 1)

The magnetic properties of uncovered Fe/ZnSe/GaAs(1 0 0) ultrathin films have been determined in situ by magneto-optical Kerr effect (MOKE). Fe films up to 10 monolayers (ML) thick were deposited on c(2×2) Zn-rich ZnSe/GaAs(0 0 1) surfaces at 180 °C. We have studied the thickness dependence of the in-plane lattice parameter of the Fe films and of the MOKE hysteresis loops in the longitudinal geometry, at 150 K, under magnetic fields up to 0.1 T applied along the [1 1 0] and [1-1 0] directions of the ZnSe(0 0 1). Reflection high energy electron diffraction show that in the low thickness regime the Fe films present an in-plane structural anisotropy characterized by an expansion along the [1 1 0] direction. Hysteretic loops were obtained only starting from ∼5 ML Fe. We found the onset of an uniaxial magnetic anisotropy with [1 1 0] magnetic easy axis at 7 ML Fe.     

Exchange coupling of Co/Cr(0 0 1) superlattices for in-plane and perpendicular anisotropy

We have investigated the exchange coupling of Co/Cr(0 0 1) superlattices by polar and longitudinal magneto-optical Kerr effect measurements, by varying both the Co and Cr film thicknesses. At a Co thickness of ≈10 Å a nearly perpendicular anisotropy is found with antiferromagnetic order in the range of 5–15 Å of Cr thickness. For these superlattices the magnetization curve starting from remanence to saturation is characterized by a surface spin-flip transition at low field, followed by domain wall nucleation and motion, and finally by a coherent spin rotation with increasing field. Antiferromagnetic coupling is also observed for superlattices with thicker Co layers and with in-plane magnetic anisotropy.     

Surface electromigration of In on vicinal Si(0 0 1)

Surface mass transport of In film on vicinal Si(0 0 1) has been systematically investigated by a scanning Auger electron microscopy (SAM), low energy electron diffraction (LEED) and atomic force microscopy (AFM). It was observed that the temperature dependence of the mass transport shows the critical phenomenon. Above a critical temperature T_c, surface electromigration of the In film toward the cathode side dominated the surface mass transport on the vicinal Si(0 0 1) surface. The LEED and AFM observations revealed that the In film surface on the vicinal Si(0 0 1) consists of 3×4 terraces and (3 1 0) facets. The area ratio of the facet to the terrace exhibited abrupt an increase at T_c. It is believed that the change of the mass transport is related to the abrupt change of the area ratio of the facet to the terrace. Both the critical temperature T_c and the spread due to the surface electromigration of the In film depended on the configuration of the DC current direction and the step edge.     

Preparation of solution-based YBCO films with BaSnO3 particles

The YBCO films with BaSnO₃ (BSO) particles were prepared on LAO (0 0 1) substrates by metal organic deposition using trifluoroacetates (TFA-MOD) via introducing SnCl₄ powders into the YBCO precursor solution. It was found that with the increase of the SnCl₄ contents, the slower decomposition and higher temperature for nucleation during the reaction were requested compared to that of pure YBCO film. The YBCO films with different contents of Sn with dense surface and well c-alignment were obtained under optimized heat treatment, and the BaSnO₃ phases were detected by XRD analysis. Litter effect of BSO particles on the T_c and J_c values of YBCO films was found. All YBCO films with BSO particles had T_c values over 90 K and J_c values over 1 MA/cm². A significant enhancement of J_c was observed for YBCO films with BSO particles compared to that of pure YBCO film by the field dependence of J_c values. The best property was obtained for YBCO film with 6 mol.% Sn at 77 K under magnetic field. The results showed that the J_c value of YBCO film with 6 mol.% Sn was enhanced by a factor of 2 in 2 T, and over a factor of 10 beyond 4 T compared to that of pure YBCO film.     

