Tuning the coercivity of synthetic antiferromagnetic nanoplatelets with perpendicular anisotropy by varying the Co1−xBx alloy composition

Abstract

Synthetic antiferromagnetic (SAF) nanoplatelets (NPs) with perpendicular magnetic anisotropy based on archetype SAF structures, e.g., Pt/Co/Ru/Co/Pt are of particular interest for torque-related bio-applications due to their large uniaxial magnetic anisotropy. However, when the diameter of the NPs is reduced, the magnetic properties change. For instance, the coercive field increases and starts to vary strongly from NP to NP in a batch due to an increased variation in the reversal probability dominated by local defects. In this study, we investigate how the concentration of boron (B) in Co1−xBx affects the switching properties of NPs, because the addition of B renders the Co layer amorphous and hence introduces more defects. Moreover, it reduces variations in local crystalline anisotropy making the magnetic properties of the NPs more soft. Specifically, we show that the coercivity of the NPs decreases with higher B concentration. This decrease is explained by two mechanisms: (i) the loss of interfacial anisotropy due to less Co–Pt hybridization at the Pt–Co interface and (ii) an increase in the surface’s magnetic domain nucleation site density leading to a narrower distribution of the coercivity of an ensemble of NPs. This understanding will greatly help the field of NP-torque related applications as the spread in applied torque from the NPs to their surroundings can be homogenized.