Decreasing Dot Size Research Articles

Formation of a Body-Centered Tetragonal Structure With High Magnetic Anisotropy and Coercivity in Fe–Co–V Films With High C Contents

A high amount of C (0–20 at.%) was added to Fe–Co–V films via sputtering to transform their body-centered cubic (bcc) structure to a face-centered cubic (fcc) lattice through a body-centered tetragonal (bct) structure. The Fe–Co films without V retained a bcc or bct crystal structure with an axial ratio ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c/a</i> ) of 1.03 even at high C contents close to 20 at.%. Meanwhile, in the Fe–Co films containing 10 at.% of V, a bct structure with 1.06 ≤ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c/a</i> ≤ 1.30 was formed after adding 5–12 at.% of C, which was subsequently transformed to an fcc structure with increasing C content. The saturation magnetization of the Fe–Co–V–C films with the bct structure was approximately 1000–1500 kA/m, and their uniaxial magnetic anisotropy constant ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> ) was approximately 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> J/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> (the films with high Fe contents exhibited high <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> values). The bct Fe–Co–V–C films with high <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> were microfabricated into nanodot array patterns via electron beam lithography. The coercivity of the pattern increased with decreasing dot size, and that of the pattern with a dot diameter of 85 nm was nearly an order of magnitude higher than that of the continuous film.

Time and spatial evolution of spin–orbit torque-induced magnetization switching in W/CoFeB/MgO structures with various sizes

We study spin–orbit torque (SOT) switching in W/CoFeB/MgO structures with various dot sizes (120–3500 nm) using pulsed current of various widths τ (800 ps–100 ms) to examine the time and spatial evolution of magnetization switching. We show that the switching behavior and the resultant threshold switching current density Jth strongly depend on device size and pulse width. The switching mode in a 3500 nm dot device changes from probabilistic switching to reproducible partial switching as τ decreases. At τ = 800 ps, Jth becomes more than 3 times larger than that in the long-pulse regime. A decrease in dot size to 700 nm does not significantly change the switching characteristics, suggesting that domain-wall propagation among the nucleated multiple domains governs switching. In contrast, devices with further reduced size (120 nm) show normal full switching with increasing probability with current and insignificant dependence of Jth on τ, indicating that nucleation governs switching.

Study on the Magnetization Dynamics of Ni–Fe Dot Arrays Estimated by the CPW-FMR Measurement Method

This paper describes the magnetization dynamics of Ni–Fe dot array estimated by the ferromagnetic measurement with the coplanar waveguide. The magnetic configuration of the dot changes from a multi-domain to a closure domain as the dot size decreases. The switching field at which a single domain is formed increases with the dot size reduction, which may be attributed to the enhanced magnetostatic energy. On the other hand, the damping constant decreases monotonically from 0.029 to 0.012 for the major axis length $ , and their values are higher than that of the Ni–Fe film ( $\alpha =0.009$ ). The reason for these differences is that the demagnetization field distribution near the dot edge becomes inhomogeneous with the dot size reduction. On the basis of these results, it is revealed that the extrinsic factor, such as the inhomogeneous demagnetization field distribution, strongly influences on the damping constant. These results also suggest that the correlation with the magnetic configuration of the dot is one of the most important to understand the magnetization dynamics in submicrometer-sized magnets in detail.

Morphology and magnetic properties of Fe3O 4 nanodot arrays using template-assisted epitaxial growth.

Arrays of epitaxial Fe3O4 nanodots were prepared using laser molecular beam epitaxy (LMBE), with the aid of ultrathin porous anodized aluminum templates. An Fe3O4 film was also prepared using LMBE. Atomic force microscopy and scanning electron microscopy images showed that the Fe3O4 nanodots existed over large areas of well-ordered hexagonal arrays with dot diameters (D) of 40, 70, and 140 nm; height of approximately 20 nm; and inter-dot distances (Dint) of 67, 110, and 160 nm. The calculated nanodot density was as high as 0.18 Tb in.−2 when D = 40 nm. X-ray diffraction patterns indicated that the as-grown Fe3O4 nanodots and the film had good textures of (004) orientation. Both the film and the nanodot arrays exhibited magnetic anisotropy; the anisotropy of the nanoarray weakened with decreasing dot size. The Verwey transition temperature of the film and nanodot arrays with D ≥ 70 nm was observed at around 120 K, similar to that of the Fe3O4 bulk; however, no clear transition was observed from the small nanodot array with D = 40 nm. Results showed that magnetic properties could be tailored through the morphology of nanodots. Therefore, Fe3O4 nanodot arrays may be applied in high-density magnetic storage and spintronic devices.

