M. Shamsuzzoha

Quantitative correlation of phase structure with the magnetic moment in rf sputtered Fe-N films

Single‐layer Fe‐N films were deposited onto DC biased glass substrates at room temperature by reactive rf magnetron sputtering in an N2‐Ar atmosphere. The Ar pressure was fixed at 6 mTorr and the properties were investigated as a function of N2 flow rate (R). X‐ray diffraction and transmission electron diffraction were used to identify phases in the films. At R=0, the saturation magnetic moment density σS measured in a vibrating‐sample magnetometer (VSM) was 210 emu/g, within experimental error equal to the value for bulk α‐Fe, and only a single phase with the bulk lattice constant of α‐Fe was detected. At R=5 sccm, σS=227 emu/g, and an α‐Fe(N) phase with a ∼1% expanded lattice constant was detected. At R=15 sccm, σS reached a maximum of 247 emu/g and x‐ray and transmission electron diffraction gave clear evidence of the Fe16N2 phase in addition to α‐Fe(N). At R=30 sccm, σS dropped to 196 emu/g, and the diffraction data showed α‐Fe, α‐Fe(N), and a significant fraction of Fe4N. The value of σS calculated f...

Electrodeposition of highly uniform magnetic nanoparticle arrays in ordered alumite

We report the fabrication of nanometer scale ordered arrays of magnetic cylindrical nanoparticles with low aspect ratio (height/radius a=0.2–7) and ultrahigh uniformity. Anodization and electrochemical deposition are employed for template synthesis and metal particle growth, respectively. Particle uniformity is achieved by an electrodeposition scheme, utilizing pulse reverse voltage wave forms to control nucleation and growth of the particles. The resulting nanoparticles are polycrystalline and grains are randomly oriented. The magnetic properties of the array are dominated by particle shape and by interparticle magnetostatic interactions. A very clear transition of the anisotropy from perpendicular to in plane is observed at an aspect ratio a of about two. The arrays exhibit good thermal stability, demonstrating a great potential of these structures as future recording media in a patterned scheme. The pulse reverse electrodeposition technique shows great promise for the synthesis of nanostructures of var...

Formation of FePt nanoparticles by organometallic synthesis

Our interest in determining the mechanism of FePt nanoparticle formation has led to this study of the evolution of particle size and composition during synthesis. FePt nanoparticles were prepared by the simultaneous reduction of platinum acetylacetonate and thermal decomposition of iron pentacarbonyl. During the course of the reaction, samples were removed and the particle structure, size, and composition were determined using x-ray diffraction, transmission electron microscopy (TEM), and scanning electron microscopy–energy dispersive x-ray spectrometry. Early in the reaction the particles were Pt rich (greater than 95at.% Pt) and as the reaction proceeded the Fe content increased to the target of 50%. The particle diameter increased from 3.1to4.6nm during the reaction. Energy dispersive x-ray spectrometry measurements of individual particle compositions using a high resolution TEM showed a broad distribution of particle compositions with a standard deviation greater than 15% of the average composition.

Scanning tunneling microscopy and high-resolution transmission electron microscopy of C16H33-Q3CNQ, hexadecylquinolinnium tricy anoquinodimethanide

Abstract Scanning tunneling microscopy (STM) of a Langmuir-Schaefer (LS) monolayer film of hexadecylquinolinium tricyanoquinodimethanide (C 16 H 33 Q3CNQ, 3 ) on graphite shows atomic resolution: the image is ascribed to the atoms in the dicyanomethylene “swallowtail” end of the molecule. The molecule is probably twisted about 20° from the normal to the graphite surface. High-resolution transmission electron microscopy (HRTEM) of a LS monolayer of 3 on a Cu grid shows a spacing of 18 A between well-defined layers, and a pseudo-hexagonal pattern.

The observation of phase formation in annealed rf-sputtered FeN films

Abstract X-ray diffraction and transmission electron microscopic studies were undertaken to characterize various magnetic phases observed prior to and during in-situ annealing of FeN films deposited on glass substrates by reactive rf-biased sputtering techniques. The concentration of nitrogen in the films was controlled by altering the nitrogen flow rate ( R ) during reactive sputtering. Experiments were carried out on three films prepared with R = 2, 5 and 8 standard cubic centimeters per minute (sccm). The films contain an α-Fe phase saturated with nitrogen atoms and a martensite phase supersaturated with nnitrogen atoms in the as-deposited state. The increase in the value of R in film deposition results in a very small change in the lattice parameter of the α-Fe phase, but allows the martensite phase to progressively increase supersaturation with nitrogen atoms. The sample prepared at R = 2 sccm, upon annealing at temperatures ranging between room temperature (RT) and 500°C, showed no structural changes. Annealing of the samples prepared at R = 5 and 8 sccm also exhibited no structural changes between RT and 400°C, but showed some new phases when heated to 500°C. The sample for R = 5 sccm contained a Fe 16 N 2 phase, while the sample for R = 8 sccm contained Fe 16 N 2 and Fe 4 N phase. During annealing, the grain size for the samples increased from 10–30 nm at RT to 60–90 nm at 500°C. The phases formed at 500°C in all samples were stable and remained unchanged when the specimens were cooled to room temperature.

