R.
A. Lukaszew, Z. Zhang, Physics and Astronomy Department, The University of
Toledo, Toledo, OH
C.
Cionca and R. Clarke, Department of Physics, University of Michigan, Ann Arbor
Understanding the correlation between film structure and its ferromagnetic properties is very important for applications. For example a combination of epitaxial and polycrystalline layers of the same magnetic material can be used in a spin-dependent tunneling device [1]. For such applications it is important to achieve very smooth interfaces to ensure the integrity of the various intervening layers. Epitaxial and smooth Ni films can be grown on MgO substrates using molecular beam epitaxy (MBE) and also sputtering. We have observed that for (001) Ni films MBE grown, the epitaxial quality of the films, as seen in-situ with RHEED, becomes evident only after 10 nm of film growth. In the case of the MBE grown films, UHV annealing at low temperature (300 C) yields significant reduction of the surface roughness, as observed with in-situ STM, and in addition, appears to induce nano-patterning of the surface via self-assembly of periodic stripes. To establish correlation between surface morphology and structure with magnetic properties, we have used STM/AFM images and high resolution XRD along with studies on the azimuthal dependence of the magnetization reversal using longitudinal Kerr effect (MOKE). Our MOKE studies of MBE grown and in-situ annealed (001) Ni films indicate uniaxial anisotropy superimposed to the expected four-fold magneto-crystalline anisotropy. The uniaxial anisotropy is absent in non-annealed MBE grown films or sputtered annealed films, which don’t exhibit a characteristic surface morphology, suggesting correlation between the observed surface nano-patterning and the observed anisotropy in the magnetic properties. Comparison with (111) films doesn’t indicate surface nano-patterning in this case and this is also evidenced in the azimuthal dependence of the magnetization reversal. High-resolution XRD reciprocal maps obtained at the APS, ANL (February 2003 run) for MBE grown and sputtered (001) Ni films on MgO indicate epitaxial cube on cube growth for both types of films, with no significant structural differences between them. Ex-situ STM/AFM images will also be shown a comparison of the surface morphology of the films. Our findings support the proposed explanation relating the magnetic anisotropy effects observed with MOKE with the surface morphology of the films.
[1]. R. A. Lukaszew, Y. Sheng, C. Uher
and R. Clarke: Use of magnetocrystalline anisotropy in spin-dependent
tunneling. Appl. Phys. Lett. 75, 1941-1943 (1999). United States Patent
6,535,365