F. Yıldız

Magnetic anisotropies in ultrathin iron films grown on the surface-reconstructed GaAs substrate

Magnetic anisotropies of epitaxial ultrathin iron films grown on the surface-reconstructed GaAs substrate were studied. Ferromagnetic resonance technique was exploited to determine magnetic parameters of the films in the temperature range of 4–300K. Extraordinary angular dependence of the FMR spectra was explained by the presence of fourfold and twofold in-plane anisotropies. A strong in-plane uniaxial anisotropy with magnetic hard axis along the [11¯0] crystallographic direction is present at the GaAs∕Fe(001) interface while a weak in-plane uniaxial anisotropy for the Fe grown on Au has its easy axis oriented along [11¯0]. A linear dependence of the magnetic anisotropies as a function of temperature suggests that the strength of the in-plane uniaxial anisotropy is affected by the magnetoelastic anisotropies and differential thermal expansion of contacting materials.

Magnetization and impedance measurements of multilayer Co/Cu structures in millimeter waveband

The saturation magnetization of Co/Cu multilayer structures and individual Co films 1.0–4.0 nm thick have been studied both by electron spin resonance technique and by vibrating sample magnetometer method. The results of static and dynamic measurements are in good agreement. It was found that the saturation magnetization of Co layers imbedded in a multilayer structure was greater than the magnetization of individual films and tended to the value of bulk material. The saturation magnetization of multilayer structure with thickness of nonmagnetic Cu interlayer varying from 1.7–10 nm have been investigated for samples both with and without Fe-buffer layer. For the first case Co saturation magnetization has decreased with increase of nonmagnetic interlayer in contrast to similar samples without buffer. The measurements of impedance characteristics have been carried out for the multilayers Fe(6 nm)/[Co(tC)/ Cu(2 nm)]8 (t=1, 2, 3 nm) in the millimeter waveband (at n ¼ 44 GHz). The transmission coefficient dependence on the external static magnetic field for a waveguide containing the multilayer specimen was found to be the same as the dependence of direct current giant magnetoresistance of the mentioned specimen. r 2003 Elsevier Science B.V. All rights reserved.

Ion synthesis and FMR studies of iron and cobalt nanoparticles in polyimides

Polyimide foils were implanted with 40 keV Fe+ and Co- to doses of 0.25-1.5×1017 ions/cm2. Electron microscopy studies showed the formation of iron and cobalt nanoparticles in the implanted polymer layer with a thickness of about 70 nm. The size and shape of the ion-synthesized metal nanoparticles depend on the implantation parameters and subsequent thermal annealing. A ferromagnetic resonance (FMR) response was found in the iron-implanted samples as well as in the annealed cobalt and iron samples. The effective magnetization values of the metal/polymer composite layers were extracted from the FMR spectra and plotted as a function of implantation dose. The magnetic properties of the iron and cobalt nanoparticles in polyimide are compared and discussed.

Magnetic and structural properties of the Fe layers in CoO/Fe/Ag(001) heterostructure

The influence of interfacial oxidation on the magnetic behaviour of CoO covered Fe/Ag(001) is reported. Coverage with CoO causes the formation of a mixed Fe2O3-Fe3O4 interfacial oxide layer. The depth of the Fe-oxide varies with the thickness of pre-covered Fe and above 8 monolayers (MLs) of Fe the oxide depth becomes constant at 2 ML. Differences in exchange bias and coercivity obtained from magnetic field and zero field cooling nearly vanish above 8 ML Fe thickness, showing a direct correlation between the magnetic behaviour of Fe and structure of the interfacial Fe-oxide layer.

Highly efficient inverted organic light emitting diodes by inserting a zinc oxide/polyethyleneimine (ZnO:PEI) nano-composite interfacial layer

The electrode/organic interface is one of the key factors in attaining superior device performance in organic electronics, and inserting a tailor-made layer can dramatically modify its properties. The use of nano-composite (NC) materials leads to many advantages by combining materials with the objective of obtaining a desirable combination of properties. In this context, zinc oxide/polyethyleneimine (ZnO:PEI) NC film was incorporated as an interfacial layer into inverted bottom-emission organic light emitting diodes (IBOLEDs) and fully optimized. For orange-red emissive MEH-PPV based IBOLEDs, a high power efficiency of 6.1 lm W-1 at a luminance of 1000 cd m-2 has been achieved. Notably, the external quantum efficiency (EQE) increased from 0.1 to 4.8% and the current efficiency (CE) increased from 0.2 to 8.7 cd A-1 with rise in luminance (L) from 1000 to above 10 000 cd m-2 levels when compared to that of pristine ZnO-based devices. An identical device architecture containing a ZnO:PEI NC layer has also been used to successfully fabricate green and blue emissive IBOLEDs. The significant enhancement in the inverted device performance, in terms of luminance and efficiency, is attributed to a good energy-level alignment between the cathode/organic interface which leads to effective carrier balance, resulting in efficient radiative-recombination.

