Positive and negative exchange bias in IrMn/NiFe bilayers

We present the observation of double shifted hysteresis loops in IrMn/NiFe bilayer structures. The bilayer structures were fabricated using high vacuum DC magnetron sputtering system. The hysteresis loops of the as deposited samples show the double shifted loops at NiFe layer thicknesses 5 nm and 6 nm, whereas the IrMn layer thickness was kept constant at 15 nm. The results were interpreted as the contribution of both positive and negative exchange bias fields. We suppose that this phenomenon is occurring due to the ferromagnetic (FM) layer exchange coupled with the antiferromagnetic (AFM) layer in two different magnetization directions. The ferromagnetic coupling of the interface spins in some regions of the film generates the hysteresis loop shift toward negative fields and antiferromagnetic coupling toward positive fields in the other regions. The double shifted hysteresis loops disappeared after magnetic field annealing of the samples above Neel temperature of the AFM layer. The X-ray diffraction patterns of the sample show the IrMn (111) crystalline growth necessary for the development of exchange bias field in this system. The correlation between the Magnetic Force Microscopy (MFM) domain structures of the as deposited sample and the magnetization reversal process of the double shifted hysteresis loops were discussed. The results suggest that the larger multidomain formation in the AFM layer with different magnetization directions was responsible for the positive and negative exchange bias fields in IrMn/NiFe bilayer samples.     

Microstructure and magnetic properties of BaTiO3-(Ni,Zn)Fe2O4 multiferroics

The microstructures of composite xBaTiO₃-(1−x)(Ni_0.5Zn_0.5)Fe₂O₄ (BT-NZF) multiferroics with various mixing ratios (x = 0.50, 0.60 and 0.70) are investigated by means of electron backscatter diffraction (EBSD) and magnetic force microscopy (MFM). The EBSD measurements reveal a change in the texture of the ferrite and the BaTiO₃ grains upon increasing the ferrite content in the sample. The sample with x = 0.70 exhibits the best ferrite texture, where only some directions are present. Furthermore, the resulting grain sizes vary from several µm (x = 0.50) to about 100 nm in the sample with x = 0.70. The MFM images reveal the presence of magnetic domains being extended over several adjacent grains, which according to the EBSD data may comprise different crystallographic orientations. In this way, we can explain the differences in the magnetic contrast obtained.     

Effect of the tip-contact interaction on the MFM image of magnetic nanocontact

MFM images of magnetic nanocontacts fabricated in nano-oxide layers are evaluated with special attention to the tip-contact interaction. We show that a sharp ring or arc signal with a diameter on the order of 100 nm appears while the diameter of the contact is on the order of 1 nm. We also show that the position of the sharp signal is determined by the competition between the forces due to the magnetic anisotropy in the contact and due to the tip-contact interaction.     

High resolution magnetic force microscopy of patterned L1(0)-FePt dot arrays by nanosphere lithography

High resolution magnetic force microscopy (MFM) has been carried out on L1(0)-FePt dot arrays patterned by plasma modified nanosphere lithography. An ex situ tip magnetization reversal experiment is carried out to determine the magnetic domains and verify the imaging stability of MFM and the mutual perturbations between the magnetic tip and the sample. We have identified that the critical size for the single domain region is about 90?nm across. Comparison with MFM image simulation also suggests that the magnetizations of the triangular dots in both single and double domain states are parallel to one edge of the dots, indicating the large uniaxial magnetocrystalline anisotropy of the L1(0)-FePt phase and the need for decreasing the magnetostatic energy.     

