The unidirectional anisotropy constant, JK, of γ-Mn-Ir / FM (FM = Fe-Co-Ni) bilayers was investigated by systematically changing the composition of the Fe-Co-Ni layers. The FM layers were grown in the crystal structures corresponding to their composition on the (111)-oriented face centered cubic (fcc) Mn-Ir layer. JK was significantly changed around the phase boundary of the FM layer between the body-centered cubic (bcc) phase and fcc-phase. The experimental results revealed a general feature where bcc-structured FM layers were more likely to induce stronger exchange anisotropy than fcc-structured FM layers. The JK values and the lattice mismatch between the Mn-Ir layer and the FM layer were increased with increasing Co concentration in the bcc-FM region. The dependence of JK on FM composition have been explained qualitatively with lattice distortion at the interface between the Mn-Ir and FM layers.
The dependence of perpendicular exchange anisotropy on the thickness of the ferromagnetic (FM) layer in Mn-Ir / [Co/Pt]n multilayers was investigated. The dependencies on temperature of saturation magnetization, Ms, in [Co / Pt]n multilayers having Co and Pt layers of various thicknesses were examined. The rate of decrease of Ms against temperature was large for thin-Co (< 1 nm) or thick-Pt layers (> 1 nm), which indicates that thermal fluctuations in magnetization were significant. The unidirectional anisotropy constant, JK, was reduced with the decreasing thickness of the interfacial Co layer in contact with the Mn-Ir layer. This suggested that perpendicular exchange bias was degraded by thermal fluctuations in FM magnetization during thermal annealing. The dependencies of JK on the thicknesses of the Pt layer and inner Co layer could be also described by the thermal fluctuations in the magnetization of the FM layer.
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