zahra abooalizadeh

Zahra Abooalizadeh, Philip Egberts
Dynamic atomic force microscopy (AFM) was employed to spatially map the elastic modulus of highly oriented pyrolytic graphite (HOPG), specifically by using force modulation microscopy (FMM) and contact resonance (CR) AFM. In both of these techniques, a variation in the amplitude signal was observed when scanning over an uncovered step edge of HOPG. In comparison, no variation in the amplitude signal was observed when scanning over a covered step on the same surface. These observations qualitatively indicate that there is a variation in the elastic modulus over uncovered steps and no variation over covered ones. The quantitative results of the elastic modulus required the use of FMM, while the CR mode better-highlighted areas of reduced elastic modulus (although it was difficult to convert the data into a quantifiable modulus). In the FMM measurements, single atomic steps of graphene with uncovered step edges showed a decrease in the elastic modulus of approximately 0.5%, which is compared with no change in the elastic modulus for covered steps. The analysis of the experimental data taken under varying normal loads and with several different tips showed that the elastic modulus determination was unaffected by these parameters.

Zahra Abooalizadeh, Brett Leedahl, Kyle LeBlanc, and Alexander Moewes
The implementation and control of room-temperature ferromagnetism (RTFM) by adding magnetic atoms to a semiconductor's lattice has been one of the most important problems in solid-state physics in the last decade. Herein we report on the mechanism that allows RTFM to be tuned by the inclusion of nonmagnetic aluminum in nickel ferrite. This material has already shown much promise for magnetic semiconductor technologies, and we are able to add to its versatility technological viability with our results. The site occupancies and valencies of Fe atoms can be methodically controlled by including aluminum. Using the fact that aluminum strongly prefers a 3+ octahedral environment, we can selectively fill iron sites with aluminum atoms, and hence specifically tune the magnetic contributions for each of the iron sites, and therefore the bulk material as well. Interestingly, the influence of the aluminum is weak on the electronic structure, allowing one to retain the desirable electronic properties while achieving desirable magnetic properties.

Zahra Abooalizadeh, Azar Beyranvand, Jamshid Amighian
Nanopowders of Y1-xBixFeO3 (x=0.0, 0.1, 0.15 and 0.2) have been synthesized by the sol gel method. Xray diffraction identifications show that all the samples have orthorhombic structure and mean crystallite sizes of the nanopowders are in the range of 40 nm, using Scherrer's formula. Mean particle sizes of the samples were obtained by TEM, which is in the range of 75 nm. The 57Fe Mössbauer spectra of Y1-xBixFeO3 nanopowders at 78 and 295 K have been recorded and the results show that all the Fe3+ ions are almost at the symmetrical positions. Room temperature magnetization measurements show that with increasing Bi content up to 0.15, the magnetization increases, whereas it decreases for the sample with x=0.2. M-T curves of the samples were recorded at applied fields of 40 and 13500 Oe. Although all samples show a metamagnetic behavior around Tk=225 °C and at higher applied field, but for the sample with x=0.2, the behavior is more clear.

Zahra Abooalizadeh, Morteza Mozaffari, Jamshid Amighian
NiFe2−xAlxO4 nanopowders, where x is from 0 to 1.5 with a step of 0.5, have been synthesized by the sol-gel method and the effect of non-magnetic aluminum content on their structural and magnetic properties were investigated. The X-ray diffraction (XRD) patterns revealed that the synthesized nanopowders are single phase with a spinel structure. The mean crystallite sizes of the samples were calculated by Scherrer's formula and were in the range 20–31 nm. The morphology of the nanopowders was investigated by TEM and the mean particle sizes of the samples were in the range 55–80 nm. Magnetic hysteresis loops were recorded at room temperature in a maximum applied field of 3000 Oe. The results show that by increasing the aluminum content, the magnetizations of the nanopowders are decreased. This reduction is caused by non-magnetic Al3+ ions, which by their substitutions the superexchange interactions between different sites will be reduced. It is also seen that the magnetizations of the nanopowders are lower than those related to their bulk counterparts. This reduction was found to be as a consequence of the surface spin disorder. M–T curves of the samples were obtained using a Faraday balance and by which the Curie temperatures of the powders were determined. The results that are obtained show that the Curie temperatures of the nanopowders are higher than those of their bulk counterparts.