Physicochemical characterization of Fe3O4/SiO2/Au multilayer nanostructure

The purpose of this research was to synthesize and characterize gold-coated Fe₃O₄/SiO₂ nanoshells for biomedical applications. Magnetite nanoparticles (NPs) were prepared using co-precipitation method. Smaller particles were synthesized by decreasing the NaOH concentration, which in our case this corresponded to 35 nm using 0.9 M of NaOH at 750 rpm with a specific surface area of 41 m² g⁻¹. For uncoated Fe₃O₄ NPs, the results showed an octahedral geometry with saturation magnetization range of 80–100 emu g⁻¹ and coercivity of 80–120 Oe for particles between 35 and 96 nm, respectively. The magnetic NPs were modified with a thin layer of silica using Stober method. Small gold colloids (1–3 nm) were synthesized using Duff method and covered the amino functionalized particle surface. Magnetic and optical properties of gold nanoshells were assessed using Brunauer–Emmett–Teller (BET), vibrating sample magnetometer (VSM), UV–Vis spectrophotometer, atomic and magnetic force microscope (AFM, MFM), and transmission electron microscope (TEM). Based on the X-ray diffraction (XRD) results, three main peaks of Au (1 1 1), (2 0 0) and (2 2 0) were identified. The formation of each layer of a nanoshell is also demonstrated by Fourier transform infrared (FTIR) results. The Fe₃O₄/SiO₂/Au nanostructures, with 85 nm as particle size, exhibited an absorption peak at ∼550 nm with a magnetization value of 1.3 emu g⁻¹ with a specific surface area of 71 m² g⁻¹.     

Growth and in-situ electrical characterization of ultrathin epitaxial TiN films on MgO

We examine the properties of ultrathin TiN films grown by reactive dc magnetron sputtering on single-crystalline MgO(100) substrates at growth temperatures ranging from 30 to 650 °C. The resistance of the films is measured in-situ, during growth, to study the thickness at which the films coalesce and become structurally continuous. Both the in-situ resistance measurements and X-ray diffraction measurements show a clear transition from polycrystalline growth to epitaxial (100) growth well below typical TiN growth temperatures, or between 100 and 200 °C. The coalescence and continuity thicknesses are 1.09 ± 0.06 nm and 5.5 ± 0.5 nm, respectively, at room temperature but reach a minimum of 0.08 ± 0.02 nm and 0.7 ± 0.1 nm, respectively, at 600 °C. A large drop in resistivity is seen with increasing growth temperature and the resistivity reaches 16.6 μΩ cm at 600 °C. Achieving epitaxy at such a low temperature and a low continuity thickness is important in a variety of applications such as device interconnects and metal-oxide-semiconductor devices.     

A low-cost method for preliminary separation of reduced graphene oxide nanosheets

Reduced graphene oxide (RGO) nanosheets were produced by chemical reduction of exfoliated graphite oxide. Atomic force microscopy (AFM) images show that the obtained RGO nanosheets vary greatly in lateral-dimensional sizes, ranging from less than 100  100 nm to more than 2000  2000 nm. In order to separate these nanosheets, one simple and low-cost method mainly based on magnetic-stirring and centrifugation treatments was proposed. Preliminary statistical analysis of RGO nanosheets, based on AFM images, shows that the dot-like RGO nanosheets (with lateral dimensions less than 100  100 nm) and leaf-like RGO nanosheets (with lateral dimensions more than 500  500 nm) were effectively separated by this simple method.     

Dielectric/metal/dielectric structures using copper as metal and MoO3 as dielectric for use as transparent electrode

Transparent conductive oxide/metal/oxide, where the oxide is MoO₃ and the metal is Cu, is realized and characterized. The films are deposited by simple joule effect. It is shown that relatively thick Cu films are necessary for achieving conductive structures, what implies a weak transmission of the light. Such large thicknesses are necessary because Cu diffuses strongly into the MoO₃ films. We show that the Cu diffusion can be strongly limited by sandwiching the Cu layer between two Al ultra-thin films (1.4 nm). The best structures are glass/MoO₃ (20 nm)/Al (1.4 nm)/Cu (18 nm)/Al (1.4 nm)/MoO₃ (35 nm). They exhibit a transmission of 70% at 590 nm and a resistivity of 5.0 · 10^− 4 Ω cm. A first attempt shows that such structures can be used as anode in organic photovoltaic cells.     

