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)Ir(24)/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 Co(80)Fe(20) and Fe(70)Ni(30), roughly corresponding to those in the bulk binary alloy systems. The saturation magnetization, M(s), of the FM layer showed a broad peak around Fe(60)Co(40) and a shallow dip at Fe(70)Ni(30), corresponding well with the Slater-Pauling curve of bulk alloys. On the other hand, the J(K) reached a maximum around Co(70)Fe(30) 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.