Optical absorption, photoluminescence and structural analysis of CdS quantum dots in weak confinement

The diffusion-controlled growth of CdS quantum dots (QDs) dispersed in a silicate glass matrix was investigated. It was found that the size of CdS QDs can be controlled by either heat treatment at various temperatures for a fixed duration or varying times at a constant temperature. Pastel yellow colored glass samples were obtained due to the presence of CdS petite crystals. X-ray diffraction (XRD) was used for determining the average dot size which varied from 3.8 to 30 nm. The typical quantum confinement effect was clearly observed from the blue shift measured in the optical absorption edge with decreasing dot size in the absorption spectroscopy. The band gap of CdS QDs ranges from 2.41 to 2.82 eV. Measured photoluminescence (PL) at an excitation wavelength of 350 nm showed the red shift of emission wavelength with increasing thermal treatment time and temperature in agreement with the increasing dot sizes. The half-width of PL spectra seems to indicate qualitatively the size distribution of dots and is consistent with the treatment parameters.

(Invited) Photoluminescence Efficiency of Germanium Dots Self-Assembled on Oxides

Self-assembled Ge quantum dots were formed by in-situ thermal annealing of a thin amorphous Ge layer deposited by molecular beam epitaxy either on a thin porous TiO2 layer grown on SiO2 on Si(001) or directly on the SiO2 layer itself. For samples with dot diameters ranging from 10 to 35 nm, the dot photoluminescence (PL) appeared primarily as a wide near-infrared band peaked near 800 meV. The peak energy of the PL band reflects the average dot size and its shape depends on the dot size distribution. Using tight binding and effective mass theoretical models, we have analyzed the PL spectrum in terms of the dot size distribution. The observed size distribution determined from transmission electron and atomic force microscopy allowed the determination of the nonlinear increase in the PL efficiency with decreasing dot diameter. Although the absolute intensities of the PL from the samples vary, the calculated efficiency curves are all well fitted by straight lines on a log-log plot, with essentially the same slope for all samples, thereby demonstrating that under the weak confinement regime investigated here there is a universal power-law increase in PL efficiency with decreasing dot size. Knowing this generic PL efficiency, we show that it is possible to evaluate the size distribution of Ge dots from their PL energy dependence.

Effects of subphase composition on the monolayer behavior of “core–coronae” type hybrid amphiphiles

As described in this paper, the effect of solutes on monolayer properties of a “core–coronae” type amphiphile was studied. The “core–coronae” type hybrid amphiphile comprises double-decker-shaped polyhedral silsesquioxane (DDSQ) as a core and four di(ethylene glycol) (DEG) units as coronae (4DEG-DDSQ). The molecule was spread onto a water subphase containing phosphoric acid, 1,3-bis(hydroxymethyl)urea, or potassium chloride. Then monolayer properties were studied using surface pressure (π)–area (A) isotherm and Brewster angle microscope measurements. Addition of phosphoric acid and 1,3-bis(hydroxymethyl)urea to the water subphase increased the limiting surface area of the 4DEG-DDSQ monolayer compared to that prepared in the pure water surface. The surface area increase is attributed to formation of strong hydrogen bonding between OH groups of DEG units and solutes. Addition of KCl showed a negligible change in the π–A isotherm, even though DEG units coordinate with K+ ions. The monolayer spread on the different subphases was transferred onto a solid substrate and the morphology of the transferred film was observed using atomic force microscopy (AFM). The AFM images show a dot-like structure, irrespective of the subphase composition, whereas the dot size decreases and the dot density increases with addition of solutes.

Hydrostatic pressure and temperature effects on the electronic energy levels of a spherical quantum dot placed at the center of a nano-wire

The effect of pressure and temperature on the electronic structure of an InAs spherical quantum dot located at the center of a GaAs cylindrical nano-wire have been determined using finite element method, within the effective mass approximation. The energy levels and transition energies are numerically calculated as a function of the dot radius, pressure and temperature. It is shown that the pressure and temperature effects are significant and should be considered in the study of low-dimensional semiconducting systems. The results show that; energy levels (i) decrease as the dot radius increases (ii) decrease as the pressure increases and (iii) increase as the temperature increases. For very small dot radii, the energy levels show unusual behavior, such that the energy levels increase as the pressure increases. We also found that the transition energy (i) increases as the dot size decreases (ii) increases as the pressure increases and (iii) decreases as the temperature increases.

