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

TL;DR

This study investigates the origin of exchange bias in orthogonal magnetic anisotropic multilayers, revealing unidirectional pinning caused by interface coupling and magnetic field annealing, using in-situ magneto-optical Kerr effect and nuclear resonance scattering.

Contribution

It combines MOKE and GINRS techniques to elucidate the interface-driven origin of exchange bias in [Co/Pt] multilayers with Fe, highlighting the role of unidirectional pinning and magnetic field annealing.

Findings

Exchange bias manifests as hysteresis loop shift and increased coercivity.

Unidirectional pinning is caused by net in-plane magnetic spins at the interface.

Magnetic field annealing modifies the interface remanent state, influencing exchange bias.

Abstract

Magnetization reversal of soft ferromagnetic Fe layer, coupled to [Co/Pt]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 remanant state of the ML is modified by magnetic field annealing, and the effect of the same 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 centre and an unusual increase in the coercivity, similar to exchange bias phenomena, is attributed to the exchange coupling at the [Co/Pt]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 2 nm thick Fe57 marker between [Co/Pt]ML and Fe layers. Interface sensitivity is further enhanced by performing measurements under the x-ray standing wave conditions. The combined MOKE and GINRS analysis revealed the 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 responsible for the origin of EB with an unusual increase in coercivity.