R. Topkaya

Size effects and origin of easy-axis in nickel nanowire arrays

High quality compact Ni nanowire (NW) arrays with aspect ratios (wire length/diameter) varying between 70–171 for a wire length of ∼6 μm, and between 3–400 for a constant wire diameter of 60 nm were successfully grown by direct current electrodeposition into free standing porous alumina templates having a lattice constant, i.e., interpore distance, of 105 nm. The NWs have been investigated using a combination of scanning- and transmission-electron microscopies, selected-area electron diffraction, x-ray diffraction analysis, ferromagnetic resonance (FMR), and vibration sample magnetometer techniques at room temperature. Microscopic and diffraction results show that the wires are uniform and mostly single-crystalline, being 220-oriented along the growth direction. Magnetic properties of samples are heavily dependent on the wire length as well as the diameter or packing factor, P (the volume fraction of wires/template). The FMR spectra and the field orientation dependence of the resonance field values were f...

Ferromagnetic resonance studies of exchange coupled ultrathin Py/Cr/Py trilayers

Magnetic properties of ultrathin Py/Cr/Py trilayers have been investigated as a function of Cr spacer layer thickness by using ferromagnetic resonance (FMR) and vibrating sample magnetometer (VSM) techniques. The Cr spacer layer thickness was increased from 4 to 40 A with 1 A steps to determine the dependence of interlayer exchange coupling between ferromagnetic layers on the spacer layer thickness. Two strong and well resolved peaks were observed which correspond to a strong (acoustic) and weak (optic) modes of magnetization precession in the effective dc field due to the exciting external microwave field as the external dc field orientation comes close to the film normal. The separation of the two modes in the field axis depends on the thickness of Cr spacer layer. An interchange in the relative positions of the acoustic and optic modes has been observed for a particular thickness of Cr spacer layer as well. A computer program for magnetically exchange coupled N magnetic layers was written to simulate t...

Sonochemical synthesis and chracterization of Mn3O4 nanoparticles

AbstractWe report on the synthesis of Mn3O4 nanoparticles (NPs) using a novel sonochemical method without requiring any pH adjustment. Synthesized material was identified as tetragonal hausmannite crystal structure model of Mn3O4 from XRD analysis. Crystallite size was estimated from x-ray line profile fitting to be 17±5 nm. FTIR analysis revealed stretching vibrations of metal ions in tetrahedral and octahedral coordination confirming the crystal structure. TEM analysis revealed a dominantly cubic morphology of NPs with an average size of ∼20 nm. Magnetic evaluation revealed a blocking temperature, TB of 40 K above which the material behaves paramagnetic. Asymmetric coercive field is attributed to the interaction between ferromagnetic Mn3O4 and antiferromagnetic Mn oxide at the surface of nanoparticles.

Ferromagnetic resonance investigation of Py/Cr multilayer system

The multilayer thin films consisting of alternating stacks of ferromagnetic Permalloy (Py) and ultra thin non-magnetic (sub-nanometer) spacer (Cr) layers have been investigated by using ferromagnetic resonance (FMR) and dc magnetization measurement techniques. The non-magnetic spacer layer thickness changes from 0.5 A to 2.5 A by 0.5 A steps. Polycrystalline composite metallic films were grown on Si substrate by usual magnetron sputtering techniques at UHV conditions. The magnetic hysteresis curves were recorded by conventional dc magnetization. The ac and dc magnetic properties were investigated by using ferromagnetic resonance. FMR measurements were carried out for different directions of external dc magnetic field in order to search magnetic anisotropy. Two strong and well defined peaks (acoustic and optic mode) were observed in FMR spectrum as the magnetic field direction approaches to the film normal. Their positions and relative intensities helped to characterize coupling species. The magnetic parameters have been deduced by using a theoretical model. It has been found that the exchange coupling parameter between ferromagnetic layers through non-magnetic Cr spacer is ferromagnetic in nature and strongly decreases with increasing spacer layer thickness. The magnetic anisotropy parameters strictly depend on magnetic layer thickness while the dc magnetization is almost constant for a few nanometer thick Permalloy as well. We observed only ferromagnetic coupling between ferromagnetic layers in all samples.The multilayer thin films consisting of alternating stacks of ferromagnetic Permalloy (Py) and ultra thin non-magnetic (sub-nanometer) spacer (Cr) layers have been investigated by using ferromagnetic resonance (FMR) and dc magnetization measurement techniques. The non-magnetic spacer layer thickness changes from 0.5 A to 2.5 A by 0.5 A steps. Polycrystalline composite metallic films were grown on Si substrate by usual magnetron sputtering techniques at UHV conditions. The magnetic hysteresis curves were recorded by conventional dc magnetization. The ac and dc magnetic properties were investigated by using ferromagnetic resonance. FMR measurements were carried out for different directions of external dc magnetic field in order to search magnetic anisotropy. Two strong and well defined peaks (acoustic and optic mode) were observed in FMR spectrum as the magnetic field direction approaches to the film normal. Their positions and relative intensities helped to characterize coupling species. The magnetic param...