Exciton dephasing in lead sulfide quantum dots byX-point phonons

Using a sensitive transient four-wave mixing technique in heterodyne detection, we measured the ground-state excitonic dephasing in PbS colloidal quantum dots of 4–6-nm diameter in the temperature range from 5 to 100 K. We observe an ultrafast (<100 fs) initial decay followed by a component of about 300 fs at 5 K, which becomes faster with decreasing dot size and increasing temperature. This dynamics is attributed to pure dephasing of phonon-assisted transitions in the peculiar band structure of PbS, which allows coupling to phonons with wave vectors both near the Brillouin zone center and near the (100) zone edge, the X point. We estimate an upper limit of 7% for the weight of the zero-phonon line with 2.6-ps spin-flip limited dephasing.

Photo-assisted STM and STM Fourier Transform Nanospectroscopy

Scanning tunneling microscopy (STM) based techniques combined with optical excitation enable us to analyze a variety of functional properties of nano-materials and nano-devices. A review of several works on photo-assisted STM is presented. It is shown that spatially resolved Fourier-transform photo-absorption spectra of individual GeSn nanodots, obtained by a technique based on STM, exhibited a distinct peak far below the absorption edge of the Si substrate, which showed a clear blue shift with decreasing dot size. The energy position of the peak was in good agreement with the optical transition energy between discrete levels predicted by the quantum-confinement size dependence.

Investigation of the magnetization reversal of a magnetic dot array of Co/Pt multilayers

The magnetization reversal behavior of a dot array consisting of Co/Pt multilayers with perpendicular magnetic anisotropy was investigated. The size of the dots was varied from 200 nm down to 40 nm, while keeping the filling factor constant at about 0.16. The structural properties were determined by scanning electron microscopy, whereas the magnetic investigation was performed using SQUID and MFM techniques. It was observed that the dot size has a severe impact on the magnetization reversal mechanism where only the smallest dots with a size of 40 nm are found to be in a magnetic single-domain state. Moreover, the patterning process leads to a degradation of the multilayer, leading to a reduction of the switching field and an increase of the switching field distribution with decreasing dot size. In addition, micromagnetic simulations were performed to understand the magnetization reversal mechanism in more detail.

Efficient photoluminescence from pulsed laser ablated nanostructured indium oxide films

Abstract Nanocrystalline indium oxide films have been deposited using pulsed laser ablation technique. The effect of post-deposition annealing on the structural, morphological, electrical and optical properties of the as-deposited indium oxide thin films prepared by non-reactive pulsed laser ablation has been studied systematically using X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), UV–vis spectra, photoluminescence spectra and DC electrical measurements. X-ray diffraction analysis reveals that at higher annealing temperatures, all the films exhibit a polycrystalline structure with a preferred orientation along (4 0 0) lattice plane instead of mostly occurring (2 2 2) reflection plane. The lattice constants, biaxial strain and lattice strain of the films are calculated and correlated with annealing temperature. Particle size calculated using Debye Scherrer's formula reveals the presence of quantum dots in the films annealed at 473 K and 773 K and confirmed via SEM and AFM analysis. Mass transport mechanisms have been proposed using SEM and AFM to account for the grain coalescence process viz., Ostwald ripening mechanism, sintering, and cluster migration. A blue shift in the band gap energy values are obtained for the films and is due to quantum confinement effect as a result of their nanostructured nature. Enhanced quantum confinement effect due to the increase in porosity in the films is also observed. Efficient photoluminescence emission is observed in all the films in the UV region and corresponds well with the band gap values obtained for the films. Electrical measurements show that the electrical conductivity in the films increases with decrease in dot size.

Measurement and analysis of optical gain spectra in 1.6 to 1.8 μm InAs/InP (100) quantum-dot amplifiers

Small signal modal gain measurements have been performed on two-section ridge waveguide InAs/InP (100) quantum-dot amplifiers that we have fabricated with a peak gain wavelength around 1.70 μm. The amplifier structure is suitable for monolithic active-passive integration, and the wavelength region and wide gain bandwidth are of interest for integrated devices in biophotonic applications. A 65 nm blue shift of the peak wavelength in the gain spectrum has been observed with an increase in injection current density from 1,000 to 3,000 A/cm2. The quantum-dot amplifier gain spectra have been analyzed using a quantum-dot rate-equation model that considers only the carrier dynamics. The comparison between measured and simulated spectra shows that two effects in the quantum-dot material introduce this large blue shift in the gain spectrum. The first effect is the carrier concentration dependent state filling with carriers of the bound excited and ground states in the dots. The second effect is the decrease in carrier escape time from the dots to the wetting layer with decreasing dot size.

Fourier-transform photoabsorption spectroscopy of quantum-confinement effects in individual GeSn nanodots

Spatially resolved Fourier-transform photoabsorption spectra of individual Ge1−xSnx nanodots, obtained by a technique based on scanning tunneling microscopy, exhibited a distinct peak far below the absorption edge of the Si substrate, which showed a clear blue shift with decreasing dot size. The energy position of the peak measured in high accuracy was in good agreement with the optical transition energy between discrete levels theoretically predicted by the size dependence due to a quantum-confinement effect, which was previously observed in scanning tunneling spectroscopic measurements.

