(Ga,Mn)As on patterned GaAs(0 0 1) substrates: Growth and magnetotransport

A new type of (Ga,Mn)As microstructures with laterally confined electronic and magnetic properties has been realized by growing (Ga,Mn)As films on -oriented ridge structures with (1 1 3)A sidewalls and (0 0 1) top layers prepared on GaAs(0 0 1) substrates. The temperature- and field-dependent magnetotransport data of the overgrown structures are compared with those obtained from planar reference samples revealing the coexistence of electronic and magnetic properties specific for (0 0 1) and (1 1 3)A (Ga,Mn)As on a single sample.     

Photoluminescence study of type-II GaAs quantum well wires grown on nano-faced (3 1 1)A surface: Quasi-1D exciton observation?

GaAs/AlAs corrugated superlattices (CSLs) grown on nano-faceted (3 1 1)A GaAs surface were studied using Raman spectroscopy and photoluminescence (PL) techniques. Raman data (splitting of localized transversal optical phonons) have proved structural anisotropy of the CSLs. The structural anisotropy leads to optical anisotropy appeared in strong polarization dependence of PL. Temperature dependence of PL has shown that the CSLs are type-II superlattices. Additional peak in PL spectrum at low (77–100 K) temperatures can be result of quasi-1D exciton appearance in the CSLs.     

[1 1 1]B-oriented GaAsSb grown by gas source molecular beam epitaxy

We report the GaAsSb bulk layers and GaAsSb/GaAs quantum wells (QWs) grown on (1 1 1)B GaAs substrates by gas source molecular beam epitaxy. We found that Sb composition in the GaAsSb epilayers is very sensitive to the substrate temperature. The composition drops from 0.35 to 0.16 as the substrate temperature increases from 450 to 550 °C. The [1 1 1]B-oriented GaAsSb epilayers show phase separation when the substrate temperature is lower than 525 °C. For a GaAsSb/GaAs multiple quantum wells (MQWs) structure composed of five periods of 5 nm GaAs_0.73Sb_0.27 QW and 30 nm GaAs barrier, the room temperature photoluminescence emission is located at 1255, 80 nm longer than the [1 0 0]-oriented sample with the same Sb composition. The peak wavelength shows significant blue shift as the excitation level increases, which evidences the type-II band alignment in this heterostructure.     

Experimental evidence and extraction of the electron mass variation in [1 1 0] uniaxially strained MOSFETs

In this paper, we investigate electron mobility enhancement in [1 1 0] uniaxially strained nMOSFETs with three different channel orientations on a [0 0 1] Si substrate. We have experimentally demonstrated that, for stress applied in a [1 1 0] direction, electrical results cannot be explained without considering that the in-plane mass m_t for the [0 0 1] two-fold valleys (Δ2) becomes anisotropic and varies with strain. For the first time, their values in the transport direction, perpendicular and parallel to an applied stress, have been extracted from electrical characterization of the MOS transistor thanks to an original technique. It has been found that the conduction mass of Δ2 along the standard [1 1 0] channel direction is reduced by a tensile uniaxial stress along [1 1 0] while it is increased by a tensile uniaxial stress along . These results reinforce several previous theoretical works.     

Self-organized GaAs patterns on misoriented GaAs (1 1 1)B substrates using dilute nitrides by molecular beam epitaxy

Recently, the growth of patterned surfaces is being used to demonstrate the site control of the three-dimensional nanostructures, and in particular quantum dots. Nevertheless the pre-patterning techniques show some disadvantages. In this work, we report a novel in situ hole-patterning technique which consists of growing by molecular beam epitaxy a dilute nitride GaAsN layer on 1° and 2° towards [2¯ 1 1] misoriented GaAs(1 1 1)B substrates. Later, we carry out a regrowth of GaAs layers on this patterned surfaces in order to improve the surface quality and the homogeneity of the characteristics of holes (size, depth, etc.). Consecutively, we use these patterned surfaces to grow InAs quantum dots, whose growth on these misorientations results in a greater difficulty. A structural characterization of the resulting samples, both hole-patterns and quantum dots, has been performed. Besides, we have realized studies of the dependence of the surface morphology on some important parameters (including substrate misorientation, thicknesses of the GaAsN and GaAs layers grown and growth conditions).     

