Origin of exchange bias in[Co/Pt]ML/Femultilayers with orthogonal magnetic anisotropies

Magnetization reversal of a soft ferromagnetic Fe layer coupled to a ${[\mathrm{Co}/\mathrm{Pt}]}_{\mathrm{ML}}$ multilayer (ML) with perpendicular magnetic anisotropy (PMA) has been studied in situ with an aim to understand the origin of exchange bias (EB) in orthogonal magnetic anisotropic systems. The interface remanent state of the ML is modified by magnetic field annealing, and its effect on the soft Fe layer is monitored using the in situ magneto-optical Kerr effect (MOKE). A considerable shift in the Fe layer hysteresis loop from the center and an unusual increase in the coercivity, similar to EB phenomena, is attributed to the exchange coupling at the ${[\mathrm{Co}/\mathrm{Pt}]}_{\mathrm{ML}}$ and Fe interface. The effect of the coupling on spin orientation at the interface is further explored precisely by performing an isotope selective grazing incident nuclear resonance scattering (GINRS) technique. Here, the interface selectivity is achieved by introducing a 15 \AA{}-thick ${\mathrm{Fe}}^{57}$ marker between the ${[\mathrm{Co}/\mathrm{Pt}]}_{\mathrm{ML}}$ and Fe layers. The interface signal from ${\mathrm{Fe}}^{57}$ is further enhanced by performing measurements under x-ray standing-wave conditions. The combined MOKE and GINRS analysis revealed a unidirectional pinning of the Fe layer due to the net in-plane magnetic spin at the interface caused by magnetic field annealing. Unidirectional exchange coupling or pinning at the interface, which may be due to the formation of asymmetrical closure domains, is found to be responsible for the origin of EB with an unusual increase in coercivity.