Soft magnetic characteristics of an ultrathin CoFeNi free layer in spin-valve films

We have investigated the soft magnetic characteristics of an ultrathin Co-rich CoFeNi free layer in spin-valve films. By addition of Ni to a Co-rich CoFe free layer, magnetostriction (λ$\lambda$) of the films increased positively with Ni concentration, in contrast to which a Co₉₀Fe₁₀ free layer showed a negatively large λ$\lambda$. However, Ni addition also caused an increase in coercivity of the easy axis direction (H_c^e.a.). To avoid this problem, a slight decrease in the Co contents of a CoFeNi free layer was found to be effective for decreasing H_c^e.a.. In order to satisfy both the small λ$\lambda$ and H_c^e.a., a free layer of (Co₈₆Fe₁₄)₈₈₋₉₄Ni₁₂₋₆ proved to be an optimum composition in spin-valve films. Moreover, the zero λ$\lambda$ composition of the CoFeNi free layer was changed by a high-conductance Cu layer deposited on the free layer, which was considered to come from the lattice strain of a free layer.