Quantum oscillation of the tunneling conductance in fully epitaxial double barrier magnetic tunnel junctions

We investigated spin-dependent tunneling conductance properties in fully epitaxial double MgO barrier magnetic tunnel junctions with layered nanoscale Fe islands as a middle layer. Clear oscillations of the tunneling conductance were observed as a function of the bias voltage. The oscillation, which depends on the middle layer thickness and the magnetization configuration, is interpreted by the modulation of tunneling conductance due to the spin-polarized quantum well states created in the middle Fe layer. This first observation of the quantum size effect in the fully epitaxial double barrier magnetic tunnel junction indicates great potential for the development of the spin-dependent resonant tunneling effect in coherent tunneling regime.     

Magnetic and magnetooptical properties of Fe/Pt and Fe/Pt/Fe thin-film magnetic structures

Results of a study of magnetic and magnetooptical properties of Fe/Pt double-layer and Fe/Pt/Fe three-layer thin-film magnetic structures are presented. A strong effect of the Pt layer on magnetic properties of the studied samples was revealed. It was established that the saturation field of three-layer magnetic structures has an oscillating magnitude with varying Pt layer thickness, and the oscillation period is a function of the Fe layer thickness. The data obtained are explained by the presence of exchange interaction between the Fe layers via the Pt layer. A strong effect of Pt on spectral dependences of the equatorial Kerr effect in the thin-film structures under study is revealed.     

Microwave and THz sensing using slab-pair-based metamaterials

In this work the sensing capability of an artificial magnetic metamaterial based on pairs of metal slabs is demonstrated, both theoretically and experimentally, in the microwave regime. The demonstration is based on transmission measurements and simulations monitoring the shift of the magnetic resonance frequency as one changes a thin dielectric layer placed between the slabs of the pairs. Strong dependence of the magnetic resonance frequency on both the permittivity and the thickness of the dielectric layer under detection was observed. The sensitivity to the dielectrics′ permittivity (ε) is larger for dielectrics of low ε values, which makes the approach suitable for sensing organic materials also in the THz regime. The capability of our approach for THz sensing is also demonstrated through simulations.     

Influence of multilayer modulation on structural and magnetic features in the Pt/Sm-Co system

In this work, we study the influence of Pt underlayer in Pt/Sm-Co/Pt trilayers and in Pt/Sm-Co multilayers. In both cases, Pt underlayer seems to impose better crystallinity to Sm-Co layer and certainly promotes the evolution of the hard-magnetic SmCo₅ phase. Particularly, in the case of multilayer form, where multiple interlayers of Pt each one serving as a dedicated underlayer for the deposition of a specific Sm-Co layer, enhanced crystallinity is observed. Moreover, post-deposition annealing facilitates these features at relatively lower temperatures (∼400 °C) than those met in thin-film cases. This behavior is also followed by enhancement of saturation magnetization, while higher temperature post-deposition thermal treatment seems to deteriorate structural and magnetic features. If annealing temperature gets over 550 °C macroscopic magnetic features depress, probably due to domination of annealing-activated processes such as Sm oxidation and formation of non-magnetic phases since Pt diffuses throughout the whole magnetic layer.     

Oscillatory magnetoconductance of narrow quantum waveguides in the presence of inhomogeneous magnetic fields

We present quantum mechanical calculations of magnetoconductance of narrow quantum waveguides in the presence of inhomogeneous perpendicular magnetic field with the use of a model of two coupled tight-binding chains and the transfer-matrix method. The variation of the magnetoconductance with the magnetic flux φ threading one unit cell in the chains for different Fermi energies of the electrons is presented. The effect of magnetically defined ‘barriers’ on the conductance as a function of the Fermi energy is studied in detail for various samples with different magnetically structural configurations. The profile of the conductance depends on the magnitude and the relative direction of the magnetic field piercing the magnetic ‘barriers’. The behaviors of the conductance for the linear-variation and other modulation functions of the magnetic field in a finite region are shown. The abrupt change of the magnetic field in the interface between two adjacent regions causes striking oscillation structures imposed upon the conductance steps. When the magnetic field is varied smoothly (adiabatically) the oscillation structures in the conductance are substantially suppressed and smeared out and finally replaced by the rounded conductance step in the corner. The presence of a magnetically defined cavity in the waveguide leads to pronounced oscillations and the appearance of resonant dip-peak pair in the conductance.     

