Influence of underlayers on the soft magnetic properties of Fe-Co-Al-O films

The soft magnetic properties of Fe-Co-Al-O films deposited by sputtering (Fe70Co30)99(Al2O3)1 on a variety of underlayers have been investigated. A remarkable reduction in coercivity, Hc, was found in Fe-Co-Al-O films sputtered on underlayer materials, such as Au, Ag or Cu, that have a surface-free energy in the region of 1-2 J/m2. All underlayers induced a BCC-(1 1 0) preferred grain orientation in the Fe-Co-Al-O films, although this effect was weak for Au, Ag and Cu. When the surface free energy of the underlayer material is higher than 2 J/m2, as in the case of Hf, Fe-Co-Al-O films can be grown with a well-defined BCC-(1 1 0) crystallographic texture. The primary effect of varying the underlayer in Fe-Co-Al-O bilayer structures is to change the crystallographic texture of the Fe-Co-Al-O films due to the difference in surface-free energy. Transmission electron microscopy performed on cross-sections revealed the presence of clear columnar grains for all underlayers, with an average size of approximately 50 nm for Hf, dropping to 5-7 nm for Au, Ag and Cu. This allows us to explain qualitatively the reduction in Hc using Hoffman's ripple theory.