Systematic Study for Magnetization Dependence of Exchange Anisotropy Strength in Mn-Ir/FM (FM ${=}$ Ni-Co, Co-Fe, Fe-Ni) Bilayer System

The unidirectional anisotropy constant, J_K, of the Mn₇₆Ir₂₄/FM (FM =Ni-Co, Co-Fe, Fe-Ni) bilayers was investigated, systematically changing the FM layer composition. On the (111)-oriented face centered cubic (fcc) Mn-Ir layer, the FM layers were grown in the respective crystal structure, corresponding to their composition. The phase boundaries between fcc and body centered cubic (bcc) structures existed around Co₈₀Fe₂₀ and Fe₇₀Ni₃₀, roughly corresponding to those in the bulk binary alloy systems. The saturation magnetization, M_s, of the FM layer showed a broad peak around Fe₆₀Co₄₀ and a shallow dip at Fe₇₀Ni₃₀ , corresponding well with the Slater-Pauling curve of bulk alloys. On the other hand, the J_K reached a maximum around Co₇₀Fe₃₀ and showed different variation from that of the M_s against the FM layer composition. Therefore, the correlation plot between J_K and M_s did not provide any clear relationship. From the experimental results, we found a general feature that bcc structured FM layers are more likely to induce strong exchange anisotropy than fcc structured FM layers.