Noel T. Nuhfer

Magnetic properties and microstructural observations of oxide coated FeCo nanocrystals before and after compaction

A radio frequency (rf) plasma torch has been used to produce FeCo nanoparticles with a thin protective oxide coating from metal powder precursors. Structural characterization by conventional and synchrotron x-ray diffraction indicated a disordered bcc α-FeCo phase. High resolution transmission electron microscopy revealed spherical particles with several monolayer thick protective oxide coatings. Thermomagnetic measurements were carried out using a superconducting quantum interference device magnetometer and a vibrating sample magnetometer at temperatures between 5 and 1050 K. Antiferromagnetic (exchange bias) coupling was observed due to the presence of the oxide layer. Relatively high coercivities were observed (280 Oe at 5 K and 250 Oe at room temperature). Neel’s surface (interface) anisotropy model was employed to explain the origin of the observed coercivities. As produced powders were hot isostatically pressed at 1023 K and 22 ksi for 2 h. Dense structures were observed and compacted particles reve...

Phase, grain structure, stress, and resistivity of sputter-deposited tungsten films

Sputter-deposited W films with nominal thicknesses between 5 and 180 nm were prepared by varying the base pressure prior to film deposition and by including or not including sputtered SiO2 encapsulation layers. X-ray and electron diffraction studies showed that single phase, polycrystalline α-W could be achieved in as-deposited films as thin as 5 nm. The stress state in the as-deposited films was found to be inhomogeneous. Annealing resulted in stress relaxation and reduction of resistivity for all films, except the thinnest, unencapsulated film, which agglomerated. In-plane film grain sizes measured for a subset of the annealed films with thicknesses between 5 and 180 nm surprisingly showed a near constant value (101–116 nm), independent of film thickness. Thick-film (≥120 nm) resistivity values as low as 8.6 μΩ cm at 301 K were obtained after annealing at 850 °C for 2 h. Film resistivities were found to increase with decreasing film thicknesses below 120 nm, even for films which are fully A2 α-W with no metastable, A15 β-W evident. Sputter-deposited W films with nominal thicknesses between 5 and 180 nm were prepared by varying the base pressure prior to film deposition and by including or not including sputtered SiO2 encapsulation layers. X-ray and electron diffraction studies showed that single phase, polycrystalline α-W could be achieved in as-deposited films as thin as 5 nm. The stress state in the as-deposited films was found to be inhomogeneous. Annealing resulted in stress relaxation and reduction of resistivity for all films, except the thinnest, unencapsulated film, which agglomerated. In-plane film grain sizes measured for a subset of the annealed films with thicknesses between 5 and 180 nm surprisingly showed a near constant value (101–116 nm), independent of film thickness. Thick-film (≥120 nm) resistivity values as low as 8.6 μΩ cm at 301 K were obtained after annealing at 850 °C for 2 h. Film resistivities were found to increase with decreasing film thicknesses below 120 nm, even for films which are fully A2 α-W with no...

Origin of domain structure in hexagonal silicon carbide boules grown by the physical vapor transport method

Transmission electron microscopy (TEM), high-resolution X-ray di!raction, and KOH etching have been used to study the dislocation structure of 4H SiC crystals grown by the physical vapor transport method. Many of the etch pits on the Si(0 0 0 1) surface form arrays extending along the S11 100 T directions. Plan view conventional and high-resolution TEM show that the arrays consist of pure edge dislocations threading along the c-axis with identical Burgers vectors of the a/3S 112 1 0T type. The dislocation arrays constitute low angle [0 0 0 1] tilt boundaries, i.e., [0 0 0 1] is the common axis lying in the boundary. Typical values of the misorientation are in the 60}200 arcsec range. Evidence is presented that such boundaries can form by polygonization of the threading edge dislocations, which have been introduced into SiC crystals by prismatic slip. ( 2000 Published by Elsevier Science B.V.