Magnetization reorientation in Ni (1 1 1) ultrathin films studied by low-temperature hysteresis measurements

Surface magneto-optical Kerr effect (SMOKE) magnetometry in the temperature range 10–300 K was exploited to investigate the magnetic properties of high-quality Cu/Ni/Cu/Si(1 1 1) epitaxial heterostructures with thickness of the Ni layer, d_Ni, between 10 and 60 Å. For a fixed temperature, the equilibrium direction of the magnetization is parallel or perpendicular to the film surface, depending on the Ni thickness, because of the competition among shape anisotropy, magnetoelastic anisotropy and interface anisotropy. No reorientation of the magnetization could be observed as a function of temperature, for any of the specimens analyzed, while a large variation of the loop squareness and coercivity was found. This last variation has been qualitatively explained using a theoretical model based on a Green's function technique, valid for a monodomain film with a coherent rotation of the magnetization.     

Chemical solution processing and properties of Sr2FeMoO6 thin films

Crack-free and oriented Sr₂FeMoO₆ (SFMO) thin film with double perovskite structure has been fabricated by the chemical solution deposition (CSD) method. A homogeneous and stable SFMO precursor solution was successfully prepared by controlling the reaction of starting metal-organic compounds in a mixture solvent of 1-propanol and 2-methoxyethanol. SFMO thin films with c-axis preferred orientation could successfully be synthesized on MgO (0 0 1) and SrTiO₃ (0 0 1) substrates by optimizing the several processing conditions. SFMO thin film prepared on SrTiO₃ (0 0 1) showed a magnetoresistance effect at a low magnetic field.     

Thickness dependence of Ic and Jc of LTG-SmBCO coated-conductor on IBAD-MgO tapes

We have reported SmBa₂Cu₃O_y (SmBCO) films on single crystalline substrates prepared by low-temperature growth (LTG) technique. The LTG-SmBCO films showed high critical current densities in magnetic fields compared with conventional SmBCO films prepared by pulsed laser deposition (PLD) method. In this study, to enhance critical current (I_c) in magnetic field, we fabricated thick LTG-SmBCO films on metal substrates with ion-beam assisted deposition (IBAD)-MgO buffer and estimated the I_c and J_c in magnetic fields.All the SmBCO films showed c-axis orientation and cube-on-cube in-plane texture. T_c of the LTG-SmBCO films were 93.1–93.4 K. J_c and I_c of a 0.5 μm-thick SmBCO film were 3.0 MA/cm² and 150 A/cm-width at 77 K in self-field, respectively. Those of a 2.0 μm-thick film were 1.6 MA/cm² and 284 A/cm-width respectively. Although I_c increased with the film thickness increasing up to 2 μm, the I_c tended to be saturated in 300 A/cm-width. From a cross sectional TEM image of the SmBCO film, we recognized a-axis oriented grains and 45° grains and Cu–O precipitates. Because these undesired grains form dead layers, I_c saturated above a certain thickness. We achieved that I_c in magnetic fields of the LTG-SmBCO films with a thickness of 2.0 μm were 88 A/cm-width at 1 T and 28 A/cm-width at 3 T.     

Strongly enhanced flux pining of GdBCO coated conductors with BaSnO3 nanorods by pulsed laser deposition

We report the effects of BSO addition on the crystallinity, texture, and the field dependency of critical current density (J_c) of GdBCO coated conductors (CCs) prepared by pulsed laser deposition (PLD). Undoped and BSO-doped GdBCO films showed only c-axis oriented growth, and the incorporated BSO nanorods exhibited epitaxial relationship with the GdBCO matrix. In comparison with undoped film, BSO-doped GdBCO film exhibited greatly enhanced J_c and higher pinning force densities in the entire field region of 0–5 T (H//c) at 77 and 65 K. The BSO-doped GdBCO film showed the maximum pinning force densities (F_p) of 6.5 GN/m³ (77 K, H//c) and 32.5 GN/m³ (65 K, H//c), ～2.8 times higher than those of the undoped sample. Cross-sectional TEM analyses exhibited nano-structured BSO nanorods roughly aligned along the c-axis of the GdBCO film, which are believed effective flux pinning centers responsible for strongly improved critical current densities in magnetic fields.     