Ultraflexible and Malleable Fe/BaTiO3 Multiferroic Heterostructures for Functional Devices

The high demand for flexible spintronics based on multiferroic heterostructures makes growing high-quality flexible, functional oxides urgently, in which needs to be deposited on lattice-matched substrates. In this paper, ultraflexible and malleable iron (Fe)/BaTiO₃ (BTO) multiferroic heterostructures are demonstrated, showing a perfect crystallinity and hetero-epitaxial growth. In terms of performance, they indicate good multiferroic properties and excellent bending tunability, as well as obvious magnetoelectric (ME) coupling effect. During the phase transformation from the rhombohedral phase to the orthorhombic phase of BTO layers in the heating process, a large ME coupling coefficient of 120 Oe  ° C⁻¹ along the out-of-plane direction is obtained. This value keeps consistent in the phase-field simulation of magnetic domain evolution, in which the biaxial compressive strain induced-magnetoelastic anisotropy facilitates the magnetic easy axis of Fe layers to the [110] or [–1–10] direction. Besides, ultraflexible Fe/BTO heterostructures are found to have a 690 Oe ferromagnetic resonance (FMR) field shift along the out-of-plane direction under the flexible tuning (R  = 5 mm). This work should pave a way toward flexible spintronic and functional devices with fast speed, portability, and low energy consumption.     

低温退火对稀磁半导体(Ga,Mn)As性质的影响

利用低温分子束外延技术在GaAs(001)上外延生长出厚度为500nm的稀磁半导体(Ga,Mn)As薄膜. 双晶X射线衍射证明其为闪锌矿结构，晶格参数为0.5683nm，据此推导出其Mn含量为7%. 磁测量结果揭示其铁磁转变温度为65K. 观察了低温退火处理对(Ga,Mn)As磁性质的影响，发现生长后退火处理显著提高了其铁磁转变温度，可以达到115K.     

Nd_(0.5)Sr_(0.5)Co_(1-x)Mn_xO_3的晶体结构和电磁性能研究

采用固相反应法制备了Nd0.5Sr0.5Co1–xMnxO3多晶样品(0≤x≤1)。利用X射线衍射仪和Rietveld方法精修程序分析了样品的晶体结构;对样品的磁性和电输运性能进行了测量。结果表明:样品均为斜方晶系单相结构,空间群为Imma。样品的晶格常数a、b、c,晶格平均扭曲率D和有效磁矩μeff随着Mn含量x的增加而增大,促使Co离子向高自旋态转变。在x≥0.6时,样品中Co离子都处于高自旋态。在样品中,Mn替代了Co,Mn—O—Co的反铁磁性超交换作用与Co3+—O2–—Co4+的铁磁性双交换作用相互竞争破坏了样品的金属导电行为,致使当x≥0.2时,样品表现出半导体导电行为。     

Highly ordered C60 films on epitaxial Fe/MgO(0 0 1) surfaces for organic spintronics

Hybrid interfaces between ferromagnetic surfaces and carbon-based molecules play an important role in organic spintronics. The fabrication of devices with well defined interfaces remains challenging, however, hampering microscopic understanding of their operation mechanisms. We have studied the crystallinity and molecular ordering of C₆₀ films on epitaxial Fe/MgO(0 0 1) surfaces, using X-ray diffraction and scanning tunneling microscopy (STM). Both techniques confirm that fcc molecular C₆₀ films with a (1 1 1)-texture can be fabricated on epitaxial bcc-Fe(0 0 1) surfaces at elevated growth temperatures (100–130 °C). STM measurements show that C₆₀ monolayers deposited at 130 °C are highly ordered, exhibiting quasi-hexagonal arrangements on the Fe(0 0 1) surface oriented along the [1 0 0] and [0 1 0] directions. The mismatch between the surface lattice of the monolayer and the bulk fcc C₆₀ lattice prevents epitaxial overgrowth of multilayers.     

Influence of substrate on the high-frequency permeability of thin iron films

The ferromagnetic resonance (FMR) spectra of thin metallic films obtained by magnetron deposition on polymeric and ceramic substrates are investigated in the strip line at frequencies of 0.13–12 GHz via frequency and external magnetic field sweeping. The influence of mechanical stresses on the FMR spectra of films deposited on an elastic (polyethylene rephthalate) substrate is discussed. The magnetostriction contribution to the anisotropy field of a film, as well as the influence of tensile stresses on the quasi-static permeability and FMR frequency, is estimated. It is demonstrated that the microwave properties of a thin metallic film are also specified by the properties of the substrate with such a film. A distinction in the magnetic properties of films with the same composition, which are deposited on different substrates, is explained in terms of the magnetostriction effect.     

