Giant injection magnetoresistance in gallium arsenide/granulated film heterostructures with nanosize cobalt inclusions

Electron transport has been studied in the gallium arsenide/granulated SiO₂ film heterostructure with Co nanoparticles and in the gallium arsenide/TiO₂ film heterostructure with Co island sublayers. When electrons are injected from a film into a semiconductor, a new phenomenon is observed, which is called injection magnetoresistance. For the SiO₂(Co)/GaAs structure with 60 at. % Co in a magnetic field of 23 kOe at a voltage of 50 V, the injection magnetoresistance reaches 5200% at room temperature.     

Electrochemical formation of intermediate layer for Co/Pd multilayered media

Pd nanocluster seeds were formed on a soft magnetic underlayer (SUL) using an electrochemical substitution reaction, and were utilized as an intermediate layer for a Co/Pd multilayered ([Co/Pd]_n) perpendicular magnetic recording medium. A CoNiFeB film prepared with electroless deposition was used as SUL, which was immersed into a PdCl₂ solution for the formation of Pd seeds. The Pd seeds were found to effectively reduce the size of magnetic domains in the [Co/Pd]_n film deposited on them. The optimization of the concentration of the PdCl₂ solution and the use of the pretreatment process with a SnCl₂ solution were effective to obtain the smooth SUL surface with fine Pd seeds as small as 5 nm. The 20 nm-thick [Co/Pd]_n film deposited on the optimized Pd seeds/CoNiFeB SUL exhibited a high coercivity of 7.8 kOe and a small magnetic domain size of 69 nm. These results indicated that the combination of the Pd seeds and the electroless-deposited SUL was desirable in terms of the improvement not only in the magnetic properties of [Co/Pd]_n media but also in the mass productivity of the underlayer.     

Enhanced growth of low-resistivity cobalt silicide by using a Co/Au/Co trilayer film on Si0.8Ge0.2 virtual substrate

Formation of Co germanosilicides on Si_0.8Ge_0.2 virtual substrates with a Co/Au/Co sandwich thin film after different heat treatments has been investigated. The sequence of phase formation is the same as the reaction of blanket Co thin film with (001)Si. The presence of thin interposing Au layers was found to significantly enhance the formation of low-resistivity CoSi₂ on (001)Si_0.8Ge_0.2 substrates. The formation temperature of CoSi₂ phase in the Co/Au/Co/(001)Si_0.8Ge_0.2 samples was lowered by about 200 °C compared to that of Co/(001)Si_0.8Ge_0.2 samples. From TEM and EDS analysis, some of Au atoms were found to diffuse from the original interface position to disperse within the CoSi₂ layers during silicidation reactions. The mechanisms for the enhanced formation of CoSi₂ in the Co/Au/Co/Si_0.8Ge_0.2 system were explained in the context of classical nucleation theory.     

Substrate orientation-induced distinct ferromagnetic moment in Co:ZnO films

Dilute (3.8 at.%) cobalt-doped ZnO thin films are deposited on LiTaO₃ (LT) substrates with three different orientations [LT(1 1 0), LT(0 1 2) and LT(0 1 8)] by direct current reactive magnetron co-sputtering. The experimental results indicate that Co atoms with 2+ chemical valence are successfully incorporated into the ZnO host matrix on various oriented substrates, and the substrate orientations have a profound influence on the crystal growth and magnetization of Co:ZnO films. A large magnetic moment of 2.42μ_B/Co at room temperature is obtained in the film deposited on LT(0 1 2), while the corresponding values of the other films deposited on LT(1 1 0) and LT(0 1 8) are 1.21μ_B/Co and 0.65μ_B/Co, respectively. Furthermore, the crystal growth mode of Co:ZnO films on various oriented LT, the relationship between the microstructures and corresponding ferromagnetic properties are also discussed.     

Magnetic properties of Co nanoparticle granular films capped with Pt

Magnetization and susceptibility measurements have been performed on Co granular (Al₂O₃/Co/Pt)₂₅ multilayers. A cusp-like anomaly is present in the susceptibility, which together with magnetization measurements suggests that the samples behave as an amorphous Correlated-Super-Spin system. The presence of CoPt partial alloying is proven by the XANES spectra. The XMCD spectra show that the interfacial Pt atoms become magnetically polarized by hybridization of the Pt 5d and Co 3d electrons, and are ferromagnetically coupled to the Co particle moments. The Pt layer is effective in transmitting interparticle exchange coupling.     

