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 Mn ₇₆ Ir₂₄/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 Co₈₀ Fe₂₀ and Fe₇₀ Ni₃₀, roughly corresponding to those in the bulk binary alloy systems. The saturation magnetization, of the FM layer showed a broad peak around Fe₆₀ Co₄₀ and a shallow dip at Fe Ni, 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.