Ezana Negusse

Investigation of structural and electronic properties of graphene oxide

The local atomic structure of graphene oxide has been probed using synchrotron radiations. Detailed investigations of recently proposed simplistic model of graphene oxide using x-ray absorption near edge spectroscopy have been performed. X-ray diffraction measurements and calculations indicate loss of coherence between graphene-like layers. However, larger in-plane structural coherence is understood to be present. Selected area electron diffraction measurements indicate the presence of graphitic regions in graphene oxide which is expected to produce interesting confinement effects in graphene oxide which could be important for the development of tunable electronic and photonic devices.

Single crystal EPR determination of the spin Hamiltonian parameters for Fe8 molecular clusters

Abstract We use a multi-high-frequency resonant cavity technique to obtain EPR spectra for single crystal samples of the biaxial molecular magnet Fe 8 [(tacn) 6 O 2 (OH) 12 ]Br 8 ·9H 2 O (Fe 8 ). By performing measurements at many closely spaced frequencies, we are able to extrapolate data back to zero magnetic field and, thereby, obtain accurate estimates of the zero-field splittings. Furthermore, from the (low-) field dependence of these splittings, with the magnetic field parallel to the easy axis, we can directly measure the g z -value. Measurements performed with the magnetic field parallel to the intermediate and hard axes may be used to constrain further the Hamiltonian parameters. Our results are in broad agreement with recent inelastic neutron scattering data. In addition, analysis of individual resonances (which we can assign to known transitions) reveals a pronounced M S dependence of the resonance line widths. Furthermore, the line positions exhibit complex (again M S dependent) temperature dependences that cannot be reconciled with the standard spin Hamiltonian.

Fe diffusion, oxidation, and reduction at the CoFeB/MgO interface studied by soft x-ray absorption spectroscopy and magnetic circular dichroism

We have studied the effect of annealing on the interface magnetization in a CoFeB/MgO structure which models the lower electrode in a magnetic tunnel junction device. We find that MgO deposition causes Fe to diffuse toward the CoFeB/MgO interface, where it preferentially bonds with oxygen to form a Fe-O-rich interfacial region with reduced magnetization. After annealing at 375 °C the compositional inhomogeneity remains; Fe is reduced back to a ferromagnetic metallic state and the full interfacial magnetization is regained.

Effect of electrode and EuO thickness on EuO-electrode interface in tunneling spin filter

The effect of electrode material and EuO film thickness on the interface between the two was studied. Of the electrodes examined, yttrium was found to decrease the formation of the nonmagnetic oxide Eu2O3. By decomposing the x-ray absorption spectroscopy (XAS) spectra of the samples with different electrodes against the reference EuO and Eu2O3 spectra, the relative fraction of these two species was quantified. Multilayers with silver electrodes had the highest amount of Eu2O3, about 41%, whereas aluminum had 28% compared to the less than 5% observed for the multilayers with yttrium electrodes. A slight decrease in the fraction of Eu2O3 with increasing EuO thickness was found. Angle dependent XAS measurements, done on 80A EuO film, indicated the presence of Eu2O3 at high grazing angles which then decreased drastically with decreasing grazing angle. This indicated that the Eu2O3 was localized at the EuO-electrode interface.

Magnetic characterization of CoFeB∕MgO and CoFe∕MgO interfaces

The use of CoFeB ferromagnetic electrodes in place of CoFe has been shown to significantly increase the tunneling magnetoresistance (TMR) of MgO based magnetic tunnel junctions (MTJs). By using soft x-ray scattering techniques, we show that the behavior of the magnetic moments located at the CoFe–MgO interface are drastically different from the rest of the CoFe film, whereas the magnetic response of the CoFeB–MgO interfacial moments is coherent with the film’s bulk. Our results support the view that the high TMR values observed in MgO based MTJs with CoFeB electrodes are due to the uniform magnetic response of the entire CoFeB electrode including the MgO interfacial moments.

Magnetostructural influences of thin Mg insert layers in crystalline CoFe(B)/MgO/CoFe(B) magnetic tunnel junctions

It is common to find a thin (∼0.5 nm) layer of Mg deposited prior to the MgO tunnel barrier in crystalline CoFe(B)/MgO/CoFe(B) magnetic tunnel junctions, due to the improved device performance that results. However, despite their common usage, the reasons why such layers are effective are unclear. We use structures that model the lower electrode of such devices to show that a suitably thick Mg insert layer enhances the crystal quality of both MgO and CoFe(B), permits interfacial oxides to reduce back to a metallic ferromagnetic state, and hence improves magnetic switching of the CoFe(B) electrode, properties which are inextricably linked to device performance.

Reversed remanent magnetic configuration in epitaxial La1−xSrxMnO3 films

The polar discontinuity which can occur at oxide interfaces can modify the properties of oxide thin films. In such a system, an unusual reversed orientation of the remanent magnetic state was observed recently for La0.7Sr0.3MnO3 on Nb-doped SrTiO3(0 0 1) deposited via pulsed laser deposition (Lee et al 2010 Phys. Rev. Lett. 105 257204). Here, we report on a similar effect for La0.7Sr0.3MnO3 grown via molecular beam epitaxy, a deposition method with different growth kinetics, onto both Nb-doped and undoped SrTiO3 (STO) substrates. The reversed magnetic state occurred in both samples, while enriched Mn3+ regions (intermediate layer) are slightly different. Intriguingly, the onset of the reversed remanent state occurred at different temperatures: ~125 K for the Nb : STO substrate and ~240 K for the undoped STO substrate. Our results point to an additional mechanism for controlling the magnetism in mixed-valence oxide films.

Exchange biased magnetoresistance based spin-transport sensor

A method for measuring processes that alter the degree of exchange bias (HEX) for a thin film by monitoring HEX indirectly through the magnetoresistance (MR) is described. In unbiased magnetic films and multilayers, the positive and negative magnetic field sweep MR spectra are symmetric about zero applied field. Introducing an exchange bias to the film shifts the intersection point of these two curves away from zero to HEX. Taking the difference in the measured MR at zero field for a positive and negative field pulse (measuring ΔMR at zero field) gives a relative measure of HEX. Any variations in the exchange bias field will result in a corresponding change in the ΔMR, which, unlike HEX, can be measured with two points rather quickly, making it ideal for sensor applications. A method for selecting material parameters for increased sensitivity or operational range is given.

Extraction of roughness parameters from specular x-ray resonant scattering

We expand on a different approach for extracting a roughness parameter (perpendicular component) from standard specular θ−2θ x-ray resonant scattering scans. Modulated in the scattering patterns are the chemical and magnetic characteristics of the thin film and interface. Continuous media-based modeling of these spectra gives sharp Bragg features unlike the broadened features of the measured spectra due to interfacial roughness. Here we examine the effect of roughness due to the incoherent contribution to the scattered light from islands with varying thicknesses. The summed spectrum is computed from the calculated spectra at different thicknesses, which are used as a basis set, added incoherently according to a thickness distribution. The mean and rms thickness values of the distribution control the periodicity and depth of the interference patterns in the spectra, respectively.

Magnetoresistance monitoring of processes that affect thin film exchange bias

A simple method where the relative change in exchange bias, introduced by external process, is measured by taking only two magnetoresistance readings at a selected field value is presented. Modeling of this measurement process showed that optimal performance can be achieved by engineering the device parameters so that the maximum in the magnetoresistance occurs near the exchange bias field and the width of the magnetoresistance (MR) curve is twice the field value of this maximum MR. Our analysis further showed that the sensitivity can be enhanced by operating the sensor at an operation field close to the exchange bias field.