Influence of underlayers on the soft properties of high magnetization FeCo films

A remarkable reduction in coercivity H-c was found in sputtered Fe65Co35(=FeCo) films on Cu, NiFe, Ru, Ta/Cu, Ta/NiFe, or Cu/IrMn underlayers. A decrease in H-c from 120 to 7-12 Oe was observed for Cu, NiFe, and Ru underlayers as thin as 2.5 nm but less for Ta. A Cu underlayer significantly reduced the maximum anisotropy fields from 2 kOe to 40 Oe, resulting in a well-defined in-plane average uniaxial anisotropy field H(k)similar to30 Oe. The saturation magnetostriction with Cu was (47+/-4)x10(-6), independent of Cu and FeCo thicknesses. In-plane tensile film stress decreased with underlayer thickness t(UL) from 2 to 0.2 GPa but much less rapidly than the reduction in H-c. All underlayers induced a (110) texture in FeCo, which was strongest with Ta. Transmission electron microscopy of cross-sections showed an unusually long range coherence with low angle grain boundaries in the FeCo without an underlayer. Clear columnar grains were visible with all underlayers with an average grain size of similar to50 nm with Ta dropping to 9-10 nm for Cu, NiFe, and Ru. This alone is sufficient to explain quantitatively the reduction in H-c using Hoffmann's ripple theory. (C) 2003 American Institute of Physics.