Structure and morphology of magnetron sputtered CoCr thin films

The growth characteristics of magnetron sputtered Co-22%Cr thin films on amorphous glass or carbon substrates have been investigated utilizing transmission electron microscopy, X-ray diffraction and electrical resistivity measurements. Results indicate that the initial deposit is "amorphous", but that small crystallites form before the film reaches 5 nm film thickness. By 10 nm, well oriented grains with hcp c-axis perpendicular to the film plane develop, and by 50 nm, a columnar microstructure is evident. A distinct subgrain structure was also observed in the thinner films (10-50 nm), and this is reflected in the electrical resistivity. These observations cannot be accounted for by an "evolutionary selection" growth scenario, but must be related to the low surface mobility of adatoms at these low substrate deposition temperatures (<50[deg]C).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28988/1/0000015.pd

Evolution of microstructure and magnetic properties in magnetron-sputtered CoCr thin films

The microstructure and magnetic-structure development in magnetron-sputtered Co-22at%Cr thin films deposited on amorphous substrates has been examined systematically and in detail. It has been found that the initial deposit is amorphous up to a thickness of [approximate]5 nm. By 10 nm thickness, a polycrystalline highly hcp c-axis textured (out of the film plane) microstructure is observed. At 50 nm thickness, a distinct columnar morphology has developed. A distinct subgrain structure was also observed in the thinner films (10-50 nm). The magnetization is initially in-plane and retains in-plane components up to a thickness of [approximate] 50 nm, after which it is predominantly out-of-plane. The effects of in-plane film stress are reflected in the effective anisotropy field and are most evident in the thinner films, where the stress is highest. The thinnest (10 nm) films display in-plane 180[deg] domain walls while thicker (50 nm) films exhibit out-of-plane "dot"-type domain structures. The "dot" domains were observed even in films that had not yet developed a columnar morphology. Intermediate-thickness films show a "feather-like" contrast, indicating that both in- and out-of-plane magnetization components are present. Intrinsic film stress was found to play a major role in determining the preferred magnetization direction and, thus, the resulting magnetic domain configurations.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29703/1/0000035.pd

Interface structure and surface morphology of (Co, Fe, Ni)/Cu/Si(100) thin films

We have examined bilayer Co/Cu, Fe/Cu, and Ni/Cu films deposited by molecular‐beam epitaxy on hydrogen‐terminated [100] silicon substrates. The magnetic metal/copper interface was examined by atomic resolution transmission electron microscopy and compared with the surface morphology as depicted by atomic force microscopy. The general orientation relationships across the magnetic metal/copper interfaces were found to be: [001]Co, Ni∥[001]Cu; (010)Co, Ni∥(010)Cu and [001]Fe∥[001]Cu; (110)Fe∥(200)Cu. The latter system is equivalent to the [11 1]Fe∥[011]Cu and (110)Fe∥(100)Cu Pitsch relationship, as has been reported earlier. Furthermore, there was a general correlation between interfacial and surface roughness, indicating that the initial interface character is propagated throughout the film during growth. © 1996 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71022/2/JAPIAU-80-9-5035-1.pd

Annealing studies of magnetron-sputtered CoCr thin films

The microstructure and magnetic properties of annealed CoCr thin films of various thicknesses, deposited on glass substrates, were examined. In general, little grain growth upon annealing was observed. Low-angle grain boundaries, present in as-sputtered films of thickness [les]50 nm, were absent in annealed films. This occurs simultaneously with a decrease in the measured electrical resistivity. A reduction in the in-plane film stress occurs with annealing, correspondingly the experimentally measured anisotropy field values change markedly. A noticeable feature of the microstructure is the appearance of Cr-rich regions in annealed films 50 nm thick and above. These regions are thought to account for the observed increases in the saturation magnetization and coercivity with annealing time.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30817/1/0000477.pd

Structural imaging of a thick-walled carbon microtubule

We analyze the structure of a thick-walled carbon microtubule based on direct electron beam imaging of the graphitic cylinders comprising the fiber. Clearly resolved six-fold symmetry of basal planes overlying the fiber core indicate zero overall fiber helicity and alignment of individual cylinders. Sidewall measurements calibrated from the {100} core fringes show uniform spacings of about 0.375 nm, which are larger than those reported for other microtubules or for crystalline graphite (0.335 nm). Short zones of local 3o helicity are observed along the fiber. Structural transitions which alter the helicity are characterized by extra atomic planes and other defects, including nested sub-tubules. We discuss implications for the fiber's growth and electrical properties.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31407/1/0000324.pd