Ellipsometric detection of GaAs(0 0 1) surface hydrogenation in H2 atmosphere

Optical properties of MBE-grown GaAs(0 0 1) surfaces have been studied by spectroscopic ellipsometry under dynamic conditions of ramp heating and cooling after desorption of passivating As-cap-layer with low pressure H₂ atmosphere (14 Torr) applied to the surface. The temperature dependence of GaAs pseudo-dielectric function with atomically smooth (0 0 1) surface carrying the fixed Ga-rich (4 × 2) reconstruction was obtained for the temperature range of 160–600 °C. It is shown ellipsometrically that GaAs(0 0 1) heating in the molecular hydrogen atmosphere results in the formation of hydrogenated layer on the surface.     

Improvement of stability and pulse energy in a diode-pumped dual-loss-modulated QML Nd:Lu0.2Y0.8VO4 laser with EO modulator and GaAs

The Q-switched and mode-locked (QML) performance in a diode-pumped Nd:Lu_0.2Y_0.8VO₄ laser with electro-optic (EO) modulator and GaAs saturaber absorber is investigated. In comparison with the solely passively QML laser with GaAs, the dual-loss-modulated QML laser with EO and GaAs can generate pulses with higher stability and shorter pulse width of Q-switched envelope, as well as higher pulse energy. At the repetition rate 1 kHz of EO, the pulse width of Q-switched pulse envelope has a compression of 89% and the pulse energy has an improvement of 24 times. The QML laser characteristics such as the pulse width, pulse peak power etc. have been measured for different small-signal transmittance (T₀) of GaAs, different reflectivity (R) of output coupler and modulation frequencies of the EO modulator (f_e). The highest peak power and the shortest pulse width of mode-locked pulses are obtained at f_e = 1 kHz, R = 90% and T₀ = 92.6%. By considering the influences of EO modulator, a developed rate equation model for the dual-loss-modulated QML laser with EO modulator and GaAs is proposed. The numerical solutions of the equations are in good agreement with the experimental results.     

Growth evolution of Bi2Sr2CaCu2O8+δ thin films deposited by infrared (1064 nm) pulsed laser deposition

Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) thin films were grown on MgO (1 0 0) using 1064 nm infrared pulsed laser deposition with post heat treatment. The material arrives at the substrate surface with characteristic spheroidal morphology. Smooth, homogeneous and highly c-axis oriented films were obtained after heat treatment. Using infrared laser as excitation, the stoichiometry of the target is preserved on the film. However, the films were apparently under-doped.     

Reduced crystallization time of YBCO in a fluorine-free MOD process using uv-lamp irradiation

We have studied the crystallization time dependence of the epitaxial YBCO films (t = 0.8 μm) grown on CeO₂-buffered SrTiO₃ substrates by fluorine-free metal–organic deposition using uv-lamp irradiation (uv-MOD). As increasing the time (T₀) for heat treatment at the reaction temperature (760 °C) from 0 to 90 min, J_c and the YBCO 0 0 l XRD intensity are steeply increased and reach their maximum values at T₀ = 10 min. This suggests that the heat treatment required for YBCO crystallization is significantly shortened in uv-MOD compared to conventional all-pyrolytic F-free MOD processes, which consume T₀ = 90–150 min for crystallizing 0.4–0.5-μm-thick films. Scanning electron microscope measurement revealed a drastic change in surface morphology between T₀ = 8 and 10 min, showing a good correspondence to the J_c and XRD data which suggest that the epitaxial growth reaches the film surface at the very early stage in the heat treatment.     