Tunable Optical Mode Ferromagnetic Resonance in FeCoB/Ru/FeCoB Synthetic Antiferromagnetic Trilayers under Uniaxial Magnetic Anisotropy

Ferromagnetic resonance (FMR) is one of the most important characteristics of soft magnetic materials, which practically sets the maximum operation speed of these materials. There are two FMR modes in exchange coupled ferromagnet/nonmagnet/ferromagnet sandwich films. The acoustic mode has relatively lower frequency and is widely used in radio‐frequency/microwave devices, while the optical mode is largely neglected due to its tiny permeability even though it supports much higher frequency. Here, a realistic method is reported to enhance the permeability in the optical mode to an applicable level. FeCoB/Ru/FeCoB trilayers are carefully engineered with both uniaxial magnetic anisotropy and antiferromagnetic interlayer exchange coupling. This special magnetic structure exhibits a high optical mode frequency up to 11.28 GHz and a maximum permeability of 200 at resonance. An abnormally low inverse switch field (<200 Oe, less than 1/5 of the single layer) is observed which can effectively switch the system from optical mode with higher frequency into acoustic mode with lower frequency. The optical mode frequency and inverse switch field can be controlled by tailoring the interlayer coupling strengths and the uniaxial anisotropy fields, respectively. The tunable optical mode resonance thus can increase operation frequency while reduce operation field overhead in FMR based devices.     

Ferromagnetism in self-assembled Ge quantum dots material followed by Mn-implantation and annealing

Ten and twenty layers of self-assembled Ge QDs with 44 and 59-nm-thick Si barrier were grown on high resistivity (1 0 0) p-type Si substrates by rapid thermal chemical vapor deposition followed by Mn ion implantation and post-annealing. A presence of ferromagnetic structure was confirmed in the insulating GeMn diluted magnetic quantum dots (DMQD) and semiconducting GeMn DMQD. The DMQD materials were found to be homogeneous, and to exhibit p-type conductivity and ferromagnetic ordering with a Curie temperature, T_C = 350 and 230 K. The X-ray diffraction (XRD) data show that there is a phase separation of Mn₅Ge₃ from MnGe nanostructure. Temperature dependent electrical resistivity in semiconducting DMQD material indicates that manganese introduces two acceptor levels in germanium at 0.14 eV from the valence band and 0.41 eV from the conduction band implying Mn substituting Ge. Therefore, it is likely that the ferromagnetic exchange coupling of DMQD material with T_C = 230 K is hole-mediated due to formation of polarons and the ferromagnetism in sample with T_C > 300 K is due to Mn₅Ge₃ phase.     

Spin dynamics in (1 1 0)-oriented quantum wells

Quantum structures of III–V semiconductors grown on (1 1 0)-oriented substrates are promising for spintronic applications because they allow us to engineer and control spin dynamics of electrons. We summarise the theoretical ideas, which are the basis for this claim and review experiments to investigate them.     

Magnetic and magnetotransport properties in the n-type (Ga,Mn)N thin films

We present the magnetic and magnetotransport properties of epitaxial (Ga,Mn)N films with nominal Mn concentration (x=0.1–0.73%) grown by plasma-enhanced molecular beam epitaxy (PEMBE). X-ray diffraction (XRD) reveals that (Ga,Mn)N has the single-phase wurtzite structure without secondary phases. The epitaxial (Ga,Mn)N films were found to exhibit n-type conductivity, ferromagnetic ordering with Curie temperature in the range 550–700 K, and in-plane magnetic anisotropy. The negative magnetoresistance (MR) was observed at temperatures below 50 K and was found to gradually increase with decreasing temperature.     

Morphological evolution and lateral ordering of uniform SiGe/Si(0 0 1) islands

By investigating the morphological evolution during epitaxial growth of Ge on Si(0 0 1) substrates, we find that highly uniform distributions of islands can be obtained. The islands are no longer domes but they consist of barns, which are bounded by steeper facets. A detailed morphological analysis indicates the presence of facets at their base, which are not stable for Ge but for Si. Finally, we show that long-range ordering of highly uniform SiGe barns can be obtained when the growth is performed on patterned Si(0 0 1) substrates.     