Improved synthesis and easy-axis alignment of L10-FePt nanoparticles

A synthesis of partially ordered FePt nanoparticles has been developed. It involves the simultaneous reduction of iron acetate (or iron chloride) and platinum acetylacetonate. The high boiling point chemical hexadecylamine or trioctylamine was used as a solvent, and oleic acid or 1-adamantanecarboxylic acid was used as a surfactant. The reflux temperature of the mixture solutions ranged from 330to360°C, where disordered FePt particles can be partially transformed into the ordered L10 phase. Compared with previous results, x-ray-diffraction patterns of as-made samples prepared with the synthesis show a higher degree of chemical ordering. The composition of the FePt nanoparticles with the synthesis can be easily tuned. The room-temperature coercivity of as-made samples ranged from 1to4kOe, depending on the particle composition as well as the refluxing temperature during synthesis. The as-made particles were aligned in a 10kOe magnetic field, giving a parallel to perpendicular remanence ratio of about 1.6.

Scanning tunneling microscopy and transmission electron microscopy of Langmuir-blodgett films of three donor-sigma-acceptor molecules: BDDAP-C-HETCNQ, BDDAP-C-HPTCNQ and BDDAP-C-HBTCNQ

Abstract Langmuir-Blodgett films of three very similar molecules, BBDAP-C-HETCNQ (1), BDDAP-C-HPTCNQ (2), and BDDAP-C-HBTCNQ (3) were studied by STM and TEM. The areas per molecule and the STM resolution decrease in the order 1 to 2 to3: it seems that disorder increases, and that a trivial change in the σ bridge linking the medium electron donor BDDAP to the strong electron acceptor TCNQ in these three molecules causes major changes in the films.

Microstructural evolution of nickel nanoparticle catalysts supported on gadolinium-doped ceria during autothermal reforming of iso-octane

The microstructure and composition of a nanoparticle Ni catalyst supported on gadolinium-doped ceria (Ce1−xGdxO(4−x)/2) were studied using transmission electron microscopy (TEM), x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS). The support of the fresh catalyst exhibits a homogenous aggregation of crystalline grains, with sizes ranging between 20 nm and 50 nm. The crystalline structure of the fresh catalyst support is of the CeO2 phase, in which gadolinium atoms exist in a solid solution of CeO2. Nickel in the fresh catalyst is highly dispersed and forms granular crystals that are 5–30 nm in size on the surface of the ceria support. The support of the used catalyst exhibits a bimodal distribution of grains in which smaller grains have similar structure and morphology as those in the fresh catalyst, while the larger sized grains appear dull and exhibit nonfaceted crystal morphology resulting either from the sintering of a number of CeO2 grains or by the occupation of highly defective crystals of Ce2O3 and CeO phases. A thin amorphous layer of carbon also covers most of the larger grains in the used catalyst. The Ni particles could not be imaged by TEM in the used catalyst, but energy dispersive x-ray spectroscopy (EDX) detected their presence. The XPS analysis of the catalyst samples suggests the participation of lattice O atoms from the ceria support in the catalytic reaction. The XPS data also show the presence of carbonate species and a higher hydrocarbon concentration in the used catalyst.

Magnetic-Semiconductors in Fe-Ti-Oxide Series and Their Potential Applications

Multifunctional nature of the Fe-Ti-oxides makes them attractive candidates for novel applications in which their coupled semiconductor, magnetic, dielectric and optical properties can be exploited. In particular, they appear to be good candidates for the emerging technologies such as spintronics, magneto-electronics, and radhard electronics. In this paper we deal mostly with pseudobrookite (Fe2 TiO5), both pure and Mn-substituted variety. Materials processing, structural, magnetic and semiconducting properties have been discussed in-depth. The temperature and magnetic field dependence on the non-liner I-V characteristics are established opening the possibilities for fabricating magnetically switched devices

Structural Characterization Of Sic Epitaxial Layers Grown On Porous Sic Substrates

A layer of porous SiC was fabricated by surface anodization of commercial 4H and 6H-SiC (0001)Si face off-axis wafers. A 8.5 μm 4H–SiC epilayer was grown on porous SiC (PSC) substrates using atmospheric pressure CVD. TEM investigation on cross-sectional specimens of the CVD epitaxial layers revealed that the presence of pores in the substrate does not lead to the formation of any micropipe in the epitaxial layer. The investigation also failed to detect a more than usual dislocation density on the basal plane of the epitaxial layer. Based upon the results of various analytical techniques applied to the CVD deposit we propose that the density of screw dislocations in the epitaxial layer is less than 5–10 4 cm −3 . It should be noted that the density of similar types of dislocations in the initial substrate as determined by the TEM was ∼10 6 cm −3 , so this preliminary investigation indicates that the epitaxial layer grown on PSC may have a reduction in dislocation density of more than an order of magnitude over those grown on conventional SiC substrates that are not porous. Synchrotron white beam x-ray topography (SWBXT) was performed on these layers. Comparison between the dislocation density on the porous and standard epitaxial layers proved to be very similar using this technique.