The magnetic anisotropy of thin epitaxial CrO2 films studied by ferromagnetic resonance

The magnetic anisotropy of thin epitaxial films of chromium dioxide (CrO2) has been studied as a function of the film thickness by the ferromagnetic resonance (FMR) technique. CrO2 films with various thicknesses in the range from 27 to 535 nm have been grown on (100)-oriented TiO2 substrates by chemical vapor deposition using CrO3 as a solid precursor. In a series of CrO2 films grown on the substrates cleaned by etching in a hydrofluoric acid solution, the FMR signal exhibits anisotropy and is strongly dependent on the film thickness. The magnetic properties of CrO2 films are determined by a competition between the magnetocrystalline and magnetoelastic anisotropy energies, the latter being related to elastic tensile stresses caused by the lattice mismatch between the film and the substrate. In the films of minimum thickness (27 nm), this strain-induced anisotropy is predominant and the easy magnetization axis switches from the [ 001] crystallographic direction (characteristic of the bulk magnet) to the [ 010] direction.

Temperature Dependence of Magnetic Properties of Single Crystalline Fe3O4 Thin Films Epitaxially Grown on MgO

Single crystalline Fe3O4 thin film of 110 angstrom in thickness grown on MgO (100) substrate have been investigated by Electron Spin Resonance technique as a function of temperature between 3 and 300K. Experimental spectra taken at different temperatures were analysed by classical Spin Wave Resonance (SWR) technique for different orientations of sample for both in-plane and out-of plane geometry. SWR spectra exhibit unidirectional character upon cooling the sample in the presence of external field, Hc. A strong unidirectional (Exchange) anisotropy field as evidenced from a shift of the resonance field for r-FC case relative to that for n-FC case at low temperature. Using a field-induced unidirectional exchange and bulk anisotropy, with the usual magneto-crystalline cubic anisotropy energy, we have simulated the experimental spectra and angular dependence of resonance field and deduced anisotropy and other magnetic parameters as a function of temperature. Verwey transition for thin film case was found to shift down to around 90 K from 120 K for bulk sample of the same compound.

Effect of MPEG Coating on Magnetic Properties of Iron Oxide Nanoparticles: An ESR Study

Metoxy polyethylene glycol (MPEG) coated and uncoated Fe3O4 (SPION) nanoparticles have been investigated by Electron Spin Resonance (ESR) technique at the temperature range between 10–300K. ESR measurements on powdered samples have been carried out by using X-band ESR Bruker EMX spectrometer. A strong and broad single ESR signal has been observed at all temperatures. Temperature dependence of ESR linewidth and resonance field is studied. The linewidth increases and the resonance field decreases by decreasing temperature while the ESR signal intensity remeains almost independent on the temperature. The shift in the resonance field value is a clear indication of the induced field (exchange anisotropy field), causing the frustration (disorder) of any magnetic system. A strong effect of coating on magnetic properties of SPION nanoparticles has been observed as well.

Solution processed ternary blend nano-composite charge regulation layer to enhance inverted OLED performances

Inverted bottom-emission organic light emitting diodes (IBOLEDs) have attracted increasing attention due to their exceptional air stability and applications in active-matrix displays. For gaining high IBOLED device efficiencies, it is crucial to develop an effective strategy to make the bottom electrode easy for charge injection and transport. Charge selectivity, blocking the carrier flow towards the unfavourable side, plays an important role in determining charge carrier balance and accordingly radiative recombination efficiency. It is therefore highly desirable to functionalize an interfacial layer which will perform many different tasks simultaneously. Here, we contribute to the hole-blocking ability of the zinc oxide/polyethyleneimine (ZnO:PEI) nano-composite (NC) interlayer with the intention of increasing the OLED device efficiency. With this purpose in mind, a small amount of 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) was added as a hole-blocking molecule into the binary blend of ZnO and P...

Effect of Interdot Distance on Magnetic Behavior of 2-D Ni Dot Arrays

Square array of circular dots with varying interdot spacing in the range of 1-4 μm has been investigated. The dots were patterned by physical mask. Magnetic properties were explored by the ferromagnetic resonance technique and a micromagnetic simulation based on metropolis algorithm. Evaluation of the resonance fields showed that magnetic anisotropy depends on interdot distance indicating an evidence of interdot coupling in system. The coupling is getting weaker with increasing separation from 1 to 4 μm. The study revealed that magnetic behavior of the patterned structures can be tuned by changing interdot distance.