Intrinsic pinning properties of FeSe0.5Te0.5

The intrinsic pinning properties of FeSe_0.5Te_0.5, which is a superconductor with a critical temperature T_c of approximately 14 K, were studied through the analysis of magnetization curves obtained using an extended critical state model. For the magnetization measurements carried out with a superconducting quantum interference device (SQUID), external magnetic fields were applied parallel and perpendicular to the c-axis of the sample. The critical current density J_c under the perpendicular magnetic field of 1 T was estimated using the Kimishima model to be equal to approximately 1.6 × 10⁴, 8.8 × 10³, 4.1 × 10³, and 1.5 × 10³ A/cm² at 5, 7, 9, and 11 K, respectively. Furthermore, the temperature dependence of J_c was fitted to the exponential law of J_c(0) × exp(?αT/T_c) up to 9 K and the power law of J_c(0) × (1 ? T/T_c)ⁿ near T_c.     

Epitaxial alloyed films out of the bulk stability domain: Surface structure and composition of Ni3Al and NiAl films on a stepped Ni(1 1 1) surface

We have studied by Spot Profile Analysis Low Energy Electron Diffraction (SPA-LEED) and Auger Electron Spectroscopy (AES) Ni–Al alloyed layers formed by annealing, around 780 K, Al deposits on a stepped Ni(1 1 1) surface. The surface structure and composition of the thin epitaxial Ni₃Al and NiAl films, obtained respectively below and above a critical Al initial coverage θ_c, differ markedly from those of corresponding bulk alloys.The Ni₃Al ordered films form in a concentration range larger than the stability domain of the L1₂ Ni₃Al phase. The NiAl films present a marked distortion with respect to the lattice unit cell of the B2 NiAl phase, which slowly decreases when the film thickness increases.It also appears that the value of θ_c depends on the morphology of the Ni(1 1 1) substrate, increasing from θ_c = 4.5 ML for a flat surface to θ_c = 10 ML for a surface with a miscut of 0.4°. This could be directly related to the presence of steps, which favour Ni–Al interdiffusion.     

Use of film thickness and Cu additive to improve (0 0 1) texture in MgO/FePtCu(C) bilayers

A multilayer structure has been proposed that demonstrates improved (0 0 1) texture for FePt-based L1₀ perpendicular media. Achieving a strong perpendicular magnetic anisotropy requires aligning the L1₀ crystallographic c-axis along the film normal. The ordered L1₀ FePt structure is tetragonal with a c/a ratio close to 0.965. This makes discriminating between the three crystallographic variants ([1 0 0], [0 1 0], and the desired [0 0 1]) difficult. Alloying FePt with Cu to reduce the c/a ratio and using a multilayer approach to keep the magnetic layers thin results in a structure with an adjustable M_rt and a strong (0 0 1) texture (rocking curve widths around 2°). This is a remarkable improvement in texture from pure FePt multilayered films or monolithic FePt(X) films. The proposed [MgO(2 nm)/Fe_50−xPt₅₀Cu_x(5 nm)]_×n structure limits grain size in the vertical (perpendicular) direction albeit not in the plane of the film. Carbon can be added to the FePtCu layer to reduce the grain size with minimal degradation of the (0 0 1) orientation.     

Homoepitaxial growth of MOD-YBCO thick films on evaporated and MOD templates

We have prepared metal organic deposition (MOD)-YBCO thick films by repeating the coating-pyrolysis-crystallization procedure onto ～100-nm-thick evaporated and MOD templates. Surface morphology of the template was found to strongly affect the homoepitaxial growth of MOD-YBCO layers on the template; namely, the epitaxial growth of MOD-YBCO on the evaporated template was much easier than that on the MOD template. A 220-nm-thick epitaxial MOD-YBCO film was successfully prepared on the 100-nm-thick evaporated-YBCO template to obtain a 320-nm-thick YBCO film, which exhibited J_c = 2.44 MA/cm² and I_c = 78 A/cm. The I_c value has significantly increased from 37 A/cm for the evaporated-template.     

