Dereje Seifu

Transmission Mössbauer spectroscopy on pulsed-laser-deposited thick Fe and Ni–Zn ferrite films

Mossbauer spectroscopy with 57Fe is used to characterize pulsed-laser-deposited (PLD) Ni0.6Zny–Fe2O4 films is demonstrated. Mossbauer spectra were recorded as a function of film thickness (3–16 μm) for films deposited in background O2 pressures of 50 and 200 mTorr, and for a zinc concentration of y=0.72. A spectrum was also recorded for y=0.4 deposited in a background pressure of 200 mTorr. The site occupancy was determined for the tetrahedral (A) and octahedral (B) sites. Results obtained here show that the effect of increasing the Zn is to shift Fe(tet) to Fe(oct). When compared with previous Mossbauer measurements on the bulk NiZn ferrites, the ferrite films give spectra of similar quality with no visible evidence of clustering or multiple phases present. The same linewidth was found for a PLD processed Fe film and a Mossbauer standard 70-μm-thick α-Fe enriched foil. The Fe film had a magnetic hyperfine field of 334 kOe and no evidence of the presence of any iron oxides. Good spectra may be obtained by...

Multilayered graphene acquires ferromagnetism in proximity with magnetite particles

Anisotropic diamagnetism of pristine graphite and graphene is well known. Here, evidence of significant induced ferromagnetism in multilayer graphene (MLG) decorated with ferrimagnetic Fe3O4 particles is reported. This MLG-Fe3O4 nano-composite was prepared by a one-step ultrasonic treatment at 75 °C in the surfactant sodium dodecyl-benzene-sulfonate. To verify the phase structure and morphology of the composite, X-ray diffraction, scanning and transmission electron microscopy, scanning tunneling electron microscopy, and Raman spectroscopy were employed. Room temperature data of magnetization versus magnetic field showed that the saturation magnetization MS = 58.6 emu/gm for pristine Fe3O4 increased to MS = 158.4 emu/gm for a 1:1 composite of Fe3O4 to MLG. These results lead to induced MS = 253 emu/gm in MLG resulting from its proximity to Fe3O4. Similar experiments on Fe3O4 to single walled carbon nanotubes (SWNT) composite did not show any induced magnetism in SWNT.

Study of mechanical alloying of Sm and Fe

Mechanical alloying of Sm and Fe with the composition of SmFe3 was studied using x-ray-diffraction (XRD), Mossbauer, and magnetization measurements. Data taken as a function of milling time for up to 20 h show significant changes occurring during ball milling. The XRD studies show that the initial crystalline Bragg reflections changed to a broad maximum, which is attributed to the formation of an amorphous phase. The initial six-line pattern in the Mossbauer spectrum, characteristic of magnetic ordering, changed to a broad singlet, characteristic of a nonmagnetic material. Magnetization measurements revealed that the coercive field was at its maximum after 5 h of milling and decreased sharply as the milling time increased. The remanent magnetization was at its maximum between 5 and 10 h of milling. The final product of the ball milling, which exhibited the characteristics of an amorphous paramagnetic material in its XRD and Mossbauer spectrum, was studied after heat treatment. The XRD and the Mossbauer sp...

Enhanced magnetic properties of carbon nanotubes and multilayer graphene decorated with Co 3 O 4

Here we report evidence of enhanced magnetic properties of carbon nanotubes (CNTs) and multilayer graphene (MLG) decorated with Co₃O₄ nanoparticles (NPs). These 1:1 CNTs/Co₃O₄ and MLG/Co₃O₄ nano-composites were prepared by a one-step ultrasonic treatment in sodium dodecylbenzene sulfonate (SDBS) surfactant. Structure and morphology of the composites were verified using X-ray diffraction, scanning and transmission electron microscopy, and Raman spectroscopy. Magnetization (M) vs. magnetic field (H) measurements at room temperature in H up to 2.5 kOe showed that the measured M= MS at 2.5 kOe in both the CNT/Co₃O₄ and MLG/Co₃O₄ nanocomposites is enhanced by a factor of about 2.5 as compared to MS expected from the simple addition of MS measured for the two components of the nanocomposites. This is surprising since Co₃O₄ is a paramagnet at room temperature. These enhanced magnetic properties in the nanocomposites likely result from contributions from Co³⁺ ions which being in the low-spin S=0 state are non-magnetic in pristine Co₃O₄ but become magnetic in the reduced crystalline symmetry of the interface layers in CNTs/Co₃O₄ and MLG/Co₃O₄.