Structure and magnetic properties of magnetron-sputtered [(Fe/Pt/Fe)/Au]n multilayer films

Using dc magnetron sputtering, Fe/Pt/Au multilayer films were prepared, and the effects of Au layer thickness and annealing temperature on structure and magnetic properties of the Fe/Pt/Au multilayer films were investigated. The as-deposited Fe/Pt/Au multilayer films have good periodic structure with composition modulation along the growth direction. The stress stored in the as-deposited films promoted the ordering of the films annealed at 400 °C. When the films were annealed at 500 °C, the thicker Au layer could restrain the order-disorder transformation region volume and lead to the decrease of the ordered volume fraction with Au layer thickness increasing.     

磁性纳米颗粒膜的微磁学模拟

用微磁学方法对磁性纳米颗粒膜的磁特性进行了模拟，采用的模型是由122个磁性纳米颗粒组成的面心立方(fcc)结构体系.结果表明：在该体系中，偶极相互作用对体系的静态磁结构的影响显著，而交换相互作用的影响表现不明显.在此基础上，本文还采用有效媒质理论计算分析了磁性合金颗粒不同体积比时颗粒膜的磁谱和表征电磁参量发生显著变化的逾渗现象和逾渗阈值.并完成了对高磁损耗磁性纳米颗粒膜的材料设计. 关键词： 微磁学 纳米颗粒膜 逾渗阈值 磁导率 材料设计     

Magnetization reversal and domains structures in (Co/Ni/Co/Pt) multilayers

Multilayers of [Co/Ni(t_Ni)/Co/Pt]_×4 are investigated for different Ni insertion layer thicknesses. The resulting magnetic properties and magnetic domain structures are compared with [Co/Ni]_×8 multilayers. As determined by magneto-optical Kerr effect microscopy and a vibrating sample magnetometer measurements, all multilayers exhibited a perpendicular magnetic anisotropy. It is found that the nucleation field and magnetic coercivity of [Co/Ni(t)/Co/Pt]_×4 multilayers are lower than (Co/Ni)_×8 and decreased with Ni thickness. Magnetization decay measurements reveal that these multilayers did not show an exponential decay behavior as was observed in rare earth transition metal alloys. Very narrow wires will remain stables for several hours even with an applied magnetic field closer to the coercivity. Insertion of very thin Ni in (Co/Pt) multilayers offers a good way to optimize the magnetic properties of the material and adjust the domain size for nanowire-based devices.     

电场作用下5CB液晶分子的近壁面层黏弹性的QCM研究

利用石英晶体微天平(quartz crystal microbalance,QCM)研究了电场对5CB液晶分子的近壁面层黏弹性的影响.对QCM结果的分析发现,电场作用对液晶的黏度影响分为两部分,通过建立含吸附膜的双层膜模型,分析了QCM的两部分结果,发现电场对近壁面吸附层及体相层的影响是不同的.根据QCM的双层膜模型,对近壁面层液晶分子的黏弹性及膜厚进行了定量的分析计算,结果表明5CB在石英晶体上电极附近有一层约100nm厚的近壁面吸附层,其复剪切黏度随电场强度的增加而减小,这与5CB液晶的体相黏度变化规 关键词： 5CB液晶 石英晶体微天平 近壁面 黏弹性     

Adsorption-induced conductance changes of thin Pt films and PtPd/TiO2 gas sensors

Chemisorption of H₂ and O₂ and resulting changes in electrical conductance of a typical gas sensing material, PtPd/TiO₂, and thin Pt films on glass are studied and compared. The activation energy of conduction increases as Pt film thickness decreases. Chemisorption of H₂ on thin Pt films causes an increase in conductance and activation energy of conduction. O₂ chemisorption results in a decrease in conductance and increase in activation energy of conduction. Alteration in the number of charge carriers and reduction in charge carrier mobility are the mechanisms proposed for the observed changes. Compared to thin Pt films, relatively large changes in electrical conductance are observed upon chemisorption of gases on TiO₂ supported PtPd. The role of the oxide substrate in the observed chemical interaction and electronic response is discussed. The electronic changes upon adsorption/desorption of gases are reversible for thin Pt films but only partially reversible for TiO₂ supported PtPd.     

