Smoothening of Cu films grown on Si(001)

We report an in situ study of the molecular-beam epitaxy growth and annealing of Cu(001) films grown on hydrogen-terminated Si(001) substrates, resulting in a promising approach to achieve smooth epitaxial morphology. Using correlated reflection high-energy electron diffraction and scanning tunneling microscopy data, we find a temperature interval below the onset of silicide formation where a dramatic smoothening of the epitaxial Cu surfaces occurs. Our measurements indicate that a reduction in roughness is possible in this regime because the annealing is controlled by lateral diffusion kinetics. © 2000 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71331/2/APPLAB-76-6-724-1.pd

Use of magnetocrystalline anisotropy in spin-dependent tunneling

Epitaxial growth techniques are used to impose in-plane magnetocrystalline anisotropy on a spin-polarized tunneling configuration. A Cu(100) buffer layer grown on a Si(100) substrate stabilizes epitaxial face-centered-cubic cobalt as one of the ferromagnetic electrodes. The negative magnetocrystalline constant of this metastable phase favors easy axes along Co 〈110〉 and, due to the single crystal nature of this layer, the coercivity is more than an order of magnitude larger than in the polycrystalline layers which form the second electrode. Our approach provides a way to access the high degree of spin polarization characteristic of the 3d3d transition metals. © 1999 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69989/2/APPLAB-75-13-1941-1.pd

Surface Morphology and Magnetic Anisotropy

Understanding the correlation between film structure and its ferromagnetic properties is very important for applications. Despite significant lattice mismatch epitaxial (001) fcc Ni films can be grown on MgO substrates using sputtering or molecular beam epitaxy (MBE). For both types of films it is observed that the average magnetization switching field is very similar but its azimuthal dependence is not. Structural characterization indicates very similar structure for both types of films where subtle differences are responsible for the striking difference in the anisotropy of the magnetic properties. © 2003 American Institute of PhysicsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87491/2/629_1.pd

Tailored Fano resonance and localized electromagnetic field enhancement in Ag gratings

Metallic gratings can support Fano resonances when illuminated with EM radiation, and their characteristic reflectivity versus incident angle lineshape can be greatly affected by the surrounding dielectric environment and the grating geometry. By using conformal oblique incidence thin film deposition onto an optical grating substrate, it is possible to increase the grating amplitude due to shadowing effects, thereby enabling tailoring of the damping processes and electromagnetic field couplings of the Fano resonances, hence optimizing the associated localized electric field intensity. To investigate these effects we compare the optical reflectivity under resonance excitation in samples prepared by oblique angle deposition (OAD) and under normal deposition (ND) onto the same patterned surfaces. We observe that by applying OAD method, the sample exhibits a deeper and narrower reflectivity dip at resonance than that obtained under ND. This can be explained in terms of a lower damping of Fano resonance on obliquely deposited sample and leads to a stronger localized electric field. This approach opens a fabrication path for applications where tailoring the electromagnetic field induced by Fano resonance can improve the figure of merit of specific device characteristics, e.g. quantum efficiency (QE) in grating-based metallic photocathodes

Optimizing the planar structure of (1 1 1) Au/Co/Au trilayers

Au/Co/Au trilayers are interesting for a range of applications which exploit their unusual optical and electronic transport behaviour in a magnetic field. Here we present a comprehensive structural and morphological study of a series of trilayers with 0–7 nm Co layer thickness fabricated on glass by ultrahigh vacuum vapour deposition. We use a combination of in situ electron diffraction, atomic force microscopy and x-ray scattering to determine the optimum deposition conditions for highly textured, flat and continuous layered structures. The 16 nm Au-on-glass buffer layer, deposited at ambient temperature, is found to develop a smooth (1 1 1) texture on annealing at 350 °C for 10 min. Subsequent growth of the Co layer at 150 °C produces a (1 1 1) textured film with lateral grain size of ∼150 nm in the 7 nm-thick Co layer. A simultaneous in-plane and out-of-plane Co lattice expansion is observed for the thinnest Co layers, converging to bulk values for the thickest films. The roughness of the Co layer is similar to that of the Au buffer layer, indicative of conformal growth. The 6 nm Au capping layer smoothens the trilayer surface, resulting in a surface roughness independent of the Co layer thickness.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/58140/2/d7_9_003.pd

Temperature dependence of the magnetization reversal in Co(fcc)–BN–Co(poly hcp) structures

The magnetic properties of multilayer structures with two magnetic layers of the same metal (Co) but with different crystallographic structures separated by an insulating BN layer have been studied. These structures were prepared on Si (001) substrates by a combination of molecular beam epitaxy (metallic layers) and electron cyclotron resonance-assisted sputtering (BN layer). An fcc Co single-crystal layer (60 Å) was first stabilized by growing it on a copper fcc buffer layer and subsequently a polycrystalline Co layer (70 Å) with hcp structure was grown on top of the insulating BN layer. A CoO antiferromagnetic layer, formed adjacent to this hcp Co layer, significantly influenced the magnetic behavior of the polycrystalline hcp Co layer. The magnetic hysteresis loops for these structures were measured at temperatures ranging from 5 to 350 K with the magnetic field applied along the easy (110) in-plane axis of the fcc Co. A very sharp flipping of the magnetization was found for the fcc Co layer with a nearly temperature-independent coercive field that increased from 14 mT below 100 K to 16 mT at 300 K. In contrast, the magnetization reversal in the hcp Co layer was smoother and its coercivity varied significantly with temperature depending on the strength of the exchange coupling with the adjacent CoO layer. At 5 K the coercivity was greater than 0.2 T and decreased with increasing temperature, becoming essentially zero above room temperature. When cooling in a magnetic field, an exchange offset was observed below 150 K that increased to about 0.1 T at 5 K.© 1999 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70977/2/JAPIAU-85-8-5765-1.pd

Development of Nb and Alternative Material Thin Films Tailored for SRF Applications

Over the years, Nb/Cu technology, despite its shortcomings due to the commonly used magnetron sputtering, has positioned itself as an alternative route for the future of superconducting structures used in accelerators. Recently, significant progress has been made in the development of energetic vacuum deposition techniques, showing promise for the production of thin films tailored for SRF applications. JLab is pursuing energetic condensation deposition via techniques such as Electron Cyclotron Resonance and High Power Impulse Magnetron Sputtering. As part of this project, the influence of the deposition energy on the material and RF properties of the Nb thin film is investigated with the characterization of their surface, structure, superconducting properties and RF response. It has been shown that the film RRR can be tuned from single digits to values greater than 400. This paper presents results on surface impedance measurements correlated with surface and material characterization for Nb films produced on various substrates, monocrystalline and polycrystalline as well as amorphous. A progress report on work on NbTiN and AlN based multilayer structures will also be presented