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Domain-wall velocities of up to 750 m s−1 driven by exchange-coupling torque in synthetic antiferromagnets

Abstract

The operation of racetrack memories1,2,3 is based on the motion of domain walls in atomically thin, perpendicularly magnetized nanowires, which are interfaced with adjacent metal layers with high spin–orbit coupling. Such domain walls have a chiral Néel structure4,5,6,7 and can be moved efficiently by electrical currents5,6. High-capacity racetrack memory requires closely packed domain walls, but their density is limited by dipolar coupling from their fringing magnetic fields3. These fields can be eliminated using a synthetic antiferromagnetic structure composed of two magnetic sub-layers, exchange-coupled via an ultrathin antiferromagnetic-coupling spacer layer8. Here, we show that nanosecond-long current pulses can move domain walls in synthetic antiferromagnetic racetracks that have almost zero net magnetization. The domain walls can be moved even more efficiently and at much higher speeds (up to 750 m s−1) compared with similar racetracks in which the sub-layers are coupled ferromagnetically. This is due to a stabilization of the Néel domain wall structure, and an exchange coupling torque that is directly proportional to the strength of the antiferromagnetic exchange coupling between the two sub-layers. Moreover, the dependence of the wall velocity on the magnetic field applied along the nanowire is distinct from that of the single-layer racetrack due to the exchange coupling torque. The high domain wall velocities in racetracks that have no net magnetization allow for densely packed yet highly efficient domain-wall-based spintronics.

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Figure 1: Current-driven DW motion in SAF and SF nanowires: experiment and schematic illustration of torques.
Figure 2: Influence of SF and SAF structure on DW velocity.
Figure 3: Longitudinal field dependence of DW velocity for SF and SAF structures.
Figure 4: Current-driven DW dynamics in nanowires formed on Au underlayers.

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Acknowledgements

The authors acknowledge partial support from the Army Research Office (contract no. W911NF-13-1-0107).

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S.P. conceived and designed the experiments. K.R. carried out the experiment and analysed the experimental data. S.Y. grew films, patterned the devices and developed the 1D model. S.Y. and S.P. wrote the manuscript. All authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to Stuart Parkin.

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The authors declare no competing financial interests.

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Yang, SH., Ryu, KS. & Parkin, S. Domain-wall velocities of up to 750 m s−1 driven by exchange-coupling torque in synthetic antiferromagnets. Nature Nanotech 10, 221–226 (2015). https://doi.org/10.1038/nnano.2014.324

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