Electron paramagnetic resonance characterization of gamma irradiation damage centers in powder of L-(+)-tartaric acid, N-acetyl-L-alanine and 1-methyl-L-histidine

The electron paramagnetic resonance (EPR) of γ-irradiated powders of l-(+)-tartaric acid, N-acetyl-l-alanine and 1-methyl-l-histidine were investigated at room temperature. Radiation damage centers are attributed to HOOCCH(OH)Ċ(OH)COOH, CH3ĊHCOOH and ‒CH2ĊHCOOH radicals. The spectra were computer-simulated. Hyperfine structure constants and g-values were determined for these three radicals. The unirradiated samples showed no EPR signal. Some spectroscopic properties and suggestions concerning the possible structure of the radicals are discussed in this paper.

Development of Halbach magnet for portable NMR device

Nuclear magnetic resonance (NMR) has enormous potential for various applications in industry as the on-line or at-line test/control device of process environments. Advantage of NMR is its non-destructive nature, because it does not require the measurement probe to have a contact with the tested media. Despite of the recent progress in this direction, application of NMR in industry is still very limited. This is related to the technical and analytical complications of NMR as a method, and high cost of NMR analyzers available at the market. However in many applications, NMR is a very useful technique to test various products and to monitor quantitatively industrial processes. Fortunately usually there is no need in a high-field superconducting magnets to obtain the high-resolution spectra with the detailed information on chemical shifts and coupling-constant. NMR analyzers are designed to obtain the relaxation parameters by measuring the NMR spectra in the time domain rather than in frequency domain. Therefore it is possible to use small magnetic field (and low frequency of 2-60 MHz) in NMR systems, based on permanent magnet technology, which are specially designed for specific at-line and on-line process applications. In this work we present the permanent magnet system developed to use in the portative NMR devices. We discuss the experimental parameters of the designed Halbach magnet system and compare them with results of theoretical modelling.

Polymer Assisted Co-precipitation Synthesis and Characterization of Polyethylene Glycol (PEG)/CoFe2O4 Nanocomposite

Polyetylene glycol (PEG)/CoFe2O4 nanocomposite have been synthesized by PEG assisted co-precipitation method. The presence of PEG on the surface of CoFe2O4 nanoparticles was confirmed by Fourier transform infrared spectroscopy. Vibrating sample magnetometer measurements revealed a saturation magnetisation (Ms) value of 90.95 emu/g, which is higher than bulk value, and coercive field (Hc) of 862 Oe that is close to the bulk value at room temperature. The temperature dependent magnetization increases initially and then decreases with increasing temperature. This anomality is attributed to the decrease of B sublattice magnetization more rapidly than A sublattice magnetization. The Mr/Ms values lower than theoretical value of 0.5 suggests that the PEG/CoFe2O4 nanocomposite has effective uniaxial anisotropy according to the Stoner-Wohlfarth model. Enhanced magnetic properties (higher Ms and bulk-like Hc value) make the PEG/CoFe2O4 nanocomposite a promising candidate for high density magnetic recording media.

Magnetic Characterization of Exchange Coupled Ultrathin Magnetic Multilayers by Ferromagnetic Resonance Technique

The magnetic–nonmagnetic multilayers have been widely used in various applications. As well known the important physical parameters depend on relevant applications. For giant magneto resistance (GMR), magnetic data storage, MRAM and spintronics applications, the most important magnetic parameters in multilayered structures are interlayer exchange coupling, magnetic anisotropy, saturation magnetization and spin relaxation time. All of these parameters strictly depend on the physical size of the elements which are continuously shrinking even down to nanometer scale for ultra high density data processes. However, as the dimensions (thickness) of the films continues to decrease the magnetic signal intensity gets so weak that its detection becomes one of the major issues. But still ferromagnetic resonance (FMR) can be powerful enough to study these multilayered structures.

Magnetisation dynamics of as quenched and pulse annealed soft metallic glasses

Abstract Pulse annealing method (PAM) is a practical and effective method to improve the magnetic characteristics of metallic glasses. Pulse annealing method overcomes oxidation problems during the annealing process. It minimises the difficulties in managing the crystallisation steps and causes low coercivity in contrast to high susceptibility in milliseconds. In this work, Fe77Cr2Si5B16 and Fe39Ni39Mo4Si6B12 metallic glasses were annealed by PAM. The ferromagnetic resonance spectra were measured, and the angular variation of resonance fields at the out of plane geometries was studied. The effective magnetisation and g factor fitting parameters were obtained as 3450 and 3750 Oe for Fe77Cr2Si5B16 and Fe39Ni39Mo4Si6B12 respectively from the angular variation of the resonance field. The M–H loops at room temperature were also plotted, and the changes of the magnetic properties were observed after pulse annealing. Amorphous to nanostructure steps can be seen from this method without any doubt.