A model for engineering the electrical conductance at nanoscale

With the emergence of nanoelectronics faster and denser circuits are being produced, this largely because the aggressive scaling to the nanometer range of the insulating film used as dielectric. Moreover, enhancements of the electrical conductivity of nanofiller based composites can be achieved by the incorporation of conductive nanofillers into polymer matrix. In such systems electron wave-function penetration into the dielectric is important as it leads to undesired or desired leakage currents by tunneling respectively. Therefore, a proper design of the electrical conductance in such structures becomes important in order to control accurately their performance. In this research, a model for engineering the electrical conductance of resistors at nanoscale is presented. The conductance at infinitesimal bias of nanoresistors is modeled within the framework of Landauer's tunneling which results in an exponential integral function for the total electrical conductance. Model takes the effects of azimuthal and inclination angles between nanocontacts into account, as well as the effect of the thickness of the dielectric layer. The model also unveils a U-shaped behavior of the electrical conductance as a function of the azimuthal angle between nanocontacts. As a result, a minimal electrical conductance is predicted when the azimuthal angle reaches 90°.     

Magnetoplasmonic crystals: Resonant linear and nonlinear magnetooptical effects

Results of spectroscopic analysis of the optical second-harmonic (SH) generation in magnetic plasmonic structures comprised of iron garnet and periodic arrays of gold stripes are presented. It is shown experimentally that, in the region of the resonant excitation of a surface plasmon on the metal–magnetic dielectric interface, an increase in the SH intensity and an alternating modulation of the magnetic contrast for the SH, reaching 40%, are observed. The results are described in terms of a nonlinear Fano resonance.     

The structure of epitaxially grown metal films on single crystal surfaces of other metals: Gold on Pt(100) and platinum on Au(100)

Gold was evaporated onto Pt(100) and platinum was evaporated onto a Au(100) single crystal surface. Deposition of gold onto Pt(100) removed the $\text{(}\text{14}\text{1}\text{1}\text{5}\text{)}$ reconstructed surface structure and at a coverage of about 0.5 monolayer a (1 × 1) pattern fully developed. This pattern remained unchanged up to 2 gold monolayers. Multilayers of gold produced (1 × 5) and (1 × 7) surface structures after annealing. These observations imply variable interatomic distances in the gold layers. The (1 × 5) and (1 × 7) surface structures can be explained by the formation of a hexagonal top atomic layer on a substrate that retains a square lattice. The well known structure of clean Au(100) did not form, even at 32 layers of gold on Pt(100). Platinum deposited onto Au(100) removed its surface reconstruction yielding a fully developed (1 × 1) pattern at about one-half layer. This pattern remained unchanged upon further platinum deposition. The absence of new reconstructions in this case may be linked with the growth mechanism that is inferred from the variation of the Auger signal intensities of the substrate and adsorbate metals with coverage of the adsorbate. It was found that platinum on Au(100) forms microcrystallites (Volmer-Weber type growth), while gold on Pt(100) grows layer-by-layer (Frank-van der Merwe growth mechanism).     

Investigation of HfO<Subscript>2</Subscript>/SiO<Subscript>2</Subscript>/<Emphasis Type="Italic">n</Emphasis>-Si(001)-based MOS structures via ballistic electron emission microscopy

(Au, Pt)/HfO₂/SiO₂/n-Si(001) metal-oxide-semiconductor structures with a thin (≈0.5 nm) SiO₂ layer, which is formed between HfO₂ and Si during atomic layer deposition of oxide layers, have been investigated via ballistic electron emission spectroscopy. The potential barrier heights at the (Au, Pt)/HfO₂ interfaces have been determined experimentally. The peculiarities observed in the curves of dependence of the collector current on the voltage between a scanning tunneling microscope probe and a metallic electrode are related to electron transport through the vacancy defect region of HfO₂ and the quantum-mechanical interference of electron waves arising from multiple reflections at the interfaces of the two-layer dielectric and at the interfaces of dielectric with a substrate and a metallic electrode.     

Structure and bonding of water on Pt(111)

We address the adsorption of water on Pt(111) using x-ray absorption, x-ray emission, and x-ray photoelectron spectroscopy along with calculations in the framework of density functional theory. Using the direct relationship between the electronic structure and adsorbate geometry, we show that in the first layer all the molecules bind directly to the surface and to each other through the in-layer H bonds without dissociation, creating a nearly flat overlayer. The water molecules are adsorbed through alternating metal-oxygen (M-O) and metal-hydrogen (M-HO) bonds.     

