Fabrication and characterization of IrMn/PtMn/PtCr/PtMn antiferromagnets in terms of the role of each layer

Herein, we investigated the exchange couplings in ferromagnet/antiferromagnet film systems of structures CoFe/Pt50Cr50, CoFe/interface-Pt50Mn50/Pt50Cr50, CoFe/interface-Pt50Mn50/Pt50Cr50/backed-Pt50Mn50, and CoFe/Ir20Mn80/interface-Pt50Mn50/Pt50Cr50/backed-Pt50Mn50. These investigations revealed that the backed(b)-PtMn(2.5–3.5 nm), the interface(i)-PtMn(2 nm), and the IrMn(0.8 nm)/i-PtMn(1.2 nm) significantly increased the unidirectional anisotropy constant (Jk) up to 0.67 erg/cm2. The insertion of an ultra-thin IrMn(0.8–1.2 nm) layer at the CoFe interface improved the CoFe hysteresis, i.e. the ratio of exchange bias field (Hex) to coercivity (Hc) was increased. The effects associated with thin b-PtMn, i-PtMn, and IrMn/i-PtMn layers were presumably due to a mismatch between lattice constant and a/c ratio of the PtCr. In other words, the larger lattice constants and lattice deformation during L10 transformation of PtMn may promote a dynamic-stress-induced PtCr ordering-assisting effect, though the PtCr thickness was reduced to 13 nm. The real cause of the effect of ultra-thin IrMn layers has not been properly explained to date. The blocking temperature (Tb) for the CoFe/i-PtMn(2 nm)/PtCr(25 nm)/b-PtMn(3 nm) film was 500 °C. Although the additional insertion of IrMn(0.8 nm) reduced Tb to 410 °C, the Tb and the normalized Hex did not depend on the PtCr thickness, which varied from 13 to 25 nm. Remarkably, an insertion of IrMn(0.8 nm) had no influence on the normalized Hex at temperatures below 300 °C.