Spin-orbitronics and Racetrack Memory
Stuart Parkin, Max Planck Institute for Microstructure Physics, Halle (Saale), Germany / Martin Luther University Halle-Wittenberg
Over the past few years there have been remarkable discoveries in spin-based phenomena that rely on spin-orbit coupling that could spur the development of advanced magnetic memory devices1-4. In particular these recent discoveries make Racetrack Memory (RTM) extremely attractive as a dense, high performance, low energy consuming, non-volatile memory technology1. Racetrack Memory stores digital data in the form of the presence or absence of chiral domain walls. These domain walls are shifted to and fro, synchronously, along magnetic nano-wires – the racetracks - using current pulses so that the data can be read or written using single reading and writing devices per racetrack. Using vertical (v) nano-wires, incorporated with conventional CMOS circuits, RTM promises densities 100 x greater than is possible with any conventional charge-based memory technology. Recent discoveries in spin-orbitronics make possible the high-speed motion of a series of chiral Néel domain walls by nanosecond long current pulses. Speeds of more than 1 km/s have been demonstrated using a combination of spin-orbit4 and giant exchange torques2. Thus, v-RTM promises very high bandwidth memories and, moreover, allows, in the same technology, a dynamic trade-off between latency and density. On the other hand a three terminal single domain wall horizontal (h) racetrack has the potential as a replacement for SRAM with similar performance but in a much smaller footprint and with non-volatility. v-RTM and h-RTM are a powerful combination of memory technologies for advancing electronics.
The same Dzyaloshinskii-Moriya exchange interactions that stablilize chiral Néel domain walls also enable the formation of topological spin textures such as skyrmions. Recently we have discovered magnetic antiskyrmions in a tetragonal Heusler compound, using Lorentz transmission electron microscopy5. Antiskyrmions are magnetic bubbles that are separated from the surrounding perpendicularly magnetized region by boundaries composed of alternating Néel and Bloch walls. Antiskyrmions have superior properties compared to skyrmions for applications in RTM.
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2 Yang, S.-H., Ryu, K.-S. & Parkin, S. S. P. Domain-wall velocities of up to 750 ms−1 driven by exchange-coupling torque in synthetic antiferromagnets. Nat. Nano. 10, 221-226, (2015).
3 Garg, C., Yang, S.-H., Phung, T., Pushp, A. & Parkin, S. S. P. Dramatic influence of curvature of nanowire on chiral domain wall velocity. Sci. Adv. 3, e1602804, (2017).
4 Ryu, K.-S., Thomas, L., Yang, S.-H. & Parkin, S. S. P. Chiral spin torque at magnetic domain walls. Nat. Nano. 8, 527–533, (2013).
5 Nayak, A. K. et al. Magnetic antiskyrmions above room temperature in tetragonal Heusler materials. Nature 548, 561-566, (2017).