Characteristics of SrBi2Ta2O9 ferroelectric films in an in situ applied low electric field prepared by metalorganic decomposition

SrBi₂Ta₂O₉ (SBT) films were prepared on Pt/TiO₂/SiO₂/Si substrates at 750 °C in oxygen by metalorganic decomposition method. A low electric field was in situ applied during the film crystallization. It was first found that a low electric field and its direction have significant influence on the microstructures and ferroelectric properties of SBT films. Under a positive electric field (assuming that the bottom electrode is electrically grounded), the films show stronger c-axis-preferred orientation than without electric field and under a negative electric field. As a possible origin is proposed that the interface-induced nucleation growth between SBT and Pt coated substrate with application of low electric field plays a key role. Above all, an in situ applied low electric field during the film crystallization is a promising technique controlling film orientation for film preparation by wet chemical method.     

Magnetic transition in Co/(Gd–Co) multilayers

[Co/Gd_0.36Co_0.64]₄/Co multilayers with Co termination layer have been prepared by rf sputtering. They form macroscopic ferrimagnets with a compensation temperature (T_comp) determined by the thickness ratio of the layers. In low fields the magnetization of Co and Gd–Co layers are along the axis of the applied field. Increasing field makes the moments of both the Co and Gd–Co layers deviate from the axis of the field giving rise to a transition into a twisted state. These magnetic transitions were studied by vibrating sample magnetometer (VSM), magneto-optic Kerr effect and magnetoresistance measurements at various temperatures. The nucleation and evolution of surface- and bulk-twisted magnetic states were also observed in these multilayers.     

Novel ultraviolet photoluminescence of ZnO/ZnGa2O4 composite layers

ZnO/ZnGa₂O₄ composite layers were synthesized by simple thermal oxidation of ZnS substrates with gallium in the air. The continuous-wave and time-resolved photoluminescence measurements for the composites were performed at room temperature. It is found that the visible deep level emission from ZnO in ZnO/ZnGa₂O₄ composite layer was almost suppressed. In addition, the UV emission with long lifetime was also observed in comparison with that of pure ZnO layer without ZnGa₂O₄.     

Giant Faraday rotation in Fe–ZnSe granular films

The giant magneto-optical Faraday effect (giant Faraday rotation) of ferromagnetic metal–semiconductor matrix Fe–ZnSe granular films prepared by radio frequency sputtering is studied. The result shows that the Faraday rotation angle θ_F value of the granular films sample with Fe volume fraction x=35% is of the order of 10⁵ °/cm at room temperature. Temperature dependence of the θ_F of Fe_0.35(ZnSe)_0.65 granular films shows that θ_F value, below 150 K, increases rapidly with the decrease of the temperature, and when T=10 K θ_F value is 6×10⁵ °/cm. Through the study of the structure and dependence of magnetic properties on temperature, it has been found that the remarkable increase of the θ_F value of Fe_0.35(ZnSe)_0.65 granular films below 150 K seems to arise from the sp–d exchange interaction inside the granular films.     

First-principles study on the metallic antiferromagnet Co[PhPO3]·H2O

The electronic structure and the magnetic properties of transition metal phosphonate Co(PhPO₃)·H₂O have been studied by first-principles within the density-functional theory (DFT) and the full potential linearized augmented plane wave (FP-LAPW) method. The total energy, the total magnetic moment, the atomic spin magnetic moments and the density of states(DOS) of Co(PhPO₃)·H₂O were all calculated. The calculations reveal that the title compound is a metallic antiferromagnet and has a metallic ferromagnetic metastable state, which are in good agreement with the experiment. The spin magnetic moment of Co(PhPO₃)·H₂O is about 4.93 _μB${\mu }_{\mathrm{B}}$ per molecule, and it is mainly assembled at the cobalt atom, at the same time, with a little contribution from the P, O1, O2, O3.     

Theoretical study on the composition dependence of Ga8−nAsn (n=0-8) clusters

Using full-potential linear-muffin-tin-orbital molecular-dynamics (FP-LMTO-MD) method, we have investigated the dependence of GaAs clusters with eight atoms on composition. It is found that the ground state structures for Ga-rich and As-rich clusters are cube structures. As the ratio between gallium atoms and arsenic atoms is close to one, structural distortion become increasingly severe, or even the clusters adopt other geometrical configurations as their ground state structures. The energy gap E_g between the highest-occupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO), and the vertical electron affinity show a certain degree of even/odd alternation with cluster composition. Among nine Ga_8−nAs_n (n=0-8) clusters, only a few of clusters have different energy orders between the ionic and neutral isomers with large binding energy. Some ionic structures would change into other configurations due to severe structural distortion.     