Nanowires of Fe/multi-walled carbon nanotubes and nanometric thin films of Fe/MgO

We observed that nanowires of Fe grown in the lumens of multi-walled carbon nanotubes required four times higher magnetic field strength to reach saturation compared to planar nanometric thin films of Fe on MgO(100). Nanowires of Fe and nanometric thin films of Fe both exhibited two fold magnetic symmetries. Structural and magnetic properties of 1-dimensional nanowires and 2-dimensional nanometric films were studied by several magnetometery techniques. The θ-2θ x-ray diffraction measurements showed that a (200) peak of Fe appeared on thin film samples deposited at higher substrate temperatures. In these samples prepared at higher temperatures, lower coercive field and highly pronounced two-fold magnetic symmetry were observed. Our results show that maximum magnetocrystalline anisotropy occurred for sample deposited at 100 °C and it decreased at higher deposition temperatures.

High Yield Magnetic Nanoparticles Filled Multiwalled Carbon Nanotubes Using Pulsed Laser Deposition

We present a high yield filling technique of multiwalled carbon nanotubes (MWNTs), grown vertically on a SiO2 substrate, with magnetic nanoparticles using pulsed laser deposition (PLD). Magnetization measurements in-plane and out- of-plane with respect to the sample surface indicate reasonable coercivity estimated at 0.4 T. The magnetic anisotropy is however found to be randomly oriented, indicating a polycrystalline structure. The unique difference between the in-plane and out-of- plane magnetizations is the sharing produced by the demagnetizing field in the perpendicular direction.

Mössbauer Study of Nano-Crystalline Thin Films of Fe—Zr—O

Transmission 57 Fe Mossbauer spectroscopy (TMS) was used to study Fe-Zr-O, high resistive soft magnetic thin films, deposited on polycrystalline alumina substrate (Al 2 O 3 ) and on Zr-Ti-Sn (ZZT) substrates by PVD (magnetron sputtering). The atomic composition (stochiometry) of the as-deposited film using ESCA was found to be Fe 54 Zr 10 O 36 . The sample spectrum of the as-deposited is a doublet with a quadrupole splitting of 0.8 mm/s and isomer shift of 0.4 mm/s. The Mossbauer spectra of films annealed at several temperatures yield a spectrum that is a superposition of magnetic and non-magnetic subspectra. The intensity of the magnetic sub-spectrum characterized by hyperfine split sextet became preponderant with annealing temperature.

Induced ferromagnetism in multilayered graphene in proximity with CoFe2O4

Composites of anisotropic diamagnetic multilayer Graphene (MLG) and ferrimagnetic CoFe2O4 (CFO) nanoparticles (NPs) were synthesized through a one-step sonication process in the presence of a surfactant. The samples were characterized at ambient using x-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy, magnetic force microscopy (MFM) and magnetometry (vibrating sample magnetometer). An induced ferromagnetism was observed in MLG with saturation magnetization MS = 18 emu/g(MLG). This induced ferromagnetism is attributed to ferrimagnetic CFO uniformly distributed on the surface of diamagnetic MLG. SEM images confirm uniform dense distribution of CFO nanoparticles on MLG. MFM images confirm the formation of larger magnetic domains in the composite sample of MLG/CFO. The measured coercivity HC = 3.6 Oe of MLG/CFO composite is much smaller than HC = 32.4 Oe of pristine CFO NPs; this difference in HC is attributed to the effect of change in surface anis...

57Fe Mössbauer Spectroscopy in Coal with Different Caloric Values

The environmental concern associated with mining and burning of coal has long been discussed in the literature. The present study deals with the four different ranks of coal with varying calorific values (CV) aiming to determine if these values are influenced by the sulfur content and/or the forms of iron-sulfur minerals associated with these types of coal. It has been established that Mossbauer spectroscopy is undoubtedly the best analytical tool for discerning iron minerals. The four different types of coal had CVs ranging from 32,403 to 16,000 kJ/kg and sulfur concentrations ranging from 0.25 to 2.5%. There was no direct correlation between the CV and the percentage of pyrites and ferrous sulphate. However, in general, as the CV increases, the percentage of sulfur increases.

Mössbauer spectra of circumstellar silicate dust analogs used as a Fischer-Tropsch catalyst

Synthetic smoke produced by nonequilibrium gas-to-solid condensation in a Fe-SiO-H2-O2 vapor served as a catalyst for Fischer-Tropsch-type catalytic production of hydrocarbons, reactions likely occurring in protostellar nebulae. The freshly-synthesized smoke gave a Mossbauer spectrum showing just a hint of sharp absorption maxima; use as a catalyst brought about deepening and narrowing of the absorption peaks along with a 100-fold increase in mass density, apparently by hydration. Predominant in both raw and post-catalytic spectra is a doublet typical of ferric silicates and oxides. We conclude that the Fischer-Tropsch-type reactions enhanced development of ferric phases similar to those in interplanetary dust particles and chondritic meteorites.