Ali R. Koymen

Apparatus for positron annihilation‐induced Auger electron spectroscopy

The apparatus used in the first direct measurement of energy spectra of positron annihilation‐induced Auger electrons is described. The apparatus consists of a magnetically guided low‐energy positron beam, a UHV surface preparation and characterization chamber, and an energy spectrometer. The spectrometer includes a trochoidal monochromator and a microchannel plate detector. A permanent magnet is mounted behind the sample to produce a field gradient which redirects the outgoing Auger electrons along the spectrometer axis. The combination of trochoidal monochromator and permanent magnet permits the measurement of the total kinetic energy of Auger electrons with an effective angular acceptance of ∼2π. The large angular acceptance and single particle detection capability of this spectrometer make it possible to perform Auger measurements using extremely low incident beam currents (∼10−15 A), and may make it useful in other low signal experiments. The spectrometer response function is modeled and compared to ...

Crystalline magnetic carbon nanoparticle assisted photothermal delivery into cells using CW near-infrared laser beam

Efficient and targeted delivery of impermeable exogenous material such as small molecules, proteins, and plasmids into cells in culture as well as in vivo is of great importance for drug, vaccine and gene delivery for different therapeutic strategies. Though advent of optoporation by ultrafast laser microbeam has allowed spatial targeting in cells, the requirement of high peak power to create holes on the cell membrane is not practical and also challenging in vivo. Here, we report development and use of uniquely non-reactive crystalline magnetic carbon nanoparticles (CMCNPs) for photothermal delivery (PTD) of impermeable dyes and plasmids encoding light-sensitive proteins into cells using low power continuous wave near-infrared (NIR) laser beam. Further, we utilized the magnetic nature of these CMCNPs to localize them in desired region by external magnetic field, thus minimizing the required number of nanoparticles. We discovered that irradiation of the CMCNPs near the desired cell(s) with NIR laser beam leads to temperature rise that not only stretch the cell-membrane to ease delivery, it also creates fluid flow to allow mobilization of exogenous substances to the delivery. Due to significant absorption properties of the CMCNPs in the NIR therapeutic window, PTD under in vivo condition is highly possible.

Elimination of the secondary electron background in Auger electron spectroscopy using low energy positron excitation

Data is presented which demonstrate that positron annihilation‐induced auger electron spectroscopy (PAES) can be used to eliminate secondary electrons in the energy range of Auger electrons. In PAES the core–hole excitations necessary for Auger electron spectroscopy (AES) are created by annihilation of core electrons using low energy (<25 eV) positrons. Secondary electrons cannot be created through collisional processes with energies in excess of an energy Ek≊Ep (the primary beam energy). Spectra obtained for a Cu (110) surface demonstrate that Auger signals can be obtained with peak‐to‐background ratios in excess of 50:1 (more than two orders of magnitude improvement over conventional electron excited Auger methods). PAES spectra obtained from Cs‐covered Cu surfaces are used to place upper limits on the relative contribution of gamma‐ray induced secondary electrons to the PAES background. Implications of the high signal‐to‐background ratios obtained using PAES for increasing the elemental sensitivity and reducing beam damage of AES are discussed.Data is presented which demonstrate that positron annihilation‐induced auger electron spectroscopy (PAES) can be used to eliminate secondary electrons in the energy range of Auger electrons. In PAES the core–hole excitations necessary for Auger electron spectroscopy (AES) are created by annihilation of core electrons using low energy (<25 eV) positrons. Secondary electrons cannot be created through collisional processes with energies in excess of an energy Ek≊Ep (the primary beam energy). Spectra obtained for a Cu (110) surface demonstrate that Auger signals can be obtained with peak‐to‐background ratios in excess of 50:1 (more than two orders of magnitude improvement over conventional electron excited Auger methods). PAES spectra obtained from Cs‐covered Cu surfaces are used to place upper limits on the relative contribution of gamma‐ray induced secondary electrons to the PAES background. Implications of the high signal‐to‐background ratios obtained using PAES for increasing the elemental sensitivity and...

Tunable thermal hysteresis in CoGd alloys

We show that tunable magnetic thermal hysteresis can be realized for a simple ferrimagnetic CoGd alloy. The magnetization of a Co0.8Gd0.2 alloy shows a bow-tie curve as a function of temperature, which indicates supercooled and superheated phase transitions. The width of the magnetic thermal hysteresis can be tuned by an external magnetic field, with values of 190 K at fields near 100 Oe and values of 20 K near 400 Oe. Experimental results are in reasonable agreement with theoretical calculations.

