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VIDEO DOI: https://doi.org/10.48448/7hrc-fv90

technical paper

MMM 2022

November 07, 2022

Minneapolis, United States

Phase Coexistence and Transitions between Anti and Ferromagnetic States in a Synthetic Antiferromagnet

Skyrmions—topologically protected vortex-like spin structures—have been proposed as the new information carriers in racetrack memory devices 1. In order to realise such devices a small size, high speed of propagation, and minimal deflection angle are required. Modelling has shown that synthetic antiferromagnets (SAFs) present the ideal materials system to realise these aims 2. However, their magnetic compensation makes observation of skyrmions difficult and indeed this was only recently achieved 3. There is thus significantly more to understand about the behaviour of synthetic antiferromagnets and the skyrmions therein. Here we present a comprehensive magnetic force microscopy (MFM) and XMCD-PEEM study of a SAF multilayer composed of 20 magnetic layers alternating between CoB and CoFeB each coupled antiferromagnetically with a Ru spacer layer to the one above and below. PEEM results show that in the SAF phase the sample is spontaneously single-domain, and when exposed to a complicated field protocol, large defect-pinned domains can be stabilized. As shown in the hysteresis loop presented in Fig. 1 the SAF undergoes a phase transition between the compensated antiferromagnetic state and its field-polarised ferromagnetic state as a field is applied. MFM shows that in our samples this is as a result of defect-driven bubble nucleation where FM ordered regions nucleate and then expand to cover the entire film. Once the FM regions exceed a critical size they collapse into a skyrmion/stripe domain pattern and so retain a net zero magnetisation. At a narrow range of fields, e.g. as shown in Fig. 2 we observe a phase coexistence between the compensated AF state and the net-zero magnetisation FM state. As the magnetic field is increased we go on to observe isolated skyrmions at fields below saturation as expected in a FM system. These results give perspective on the ferromagnetic nature of skyrmions observed in systems at fields just below saturation and can help to inform the observation of true antiferromagnetic skyrmions.


Transcript English (automatic)

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Swarnali Hait and 1 other author

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