W. Vavra

Interface anisotropy in cobalt‐based epitaxial superlattices

We have measured the magnetic anisotropy in a series of Co‐Au and Co‐Cu superlattices prepared by molecular‐beam epitaxy. Significant epitaxial strains give rise to a magnetoelastic contribution and a large crossover thickness (∼19 A) for perpendicular easy magnetization. The results are discussed in the context of a careful analysis of the interfacial strains and coherence determined by in situ. time‐resolved reflection high‐energy electron diffraction techniques and x‐ray scattering.

Epitaxial strain, metastable structure, and magnetic anisotropy in Co-based superlattices (invited)

We explore the relationship between interface structure and magnetic anisotropy in three types of Co‐based superlattices: Cohcp‐Au; Cofcc‐Cu; and Cohcp‐Cr, grown epitaxially on GaAs(110). For very thin layers of Co, Co‐Au, and Co‐Cu superlattices exhibit a perpendicular easy axis due to magnetoelastic contributions to the anisotropy energy. The magnetic anisotropy in Co‐Cr is found to be strongly dependent on growth conditions. At slow deposition rates of Co the interface between Co and Cr becomes diffuse as is evidenced by a low saturation moment and a shift toward perpendicular anisotropy whereas samples with abrupt interfaces show predominantly parallel anisotropy. The Cr layers grow in a metastable hcp phase which appears to be paramagnetic. The results illustrate the influence of the heterointerface on magnetic properties.

The microstructure and electrical properties of nonalloyed epitaxial Au-Ge ohmic contacts to n-GaAs

The microstructure and electrical properties of nonalloyed epitaxial Au‐Ge contacts were studied. Ohmic behavior was obtained after a 3 h anneal at 320 °C with the lowest average contact resistance and specific contact resistivity found to be ∼0.28 Ω mm and ∼7×10−6 Ω cm2, respectively. Localized reactions in the form of islands were observed across the surface of the contact after annealing and were composed of Au, Ge, and As, as determined by secondary ion mass spectroscopy (SIMS) imaging and Auger depth profiling. Back side SIMS profiles indicate deep Ge and Au diffusion into the GaAs substrate in the island regions. Ohmic contact behavior was found to depend upon both the kinetics of the reactions (localized reactions and island growth) and the thermodynamics (substantial diffusion of both Au and Ge) of the system. A model describing the coupled Au and Ge in‐diffusion with respect to the GaAs substrate is presented.

Real-Time Rheed Studies of Epitaxial Co-Cr Superlattices

The epitaxial growth1,2 of a bcc form of cobalt, on (110) GaAs, has stimulated a great deal of interest in metastable phases of the magnetic transition metal elements. An issue of general importance in this kind of system concerns the accommodation of lattice mismatch, including possible differences in symmetry, at the heterostructure interfaces. The resulting interface structure is known to play a crucial role in the magnetic properties of such materials, especially with regard to anisotropy3, interlayer coupling,4,5 and giant magnetoresistance effects6.

Epitaxy of Co-Metal Superilattices

Magnetic superlattices are of great current interest due to the improvement of crystal growth techniques for their preparation and their potential for practical applications in high density magnetic recording. It is apparent that a strong correlation exists between structural and magnetic properties of these supcrlattices. Careful sample preparation and structural characterization arc necessary in order to tailor their magnetic properties. Molecular Beam Epitaxy (MBE) provides the capability for controlled crystal growth and in-situ strtuctoral, chemical and physical characterization. Ex-situ techniques such as X-ray diffraction and high-resolution TEM, also provide insight into interfaces in these structures. Co layers sandwiched with Au, Cu and Pt layers of various thickness are discussed in this paper.

MBE-Grown Epitaxial Co/Cr Superlattices

A series of epitaxial Co/Cr superlattices has been grown by molecular beam epitaxy. The Cr is in a metastable hcp phase as confirmed by transmission electron microscopy, selected area diffraction, and reflection high energy electron diffraction. The Cr layers are 10A thick in all samples while the Co layers are varied from 12A to 40A. The diffusion between Co and Cr is studied by SQUID magnetometry and indicates step-like interfaces in the best samples. Interfacial sharpness has also been found to be unusually sensitive to Co deposition rates, and in contrast with other superlattice systems, we find that sharper interfaces enhance parallel anisotropy. Hall effect measurements of the saturation field are within 10% of SQUID values. Magnetoresistance at 4.2K is only 1/3% which we believe is a consequence of the high density of states at the Fermi level of hcp Cr.

Structural and Magnetic Properties of Epitaxial Co-Au Superlattices

We describe measurements on the structural and magnetic properties of Co–Au superlattices grown on Ge–buffered (110) GaAs by molecular beam epitaxy. Samples have been prepared with Co layer thicknesses ranging from 5–40A and Au spacer layers of constant thickness, ∼ 16A. X–ray scattering and high-resolution transmission electron microscopy show that the hcp Co layers grow epitaxially with the (0001) axis parallel to (111)Au and with the in–plane Co[11 2 0] axis parallel to GaAs[001]. SQUID magnetometer measurements reveal a crossover in the magnetic anisotropy of the as–grown samples such that the easy axis is perpendicular to the substrate plane when the Co layer thickness is less than ∼19A.