Surface structure and composition of high-surface-area molybdenum nitrides

The determination of surface structure is critical in elucidating structure-function relationships for catalytic materials. However, such materials often consist of active regions of very limited spatial extent, rendering conventional bulk structural characterization techniques of limited utility. In this work, we have employed high-resolution transmission electron microscopy coupled with Fourier analysis, and X-ray photoelectron spectroscopy to determine the near-surface structures and compositions of a series of molybdenum nitride catalysts. The results show the near-surface to differ both in crystal structure and composition from the bulk. The bulk structure was [gamma]-Mo2N (fcc) while the lattice structure near the surface was body-centered. Many of the materials contained nitrogen in excess of that expected for phases in the Mo-N phase diagram. As the amount of nitrogen decreased, the oxygen content increased, and the calculated lattice parameter increased. Taken together, the results suggested the presence of Mo2N3-xOx, a hypothetical primitive cubic structure, near the surface. This structure, which would produce a diffraction pattern that approximates that of a body-centered lattice, accounts for the near-surface structural and compositional properties of the high-surface-area molybdenum nitrides.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31593/1/0000522.pd

Electric Polarization of Heteropolar Nanotubes as a Geometric Phase

The three-fold symmetry of planar boron nitride, the III-V analog to graphene, prohibits an electric polarization in its ground state, but this symmetry is broken when the sheet is wrapped to form a BN nanotube. We show that this leads to an electric polarization along the nanotube axis which is controlled by the quantum mechanical boundary conditions on its electronic states around the tube circumference. Thus the macroscopic dipole moment has an {\it intrinsically nonlocal quantum} mechanical origin from the wrapped dimension. We formulate this novel phenomenon using the Berry's phase approach and discuss its experimental consequences.Comment: 4 pages with 3 eps figures, updated with correction to Eqn (9

Local Defect in Metallic Quantum Critical Systems

We present a theory of a single point, line or plane defect coupling to the square of the order parameter in a metallic system near a quantum critical point at or above its upper critical dimension. At criticality, a spin droplet is nucleated around the defect with droplet core size determined by the strength of the defect potential. Outside the core a universal slowly decaying tail of the droplet is found, leading to many dissipative channels coupling to the droplet and to a complete suppression of quantum tunneling. We propose an NMR experiment to measure the impurity-induced changes in the local spin susceptibility.Comment: 2 figures; 5 page

Antiferromagnetic Domains and Superconductivity in UPt3

We explore the response of an unconventional superconductor to spatially inhomogeneous antiferromagnetism (SIAFM). Symmetry allows the superconducting order parameter in the E-representation models for UPt3 to couple directly to the AFM order parameter. The Ginzburg-Landau equations for coupled superconductivity and SIAFM are solved numerically for two possible SIAFM configurations: (I) abutting antiferromagnetic domains of uniform size, and (II) quenched random disorder of `nanodomains' in a uniform AFM background. We discuss the contributions to the free energy, specific heat, and order parameter for these models. Neither model provides a satisfactory account of experiment, but results from the two models differ significantly. Our results demonstrate that the response of an E_{2u} superconductor to SIAFM is strongly dependent on the spatial dependence of AFM order; no conclusion can be drawn regarding the compatibility of E_{2u} superconductivity with UPt3 that is independent of assumptions on the spatial dependence of AFMComment: 12 pages, 13 figures, to appear in Phys. Rev.

Synthesis and characterization of molybdenum nitride hydrodenitrogenation catalysts

Details concerning the relationships between the structural, chemical and catalytic properties of Mo nitrides have been elucidated. A series of Mo nitride catalysts were prepared by the temperature programmed reaction of MoO3 with NH3. The structural properties of these nitrides were complex functions of the heating rates and space velocities employed. Two reaction sequences were proposed to account for the synthesis of high, medium and low surface area materials. An interesting conclusion was that the degree of reduction of the molybdate precursor or intermediate governed the structural properties of the product. Some evidence is also presented to suggest that the nucleation and growth rates involved in the transformation of the oxide to the nitride were significantly influenced by the synthesis conditions. The Mo nitrides proved to be exceptional pyridine hydrodenitrogenation catalysts. Their catalytic properties were superior to those of a commercial sulfided Co-Mo hydrotreatment catalyst, having higher activities and better C-N bond hydrogenolysis selectivities. Hydrodenitrogenation over the Mo nitrides appeared to be structure-sensitive. While detailed relationships between the catalytic activity and surface stoichiometry could not be ascertained, there did appear to be a correlation between the activity, and the particle size and grain boundary length. We proposed that at least two types of HDN sites existed on the Mo nitride surfaces; modest activity sites on the particles and high activity sites at grain boundaries. The N/Mo stoichiometry of the highest activity catalyst was near unity suggesting that MoN was present perhaps localized at the grain boundaries. Finally structures near or at the surface were markedly different from those of the bulk. While the predominant bulk phase was [gamma]-Mo2N, the surface appeared to consist of either non-stoichiometric [beta]-Mo16N7 or mixtures of Mo and [beta]-Mo16N7.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29977/1/0000340.pd