Molecular beam epitaxy grown GeSn p-i-n photodetectors integrated on Si

GeSn p-i-n photodetectors with a low Sn mole fraction made by molecular beam epitaxy on Si substrates show higher optical responsivities for wavelength λ > 1400 nm compared with p-i-n photodetectors made from pure Ge. The Sn incorporation in Ge is done by a low temperature growth step in order to minimize Sn segregation. The Sn incorporation and the alloy content are investigated by μ-Raman spectroscopy and calibrated Secondary Ion Mass Spectrometry. The photodetectors are manufactured with sharp doping transitions and are realized as double mesa structures with diameters from 1.5 μm up to 80 μm. The optical measurements are carried out with a broadband super continuum laser from λ = 1200 nm up to λ = 1700 nm. At a wavelength of λ = 1550 nm the optical responsivity of these vertical GeSn diodes is 0.1 A/W. In comparison with a pure Ge detector of the same geometrical dimensions the optical responsivity is increased by factor of three as a result of Sn caused band gap reduction.     

Preparation and luminescence properties of spin-coated PMMA films containing Si nanocrystallites

We report on the synthesis of Si nanocrystallites by pulsed laser ablation in toluene followed by the preparation of composite films with PMMA and their luminescence studies. Transmission electron microscopy images show that the sizes of silicon nanocrystallites vary from about 4 nm down to below 1 nm. The composite films exhibit strong emissions with their spectral peaks continuously moving from 387 to 506 nm when the excitation wavelength varies from 300 to 440 nm, in accordance with the quantum confinement effect. Their time-resolved photoluminescence spectra reveal a multi-exponential decay, implying that the light emission may be also related to some surface states.     

Effect of TiO2 nanopatterns on the performance of hydrogenated amorphous silicon thin-film solar cells

We investigate how TiO₂ nanopatterns formed onto ZnO:Al (AZO) films affect the performance of hydrogenated amorphous silicon (a-Si:H) solar cells. Scanning electron microscopy results show that the dome-shaped TiO₂ nanopatterns (300 nm in diameter) having a period of 500 nm are formed onto AZO films and vary from 60 to 180 nm in height. Haze factor increases with an increase in the height of the nanopatterns in the wavelength region below 530 nm. Short circuit current density also increases with an increase in the height of the nanopatterns. As the nanopatterns increases in height, the fill factor of the cells slightly increases, reaches maximum (0.64) at 100 nm, and then decreases. Measurements show that a-Si:H solar cells fabricated with 100 nm-high TiO₂ nanopatterns exhibit the highest conversion efficiency (6.34%) among the solar cells with the nanopatterns and flat AZO sample.     

Inversion of diblock copolymer micelles by selective solvents for conversion of gold nanopatterns

We report the creation of spherical and donut-like nanostructures by the inversion of diblock copolymers. We synthesized a diblock copolymer, consisting of one block which allows the synthesis of nanoparticles and the other block which is selectively removable. Using a selective solvent for each block, we produced two types of micelles, which have the inversed position of two blocks, that is, the core of the nanoparticle-synthesizable block and the corona of the removable block (~ 29 nm in diameter), and vice versa (~ 53 nm in diameter). Single layers of these two micelles on substrates were converted to spherical or donut-like nanostructures by selective elimination of the removable coronas or cores. Moreover, these nanostructures were employed as templates for the synthesis and arrangement of spherical nanoparticles (~ 24 nm) and their ring-like configuration in large area.     

Synthesis and photoluminescence of ultra-pure germanium nanoparticles

We have used aerosol deposition to synthesize defect and micro-strain free, ultra-pure germanium nanoparticles. Transmission electron microscopy images show a core-shell configuration with highly crystalline core material. Powder X-ray diffraction measurements verify the presence of highly pure, nano-scale germanium with average crystallite size of 30 nm and micro-strain of 0.058%. X-ray photoelectron spectroscopy demonstrates that GeO_x (x ? 2) shells cover the surfaces of the nanoparticles. Under optical excitation, these nanoparticles exhibit two separate emission bands at room temperature: a visible emission at 500 nm with 0.5-1 ns decay times and an intense near-infrared emission at 1575 nm with up to ∼20 μs lifetime.     

Magnetization reversal in nanotriangles fabricated by nanosphere lithography

We report on the magnetization reversal behavior of sub 100-nm triangular shaped Ni₈₀Fe₂₀ dot array fabricated by nanosphere lithography. Hysteresis loops measured by magneto-optical Kerr effect magnetometry are classified into single and double-switched loops in 45 nm, 80 nm and 100 nm triangular nanomagnets. Micromagnetic simulations show that a plateau observed in the double-switched loop in the 100-nm triangular nanomagnet is due to the formation of a metastable mediating V state.     