Visible-Light Photochemical Activity of Heterostructured Core–Shell Materials Composed of Selected Ternary Titanates and Ferrites Coated by TiO2

Heterostructured photocatalysts comprised of microcrystalline (mc-) cores and nanostructured (ns-) shells were prepared by the sol-gel method. The ability of titania-coated ATiO3 (A = Fe, Pb) and AFeO3 (A = Bi, La, Y) catalysts to degrade methylene blue in visible light (λ > 420 nm) was compared. The catalysts with the titanate cores had enhanced photocatalytic activities for methylene blue degradation compared to their components alone, whereas the catalysts with ferrite cores did not. The temperature at which the ns-titania shell is crystallized influences the photocatalytic dye degradation. mc-FeTiO3/ns-TiO2 annealed at 500 °C shows the highest reaction rate. Fe-doped TiO2, which absorbs visible light, did not show enhanced photocatalytic activity for methylene blue degradation. This result indicates that iron contamination is not a decisive factor in the reduced reactivity of the titania coated ferrite catalysts. The higher reactivity of materials with the titanate cores suggests that photogenerated charge carriers are more easily transported across the titanate-titanate interface than the ferrite-titanate interface and this provides guidance for materials selection in composite catalyst design.

Magnetic properties of carbon‐coated rare‐earth carbide nanocrystallites produced by a carbon arc method

Carbon‐coated gadolinum and holmium carbide nanocrystallites have been generated using a modification of the Huffman‐Kratschmer carbon arc process. Bulk amounts of these particles were isolated from the other by‐products using a magnetic field gradient. Transmission electron microscopy revealed the presence of 10–50 nm diameter crystallites coated with numerous graphitic layers. The nanocrystallite phases were identified as Gd2C3 and Ho2C3, respectively, by x‐ray and electron diffraction. Magnetization measurements were performed using a superconducting quantum interference device magnetometer between ±5 T at temperatures ranging from 4 to 200 K. The magnetization curves were shown to scale as a function of H/T. The RE3+ sites in RE2C3 have C3 site symmetry. For Gd2C3 the universal curve was fit with a Brillouin function consistent with the Gd3+ free‐ion ground‐state values of J=7/2 and g=2. The 5I8 Ho3+ free‐ion ground state is split, presumably due to a C3 symmetry crystal field. Consequently, for Ho2C3...

Future Prospects for Defect and Strain Analysis in the SEM via Electron Channeling

Electron diffraction in both SEM and TEM provides a contrast mechanism for imaging defects as well as a means for quantifying elastic strain. Electron backscatter diffraction (EBSD) is the commercially established method for SEM-based diffraction analysis. In EBSD, Kikuchi patterns are acquired by a charge-coupled device (CCD) camera and indexed using commercial software. Phase and crystallographic orientation information can be extracted from these Kikuchi patterns, and researchers have developed cross-correlation methods to measure strain as well.

Postannealing effects on magnetic properties and microstructure of CoCrPt/Ti perpendicular recording media

In this work, we investigate the postannealing effects on the magnetic and structural properties of CoCrPt perpendicular films. We observe a coercivity of 5000 Oe in the films with a 2 nm CrMn overlayer, which is about two times larger than the coercivity of similar films without a CrMn overlayer. This increment is attributed to the decoupling of grains by diffusion of CrMn from the top layer through the grain boundaries. An increase in the negative nucleation field and a decrease in intergranular exchange coupling with annealing temperatures was observed for the films with a CrMn overlayer. On the other hand, the films without a CrMn overlayer show the opposite trends except at high annealing temperature (450 °C). We observe a coercivity of ∼7600 Oe and a negative nucleation field of ∼2400 Oe for a film with a CrMn overlayer annealed at 450 °C for 5 min.