Effects of Doping Mn on Nd_(1.85)Ce_(0.15)CuO_4 System

Nd1.85Ce0.15Cu1-xMnxO4 samples with doping level up to 20% have been synthesized by solid-state reaction method. The influence of Mn on their normal-state transport, crystal structure, superconductivity and magnetic properties has been investigated. For the samples with x〉0.03, magnetization under zero-field cooling indicates that the magnetic state changes from ferromagnetic to paramagnetic at T≈100 K, which can be explained with the interaction between Mn4＋and Mn3＋. The electrical resistivity p of samples increases with Mn doping. For the samples with doping level lower than 0.20, p initially increases with the decrease of temperature, i.e., dp/dt〈0, and then shows superconductivity transition at ≈20 K. The results suggest the coexistence of superconductivity and ferromagnetic ordering in Mn doped Nd1.85Ce0.15CuO4.     

声表面波器件用Pb[(Mn_(1/3)Nb_(2/3))_xZr_yTi_(1-x-y)]O_3压电陶瓷材料的研制

声表面波器件用铌锰锆钛酸铅系压电陶瓷材料,采用真空烧结工艺来提高陶瓷致密度,改善表面光洁度。添加CeO_2抑制晶粒生长,制得了小于2μm微晶压电陶瓷材料。进一步改善陶瓷材料的表面光洁度,样品经抛光后得表面最大孔洞小于0.5μm,适宜制作38MHz的声表面波滤波器件。     

Enhanced Collective Magnetic Properties Induced by the Controlled Assembly of Iron Oxide Nanoparticles in Chains

1D assemblies of magnetic nanoparticles are of great potential for designing novel nanostructured materials with enhanced collective magnetic properties. In that challenging context, a new assembly strategy is presented to prepare chains of magnetic nanoparticles that are well‐defined in structure and in spatial arrangement. The 1D assembly of iron oxide nanoparticles onto a substrate is controlled using “click” chemistry under an external magnetic field. Co‐aligned single nanoparticle chains separated by regular distances can be obtained by this strategy. The intrinsic high uniaxial anisotropy results in a strong enhancement of magnetic collective properties in comparison to 2D monolayers or isolated nanoparticles. In contrast to the intensively studied bundle chains of nanoparticles, the finely tuned chain structure reported here allows evidencing a first order intrachain dipolar interaction and a second order interchain magnetic coupling. This study offers new insights into the collective magnetic properties of highly anisotropic particulate assemblies which have been investigated by combining superconducting quantum interference device magnetometry, magnetic force microscopy, and ferromagnetic resonance.     

Temperature-controlled self-organized InP nanostructures grown on GaAs(1 0 0) substrate by MOCVD

The self-organized InP nanostructures grown on GaAs(0 0 1) substrates by metalorganic vapor deposition were examined in detail using atomic force microscopy. By properly selecting growth temperature, three kinds of nanostructure, islands, pits and ripples were formed. For growth temperature of 400–450 °C, the surface morphologies were governed by islands; but, for the growth temperature of 500 °C, the formation of surface ripples instead of islands was presumably due to the combination effect of temperature-controlled surface kinetics and strain effect. On the other hand, the observation of enhanced growth of pits upon a high-temperature annealing (at 685 °C for 90 s) indicated that the strained InP epitaxial film would be morphologically stabilized by taking the form of pits formation.     

Fabrication Method,Ferromagnetic Resonance Spectroscopy and Spintronics Devices Based on the Organic-Based Ferrimagnet Vanadium Tetracyanoethylene

Organic-based magnetic materials have been used for spintronic device applications as electrodes of spin aligned carriers and spin-pumping substrates. Their advantages over more traditional inorganic magnets include reduced magnetic damping and lower fabrication costs. Vanadium tetracyanoethylene, V[TCNE]_x (x ≈ 2), is an organic-based ferrimagnet with an above room-temperature magnetic order temperature (T_c ≈ 400 K). V[TCNE]_x has deposition flexibility and can be grown on a variety of substrates via low-temperature chemical vapor deposition (CVD). A systematic study of V[TCNE]_x thin-film CVD parameters to achieve optimal film quality, reproducibility, and excellent magnetic properties is reported. This is assessed by broadband ferromagnetic resonance (FMR) that shows most narrow linewidth of ≈1.5 Gauss and an extremely low Gilbert damping coefficient. The neat V[TCNE]_x films are shown to be efficient spin injectors via spin pumping into an adjacent platinum layer. Also, under an optimized FMR linewidth, the V[TCNE]_x films exhibit Fano-type resonance with a continuum broadband absorption in the microwave range, which can be readily tuned by the microwave frequency.