Theoretical study of p-d hybridization in Co perovskite

The real-space recursion method and unrestricted Hartree-Fock approximation have been applied to calculate the density of states of various Co perovskite, CeCoO₃, SrCoO₃ and Sr_1−xCe_xCoO₃. We have studied the magnetically ordered states of these Co perovskites in an enlarged double cell, and find its various magnetic structures due to the occupancy of 3d band and its interaction with neighboring Co ions. In this study, we have studied the p-d hybridization of the three Co perovskites, we find t_2g electrons are localized and the flat e_g band is responsible for the itinerant behavior, and although the rare earth elements itself contribute little to the DOS at the Fermi energy, the DOS at Fermi energy and the magnetic moment changed consequently because of different valence of Co ions in these compounds and p-d hybridization effect is very important.     

Effect of Ni interlayer on stress level of CoSi2 films in Co/Ni/Si(1 0 0) bi-layered system

The effect of Ni interlayer on stress level of cobalt silicides was investigated. The X-ray diffraction patterns (XRD) show that low temperature formation of Co_1−xNi_xSi₂ solid solution was obtained while Ni interlayer was present in Co/Si system, which was confirmed by Auger electron spectrum (AES) and sheet resistance measurement. XRD was also used to measure the internal stress in CoSi₂ films by a 2θ_ψ − sin²ψ method. The result shows that the tensile stress in CoSi₂ films evidently decreased in Co/Ni/Si(1 0 0) system. The reduction of lattice mismatch, due to the presence of Ni in Co_xNi_1−xSi₂ solid solution, is proposed to explain this phenomenon.     

Preparation and photocatalytic activity of CeO2/TiO2 interface composite film

The CeO₂/TiO₂ and TiO₂/CeO₂ interface composite films were prepared on glass substrates by the sol-gel process via dip-coating and calcining technique. The scanning electron microscopy (SEM) revealed that the TiO₂ layer has a compact and uniformity glasslike surface with 200 nm in thickness, and the CeO₂ layer has a coarse surface with 240 nm in thickness. The X-ray diffractometer (XRD) analysis showed that the TiO₂ layer is made up of anatase phase, and the CeO₂ layer is structured by cubic fluorite phase. Through a series of photo-degradation experiments, the relationship of the photocatalytic activity with the constituents of the films was studied. In virtue of the efficient interfacial charge separation via the process of electron transfer from TiO₂ to CeO₂, the photocatalytic activity of the CeO₂/TiO₂ composite film is high. Contrarily, the photocatalytic activity of the TiO₂/CeO₂ composite film is low, due to its inert surface made up of CeO₂ with broad bandwidth. Apart from the effect of the film structure, the effect of film thickness on photocatalytic activity was also discussed.     

Electrical properties of highly (111)-oriented lead zirconate thin films

Antiferroelectric PbZrO₃ thin films were grown on Pt/Ti/SiO₂/Si substrates with predominant (111) orientation using a sol-gel process. The Pt/PbZrO₃/Pt film capacitor showed well-saturated hysteresis loops at an applied voltage of 5 V with remanent polarisation (P_r) and coercive electric field (E_c) values of 8.97 μC/cm² and 162 kV/cm, respectively. The leakage current density of the highly (111)-oriented PbZrO₃ film was less than 1.0×10⁻⁷ A/cm² over electric field ranges from 0 to 105 kV/cm. The conduction current depended on the voltage polarity. The PbZrO₃/Pt interface forms a Schottky barrier at electric fields from 20 to 160 kV/cm. The dielectric relaxation current behaviour of Pt/PbZrO₃/Pt capacitor obeys the well-known Curie-Von Schweidler law at electric field of 20-80 kV/cm, the currents have contributions of both dielectric relaxation current and leakage current.     

Charge transient spectroscopy measurements of GaAs metal-insulator-semiconductor structures

The Au/Pd/Ti-SiO₂-(n)GaAs structures with and without (NH₄)₂S_x treated gallium arsenide surface, previously analysed by impedance spectroscopy (IS) method, have been investigated using charge transient spectroscopy (QTS) technique. The isothermal QTS spectra of MIS structures kept at room temperature under set of quiescent biases have been recorded in response to both negative and positive pulses of fixed small amplitudes. Two types of charge relaxation characterized by time constant values have been evidenced. The attempt to compare QTS results with ones obtained by impedance spectroscopy method has been presented.     

