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) of the films increased positively with Ni concentration, in contrast to which a Co90Fe10 free layer showed a negatively large λ. However, Ni addition also caused an increase in coercivity of the easy axis direction (Hc 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 Hc e.a.. In order to satisfy both the small λ and H ce.a., a free layer of (Co86Fe 14)88-94Ni12-6 proved to be an optimum composition in spin-valve films. Moreover, the zero λ 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.
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