Magnetic and structural properties of FeCoB thin films

The magnetic and structural properties of FeCoB thin films as a function of boron content have been investigated for applications requiring soft, high moment materials. The films were either co-sputtered from separate FeCo-35 and B targets or from a (Fe65Co35)(90)B-10 alloy target. A peak in the coercivity was observed for small amounts of boron. Measurements of coercivity with fields applied along the radial and circumferential directions of the Si wafer showed isotropic magnetic behavior. The coercivity and saturation induction decreased with increasing amounts of boron above 2 at.%. High angle x-ray diffraction showed a broadening of the (110) peak with increasing boron, indicative of either increased microstrains or smaller average grain size perpendicular to the film. The (110) peak, however, did not shift with added boron, suggesting that on average the boron does not expand the FeCo lattice. The drop in coercivity around 10 at.% B was identified with a transition to a primarily amorphous state. Since these alloys have a high positive magnetostriction, their magnetic properties are particularly sensitive to film stress/strain. Using sputtering gas pressure as a variable, differences in coercivity were also correlated with changes in film strain.