Influence of Zr and Ce doping on electromagnetic properties of (Gd,Y)–Ba–Cu–O superconducting tapes fabricated by metal organic chemical vapor deposition

(Gd,Y)Ba₂Cu₃O_x tapes have been fabricated by metal organic chemical vapor deposition (MOCVD) with Zr-doping levels of 0–15 mol.% and Ce doping levels of 0–10 mol.% in 0.4 μm thick films. The critical current density (J_c) of Zr-doped samples at 77 K, 1 T applied in the orientation of H 6 c is found to increase with Zr content and shows a maximum at 7.5% Zr doping. The 7.5% Zr-doped sample exhibits a critical current density (J_c) of 0.95 MA/cm² at H 6 c which is more than 70% higher than the J_c of the undoped sample. The peak in J_c at H 6 c is 83% of that at H 6 a–b in the 7.5% Zr-doped sample which is more than twice as that in the undoped sample. Superconducting transition temperature (T_c) values as high as about 89 K have been achieved in samples even with 15% Zr and 10% Ce. Ce-doped samples with and without Ba compensation are found to exhibit substantially different J_c values as well as angular dependence characteristics.     

Long RE123 coated conductors with high critical current over 500 A/cm by IBAD/PLD technique

We have developed long RE₁Ba₂Cu₃O_7?X (RE123) coated conductors with large current capacity by the ion beam assisted deposition (IBAD) and the pulsed laser deposition using hot wall heating (HW-PLD) technique. As a result, we could fabricate an 8 m-long Gd₁Ba₂Cu₃O_7?X (Gd123) coated wire with the minimum and maximum critical current (I_c) of 951 A/cm-w and 1003 A/cm-w at 77 K, 0 T, respectively, measured in 0.7 m-long sections by the standard 4-probe technique. Furthermore, we succeeded in preparation of over 600 m-long Gd123 coated wire with the uniform I_c distribution over 600 A/cm-w. It had average, maximum and minimum I_c of 665, 698, 609 A/cm-w, respectively. The n-values of the sample showed the maximum I_c and minimum I_c were 40 and 36, respectively. As a result, we set the new world record of I_c × L value as 374535 A m (= 609 A × 615 m). The in-field performance of this long wire was quite high as well; the minimum I_c exceeded 50 A/cm-w at 77 K, 3 T.     

Effect of the composition on the superconducting properties of IBAD-MgO SmBCO coated conductors with superconducting film directly deposited on epi-MgO layer

Sm_1+xBa_2?xCu_3+yO_7?δ (SmBCO) films were directly deposited on the epi-MgO/IBAD-MgO/Y₂O₃/Al₂O₃/Hastelloy template by co-evaporation using the evaporation using drum in dual chambers (EDDC) system without the buffer layer in order to investigate the effect of the composition ratios on superconducting property, microstructure and texture of SmBCO film. The films with gradient composition ratios of Sm:Ba:Cu were deposited using a shield with an opening which was placed between the substrate and the boats. The highest I_c of 52 A (corresponding to J_c = 1.6 MA/cm² and a thickness of 800 nm) was observed at 77 K in self field at a composition x = 0.01–0.05 and y = ?0.23 to ?0.46. When the composition ratio is outside this range, the I_c value rapidly decreased. The superconducting critical current was highly dependent on the composition ratio. As the composition ratio is farther away from that of the highest I_c, the SmBCO (1 0 3) peak intensity increased and the amount of a-axis oriented parts increased. A dense microstructure with round-shape grains was observed in the region showing the highest I_c. The optimum composition ratio can be found by analyzing films deposited with variable deposition rates of each depositing element.     

Properties of thin manganite films grown on semiconducting substrates for spintronics applications

La_0.7Sr_0.3MnO₃ (LSMO) manganite thin films were grown by pulsed plasma deposition on silicon (Si) and gallium arsenide (GaAs) substrates covered by an amorphous oxide. Manganite films are characterized by polycrystalline structure. Ferromagnetic transition is above room temperature and for 50 nm thick film the Curie temperature was as high as 325 K and 305 K for LSMO/SiO_x/Si and LSMO/AlO_x/GaAs, respectively.