Magnetism in nanometer-thick TiOx/Co/TiOx/TiN/Si multilayered structures

Most studies on Co-doped TiO₂ system were focused on thin films grown by MBE-based methods. In this work we report the ferromagnetism of nanometer-thick-layered TiO₂/Co/TiO₂/TiN film grown on Si substrate by conventional magnetron sputtering. For the growth of TiO₂ on silicon, a non-oxide thermally stable material, TiN, was introduced to prevent Ti penetration into the Si substrate. Structural, magnetic, and transport measurements respectively by Raman, SQUID and Hall effect show that our samples are n-type semiconductors and exchange bias effect due to exchange coupling between Co and interfacial CoO. For the rapid vacuum annealed specimen, we found an enhanced loss and a Perminvar-type constricted hysteresis loop, which attributed to pinning of domain walls due to an induced anisotropy by the pair ordering in the metallic alloy of Co-Ti-Si.     

Structure, magnetic and electrical transport properties of cosputtered Fe0.5Ge0.5 nanocomposite films

Fe_0.5Ge_0.5 nanocomposite films with different film thicknesses were fabricated using cosputtering. The films are composed of Ge, Fe and Fe₃Ge₂, and are ferromagnetic at room temperature. The saturation magnetization and magnetic interaction including dipolar interaction and exchange coupling increase with the increasing film thickness. The electrical conductance mechanism turns from metallic to semiconducting and the saturation Hall resistivity ρ_xys increases with the decreasing film thickness. At 28 nm, ρ_xys is ∼137 μΩ cm at 2 K, about 150 times larger than that of pure Fe film (0.9 μΩ cm) and four orders larger than that of bulk Fe. The ρ_xy-H curves of all the films show the same linearity character in low-field range even though the temperature-independent slope is different at different film thicknesses. At high temperatures, the skew scattering mechanism is dominant. At low temperatures, side-jump effect should be dominant at large resistivity ρ_xx regime for the thin films, and the skew scattering is dominant at small ρ_xx regime for the thick films.     

Nano-star formation in Al-doped ZnO thin film deposited by dip-dry method and its characterization using atomic force microscopy, electron probe microscopy, photoluminescence and laser Raman spectroscopy

Zinc oxide doped with Al (AZO) thin films were prepared on borosilicate glass substrates by dip and dry technique using sodium zincate bath. Effects of doping on the structural and optical properties of ZnO film were investigated by XRD, EPMA, AFM, optical transmittance, PL and Raman spectroscopy. The band gap for ZnO:Al (5.0 at. wt.%) film was found to be 3.29 eV compared with 3.25 eV band gap for pure ZnO film. Doping with Al introduces aggregation of crystallites to form micro-size clusters affecting the smoothness of the film surface. Al³⁺ ion was found to promote chemisorption of oxygen into the film, which in turn affects the roughness of the sample. Six photoluminescence bands were observed at 390, 419, 449, 480, 525 and 574 nm in the emission spectra. Excitation spectra of ZnO film showed bands at 200, 217, 232 and 328 nm, whereas bands at 200, 235, 257 and 267 nm were observed for ZnO:Al film. On the basis of transitions from conduction band or deep donors (CB, Zn_i or V_OZn_i) to valence band and/or deep acceptor states (VB, V_Zn or O_i or O_Zn), a tentative model has been proposed to explain the PL spectra. Doping with Al³⁺ ions reduced the polar character of the film. This has been confirmed from laser Raman studies.     

Highly efficient transparent Zn2SiO4:Mn phosphor film on quartz glass

Highly efficient transparent Zn₂SiO₄:Mn²⁺ film phosphors on quartz substrates were deposited by the thermal diffusion of sputtered ZnO:Mn film. They show a textured structure with some preferred orientations. Our film phosphor shows, for the best photoluminescence (PL) brightness, a green PL brightness of about 20% of a commercial Zn₂SiO₄:Mn²⁺ powder phosphor screen. The film shows a high transmittance of more than 10% at the red-color region. The excellence in PL brightness and transmittance can be explained in terms of the textured crystal growth with a continuous gradient of Zn₂SiO₄: Mn²⁺ crystals.