Resistivity of sub-50 nm copper lines epitaxially grown on Si(100) substrate

We report the creation of 50 nm thick epitaxial Cu lines with line widths ranging from 20 nm to 120 nm on Si(100) substrate using a combination of electron beam lithography, oblique angle deposition, and lift-off techniques. The increase of measured resistivity as a function of decreasing line width is dominated by surface scattering that is completely diffuse. The measured resistivity of the 20 nm wide lines is ~ 4 μΩ-cm.     

Characterization of nano-crystalline ZrO2 synthesized via reactive plasma processing

Nano-crystalline ZrO₂ powder has been synthesized via reactive plasma processing. The synthesized ZrO₂ powders were characterized by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM) and FTIR spectroscopy. The synthesized powder consists of a mixture of tetragonal and monoclinic phases of zirconia. Average crystallite size calculated from the XRD pattern shows that particles with crystallite size 20 nm or less than 20 nm are in tetragonal phase, whereas particles greater than 20 nm are in the monoclinic phase. TEM results show that particles have spherical morphology with maximum percentage of particles distributed in a narrow size from about 15 nm to 30 nm.     

Cathodoluminescence of TiO2 nanotubes prepared by low-temperature anodization of Ti foils

We show that anodization of Ti sheets in an ethylene glycol and HF containing electrolyte at temperatures under 0 °C results in the formation of a self-arranged ordered porous structure at the top surface of the sample. This perforated surface structure initiates the growth of an ordered array of titania nanotubes. The inner diameter of nanotubes can be modified in a controlled fashion in the range from 10 nm to more than 250 nm through the change of the electrolyte temperature from −20 °C to + 50 °C. The spectral distribution of cathodoluminescence from a cluster of nanotubes clearly demonstrates the formation of resonator modes which are separated from each other by around 200 meV.     

Synthesis of hollow silica spheres SBA-16 with large-pore diameter

Hollow silica SBA-16 spheres with cubic ordered mesoporous shells were synthesized by an emulsion-templating method, using Pluronic F127 as a structure-directing agent, tetraethyl orthosilicateas as a silica source and heptane as a cosolvent in the presence of NH₄F. The size of these spheres is in the range of 10 to 30 μm. The shell is about 700 nm thick and consists of large pores, ~ 9 nm in diameter, arranged in a cubic order. After calcination, the spheres maintain their mesoporosity and show a high surface area of 822 m²/g. The formation mechanism of the silica hollow spheres is discussed.     

Highly transparent o-PDA functionalized ZnS-polymer nanocomposite thin films with high refractive index

We prepared highly transparent nanocomposite films with high refractive index using fluorescent nanocrystal quantum dots (NQDs). The as synthesized transparent solution of ZnS NQDs was blended with poly(vinylpyrrolidone) (PVP) to prepare nanocomposite thin films. Morphological data, studied by atomic force microscopy (AFM) and X-ray diffraction (XRD), revealed that NQDs were impregnated with polymer matrix and the size distributions (3.0 ± 0.30 nm) of them were preserved in the composite films. The nanocomposite films show high optical transparency (T > 95% at 400 nm and T > 98.5% at 750 nm) and the refractive index is satisfactorily increased (1.565 at 550 nm, 15 wt.% ZnS) compared to the base polymer (1.480 at 550 nm). The nanocomposite films show defectless fluorescence emissions as observed from NQDs before impregnation.     

One-step synthesis of tenorite (CuO) nano-particles from Cu4 (SO4) (OH) 6 by direct thermal-decomposition method

In this research work, CuO nano-particles were synthesized at 750 °C (for 2 h) by the direct thermal-decomposition method using the brochantite as precursor. The nano-particles were characterized using X-ray diffraction (XRD), energy dispersive spectra (EDS), infrared spectrum (IR), and scanning electron microscope (SEM). SEM image showed that the CuO nano-particles were of rod shape with diameter and length of 235 ± 5 nm and 856 ± 5 nm, respectively. As a result, this method could be used at an industrial scale as a cheap and convenient way in production of pure tenorite nano-particles.