Temperature and Size Dependence of Nonradiative Relaxation and Exciton−Phonon Coupling in Colloidal CdTe Quantum Dots

We report on the temperature and size dependence of the photoluminescence of core CdTe colloidal quantum dots (QDs). We show that at temperatures lower than 170 K a thermally activated transition between two different states separated by about 12−20 meV takes place. At temperatures higher than 170 K, the main nonradiative process is thermal escape assisted by multiple longitudinal optical (LO) phonons absorption. Moreover, we show that quantum confinement affects both the exciton−acoustic phonons and the exciton−LO phonons coupling. The coupling constant with acoustic phonons is strongly enhanced in QDs (up to 31μeV/K) with respect bulk CdTe (0.7μeV/K). On the contrary, the exciton-LO phonons coupling constant decreases as the dot size decreases (down to 14 meV with respect 24.5 meV in the bulk).

Size effects on exchange bias in polycrystalline Ni–Fe/Fe–Mn square dots

The effects of the dot size on the exchange bias in the Ni–Fe/Fe–Mn square dots have been investigated. The exchange bias field and the blocking temperature of the square dots decrease as the dot size decreases. We consider this behavior is because the spins are fluctuated by the thermal activation in the laterally constricted antiferromagnetic grains which are located at the edges of the dots.

Dot size dependence of magnetic properties in microfabricated L10-FePt (001) and L10-FePt (110) dot arrays

L 1 0 - Fe Pt (001) and L10-FePt (110) dot arrays with well-defined geometry were fabricated through the use of electron beam lithography and Ar ion etching. The lateral size of dots was varied in the range from 0.2×0.2to5×5μm2. Coercivity (Hc) for the perpendicularly magnetized FePt (001) dots increases with decreasing the dot size. In the case of the FePt (110) dots with in-plane magnetization, on the other hand, the dot size dependence of Hc is completely different from that for FePt (001) dots: Hc shows a slight decrease as the dot size decreases. After annealing at 600°C, the values of Hc for both FePt (001) and FePt (110) dots are remarkably enhanced although the dot size dependence shows similar behavior to that before annealing. The magnetization reversal for all the dots occurs through the nucleation of reversed domains and subsequent domain wall propagation.

Exciton fine structure and biexciton binding energy in single self-assembled InAs∕AlAs quantum dots

The exciton and biexciton emissions of a series of single quantum dots of InAs in an AlAs matrix have been studied. These emissions consist of linear cross polarized doublets showing large values of both the biexciton binding energy and the fine-structure splitting. At increasing exciton emission energy, corresponding to decreasing dot size, the biexciton binding energy of 9meV decreases down to zero, reflecting a possible crossover to an antibinding regime. Simultaneously the fine-structure splitting diminishes from a value of 0.3meV down to zero, at the same energy, suggesting a common origin for the two effects.

Fine structure splitting and biexciton binding energy in single self-assembled InAs/AlAs quantum dots

We report on photoluminescence investigations of individual InAs quantum dots embedded in an AlAs matrix which emit in the visible region, in contrast to the more traditional InAs/GaAs system. Biexciton binding energies, considerably larger than for InAs/GaAs dots, up to 9 meV are observed. The biexciton binding energy decreases with decreasing dot size, reflecting a possible crossover to an antibinding regime. Exciton and biexciton emission consists of linearly cross polarized doublets due to a large fine structure splitting up to 0.3 meV of the bright exciton state. With increasing exciton transition energy the fine structure splitting decreases down to zero at about 1.63 eV. Differences with InAs/GaAs QDs may be attributed to major dot shape anisotropy and/or larger confinement due to higher AlAs barriers.

軟磁性裏打ち層を有する磁気ドットアレイの磁気特性

Magnetic dot arrays were fabricated by using focused ion beam (FIB) lithography to pattern a continuous CoPt film with the aim of establishing design guidelines for patterned media with a soft magnetic underlayer. First, the magnetic influences of Ga ion irradiation were investigated in order to understand the characteristics of FIB milling. It was found that the implantation of Ga ions in the recording layer or the soft magnetic underlayer seriously affected the magnetic properties. Magnetic dot arrays were then fabricated, taking account of this effect. The switching field of the film drastically increased with the patterning and exhibited a further increase with decreasing dot size. The slope of the remanent magnetization curve decreased considerably as the dot spacing was decreased. The results suggest that patterned media should have a smaller dot size and larger spacing.