Topology and elemental distribution in Co alloy:oxide perpendicular media

This analytical electron microscopy study was performed to understand better the microstructure of CoCrPt:SiO2 media. The topology of the magnetic Co alloy grains and the distribution of the elements in the thin films were of special interest. High angle annular dark field images revealed that many grains in the range 5–8 nm have a cavity in their center, observed by low mass-thickness contrast. Electron energy loss spectroscopy Co elemental mapping shows Co to be concentrated mainly in the grains with a depleted region in the center. Si, O, and Cr appear to strongly segregate to the grain boundaries and somewhat to the cavities. In this paper we discuss the possible grain growth mechanisms and the effect of the presence of such cavities on recording properties.

Structure and Optical Properties of Some Layered Two-Dimensional Transition-Metal Dichalcogenides: Molybdenum Disulfide, Molybdenum Diselenide, and Tungsten Diselenide

This paper presents a systematic study of intrinsic characteristics of layered two-dimensional transitionmetal dichalcogenides, with a focus on molybdenum disulfide, molybdenum diselenide, and tungsten diselenide. Recent advances in high-resolution transmission electron microscopy (HRTEM) have made atomic-resolution imaging possible, thus enabling one to closely examine real and reciprocal space structures of two-dimensional materials. Our HRTEM micrographs, for example, confirm the expected hexagonal structure of the materials. Furthermore, lattice constant values with resolution on the order of tenths of angstroms could be extracted from the micrographs. Raman and photoluminescence spectroscopies are used to investigate the optical characteristics of the materials. These measurements provide insight into the properties of this class of materials, and their potential use in promising applications in electronic and photonic devices [1-5].

3-dimensional Morphologies of Truncated Ferrite Nanoparticles

In nanoparticles, there is an increased fraction of atoms at the surface that are not fully coordinated and therefore contribute differently to properties than do atoms in the bulk material. The justification for the relative importance of surfaces in nanoparticles must be naturally extended to consider the crystallography of the surfaces present. NiZn ferrite nanoparticles synthesized using an RF plasma torch [1], exhibit cuboctahedral morphologies with exclusively (100) and (111) type surfaces. These surfaces contribute differently [2] to surface magnetic anisotropy, which determines the high frequency magnetic response in these materials. In this paper, we explore the 3-dimensional morphologies of the ferrite nanoparticles using electron tomographic techniques. The 2-dimensional TEM projection images indicated that the small particles (< 20 nm) were octahedral (only (111) surfaces) while the larger particles (20-50 nm) were truncated octahedral (having both (100) and (111) surfaces) in shape. To confirm the 3-dimensional nanoparticle morphologies, tilt experiments were performed on selected areas of the sample using a TECNAI FEG TEM (F20 model) with a single-tilt specimen holder. A series of 2D tilt images (at 2° tilt intervals) over a tilt range of ± 45° (with respect to the primary electron beam) were recorded using the attached CCD camera. Image shifts and focus changes were performed for all tilt angles to always keep the specimen in the field of view in proper focus. The ferrite nanoparticles were quite stable to electron exposure over long time periods, and hence, electron irradiation damage to the sample is not a critical issue while obtaining a series of around 45 images in a specific area. The recorded series of 2D images was manually aligned, stacked and then rotated in the corresponding tilt axis to obtain the 3D perspective of the particle morphology, instead of using conventional 3D reconstruction routines [3]. These image series for the small [Fig. 1 (a)-(i)] and large [Fig. 2 (a)-(i)] particles were then compared to the 3D rotation models of the octahedron and truncated octahedron (rotated in 2° steps) respectively. The different projections of the model octahedron [Fig. 1 (a1), (e1) and (i1)] and the truncated octahedron [Fig. 2 (a1), (d1) and (i1)], along with the corresponding projection axes, are shown for comparison. For example, Fig. 1(a1) is the projection image of the model octahedron representing the particle in Fig. 1(a). The 3D nanoparticle morphological observations are consistent with the nucleation and growth models [4] of the ferrite particles in plasma. These models predict that the critical nucleus shape (corresponding to the small particles) is a perfect octahedron while the growth forms (corresponding to the large particles) are truncated octahedral in shape.