Anisotropic magnetoresistive model for saturated sensor elements

Presented is a model that predicts the resistive behavior of an anisotropic magnetoresistive (AMR) sensor element in magnetic saturation. Both the experimental data and the model concur with a high degree of accuracy. The model builds on the work of other investigators and it is shown to track the behavior of actual magnetoresistive elements. This paper shows that, with a minor modification to previous models, the resistor should and can be divided into isotropic and magnetically affected components that can give some new insights into the AMR effect. With this model, one can extract the parameters that have magnetic effects from the ones that are independent of the magnetic effects.

Discovery of cross-tie walls at saw-tooth magnetic domain boundaries in permalloy films

Scanning electron microscopy with polarization analysis was used to image the surface magnetic domain structure of permalloy films (350 A Ni83Fe17) in ultrahigh vacuum. These thin-film materials are used in a variety of magnetoresistive sensing applications, which depend on the state of the domains present in the film. In the demagnetized state of the films, saw-tooth domain boundaries separating two large approximately head-on domains were observed. At high magnification, cross-tie walls were seen with a periodic vortex structure along the straight edges of the saw-tooth domain boundaries.

Magnetic-field-assisted photothermal therapy of cancer cells using Fe-doped carbon nanoparticles

Photothermal therapy with assistance of nanoparticles offers a solution for the destruction of cancer cells without significant collateral damage to otherwise healthy cells. However, minimizing the required number of injected nanoparticles is a major challenge. Here, we introduce the use of magnetic carbon nanoparticles (MCNPs), localizing them in a desired region by applying an external magnetic-field, and irradiating the targeted cancer cells with a near-infrared laser beam. The MCNPs were prepared in benzene, using an electric plasma discharge, generated in the cavitation field of an ultrasonic horn. The CNPs were made ferromagnetic by use of Fe-electrodes to dope the CNPs, as confirmed by magnetometry. Transmission electron microscopy measurements showed the size distribution of these MCNPs to be in the range of 5 to 10 nm. For photothermal irradiation, a tunable continuous wave Ti: Sapphire laser beam was weakly focused on to the cell monolayer under an inverted fluorescence microscope. The response of different cell types to photothermal irradiation was investigated. Cell death in the presence of both MCNPs and laser beam was confirmed by morphological changes and propidium iodide fluorescence inclusion assay. The results of our study suggest that MCNP based photothermal therapy is a promising approach to remotely guide photothermal therapy.

Study of the structure of the Rh/Ag surface using positron annihilation induced Auger electron spectroscopy

Positron annihilation induced Auger electron spectroscopy (PAES), electron induced Auger electron spectroscopy (EAES), and low‐energy electron diffraction have been used to study the temperature dependent composition of vapor‐deposited of Rh on Ag(100). Earlier work using AES, ion scattering spectroscopy, and TDS has shown that a Ag layer diffuses to the Rh surface upon annealing to form a structure in which Rh is sandwiched between a Ag cap layer and the Ag substrate. In this work, the top layer selectivity of PAES was utilized to study the diffusion of Ag to the surface as Rh films deposited at 173 K were heated to 573 K. Analysis of the PAES spectra indicates that Rh remains in the top layer during the ∼3 h required to take PAES data when the sample is maintained at the 173 K temperature of deposition. There is a clear indication that significant migration of Ag to the surface takes place by 373 K. The Ag content of the top layer increases to approximately 100% above 473 K.

Enhanced piezoresistive characteristics of Nb2O5 modified La0.8Sr0.2MnO3 ceramics

This letter reports the giant piezoresistance phenomenon in Nb2O5 modified La0.8Sr0.2MnO3 (LSMO) polycrystalline ceramics. LSMO modified with 2mol% Nb exhibited ∼2% change in resistivity at 19.2MPa pressure as compared to 0.5% for the pure LSMO. A dramatic improvement was obtained for 5mol% Nb-modified LSMO composition, which exhibited a linear change in resistivity with uniaxial stress and the fractional change was of the order of 3% at 19.2MPa stress. The origin of the giant piezoresistance was found to be related to Nb substitution on the Mn sites which distorts the Mn–O bond, lowering the magnitude of stress required for the manganite lattice transformation.

Oscillatory temperature dependence of exchange bias for Fe/Gd ferrimagnets

We investigated the exchange bias effect for Fe/Gd ferrimagnetic multilayers. Our results show that there is a −cosine type of oscillatory exchange bias with temperature for the [(Fe 4 nm/Gd 4 nm)4/Gd 16 nm] multilayer. For the first time, exchange bias was observed to oscillate between positive and negative fields with a period of approximately 130 K. In addition, the coercive field was observed to oscillate within itself for the same temperature range studied. We speculate that oscillatory exchange bias is related to the underlying magnetic phase diagram of the ferrimagnetic Fe/Gd multilayer. When the [(Fe 4 nm/Gd 4 nm)4/Gd 16 nm] system is cooled under a high magnetic field, negative exchange bias is forced to shift to positive values irregularly. Nevertheless the system shows oscillatory behaviour.