Bekir Aktaş

Effect of doping and disorder on the half metallicity of full Heusler alloys

Heusler alloys containing Co and Mn are amongst the most heavily studied half metallic ferromagnets for future applications in spintronics. Using state-of-the-art electronic structure calculations, we investigate the effect of doping and disorder on their electronic and magnetic properties. Small degrees of doping by substituting Fe or Cr for Mn scarcely affect the half metallicity. A similar effect is also achieved by mixing the sublattices occupied by the Mn and sp atoms. Thus the half metallicity is a robust property of these alloys.

FMR properties of epitaxial Fe3O4 films on MgO(100)

Abstract The magnetic properties of epitaxial Fe3O4 films on MgO(100) substrate have been investigated. Spin wave resonance (SWR) technique has been used to study the magnetic anisotropy as a function of temperature between 4–300 K. The ferromagnetic resonance (FMR) spectra exhibit unusual unidirectional character upon cooling the sample in an external field Hc. At higher temperatures, generally both the bulk and the surface modes appear as long as the external field is applied within a small angular interval with respect to the film normal. The bulk and the surface FMR modes are well separated from each other when the field, Hap, is parallel to the cooling field, Hc, which is normal to the film plane. However these FMR modes surprisingly overlap and become only a single and broader absorption line when the applied field is antiparallel to the cooling field. Using a field-induced unidirectional exchange surface and bulk anisotropy, with the usual magneto-crystalline anisotropy energy, we have simulated the experimental spectra and deduced anisotropy and magnetic stiffness parameters as a function of temperature.

High Curie-temperature ferromagnetism in cobalt-implanted single-crystalline rutile

The ion implantation technique has been used to fabricate a Co-rich layer in rutile: single-crystalline TiO2 substrates were heavily irradiated by Co+ ions with energy of 40?keV. The magnetic properties of as-prepared and post-annealed samples were studied by both inductive and Faraday magnetometry as well as ferromagnetic resonance (FMR). A ferromagnetic Curie temperature as high as 700?K was measured in our samples. The analysis of the magnetic hysteresis loop, the temperature dependence of the saturation magnetization, and strong out-of-plane anisotropy of the FMR spectra allow us to suppose that the origin of the macroscopic high-temperature ferromagnetism is the exchange interaction mediated by oxygen vacancies.

Negative Permittivity of Polyaniline–Fe3O4 Nanocomposite

Polyaniline–Fe3O4 nanocomposite with and without ionic liquid were successfully synthesized via in situ polymerization using cetyl trimethylammonium bromide (CTAB) as surfactant. Both TG analysis and FT-IR measurements proved the presence of organic layer on the surface of Fe3O4 nanoparticles. The influence of 1-butyl-3-methyl-imidazolium bromide (BMIMBr) as ionic liquid on the structure, conductivity, and magnetic property of PANI–Fe3O4–CTAB nanocomposite were studied in detail. The results show that imidazolium-based ionic liquids BMIMBr acts as an anchor agent during the formation of PANI–Fe3O4–CTAB nanocomposite. Ionic liquid significantly deteriorated nanocomposite’s magnetic properties, and contributed to non-saturated M–H curve due to the disappearance of antiferromagnetic interactions. It has also an improving effect on AC and DC conductivities. The most important effect of IL is observed in real part of permittivity of PANI–Fe3O4–CTAB that it has negative high values at low frequency low temperature region. Due to the negative dielectric constant, material exhibits uncommon properties in electromagnetic waves scattering and attraction between similar charges. This possibility provokes research on these composites as high T superconductors, negative index materials and microwave absorbers.

Anisotropy of ferromagnetism in Co-implanted rutile

Magnetic anisotropy of cobalt implanted single-crystalline rutile has been studied by means of magneto-optical Kerr effect (MOKE) and superconducting quantum interference device (SQUID) techniques. We observed for the first time strong angular dependence of the remanent magnetization and coercive field in the plane of the implanted surface: twofold anisotropy for the (100)-substrate and fourfold anisotropy for the (001)-substrate samples. The observation opens up new possibilities to tailor magnetic anisotropies of the material. Possible origins of ferromagnetism and anisotropies in dielectric and diamagnetic single-crystalline TiO2 samples after Co-ion implantation are discussed.

Synthesis, Characterization and EPR Studies of Supramolecular Porphyrazines

Supramolecular porphyrazines with eight pyridine donor groups bound through ethylthio ester bridges on the periphery have been prepared. The pyridine donors were quaternized with iodomethane to octacationic porphyrazine. The nonanuclear supramolecular porphyrazine was prepared by the coordination of peripheral pyridine donors with VO(acac)2. The paramagnetic nonanuclear structure was studied in powder and in solution form by EPR. EPR studies together with the other spectral data confirmed the presence of identical pyridine-coordinated VO(acac)2 paramagnetic centers attached to the peripheral positions of the porphyrazine core.

Ion synthesis of iron granular films in polyimide

Abstract 40 keV Fe+ ions were implanted into polyimide (PI) films with doses of (0.25–1.25)×1017 ions/cm2 at ion current densities of 4, 8 and 12 μA/cm2. Transmission electron microscopy study showed that iron granular films consisting of isolated metal particles with lateral sizes of 25–300 nm were formed in the irradiated (carbonised) layer of polymer. An influence of implantation parameters and thermal annealing on magnetic properties of the synthesised iron films was investigated by the ferromagnetic resonance technique. It was found that the metal/polymer composite layer is formed at highest dose of 1.25×1017 ions/cm2 and it reveals the characteristics of a continuous magnetic film: remanent magnetisation and coercive field.

SYNTHESIS AND ESR STUDIES OF A SOLUBLE VANADYL PORPHYRAZINE

Octakis(hexylthio)vanadyl porphyrazinate, (VOPz), was prepared from vanadyl(IV) sulfate and metal-free porphyrazine (H2Pz), which was synthesized by the cyclotetramerization of the corresponding dinitrile derivative and further treatment with trifluoroacetic acid. Intense absorption peaks at 699 and 352 nm in the electronic spectrum indicated the C4v symmetry of VOPz. ESR studies, both in solution and in powder form, confirmed the proposed structure.

Six-fold in-plane magnetic anisotropy in Co-implanted ZnO (0001)

Magnetic anisotropies of Co-implanted ZnO (0001) films grown on single-crystalline Al2O3 (112¯0) substrates have been studied by ferromagnetic resonance (FMR) technique for different cobalt implantation doses. The FMR data show that the easy and hard axes have a periodicity of 60° in the film plane, in agreement with the hexagonal structure of the ZnO films. This six-fold in-plane magnetic anisotropy, which is observed for the first time in ZnO-based diluted magnetic semiconductors, is attributed to the substitution of cobalt on Zn sites in the ZnO structure, and a clear indication for long range ferromagnetic ordering between substitutional cobalt ions in the single-crystalline ZnO films.

Defects in CrAs and related compounds as a route to half-metallic ferrimagnetism

Half-metallic ferrimagnetism is crucial for spintronic applications with respect to ferromagnets due to the lower stray fields created by these materials. Studying the effect of defects in CrAs and related transition-metal chalcogenides and pnictides crystallizing in the zinc-blende structure, we reveal that the excess of the transition-metal atoms leads to half-metallic ferrimagnetism. The surplus of these atoms are antiferromagnetically coupled to the transition-metal atoms sitting at the perfect lattice sites. The needed condition to achieve half-metallic ferrimagnetism is to prevent the migration of the