Structural,electronic,and magnetic behaviors of 5d transition metal atom substituted divacancy graphene:A first-principles study

Structural, electronic, and magnetic behaviors of 5d transition metal(TM) atom substituted divacancy(DV) graphene are investigated using first-principles calculations. Different 5d TM atoms(Hf, Ta, W, Re, Os, Ir, and Pt) are embedded in graphene, these impurity atoms replace 2 carbon atoms in the graphene sheet. It is revealed that the charge transfer occurs from 5d TM atoms to the graphene layer. Hf, Ta, and W substituted graphene structures exhibit a finite band gap at high symmetric K-point in their spin up and spin down channels with 0.783 μB, 1.65 μB, and 1.78 μB magnetic moments,respectively. Ir and Pt substituted graphene structures display indirect band gap semiconductor behavior. Interestingly, Os substituted graphene shows direct band gap semiconductor behavior having a band gap of approximately 0.4 e V in their spin up channel with 1.5 μB magnetic moment. Through density of states(DOS) analysis, we can predict that d orbitals of 5d TM atoms could be responsible for introducing ferromagnetism in the graphene layer. We believe that our obtained results provide a new route for potential applications of dilute magnetic semiconductors and half-metals in spintronic devices by employing 5d transition metal atom-doped graphene complexes.     

Fe–Pt–MgO stacked films for perpendicular magnetic recording

Fe–Pt–MgO stacked storage layer constructed by [Fe–Pt/Fe–Pt–MgO/Fe–Pt] trilayered structure was proposed for a next-generation high-density perpendicular magnetic recording medium. The Fe–Pt–MgO composite middle layer was prepared by sputtering the Fe–Pt–MgO composite-type target including relatively large MgO content of 50 vol%. The Fe–Pt(0 0 1) seed layer deposited on MgO underlayer was effective in forming the ordered fct(0 0 1) phase for the Fe–Pt–MgO composite film. The reduction of transition jitter noise and the suppression of signal overlap were observed in the stacked-type medium with the Fe–Pt–MgO middle layer of 1 nm thickness. The improvement of recording properties is attributed to the pinning effect of magnetic domain wall by the Fe–Pt–MgO composite layer inserted into the middle of pure Fe–Pt storage layer.     

含stubs量子波导系统的电子自旋极化输运性质

理论上研究了含stubs的Rashba自旋轨道耦合（spin-orbit coupling, SOC）量子波导系统的自旋极化输运性质. 利用晶格格林函数方法,发现由于stubs和SOC产生的势阱使系统中出现束缚态,这些束缚态与传播态之间相互干涉导致电导中出现Fano共振结构,同时在对应的自旋极化率中也出现Fano共振或反共振结构. 此外,由于系统结构的突变使电子被反向散射和量子干涉效应,电导中出现一系列的共振峰. 但是,当系统加上外磁场后,所有这些效应都被抑制, 系统重新出现量子化电导, 同时自旋电导也出 关键词： 量子波导 自旋极化输运 自旋轨道耦合     

正常金属-铁磁绝缘层-dx2-y2+idxy混合波超导结中的磁散射对隧道谱的影响

在正常金属铁磁绝缘层dx2-y2 idxy混合波超导隧道结中,考虑到铁磁绝缘层的磁散射和界面的粗糙散射效应,运用BogoliubovdeGennes(BdG)方程和BlonderTinkhamKlapwijk(BTK)理论,计算了隧道结中的准粒子传输系数和微分电导.研究表明:(1)磁散射和界面粗糙散射均可以压低电导峰,其中磁散射能使电导峰滑移,而粗糙界面散射却能阻止这种滑移,且两散射的共同作用可抑制由混合波两序参数的幅值比不同所导致的电导峰滑移;(2)随铁磁层离超导表面距离的增加,隧道谱在零偏压处由凹陷变成了零偏压电导峰,继而又演化为凹陷中的中心峰;(3)当铁磁层离开超导表面有若干相干长度时,隧道谱中将呈现一些子能级谐振峰.     

Structure and magnetism of Fe/MgO/Fe multilayered nanosystems

The dependences of the structural and magnetic properties of a nanoscale Fe/MgO/Fe planar system on the thickness of the dielectric MgO layer are reported. X-ray crystallographic analysis reveals a high-quality layered structure with abrupt interlayer boundaries and a continuous MgO-insulator layer. Fourth-order magnetocrystalline anisotropy is found in the synthesized structures. A new way to provide antiferromagnetic ordering in the nanostructure is proposed by applying a magnetic field to the investigated structure at an angle of 22° with respect to the easy magnetization axis. In this case, the antiferromagnetic ordering of magnetic moments is established in the field range of 20–50 Oe.